Karl L. Skorecki Joseph Winaver Zaid A. Abassi
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Control of Extracellular Fluid Volume, 398 |
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Afferent Limb: Sensors for Fluid Homeostasis, 398 |
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Efferent Limb: Effectors for Fluid Homeostasis, 402 |
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Pathophysiology of Edema Formation, 419 |
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Local Mechanisms in Interstitial Fluid Accumulation, 420 |
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Renal Sodium Retention and Edema Formation in Congestive Heart Failure, 420 |
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Renal Sodium Retention and Edema Formation in Cirrhosis with Ascites, 437 |
CONTROL OF EXTRACELLULAR FLUID VOLUME
The volume of extracellular fluid (ECF) is maintained within narrow limits in normal human subjects, despite day-to-day variations in dietary intake of salt and water over a wide range. Plasma volume, in turn determined by the total ECF volume and the partitioning of this volume between extravascular and intravascular compartments according to the dictates of the Starling relationship, also remains remarkably constant despite alterations in dietary salt intake (Fig. 12-1 ). The relationship of ECF volume and, in particular, the volume of the plasma compartment to overall vascular capacitance determines such fundamental indices of cardiovascular performance as mean arterial blood pressure and left ventricular filling volume. Given the rigorous defense of ECF sodium (Na+) concentration, mediated mainly by osmoregulatory mechanisms concerned with external water balance (see Chapter 13 ), the quantity of Na+ determines the volume of this compartment. Surfeits or deficits of total body water alter serum Na+ concentration and osmolality but contribute little to determining the volume of the ECF. As vascular capacitance and Na+intake change in response to a given physiologic or pathologic stimulus, the renal excretion of Na+ adjusts to restore ECF volume to a level appropriate to the renewed setting of vascular capacitance.
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FIGURE 12-1 Overall scheme for body Na+ balance and partitioning of extracellular fluid volume (ECFV). In the setting of normal osmoregulation, extracellular Na+ content is the primary determinant of ECFV. Overall Na+ homeostasis depends on the balance between losses (extrarenal and renal) and intake. Renal Na+ excretion is determined by the balance between filtered load and tubule reabsorption. This latter balance is modulated under the influence of effector mechanisms, which, in turn, are responsive to sensing mechanisms that monitor the relation between ECFV and capacitance. |
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The overall relationship among Na+ intake, ECF volume, and Na+ excretion can be considered in pharmacokinetic terms. Such consideration has led to a shifting steady-state model for overall Na+ homeostasis,[1] as opposed to the constant “set-point” model (see also Reinhardt and Seeliger[2] and references therein). According to the shifting steady-state model, in any given steady state, total daily Na+ intake and excretion are equal. Acute deviations from a preexisting steady state, consequent to an alteration in Na+ intake or extrarenal excretion, results in an adjustment in renal Na+ excretion. This adjustment in renal Na+ excretion occurs as a result of a new total body Na+ content and ECF volume, and it aims to restore the preexisting steady state. This differs from the “set-point” model, in which the control system aims to reach a constant total body Na+ (“set-point”).[2] The establishment of a new steady-state level of Na+ intake and excretion in the shifting steady-state model reflects a new total body Na+ content and ECF volume. Alternatively, an alteration in the capacitance of the extracellular compartment can also result in an adjustment in renal Na+ excretion, whose aim is to restore the preexisting relationship of volume to capacitance. It is clear that the operation of such a system for Na+ homeostasis requires (1) sensors that detect changes in ECF volume relative to vascular and interstitial capacitance and (2) effector mechanisms that ultimately modify the rate of Na+ excretion by the kidney to meet the demands of volume homeostasis. Adjustments in effector mechanisms occur in response to perceived alterations in sensor input, with the aim of optimizing circulatory performance. Derangements in either sensor or effector mechanisms can lead to disordered Na+ balance and disruption of circulatory integrity. Thus, inability of the kidney to precisely adjust the rate of Na+ excretion to a given Na+ intake may result in the development of positive or negative total body Na+ balance. These perturbations, when present over extended periods, may be clinically manifested as hypertension or edema formation in the case of positive Na+ balance or hypotension and hypovolemia in the case of negative Na+ balance.
The purpose of this chapter is to summarize current understanding of the various sensor and effector mechanisms thought to be involved in the normal regulation of ECF volume and the disturbances in the mechanisms that occur in edema-forming states, namely, congestive heart failure (CHF) and cirrhosis with ascites.
Afferent Limb: Sensors for Fluid Homeostasis
Fluid homeostasis is essential to the maintenance of circulatory stability, so it is hardly surprising that volume detectors reside at several sites within the vascular bed ( Table 12-1 ). For this discussion, it is useful to consider the afferent sensing sites as comprising cardiopulmonary and arterial baroreceptors as well as renal, central nervous system (CNS), and hepatic sensors. Each compartment can be viewed as reflecting a unique characteristic of overall circulatory function, such as cardiac filling, cardiac output, renal perfusion, and fluid transudation into the interstitial space. Sensors within each compartment monitor a physical parameter (e.g., stretch, tension) that serves as an index of circulatory function within that compartment. The mechanisms by which these sensors operate are not fully elucidated, though our understanding has progressed significantly in recent years. In the past, it was assumed that mechanosensing is performed by afferent sensory nerve endings found primarily in blood vessel walls. However, it is now known that the endothelium may also participate in the process of mechanosensing.[3] The mechanisms involved include stretch-activated ion channels, protein kinases associated with the cytoskeleton, integrin-cytoskeletal interactions, cytoskeletal-nuclear interactions, and generation of reactive oxygen species.[3] It is also known that mechanical stretch and tension impinging on blood vessel cells can result in altered gene expression, mediated through specific recognition motifs within the upstream promoter elements of responsive genes.[4] In turn, signals emanating from afferent sites engage efferent mechanisms that effect compensatory changes in renal Na+ excretion. Volume expansion results in an integrated sequence of neural reflexes and hormonal responses that enhances the renal excretion of salt and water. Conversely, the reflex response to volume contraction is renal conservation of salt and water.
TABLE 12-1 -- Mechanisms for Sensing Regional Changes in Body Fludi Volume
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Central nervous system sensors |
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Hepatic volume sensors |
Cardiopulmonary Volume Sensors
Atrial Sensors
The cardiac atria possess the distensibility and the compliance necessary to monitor changes in intrathoracic venous volume. Henry and colleagues[5] demonstrated that left atrial distention induces diuresis, as a part of a “volume reflex.” Goetz and associates[6] provided a clear demonstration of the effectiveness of changes in atrial transmural pressure in controlling Na+ and water excretion in conscious dogs. Since that time, diuresis and natriuresis as a consequence of increasing atrial wall tension and the role of the atria in overall volume homeostasis have been clearly established. In humans, the role of the atria in volume homeostasis can best be illustrated by studies utilizing maneuvers that alter atrial volume and size, such as head-out water immersion (HWI) and exposure to head-down tilt, which causes a redistribution of blood and fluid from the peripheral to the central circulation and nonhypotensive lower body negative pressure (LBNP) that unloads the cardiopulmonary baroreceptors. [7] [8] [9] HWI results in increases in central blood volume, central venous pressure, and right atrial and pulmonary arterial transmural pressure gradients. After immersion, brisk natriuresis and diuresis ensue, with an increase in fractional excretion of Na+ comparable with that resulting from saline loading.[7] In contrast, the application of nonhypotensive LBNP results in a redistribution of blood to the lower limbs, thereby reducing central venous pressure and cardiac filling pressures without causing detectable changes in arterial pressure or heart rate. This maneuver has been shown by echocardiography to reduce atrial diameter. In normal human subjects, LBNP has been shown to result in antidiuresis and antinatriuresis without a significant change in renal plasma flow (RPF).[8] These findings point clearly to an atrial sensing mechanism for central venous volume that influences renal Na+ and water excretion. Higher degrees of LBNP, at the hypotensive range, have been used as a model to study the cardiovascular adjustments during progression of acute hemorrhagic shock in humans.[10] Nevertheless, the use of these maneuvers to selectively load or unload the cardiopulmonary volume receptors must be faced with caution. In particular, HWI has been shown to cause, in addition to central hypervolemia, a significant degree of hemodilution.[11] The external hydrostatic pressure of the water reduces the hydrostatic pressure gradient across the capillary wall in the legs, resulting in a net transfer of fluids from the interstitial compartment to the intravascular compartment. The central blood volume expansion is therefore associated with hemodilution and a significant decrease in the colloid osmotic pressure (COP). The importance of hemodilution and the resultant decrease in COP in mediating the natriuresis of volume expansion was underscored by Cowley and Skelton.[12] They suggested that the decrease in COP, rather than stimulation of the cardiopulmonary volume receptors, was the predominant cause of natriuresis during saline infusions in dogs. Supporting this notion are the findings of Johansen and co-workers,[13] who demonstrated that preventing hemodilution by placing an inflated (80 mm Hg), tight cuff during HWI abolished the natriuresis. These findings suggest that, during HWI, the combined effects of hemodilution and central blood volume expansion, with their associated neuronal and endocrine changes, play a pivotal role in the initiation of natriuresis.
Neural Pathways.
Neural receptors responsive to mechanical stretch or transmural pressure have been described in the atria. These are thought to be branching ends of small medullated fibers running in the vagus nerve.[14] Two populations have been described. Type A receptors, concentrated at the entrance of the great veins into the atria, discharge once per cardiac cycle, beginning with atrial systole. The activity of these receptors is not affected by atrial volume. On the other hand, the activity of type B receptors, which discharge with atrial filling, correlates well with atrial size.[14] Stretch and tension signals detected at these sites are believed to travel along cranial nerves IX and X to the hypothalamic and medullary centers, in which a series of responses are initiated: inhibition of release of antidiuretic hormone (ADH), mostly left atrium[15]; a selective decrease in renal but not lumbar sympathetic nerve discharge [16] [17]; and decreased tone in precapillary and postcapillary resistance vessels in the peripheral vascular bed, the latter influencing the magnitude of transudation of interstitial fluid. Reduction in central venous pressure and atrial size by LBNP exerts a stimulatory effect on renal nerve activity in humans, as assessed by renal norepinephrine (NE) spillover and plasma NE concentration. [9] [18] Chronic atrial stretch results in adaptation and downward resetting of these neural responses. Thus, it was demonstrated in rhesus monkeys exposed to 10 degrees of head-down tilt that such an adaptation was responsible for a “shift to the left” in the relationship of urinary Na+ excretion versus central venous pressure during saline infusion.[19] This suggests that the kidney responds with natriuresis at a significantly lower cardiac filling pressure under these conditions. Of note, cardiac denervation studies in canine models have shown that cardiac nerves are not essential for stimulating plasma renin activity and Na+ retention after an acute deficit, but they are of importance in the restoration of steady-state Na+ balance after repletion.[20] Similarly, disruption of long-term suppression of the renin-angiotensin-aldosterone system (RAAS) in response to chronic volume expansion occurs after cardiac transplantation in humans.[21]
Humoral Pathways.
Early experiments showed that interruption of neural pathways during atrial distention did not completely abolish the natriuresis and diuresis associated with this maneuver, indicating that additional factors were operative. These studies suggested a direct humoral mechanism that emanates from the heart and responds to fullness of the circulation. These findings, and the subsequent discovery by de Bold and colleagues[22] in 1981 of a factor in atrial extracts with strong natriuretic and vasodepressor activity, led to the eventual isolation and characterization of natriuretic peptides (NPs) of cardiac origin.[23] The first and best characterized of these is atrial natriuretic peptide (ANP). This 28–amino acid peptide belongs to the NP family, which comprises at least two additional structurally related peptides B- and C-type NPs (BNP and CNP, respectively) encoded by different genes. [24] [25] [26] The NPs are discussed in later sections of this chapter, as are effector mechanism for natriuresis induced by these peptides.
Numerous studies in animal models and in human subjects confirmed that a directly induced increment in atrial pressure or stretch results in a sharp release of ANP. It has been estimated that, for each rise of 1 mm Hg in atrial pressure, there is an associated rise of approximately 10 to 15 pmol/L in plasma ANP concentration.[27] This release occurs by a process of cleavage of mature circulating 28–amino acid COOH-terminus peptide, from prohormone located in preformed stores within atrial granules. Stretch-activated ANP release from atrial myocytes is thought to occur in two steps: a Ca2+-sensitive and K+ channel-dependent release of ANP from myocytes into the surrounding intercellular space, followed by a Ca2+-independent translocation of the released ANP into the atrial lumen.[28] K+-adenosine triphosphate (ATP) channel blockers such as glibenclamide can block stretch-activated ANP release.[29]Maneuvers that activate the afferent mechanism for release of ANP include intravascular volume expansion by supine posture, HWI, saline administration, exercise, angiotensin II (AII) administration, tachycardia, and ventricular dysfunction. [25] [27] In contrast, a decline in plasma ANP concentration follows volume-depleting maneuvers such as Na+ restriction, furosemide administration, and the reduction in central venous pressure associated with application of LBNP. Whereas the understanding of effects of acute alterations in atrial pressure/volume on ANP release is well established, the role of this peptide in the long-term regulation of volume homeostasis remains controversial. [30] [31] In particular, a role for the NPs in the long-term of chronic response to changes in dietary salt intake could not be demonstrated. In a study in humans with incremental levels of dietary Na+ intake, it was demonstrated that plasma ANP reflected the steady-state Na+ balance, so that the higher the salt intake, the greater the initial plasma ANP level.[32] However, the main finding in this study was the contrasting ANP response to acute oral compared with intravenous Na+ loading: Plasma ANP increased significantly after intravenous saline infusion but not after the oral Na+ loading.[32] In other studies in humans exposed to intravenous volume expansion and oral Na+ loading, no direct correlation could be found between the change in plasma ANP level and the degree of natriuresis. [30] [33] [34] The application of gene-targeting technology in mice provided novel insights regarding the diverse biologic functions of the NP family and their receptors, guanylate cyclases A and B (GC-A and GC-B, respectively). John and co-workers[35] demonstrated that ANP-gene knockout mice displayed a reduced natriuretic response to acute ECF volume expansion compared with the wild-type mice. However, when the mice were maintained on a high-NaCl (8.0%) or low-NaCl (0.008%) diet for 1 week, their cumulative Na+ and water excretions were comparable with those of wild-type mice. The main perturbation observed in mice with ANP-gene disruption was a significant increase in mean arterial pressure (MAP).[35] Additional studies demonstrated that disruption of the gene for ANP or its receptor, GC-A, demonstrated that this system is essential for maintenance of normal blood pressure but, in addition, exerts local antihypertrophic effects on the heart. Disruption of the genes encoding for the other members of the NP family, BNP and CNP or the GC-B, demonstrated that these peptides are probably not involved in the physiologic regulation of renal Na+ excretion, but instead exert local paracrine/autocrine cyclic guanosine monophosphate (cGMP)–mediated effects on cellular proliferation and differentiation in various tissues (for recent reviews, see Kuhn [26] [36]). It appears, therefore, that regulation of ECF volume and blood pressure is only one facet of the diverse biologic actions of the NP family.
Ventricular and Pulmonary Sensors
Ventricular receptors have usually been regarded solely in the context of reflex changes in heart rate and peripheral vascular resistance (PVR). However, several studies in the past suggested that nerve terminals in ventricles and in the pulmonary vasculature may be involved in sensing changes in blood volume. Increased left ventricular pressure in conscious dogs was found to cause a reflex inhibition of plasma renin activity,[37] and a coronary baroreceptor reflex, linking increased coronary artery pressure to decreased lumbar and renal sympathetic discharge, has also been detected.[38] In the lung, unmyelinated juxtapulmonary capillary (J) receptors have been found (adjacent to pulmonary capillaries) in the interstitium of the lungs.[14] The position of these receptors makes them ideally suited to detect interstitial edema before fluid enters the alveolar space. These afferent nerves join those from the atria in cranial nerves IX and X. Nevertheless, the role played by these ventricular and pulmonary receptors to overall regulation of ECF volume and Na+ remains to be determined.
Arterial Sensors
The low-pressure receptors described previously assess the fullness of the capacitance system of the vascular tree and may be geared to defend against excessive ECF expansion with the attendant deleterious consequences of pulmonary and systemic venous congestion. However, a primary role of the cardiovascular system is to optimize tissue perfusion. Therefore, it seems logical that sensing mechanisms within the arterial circuit should also have input into overall volume homeostasis and serve to defend primarily against perceived depletion of ECF volume relative to capacitance. An increase in arterial pressure causes vascular distention and baroreceptor deformation. This depolarizes the nerve endings by opening mechanosensitive ion channels and triggers action potential discharge. Baroreceptor activity and sensitivity can be further modified during sustained increases in arterial pressure and in pathologic states associated with endothelial dysfunction, oxidative stress, and platelet activation. [39] [40] Evidence favoring the presence of volume-sensitive receptors in the arterial circuit in humans originally derived from the classic observations by Epstein and co-workers[41] in subjects with arteriovenous (AV) fistulas. Closure of fistulas resulted in prompt natriuresis without changes in glomerular filtration rate (GFR) or RPF, whereas re-establishment of fistula patency reduced urinary Na+ excretion. These responses occurred in spite of a decline in hydraulic pressures in the atria and pulmonary vasculature with fistula closure, suggesting that underfilling of the arterial tree signals the kidney to retain Na+ and vice versa. Such sensors in the arterial (high-pressure) circulation exist in the carotid sinus and aortic arch as well as in the renal vasculature.
Carotid Baroreceptor
Histologic and molecular analysis of the carotid baroreceptor has indicated a large content of elastic tissue in the tunica media, which renders the vessel wall in the region highly distensible to changes in intraluminal pressure, thereby facilitating transmission of the stimulus intensity to sensory nerve terminals. Afferent signals from the baroreceptors are integrated in the nucleus tractus solitarius (NTS).[42] Mapping of the neural projections emanating from the carotid baroreceptor in the NTS of the medulla has been greatly facilitated by measurement of changes in the level of expression of the c-fos proto-oncogene after selective baroreceptor stimulation, which may vary according to different pressure thresholds.[43]
Occlusion of the common carotid artery was used in the past to alter renal sympathetic activity and Na+ excretion by the kidney in experimental animals. Common carotid arterial occlusion enhances the activity of the sympathetic nervous system (SNS) and augments renal sympathetic nerve activity. Interestingly, carotid occlusion is sometimes associated with a large natriuresis despite augmented renal sympathetic activation. This is most likely secondary to increases in arterial pressure that result in pressure natriuresis. Moreover, it has been demonstrated in humans that carotid baroreflexes may be modified by maneuvers that alter vascular volume. For example, in normal human subjects, high salt intake blunts the carotid baroreceptors.[44]
Renal Sensors
In addition to its role as a major effector target responding to signals indicating the need for adjustments in Na+ excretion, the kidney participates in the afferent limb of volume homeostasis. The sensor and effector limbs for volume homeostasis are juxtaposed in the kidney. Therefore, volume expansion and depletion may be sensed through alterations in glomerular hemodynamics and possibly renal interstitial pressure that result simultaneously in adjustments in physical forces governing tubule Na+ handling. These are described in greater detail later.
The kidney, along with other organs, has the ability to maintain constant blood flow and GFR at varying arterial pressures. This phenomenon, termed autoregulation (see also Chapter 3 ), operates over a wide range of alterations in renal perfusion pressure (RPP). Changes in RPP are “sensed” by smooth muscle elements that serve as baroreceptors in the afferent glomerular arteriole and respond by adjusting transmural pressure and tension across the arteriolar wall (myogenic response).[45] In addition to this myogenic reflex component, the juxtaglomerular apparatus-dependent tubuloglomerular feedback (TGF) contributes to the maintenance of volume homeostasis. [45] [46] [47] These mechanisms serve to minimize the changes in RPF and GFR when renal perfusion pressure is altered and thus maintain the filtered load of Na+. The juxtaglomerular apparatus is important not only because of the TGF mechanism but also because of its involvement in the generation and release of renin from the kidney. [45] [47] The physiologic control of renin release from the cells in the juxtaglomerular apparatus is exerted in three ways, all of which vary with ECF volume, thereby defining the juxtaglomerular apparatus as an important sensing site for volume homeostasis. First, renin secretion has been shown to be inversely related to perfusion pressure and directly related to intrarenal tissue pressure. The release of renin is further augmented when RPP falls below the autoregulatory range. A second mechanism influencing renin secretion is solute delivery to the macula densa. An increase in NaCl delivery passing the macula densa results in inhibition of renin release, whereas a decrease has the opposite effect. Sensing at the macula densa site is mediated by the entry of NaCl, through the Na,K,2Cl co-transport mechanism, which further leads to alterations in intracellular calcium concentration.[48] Prostaglandin E2 (PGE2) and adenosine are also involved in the release of renin.[48] A third mechanism involved in renin secretion concerns the influence of renal nerves.[49] Renal nerve stimulation increases the release of renin via direct activation of β-adrenoceptors on juxtaglomerular granular cells. This activation is followed promptly by release of renin, an effect that can be dissociated from major changes in renal hemodynamics. Sympathetic stimulation also affects intrarenal baroreceptor input, the composition of the fluid delivered to the macula densa, and the renal actions of AII, such that renal nerves may serve primarily to potentiate other controlling signals.[49]
Central Nervous System Sensors
Several studies in the past suggested that certain areas in the CNS may act as sensors to detect alterations in body salt balance. This hypothesis was based primarily on experiments showing that intracerebral administration of hypertonic saline solutions was associated with alterations in renal salt excretion (CNS-induced natriuresis) or in renal nerve activity. [50] [51] The activity of various neuroendocrine systems in the CNS, in particular the RAAS and ANP, may be also influenced by alterations in Na+ balance. Thus, it was demonstrated that alterations in dietary Na+ intake may regulate the contribution of brain AII in the modulation of baroreflex regulation of renal sympathetic nerve activity.[52] Indeed, administration of AII into the cerebral ventricles impairs baroreflex regulation of renal sympathetic nerve activity.[52] Neurons that release ANP (ANPergic neurons) are located in the paraventricular nucleus and in a region extending to the anteroventral third ventricle (AV3V). These neurons act to inhibit water and salt intake by blocking the action of AII. [53] [54] Stimulation of neurons in the AV3V region causes natriuresis and an increase in circulating ANP, whereas lesions in the AV3V region and caudally in the median eminence or neural lobe decrease resting ANP release and the response to blood volume expansion. [53] [54]
However, despite the substantial evidence linking the CNS with the regulation of ECF volume homeostasis, the nature of the sensing mechanisms and their mode of operation remain largely unknown.
Role of the Gastrointestinal Tract in the Regulation of Extracellular Fluid Volume and Sodium Balance
Under normal physiologic conditions, intake of Na+ and water reaches the ECF by absorption from the gastrointestinal tract (GIT). It is, therefore, reasonable to assume that some sensing and controlling mechanisms may exist within the GIT itself that participate in the regulation of ECF volume and Na+ balance. Experimental evidence supporting the latter contention has accumulated in the past 4 decades. Early studies in humans suggested that urinary excretion of an oral Na+ load may be faster and more pronounced than the response to the same Na+ load given by intravenous infusion.[55] Similarly, studies in experimental animals demonstrated that infusions of hypertonic NaCl directly into the portal vein caused a greater natriuresis than a similar infusion into the femoral vein. These findings were interpreted to suggest the presence of Na+-sensing mechanisms in the splanchnic and/or portal circulations in the GIT. Several mechanisms of neural and hormonal origin have been proposed.
Hepatic Receptors
Two main neural reflexes have been described. [56] [57] The “hepatorenal” and “hepatointestinal” reflexes originate from receptors in the hepatoportal region. They transduce portal plasma Na+ concentration into hepatic afferent nerve activity and reflexively augment renal Na+ excretion and attenuate intestinal Na+ absorption before a measurable increase in systemic Na+ concentration takes place. Studies in conscious rabbits subjected to intravenous infusion of 20% NaCl solution demonstrated that this procedure caused a marked decrease in renal nerve activity and increased urinary Na+ excretion.[58] Similar findings were reported in other species, supporting a role of the “hepatorenal reflex” in the regulation of renal nerve activity and augmentation of urinary Na+ excretion. [56] [57] In addition, signals originating in hepatoportal sensors can also control the intestinal absorption of an Na+ load. The intraportal infusion of 9% NaCl solution causes depression of Na+ absorption across the jejunum.[59] The afferent limbs of this reflex, referred to as the hepatointestinal reflex, are the hepatic nerves, and the efferent limbs travel through the vagus nerve. The chemical inactivation of the NTS abolishes the depressing effect of intraportal NaCl infusion on jejunal absorption, suggesting that the NTS is involved in the hepatointestinal reflex. In conclusion, specialized receptors in the hepatoportal region transfer the signal of an increased portal plasma Na+ concentration into an increase in hepatic afferent nerve activity. These afferent signals, in turn, activate the hepatorenal reflex (augmentation of renal Na+ excretion) and the hepatointestinal reflex (suppression of salt absorp-tion across the intestine). It has been suggested that the hepatoportal receptor senses the Na+ concentration via the bumetanide-sensitive Na+K+2Cl- cotransporter, because the responses of hepatic afferent nerve activity to intraportal hypertonic NaCl injection were suppressed by intraportal infusion of furosemide or bumetanide.[60]
In addition to chemoreceptors (i.e., Na+ sensors) in the hepatoportal area, the normal human liver also contains mechanoreceptors (baroreceptors). Increased intrahepatic hydrostatic pressure has been shown in the past to be associated with enhanced renal sympathetic activity and renal Na+ retention in various experimental models. [61] [62] Convincing evidence for a role of the intrahepatic baroreceptors in the modulation of renal salt retention was provided in 1987 by Levy and Wexler.[61] These investigators used the model of thoracic caval constriction in dogs to raise intrahepatic pressure without driving fluid from the vascular space as ascites. When venous pressure was increased by 6.6 cm H2O, Na+ balance studies showed a positive cumulative balance that could be prevented by liver denervation.[61] Although the nature of the volume- and Na+-sensing mechanism has not been clarified, it is thought to play an important role in the pathogenesis of primary renal Na+ retention associated with intrahepatic hypertension (see Renal Sodium Retention and Edema Formation in Cirrhosis with Ascites).
Guanylin Peptides: Intestinal Natriuretic Hormones
In addition to the Na+-sensing mechanisms in the liver and GIT, an effector hormonal mechanism linking the gut with the kidney has been sought to account for the phenomenon of postprandial natriuresis. As pointed out previously, it has been suggested in the past that the natriuretic response of the kidney to an Na+ load is more rapid when the load is delivered orally than when the same load is administered intravenously.[55] Although the latter finding remains controversial,[63] in the past decade, a novel family of cGMP-regulating peptides has been identified that may act as “intestinal natriuretic hormones.” [36] [64] [65] [66] [67] Guanylin and uroguanylin, the main representatives of the family, are small, heat-stable peptides with intramolecular disulfide bridges that share similarity with the bacterial heat-stable enterotoxins that cause “traveler's diarrhea.” Guanylin and uroguanylin were first isolated from rat jejunum and opossum urine, respectively. [68] [69] In the intestine, guanylin and uroguanylin modulate epithelial ion and water transport by a local mechanism of action, which involves binding to and activation of the receptor guanylyl cyclase-C (GC-C), a transmembrane 1050– to 1053–amino acid protein that is present in the intestinal brush border. Guanylin apparently plays a regulatory role in intestinal fluid and electrolyte transport through the second messenger cGMP. GC-C is structurally similar to the membrane-bound GC-A and GC-B, which serve as receptors of the NP family. [64] [67] In addition to these secretory effects, studies in mice with targeted inactivation of the guanylin gene suggest that this intestinal peptide has an important role in controlling intestinal epithelial cell proliferation and differentiation, via GC-C.[36]
It has been suggested that guanylin peptides, in particular uroguanylin, may also serve in intrarenal signaling pathways influencing cGMP production in renal cells, thus linking the digestive system and the kidney in the control of Na+ homeostasis. [65] [66] [67] [70] The following arguments favor the latter hypothesis. [34] [36] First, intestinal guanylin and uroguanylin mRNA levels are modulated by oral salt intake. Second, both peptides may be detected in the circulation, and high concentrations of uroguanylin are excreted in the urine. Moreover, these hormones stimulate renal electrolyte excretion by inducing natriuresis, kaliuresis, and diuresis.[67] Finally, Lorenz and co-workers[71]recently showed that mice lacking the uroguanylin gene displayed an impaired natriuretic response to oral salt loading, but not to intravenous NaCl infusion. Interestingly, uroguanylin knockout mice exhibited an increase of 10 to 15 mm Hg in their MAP, regardless of the level of dietary salt intake. Taken together, these data highly suggest a role, at least for uroguanylin, as a natriuretic hormone, which adjusts urinary Na+ excretion to balance the levels of NaCl absorbed via the GIT.[66] The importance of this system in the control of renal Na+ excretion in humans awaits further clarification.
Efferent Limb: Effectors for Fluid Homeostasis
Major renal effector mechanisms include glomerular filtration, peritubular and luminal factors, and humoral and neural mechanisms ( Table 12-2 ).
TABLE 12-2 -- Major Renal Effector Mechanisms for Body Fluid Volume Homeostasis
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Glomerular filtration rate |
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Renal nerves |
In humans, normal glomerular filtration leads to the delivery of approximately 4000 mmol of Na+/day for downstream processing by the tubules. Of this quantity, the vast majority (>99%) is reabsorbed, leaving the small remainder to escape into the final urine. It is clear from this simple calculation that even minute changes in the relationship between filtered load and fraction of Na+ reabsorbed can exert a profound cumulative influence on overall Na+balance. However, even marked perturbations in GFR are not necessarily associated with drastic alterations in urinary Na+ excretion, and hence, overall Na+ balance is most often preserved. This preservation of Na+ homeostasis is the consequence of appropriate adjustments in two important protective mechanisms, namely, TGF control of GFR acting through macula densa[47] and glomerular-tubule balance (GTB). The latter term describes the ability of proximal tubular reabsorption to adapt proportionally to the changes in filtered load.
Indeed, numerous studies in the past revealed that the modest changes in GFR that accompany volume expansion and depletion maneuvers are not sufficient to explain the accompanying adjustments in urinary Na+ excretion. Rather, these studies suggested that local intrarenal factors, acting at the level of the coupling of tubule reabsorption to glomerular filtration, are responsible for regulating urinary Na+ excretion, responding to afferent limb signals that are responsive to volume perturbation. In the following sections, these intrarenal physical factors, acting at the level of the proximal tubule and beyond, are discussed. In addition, the neural and humoral factors that modulate tubule transport, through these physical factors or through direct epithelial transport effects, are considered.
Intrarenal Physical Factors
Peritubular Factors
Infusions of saline or albumin solutions to experimental animals and humans have been frequently used as a tool to study the mechanisms of the natriuretic response to ECF volume expansion. These experiments were performed usually on an acute basis and, therefore, may bear little relevance to the chronic regulation of ECF Na+ balance. Nevertheless, the findings in many of these investigations led to the notion that alterations in hydraulic and oncotic pressures (Starling forces) in the peritubular capillary play an important role in the regulation of Na+ and water transport, in particular at the proximal nephron.
The peritubular capillary network is anatomically connected in series with the glomerular capillary bed through the efferent arteriole, so that changes in the physical determinants of GFR critically influence Starling forces in the peritubular capillaries. In the proximal tubule, the relation of hydraulic and oncotic driving forces to the transcapillary fluid flux is given by the Starling relationship: APR = Kr[(πc-πi)-(Pc-Pi)], in which APR is the absolute rate of reabsorption of proximal tubule absorbate by the peritubular capillary; Kr is the capillary reabsorption coefficient (the product of capillary hydraulic conductivity and absorptive surface area); πc and Pc are the local capillary oncotic and hydraulic pressures, respectively; and πi and Pi are the corresponding interstitial pressures. πi and Pc are forces that oppose fluid absorption, whereas πc and Pi tend to favor uptake of reabsorbate. The simultaneous determination of these driving forces allows an analysis of the net pressure favoring fluid absorption or filtration.
As a consequence of the anatomic relationship of the postglomerular efferent arteriole to the peritubular capillary, the hydraulic pressure in the peritubular capillary is significantly lower than in the glomerular capillary. The function of the efferent arteriole as a resistance vessel contributes to a decrease in hydraulic pressure between the glomerulus and the peritubular capillary. Also, because the peritubular capillary receives blood from the glomerulus, the plasma oncotic pressure is high at the outset as a result of prior filtration of protein-free fluid. It follows that the greater the GFR relative to plasma flow rate (the filtration fraction), the greater the efferent arteriolar plasma protein concentration and the lower the proximal peritubule capillary hydraulic pressure, consequently favoring enhanced proximal fluid reabsorption ( Fig. 12-2 ). Therefore, in contradistinction to the glomerular and peripheral capillary, the peritubular capillary is characterized by high values of (πc-πi), which greatly exceed (Pc-Pi), resulting in net reabsorption of fluid. The relation of proximal reabsorption to filtration fraction may contribute to Na+-retaining and edema-forming states, such as CHF (see Fig. 12-2 ).
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FIGURE 12-2 The glomerular and peritubular microcirculations. Left, Approximate transcapillary pressure profiles for the glomerular and peritubular capillaries in normal humans. Vessel lengths are given in normalized, nondimensional terms, with 0 being the most proximal portion of the capillary bed and 1 the most distal portion. Thus, 0 for the glomerulus corresponds to the afferent arteriolar end of the capillary bed, and 1 to the efferent arteriolar end. The transcapillary hydraulic pressure difference (ΔP) is relatively constant with distance along the glomerular capillary, and the net driving force for ultrafiltration (Δ-Δp) diminishes primarily as a consequence of the increase in the opposing colloid osmotic pressure difference (Δπ), the latter resulting from the formation of an essentially protein-free ultrafiltrate. As a result of the hydraulic pressure drop along the efferent arteriole, the net driving pressure in the peritubular capillaries (Δ-Δp) becomes negative, favoring reabsorption. Right, The hemodynamic alterations that are believed to occur in the renal microcirculation in congestive heart failure (CHF). The fall in renal plasma flow (RPF) in CHF is associated with a compensatory increase in ΔP for the glomerular capillary, favoring a greater-than-normal rise in the plasma protein concentration and, hence, Δπ along the glomerular capillary. This increase in the value of Δπ by the distal end of the glomerular capillary also translates to an increase in Δπ in the peritubular capillaries, resulting in the increase in the net driving pressure, responsible for enhanced proximal tubule fluid absorption, that is believed to take place in CHF. The increased peritubular capillary absorptive force in CHF also probably results from the decline in ΔP, a presumed consequence of the rise in renal vascular resistance. (From Humes HD, Gottlieb M, Brenner BM: The Kidney in Congestive Heart Failure: Contemporary Issues in Nephrology, Vol 1. New York, Churchill Livingstone, 1978, pp 51-72.) |
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Compelling experimental evidence for the relationship between proximal peritubular Starling forces and proximal fluid reabsorption came from a series of a series of in vivo micropuncture and microperfusion studies by Brenner and colleagues. [72] [73] [74] [75] In the earlier studies, rat efferent arterioles were perfused with various oncotic solutions, and it was shown that APR varied directly with the oncotic force of the perfusate and with constancy of GFR, thus providing evidence that changes in efferent arteriolar protein concentration directly modify proximal reabsorption independent of GFR.[72] To determine whether primary decreases in GFR regulate APR through effects on efferent arteriolar protein concentration, rats were studied after partial aortic constriction, a maneuver that reduced the single-nephron GFR (SNGFR) and the APR proportionately and decreased filtration fraction. APR was maintained at control levels with iso-oncotic albumin infusions that returned the efferent arteriolar plasma protein concentration to normal without changing GFR. In this way, the GTB could be modified by the prevailing peritubular oncotic pressure, with the link between GFR and APR again being related to changes in filtration fraction and peritubular capillary oncotic pressure. From these studies, the role of peritubular forces in the setting of increased ECF volume can be summarized as follows:
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Acute saline expansion results in dilution of plasma proteins and reduction in efferent arteriolar oncotic pressure. SNGFR and peritubular capillary hydraulic pressures may be increased as well, but the decrease in peritubular oncotic pressure in itself results in a decreased net peritubular capillary reabsorptive force and decreased APR. GTB is disrupted because APR falls despite the tendency for SNGFR to rise. |
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Iso-oncotic plasma infusions tend to raise SNGFR and peritubular capillary hydraulic pressures but lead to relative constancy of efferent arteriolar oncotic pressure. APR may therefore decrease slightly, resulting in less disruption of GTB and natriuresis of lesser magnitude than that observed with saline expansion. |
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Hyperoncotic expansion usually increases SNGFR (because of volume expansion) as well as APR, the latter resulting from increased efferent arteriolar oncotic pressure. GTB therefore tends to be better preserved than with iso-oncotic plasma or saline expansion. |
The possibility that changes in peritubular COP may alter proximal fluid reabsorption was also demonstrated in several studies using the in vitro isolated perfused tubule model.[76] Thus, an extensive literature from several laboratories supported the view that movements of fluid and electrolytes across the peritubular basement membrane into the surrounding capillary bed could be modulated by alterations in proximal peritubular capillary Starling forces. Moreover, these studies indicated that the effects might be mediated through corresponding alterations in physical parameters in the peritubular interstitial compartment. Ultrastructural data for the rat suggest that the peritubular capillary wall is in tight apposition to the tubule basement membrane for about 60% of the tubule basal surface. However, irregularly shaped wide portions of peritubular interstitium also exist over about 42% of the tubule basal surface, so a major part of reabsorbed fluid has to cross a true interstitial space before entering the peritubular capillaries. Alterations in the physical properties of the interstitial compartment could conceivably modulate either passive or active components of net proximal tubule fluid transport. The accepted formulation had been that Starling forces in the peritubular capillary regulate the rate of volume entry from the peritubular interstitium into the capillary. Any change in this rate of flux could lead to changes in interstitial pressure that secondarily modify proximal tubule solute transport. This formulation could explain why experimental maneuvers that were known to raise renal interstitial hydrostatic pressure (e.g., infusion of renal vasodilators, renal venous constriction, renal lymph ligation), were associated with a natriuretic response, whereas the opposite effect was obtained with renal decapsulation (see also the section on the role of renal interstitial pressure the mechanism of pressure natriuresis).
In theory, interstitial forces could influence active reabsorption of Na+, passive reabsorption, or the rate of back-flux through the paracellular shunt pathway. Because of the relatively highly permeability of the proximal, alterations in bidirectional paracellular flux have been thought to play a dominant role in transducing the effect of alterations in Starling forces on proximal tubular net reabsorption, though the magnitude of these effects and the mechanisms involved were not fully elucidated.[77] The discovery of the claudin family of adhesion molecules as an integral component of the tight junction has shed additional light on these processes. [78] [79] [80] Instead of a “passive” structure, the tight junction is now considered to be a dynamic, multifunctional complex that may be amenable to physiologic regulation by cellular second messengers or in pathologic states. [78] [79] Among the 24 known mammalian claudin-family members, at least 3, claudin-2, -10, and -11 are located in the proximal nephron of the mouse and others at more distal nephron sites. [80] [81] Claudin-2 is selectively expressed in the proximal nephron.[82]The claudin-family members are thus important candidates for the future study of the influence of Starling forces on fluid reasbsorption.
Although paracellular transport was believed to be mediated primarily through passive forces, some evidence also suggests the contribution of active transport processes. Thus, in the presence of active transport, the effects of proteins on fluid transport are enhanced. In addition, studies by Berry and colleagues[83] demonstrated no effect on the permeability properties of the proximal tubule and no effect on passive water and solute fluxes. The only active flux modulated by changes in peritubular protein was that of NaCl. On the basis of these observations, peritubular protein concentration would not likely be affecting Na+,K+-ATPase or the Na+/H+ antiporter because one would expect consequent effects on Na+ bicarbonate absorption.[84]
Luminal Factors in Glomerular-Tubular Balance
Although a considerable amount of data supports the role of peritubular capillary and interstitial Starling forces in the regulation of proximal tubule transport, some studies either have not found such effects or have suggested the presence of additional mechanisms.
Since the early 1970s, studies utilizing tubular perfusion with plasma ultrafiltrate or native tubular fluid suggested that some constituents of this fluid or intraluminal flow rate per se may be important modulators of proximal fluid reabsorption, independent of peritubular Starling forces (see Romano and co-workers[85] and references therein). The flow-dependence of proximal reabsorption was likewise supported by studies in isolated perfused proximal tubules of the rabbit nephron.[86] A key observation indicated that the presence of a transtubular anion gradient, normally present in the late portion of the proximal nephron, was necessary for the flow-dependence to occur.[87] A potential mechanism for modulation of proximal Na+ reabsorption in response to changes in filtered load depends on the close coupling of Na+ transport with the cotransport of glucose, amino acids, and other organic solutes. The increased delivery of organic solutes that accompanies increases in GFR might help to augment the rates at which both the solutes and the Na+ chloride are reabsorbed. The dependence of GTB on transtubular anion gradient was explained by the ability of the Cl-HCO3- gradient, generated by the preferential reabsorption of Na+ with bicarbonate in the early proximal tubule, to enhance the “passive” component of Na+ and fluid reabsorption in the proximal nephron. Irrespective of the exact mechanism, an important notion emerged, namely, that states of ECF volume expansion impaired the integrity of the GTB, thus allowing increased delivery of salt and fluid to more distal parts of the nephron.
Figure 12-3 presents a schematic outline of the major factors acting on the proximal nephron during a decrease in ECF and effective circulating volume.
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FIGURE 12-3 Effects of hemodynamic changes on proximal tubule solute transport: a summary. (From Seldin DW, Preisig PA, Alpern RJ: Regulation of proximal reabsorption by effective arterial blood volume. Semin Nephrol 11:212-219, 1991.) |
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Physical Factors Acting beyond the Proximal Tubule
An extensive series of experimental studies showed that the final urinary excretion of Na+, in response to volume expansion or depletion, can be dissociated from the amount delivered out of the superficial proximal nephron, suggesting that more distal and/or deeper segments of the nephron contribute to the modulation of Na+ and water excretion. Several sites along the nephron, such as loop of Henle, distal nephron, and cortical and papillary collecting ducts, were found, by micropuncture and microcatheterization techniques, to increase or decrease the rate of Na+ reabsorption in response to enhanced delivery from early segments of the nephron. However, direct evidence that these transport processes are mediated by changes in Starling forces per se is lacking. A detailed description of these experiments is not given in the present chapter, but may be found in previous editions of this book as well as in review articles.[88]
In summary, the following generalizations regarding the intrarenal control of Na+ excretion apply. Provided that ECF volume is held relatively constant, an increase in GFR leads to little or no increase in salt excretion because of a close coupling between the GFR and the intrarenal physical forces acting at the peritubular capillary to control APR. In addition, changes in the filtered load of small organic solutes, and perhaps other as-yet-uncharacterized glomerulus-borne substances in tubule fluid, may influence APR. To the extent that changes, if any, in the load of Na+ delivered to more distal segments also occur, these are matched by more or less parallel changes in distal reabsorptive rates, to ensure a high degree of GTB for the kidney as a whole. Conversely, ECF volume expansion leads to large increases in Na+ excretion even in the presence of reduced GFR. Changes in Na+ reabsorption in the proximal tubule alone cannot account for this natriuresis of volume expansion, and a variety of mechanisms for suppressing Na+ reabsorption at more distal sites have been invoked.
Mechanism of Pressure Natriuresis: Role of Renal Medullary Hemodynamics and Interstitial Pressure in Control of Sodium Excretion
The idea that changes in renal medullary hemodynamics may be involved in the natriuresis evoked by volume expansion was initially proposed in the 1960s by Earley and Friedler.[89] According to their theory, ECF volume expansion results in an increase in medullary plasma flow (MPF) with a subsequent loss of medullary hypertonicity, thereby decreasing water reabsorption in the thin descending loop of Henle. The decrease in water reabsorption in the thin descending limb lowers the Na+ concentration in the fluid entering the ascending loop of Henle, thus decreasing the transepithelial driving force for salt transport in this nephron segment. At the same time, a similar mechanism was proposed to explain the natriuresis involved in the pressure-natriuresis phenomenon. It was reasoned that increases in RPP produce a parallel increase in MPF that eliminates the medullary osmotic gradient.
The concept that alterations in the solute composition of the renal medulla and papilla play a key role in regulation of Na+ transport gained significant support in the 1970s and 1980s, when several micropuncture studies suggested that volume expansion, renal vasodilatation, and increased RPP produced a greater inhibition of salt reabsorption in the loops of Henle of juxtamedullary nephrons than in superficial nephrons. Although measurements of MPF in experimental animals undergoing volume expansion and renal vasodilatation supported the possibility of redistribution of intrarenal blood flow toward the medulla, the validity of the methodologies for intrarenal blood flow measurements used at that time was questioned. The application of newer techniques that allowed a more reliable estimation of changes in medullary blood flow, such as laser-Doppler flowmetry and videomicroscopy, resulted in a renewal of interest in the role of medullary hemodynamics in the control of Na+ excretion, especially in the context of the pressure-natriuresis relationship. [90] [91] [92] [93]
Elevation in blood pressure has been recorded, although not always, following expansion of the ECF and salt loading, though it is not a consistent observation in all studies. [30] [93] This increase in blood pressure and RPP may lead to an increase in Na+ excretion by the kidney, a phenomenon termed pressure-natriuresis. The importance of pressure-natriuresis in the long-term control of arterial blood pressure and ECF volume regulation was first recognized by Guyton and associates. [93] [94] According to this view, the kidneys play a dominant role in controlling arterial pressure by virtue of their ability to alter Na+ excretion in response to changes in arterial blood pressure. For instance, an increase in RPP results in a concomitant increase in Na+ excretion, thereby decreasing circulating blood volume and restoring arterial pressure. It was soon recognized that the coupling between arterial pressure and Na+ excretion occurred in the setting of preserved autoregulation (i.e., in the absence of changes in total RPF, GFR, or filtered load of Na+). Although the mechanism responsible for pressure-natriuresis in a setting of high-efficiency autoregulation is unclear, the possibility that the pressure-natriuresis mechanism is triggered by changes in medullary circulation received considerable attention. [89] [91] [95] [96] Laser-Doppler flowmetry and servonull measurements of capillary pressure in volume-expanded rats revealed that papillary blood flow was directly related to RPP over a wide range of pressures studied. In contrast, cortical blood flow was well autoregulated, indicating that during alterations in RPP renal medullary blood flow may not be autoregulated to the same extent as cortical blood flow. As mentioned earlier, increase in medullary plasma flow might lead to medullary “washout” with a consequent reduction in the driving force for Na+ reabsorption in the ascending loop of Henle, particularly in the deep nephrons. In addition, the increase in medullary perfusion may be associated with a rise in renal medullary interstitial hydrostatic pressure. Indeed, various physiologic and pharmacologic maneuvers that increase Pi, such as ECF volume expansion, infusion of renal vasodilatory agents, long-term mineralocorticoid escape, and hilar lymph ligation, result in a significant increase in Na+ excretion. Numerous studies established that pressure-natriuresis is associated with elevated Pi that is most evident in the volume-expanded state (see reviews by Granger and colleagues [97] [98] and references therein). Moreover, prevention of the increase in Pi by removal of the renal capsule significantly attenuated, but did not completely block, the natriuretic response to elevations in RPP. Thus, as depicted in Figure 12-4 , elevation in renal perfusion pressure is associated with an increase in medullary blood flow and increased vasa recta capillary pressure, which result in an increase in medullary Pi. This increase of interstitial pressure is thought to be transmitted to the renal cortex in the encapsulated kidney and to provide a signal that inhibits Na+ reabsorption along the nephron. In that regard, the renal medulla may be viewed as a sensor that can detect changes in RPP and initiate the pressure-natriuresis mechanism.
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FIGURE 12-4 Role of the renal medulla in modulating tubular reabsorption of Na+ in response to changes in renal perfusion pressure (RPP). (Adapted from Cowley AW Jr: Role of the renal medulla in volume and arterial pressure regulation. Am J Physiol 273:R1–R15, 1997.) |
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It has been suggested that changes in systemic pressure are transmitted to the medullary circulation via shunt pathways connecting preglomerular vessels of juxtamedullary nephrons directly to the postglomerular vasa recta capillaries.[89] This might explain how changes in systemic pressures are transmitted to the medulla when RPF and GFR are well autoregulated. Alternatively, it has been suggested more recently that the process of autoregulation of renal blood flow (RBF) leads to increased shear stress in the preglomerular vasculature, so that release of nitric oxide (NO) (see later) and perhaps cytochrome P-450 products of arachidonic acid metabolism, drive the cascade of events that inhibit Na+ reabsorption. [99] [100]
Although a substantial amount of experimental data supports the association between changes in Pi and urinary Na+ excretion, the mechanisms by which these changes decrease tubular Na+ reabsorption, as well as the nephron sites responding to the alterations in Pi, have not been fully clarified.[98] As pointed out earlier, it was postulated that elevations in Pi may increase passive back-leak or the paracellular pathway hydraulic conductivity, with a resultant increase in back-flux of Na+ through the paracellular pathways.[97] However, the absolute changes in Pi, in the range of 3 to 8 mm Hg in response to increments of about 50 to 90 mm Hg in RPP, are probably not sufficient to account for the decrease in tubular Na+ reabsorption even in the proximal tubule, the nephron segment with the highest transepithelial hydraulic conductivity.[90] Nevertheless, considerable evidence from micropuncture studies indicate that pressure-natriuresis is associated with significant changes in proximal fluid reabsorption particularly in deep nephrons, with enhanced delivery to the loop of Henle, although alterations in the pars recta and thin descending limb must also be considered.[97] Pressure-induced changes in tubular reabsorption may also occur in more distal parts of the nephron, such as the ascending loop of Henle, distal nephron, and collecting duct.[101]Therefore, elevations in RPP can affect tubular Na+ reabsorption by both proximal and distal mechanisms.
The finding that small changes in Pi are associated with significant alterations in tubular Na+ reabsorption led to the hypothesis that the changes in Pi may be amplified by various hemodynamic, hormonal, and paracrine factors. [85] [89] [91] [95] [97] Specifically, the phenomenon of pressure-natriuresis is particularly demonstrable in states of volume expansion and renal vasodilatation and is significantly attenuated in states of volume depletion.[97] Among a variety of hormonal and paracrine systems that have been documented to play a role in modulating the pressure-natriuresis relationship, changes in the activity of the RAAS and local production of prostaglandins (PGs) within the kidney received considerable attention over the years.[97] Removal of the influence of AII, by either angiotensin-converting enzyme (ACE) inhibitors or angiotensin type 1 (AT1) receptor antagonists, potentiates the pressure-natriuretic response, and inhibitors of cyclooxygenase attenuate it. [97] [102] It is important to note, however, that pharmacologic blockade of these systems only attenuates but does not completely eliminate the pressure-natriuresis response, indicating that they act as modulators and not as mediators of the phenomenon.
In recent years, the importance of the endothelial-derived factors in the regulation of renal circulation and excretory function has been recognized. Evidence suggests a role of endothelium-derived NO and P-450 eicosanoids in the mechanism of pressure-natriuresis. [90] [95] [99] NO, generated in large amounts in the renal medulla, appears to play a critical role in the regulation of medullary blood flow and Na+ excretion. [96] [103] Several studies showed that inhibition of intrarenal NO production can reduce Na+ excretion and markedly suppress the pressure-natriuretic response, whereas administration of the NO precursor improves transmission of perfusion pressure into the renal interstitium and normalizes the defect in pressure-natriuresis response in Dahl salt-sensitive rats. [90] [101] [104] [105] Likewise, a positive correlation between urinary excretion of nitrites and nitrates (metabolites of NO) and changes in renal arterial pressure or urinary Na+ excretion was observed in the dog.[106] The P-450 eicosanoids are additional endothelium-derived factor(s) that may participate in the mechanism of pressure-natriuresis. [99] [100] The importance of these agents in the regulation of renal Na+ transport and of renal and systemic hemodynamics, has been recently underscored.[107] Taken together, these observations support the hypothesis that alterations in the production of renal NO and eicosanoids may be involved in mediation of the pressure-induced natriuretic response. It is tempting to speculate that acute elevation in RPP in the autoregulatory range results in increased blood flow velocity and shear stress, leading to increased endothelial release of NO. Enhanced renal NO production may increase urinary Na+ excretion either by acting directly on tubular Na+ reabsorption or through its vasodilatory effect on renal vasculature.
Finally, McDonough and co-workers [108] [109] reported that, in response to an increase in RPP, the apical Na+/H+ exchanger in the proximal tubules may be redistributed out of the brush border into intracellular compartments. Concomitantly, basolateral Na+-K+-ATPase activity significantly decreased. Although not fully elucidated, the mechanisms of these cellular events may be related directly to the change in Pi or to changes in intrarenal paracrine agents described previously.
A major assumption of the pressure-natriuresis theory indicates that changes in systemic and renal perfusion pressure mediate the natriuretic response by the kidney. As pointed out in a recent comprehensive review by Bie,[30] acute regulatory changes in renal salt excretion may occur without a measurable elevation in arterial blood pressure. [2] [30] [110] [111] Interestingly, in many of these studies, the natriuresis was accompanied by a decrease in the activity of the RAAS without changes in plasma ANP levels. [2] [30] [110] [111] [112] Thus, whereas increases in arterial blood pressure can drive renal Na+ excretion, other “pressure-independent” control mechanisms must operate as well to mediate the “volume-natriuresis.”[30]
Humoral Mechanisms
Renin-Angiotensin-Aldosterone System
The RAAS plays an integral role in the regulation of ECF volume, Na+ homeostasis, and cardiac function.[113] This system is activated in situations that compromise hemodynamic stability, such as loss of blood volume, reduced ECF volume, low Na+ intake, hypotension, and increase in sympathetic nerve activity. The RAAS comprises of a coordinated hormonal cascade whose synthesis is initiated by the release of renin from the juxtaglomerular apparatus in response to reduced renal perfusion or fall in arterial pressure.[114] Messenger RNA (mRNA) for renin exists in juxtaglomerular cells and in renal tubule cells.[115] Renin acts on its circulating substrate, namely angiotensinogen, which is produced and secreted mainly by the liver but also by the kidney.[113] ACE, which cleaves angiotensin I (AI) to AII, exists in large amounts in the lung but also on endothelial cells of the vasculature and cell membrane of the brush border of the proximal nephron, heart, and brain.[113] AII is considered to be the principal effector of the RAAS, although it is recognized that few smaller metabolic products of AII may have biologic activities.[116]Nonrenin (cathepsin G, plasminogen-activating factor, tonin) and non-ACE pathways (chymase, cathepsin G) also exist in these tissues and may contribute to tissue AII synthesis. [113] [114] In addition to its important function as a circulating hormone, AII produced locally acts as a paracrine agent in an organ-specific mode, which might be dissociated from its systemic vasoconstrictor action. [117] [118] In that respect, the properties of AII as a growth-promoting agent in the cardiovascular system and the kidney have been increasingly appreciated. [113] [119] [120] For instance, local generation of AII in the kidney results in higher intrarenal levels of this peptide in proximal tubular fluid, interstitial fluid, and renal medulla compared with circulating levels. The epithelial cells of the proximal nephron may be an important source for the in situ generation of AII, because these cells show abundant expression of the mRNA for angiotensinogen. [121] [122] Furthermore, AII is apparently secreted from tubular epithelial cells into the lumen of the proximal nephron.[123] This may account for the high proximal tubular fluid concentrations of AII—approximately 1000 times higher than the plasma levels of the peptide. [123] [124] Moreover, recent data demonstrated that the mechanisms regulating intrarenal levels of AII may be dissociated from those controlling the systemic concentrations of the peptide.[125]
The biologic actions of AII are mediated through activation of at least two receptors subtypes, AT1 and AT2, encoded by different genes residing on different chromosomes. [126] [127] Both receptors have been cloned and were found to be G-protein-coupled, seven-transmembrane polypeptides containing approximately 360 amino acids. [113] [127] [128] [129] In the adult organism, the AT1 receptor subtype mediates most of the biologic activities of AII, whereas the AT2 receptor, expressed primarily in the fetal life, appears to play an important role in cell development and apoptosis. [130] [131] AT1 is expressed in the vascular poles of glomeruli, juxtaglomerular apparatus, and mesangial cells, whereas AT2 is localized to renal arteries and tubular structure, at a small population.[132] Besides their functional distinction, the two receptor types employ different signal transduction pathways. Stimulation of the AT1 receptor activates phospholipase A2, C, and D, resulting in increased cytosolic Ca2+ and inositol triphosphate (IP3) and inhibition of adenylate cyclase. In contrast, activation of the AT2 receptor results in increased NO and bradykinin (BK) levels, leading to elevated cGMP concentrations and vasodilation.[133]
Besides being an important source of several components of the RAAS, the kidney acts a major target organ to the principal hormonal mediators of this cascade, AII and aldosterone. In the past, it was believed that the major contribution of AII to Na+ homeostasis was the result of its actions as a circulating vasoconstrictor hormone and through stimulation of aldosterone release with subsequent tubular action of aldosterone. However, evidence indicates that AII, via AT1 receptors, exerts multiple direct intrarenal influences, including renal vasoconstriction, stimulation of tubular epithelial Na+ reabsorption, augmentation of TGF sensitivity, modulation of pressure-natriuresis, and stimulation of mitogenic pathways.[113] Moreover, exogenous infusion of AII that results in relatively low circulating levels of AII (picomolar range) is highly effective in modulating renal hemodynamic and tubular function compared with the 10- to 100-fold higher concentrations required for its extrarenal effects. Thus, the kidney appears to be uniquely sensitive to the actions of AII. Furthermore, the synergistic interactions that exist between the renal vascular and tubular actions of AII significantly amplify the influence of AII on Na+ excretion.[134] Among the direct renal actions of AII, the effect of the peptide on renal hemodynamics appears to be of critical importance. AII elicits a dose-dependent decrease in RBF but slightly augments GFR, owing to its preferencial vasoconstrictive effect on efferent arteriole, and therefore increases filtration fraction. As noted previously, the increase in filtration fraction in response to AII can be attributed to a predominant increase in efferent arteriolar resistance exerted by the peptide,[135] which may further modulate peritubular Starling forces, such as decreasing hyraulic pressure and increasing COP in the interstitium. These peritubular changes eventually lead to enhanced proximal Na+ and fluid reabsorption. It is important to note, however, that changes in preglomerular resistance have also been described during AII infusion or blockade. [136] [137] These may be secondary to changes in either systemic arterial pressure (myogenic reflex) or increased sensitivity of TGF, because AII does not alter preglomerular resistance when RPP is clamped or adjustments in TGF are prevented.[137] In addition, AII may affect GFR by reducing glomerular ultrafiltration coefficient, thereby altering the filtered load of Na+. [138] [139] This effect is believed to reflect the action of the hormone on mesangial cell contractility and increasing permeability to macromolecules.[136] Finally, AII may also influence Na+ excretion through its action on medullary circulation. Because AII receptors are present in high abundance in the renal medulla, this peptide may contribute significantly to the regulation of medullary blood flow. [136] [140] Indeed, use of fiberoptic probes revealed that AII usually reduces cortical blood flow and medullary blood flow and decreases Na+ and water excretion. [113] [132] As pointed out earlier, changes in medullary blood flow may affect medullary tonicity, which determines the magnitude of passive salt reabsorption in the loop of Henle, and also modulate pressure-natriuresis through renal interstitial pressure. [89] [91]
The other well-characterized renal effect of AII is a direct action on tubule epithelial transport systems. Infusions of AII to achieve systemic concentrations of 10-12 to 10-11 M markedly stimulated Na+ and water transport, independent of changes in renal or systemic hemodynamics. [113] [141] AII exerts a dose-dependent biphasic effect on proximal Na+ reabsorption. Peritubular capillary infusion with solutions containing low concentrations of AII (10-12–10-10 M) stimulated Na+ reabsorption, whereas perfusion with solutions containing higher concentrations of AII (>10-7 M) inhibited proximal Na+ reabsorption rate. Similar observations were reported in vitro in isolated perfused rabbit proximal tubule. Quan and Baum[142] demonstrated that addition of either the AT1 receptor antagonist losartan or the ACE inhibitor enalaprilat directly into the luminal fluid of the proximal nephron resulted in a significant decrease in proximal fluid reabsorption, indicating tonic regulation of proximal tubule transport by endogenous AII. Several studies provided insight into the specific mechanisms by which AII influences proximal tubule transport. These studies showed that AII enhances proximal tubular Na+ transport through actions at both luminal and basolateral membrane sites. AII increases Na+ and HCO3- reabsorption by stimulation of the apical membrane Na+/H+ exchanger, basolateral membrane Na+/(3)HCO3- symporter, and Na+,K+-ATPase. [143] [144] Thus, AII can affect Na+ chloride absorption by two mechanisms. Activation of the Na+/H+ antiporter can directly increase Na+ chloride absorption. In addition, conditions that increase the rate of Na+ bicarbonate absorption can stimulate passive Na+ chloride absorption by increasing the concentration gradient for passive Cl- diffusion.[84] Sodium reabsorption is further promoted by the action of AII on the Na+,K+-ATPase in the medullary thick ascending limb of henle's loop (TALH).[113] Although the issue of distal action of AII was controversial in the past, more recent studies indicated that AII also regulates Na+ and bicarbonate reabsorption in distal segments of the nephron by modulating Na+/H+ exchange and the amiloride-sensitive Na+ channel. [142] [145] [146] [147] In this context, Wang and Giebisch[147] demonstrated that AII stimulates volume reabsorption in the late distal tubule not only via the acid-base transporter but also via Na+ channels.[147] Most recently, using isolated perfused cortical collecting duct segments dissected from rabbit kidneys, Peti-Peterdi and colleagues[148] clearly showed that AII directly stimulates the Na+ channel activity in this segment.
Two additional mechanisms may amplify the antinatriuretic effects of AII mediated by the direct actions of the peptide on renal hemodynamics and tubular transport. The first relates to the increased sensitivity of the TGF mechanism in the presence of AII, and the second to the effect of AII on the pressure-natriuresis relationship. The decrease in distal delivery produced by the action of AII on renal hemodynamics and proximal fluid reabsorption could elicit afferent arteriolar vasodilation via the TGF mechanism, which, in turn, could antagonize the AII-mediated increase in proximal reabsorption. This effect, however, is minimized because AII increases the responsiveness of the TGF mechanism, thus maintaining GFR at a lower delivery rate to the macula densa.[149] In addition, infusions of AII have been shown to blunt the pressure-natriuresis relationship and to shift the relationship between Na+ excretion and arterial pressure toward higher pressures. [113] [150] This “shift to the right” in the pressure natriuresis curve may be viewed as an important Na+-conserving mechanism in situations of elevated arterial pressure.
The pharmacologic development of ACE inhibitors and highly specific AII receptor antagonists provided additional insight into the mechanisms of action of AII in the kidney and further suggested that most of the known intrarenal actions of AII, particularly regulation of renal hemodynamics and proximal tubule reabsorption of Na+, HCO3-, are mediated by the AT1 receptor. [142] [151] However, recent functional studies showed that some part of the AII at the renal level is mediated by AT2 receptors.[133] AT2 receptor subtype plays a counteregulatory protective role against the AT1 receptor-mediated antinatriuretic and pressor actions of AII. The accepted concept that AI is merely converted to AII was revised through the demonstration that AI is also a substrate for the formation of Ang-(1-7).[152] Moreover, a recently discovered homolog of ACE, ACE2, is responsible for the formation of Ang-(1-7) from AII and for the conversion of AI to Ang-(1-9), which may be converted to Ang-(1-7) by ACE. [152] [153] Ang-(1-7) may play an important role as regulator of cardiovascular and renal function. Ang-(1-7) possesses opposite effects to those of AII, including vasodilatation, diuresis, and antihypertrophic action.[154] Thus, these relatively newly discovered components of the RAAS—ACE2 and Ang-(1-7)—may play a role as negative regulators of the classic ACE system.[153]
Finally, aldosterone, the second active component of the RAAS, plays an important physiologic role in the regulation of ECF and Na+ homeostasis.[155] The primary sites of aldosterone action are the principal cells of the cortical collecting tubule and convoluted distal tubule, inwhich this hormone promotes the reabsorption of Na+ and the secretion of K+ and protons. [155] [156] Mineralocorticoids may also enhance electrogenic Na+ transport, but not K+secretion, in the inner medullary collecting duct (IMCD).[157] Aldosterone exerts its effects on ionic transport by increasing the number of open Na+ and K+ channels in the luminal membrane and the activity of Na+-K+-ATPase in the basolateral membrane.[158] The effect of aldosterone on Na+ permeability appears to be the primary event because blockade of Na+ channels with amiloride prevents the initial increase in Na+ permeability and Na+-K+-ATPase activity. [159] [160] [161] This effect on Na+ permeability is mediated by several potential mediators including changes in intracellular Ca2+ levels, changes in intracellular pH, and methylation of channel proteins, thus increasing mean open probability of the Na+ channel. [159] [160] However, the long-term effect of aldosterone on Na+-K+-ATPase activity involves de novo protein synthesis and is regulated at the transcriptional level by serum and glucocorticoid-induced kinase-1 (SGK-1). [162] [163] The Na+-retaining effect of aldosterone in the collecting tubule occurs in association with an increase in the transepithelial potential difference, which favors K+ excretion. In terms of overall body fluid homeostasis, the actions of aldosterone in the defense of ECF result from the net loss of an osmotically active particle primarily confined to the intracellular compartment (K+) and its replacement with a corresponding particle primarily confined to the ECF (Na+). The effect of a given circulating level of aldosterone on overall Na+ excretion depends on the volume of filtrate reaching the collecting duct and the composition of luminal and intracellular fluids. As noted earlier, this delivery of filtrate is in turn determined by other effector mechanisms (AII, sympathetic nerve activity, and peritubular physical forces) acting at more proximal nephron sites. It is not surprising that Na+ balance can be regulated for a wide range of intake, even in subjects without adrenal glands, and despite fixed low or high supplemental doses of mineralocorticoids. Under these circumstances, other effector mechanisms predominate in controlling urinary Na+ excretion, although often in a setting of altered ECF volume and/or K+ concentration.
In terms of blood pressure maintenance, systemic vasoconstriction—another major extrarenal action of AII—may be considered the appropriate response to perceived ECF volume contraction. As mentioned previously, higher concentrations of AII are required to elicit this response than those that govern renal antinatriuretic actions of AII, a situation analogous to the discrepancy between antidiuretic and pressor actions of vasopressin. Transition from an antinatriuretic to a natriuretic action of AII at high infusion rates can be attributed almost entirely to a concomitant rise in blood pressure.[150] Over the past few years, increasing evidence suggests that, besides the adrenal glomerulosa, aldosterone may also be produced by the heart and vasculature. It exerts powerful effects on blood vessels,[164] independent of actions that can be attributed to the blood pressure rise via regulation of salt and water balance. As observed with AII, aldosterone also possesses significant mitogenic properties. It directly increases the expression and production of transforming growth factor-β and thus is involved in the development of glomerulosclerosis, hypertension, and cardiac injury/hypertrophy. [113] [155] [164]
In summary, AII, the principal effector of the RAAS, regulates extracellular volume and renal Na+ excretion through intrarenal and extrarenal mechanisms. The intrarenal hemodynamic and tubular actions of the peptide and its main extrarenal actions (systemic vasoconstriction and aldosterone release) act in concert to adjust urinary Na+ excretion under a variety of circumstances associated with alterations in ECF volume. Many of these mechanisms are synergistic and tend to amplify the overall influence of the RAAS.
Antidiuretic Hormone
ADH is a nonapeptide (9 a.a) hormone, synthesized in the brain and secreted from the posterior pituitary gland into the circulation in response to an increase in plasma osmolality (via osmoreceptor stimulation) or a decrease in effective circulating volume and blood pressure (via baroreceptor stimulation).[165] Thus, ADH plays a major role in the regulation of water balance and the support of blood pressure and circulating volume. ADH exerts its biologic actions through at least three different G-protein-coupled receptors.[166] Two of these receptors, V1A and V2, are abundantly expressed in the cardiovascular system and the kidney and mediate the two main biologic actions of the hormone, namely, vasoconstriction and increased water reabsorption by the kidney. The V1A receptor operates through the phosphoinositide signaling pathway, causing release of intracellular Ca2+ ions. Found in the vascular smooth muscle cells, hepatocytes, and platelets, it mediates vasoconstriction, glycogenolysis, and platelet aggregation, respectively. The V2 receptor, found mainly in the renal collecting duct epithelial cells, is linked to the adenylate cyclase pathway, utilizing cyclic adenosine monophosphate (cAMP) as its second messenger. Activation of this receptor leads to increased synthesis and recruitment of aquaporin II (AQP II) water channels into the apical membrane of collecting duct cells, thus increasing the water permeability of the collecting duct.[167]
Under physiologic conditions, the ADH primarily functions to regulate water content in the body by adjusting water reabsorption in the collecting duct according to plasma tonicity. A change in plasma tonicity by as little as 1% causes a parallel change in ADH release. In turn, this alters the water permeability of the collecting duct. ADH's antidiuretic action results from complex effects of this hormone on principal cells of the collecting duct.[167] (1) ADH provokes the insertion of AQP II water channels into the luminal membrane (short-term response) and increases synthesis of AQP II mRNA and protein (long-term response)—both responses increase water permeability along the collecting duct. [167] [168] This is considered in detail in Chapter 9 . Briefly, activation of V2 receptors localized to the basolateral membrane of the principal cells increases cytosolic cAMP, which stimulates the activity of protein kinase A. The latter triggers an unidentified phosphorylation cascade that promotes the translocation of AQP II from intracellular stores to the apical membrane, which allows the reabsorption of water from the lumen to the cells. Then, the water exits the cell to the hypertonic interstitium via AQP III and AQP IV, localized at the basolateral membrane. [169] [170] (2) ADH increases the permeability of the IMCD to urea, via activation of the urea transporter (UT-A1), enabling the accumulation of the urea in the interstitium, where it contributes along with Na+ to the hypertonicity of the medullary interstitium, which is a prerequisite for maximum urine concentration and water reabsortpion.[167] ADH exerts several effects on Na+ handling at different segments of the nephron, where it increases the Na+ reabsorption via activation of epithelial Na+ channel (EnaC) mainly in the cortical and outer medullary collecting duct (OMCD).[167] In addition, ADH may influence renal hemodynamics and reduce RPF, especiallyto the inner medulla.[171] The latter is mediated by the V1A-receptor and may be modulated by the local release of NO and PGs. At higher concentrations (pathophysiologic range), ADH may also decrease total RPF and GFR, as a part of the generalized vasoconstriction induced by the peptide.[167]
A third receptor for ADH, V3 (V1β), is found predominantly in the anterior pituitary and is involved in the regulation of adrenocorticotropic hormone (ACTH) release.
In addition to its renal effects, ADH also regulates extrarenal vascular tone through the V1A receptor. Stimulation of this receptor by ADH results in a potent arteriolar vasoconstriction in various vascular beds with a significant increase in systemic vascular resistance (SVR). However, physiologic increases in ADH do not usually cause a significant increase in blood pressure, because ADH also potentiates the sinoaortic baroreflexes that subsequently reduce heart rate and cardiac output.[172] Nevertheless, at supraphysiologic concentrations of ADH, such as occur when effective circulating volume is severely compromised (e.g., shock, CHF), ADH plays an important role in supporting arterial pressure and maintaining adequate perfusion to vital organs such as the brain and myocardium. ADH also has a direct, V1 receptor-mediated, inotropic effect in the isolated heart.[173] In vivo, however, ADH has been reported to decrease myocardial function,[174] the latter attributed due to either cardioinhibitory reflexes or coronary vasoconstriction induced by the peptide. More importantly, ADH has been shown to stimulate cardiomyocyte hypertrophy and protein synthesis in neonatal rat cardiomyocytes and in intact myocardium through a V1-dependent mechanism. [175] [176] These effects are very similar to those obtained with exposure of cardiomyocytes to AII or catecholamines, although not necessarily through the same cellular mechanisms. By this growth-promoting property, ADH may contribute to the induction of cardiac hypertrophy and remodeling.
Controversy exists regarding the effect of ADH on natriuresis, with some authors finding a natriuretic response with infusions and others finding Na+ retention. [177] [178] These variations may be due to species differences. Blandford and colleagues[179] infused rats with a specific antagonist of V2 receptors, resulting in increased Na+ and water excretion, and suggested that the endogenous activity of ADH is one of Na+ retention. However, in terms of overall volume homeostasis, the predominant influence related to ADH arises indirectly from water accumulation or blood pressure changes. The systemic vasoconstrictor actions of ADH are the effects that would be expected to defend blood pressure in the presence of perceived ECF volume contraction. However, in this regard, potential hypertensive effects of ADH are buffered by a concomitant increase in baroreflex-mediated sympathoinhibition or by an increase in PGE2, resulting in a blunting of vasoconstriction, and by a direct vasodepressor action of V2 receptor activation. [178] [180] [181]
Prostaglandins (see also Chapter 11 )
PGs in the kidney regulate renal function including hemodynamics, renin secretion, growth response, tubular transport processes, and immune response in both health and disease states ( Table 12-3 ).[182] Currently, two known principal isoforms of cycloxygenase (COX-1 and COX-2) metabolize arachidonic acid released from membrane phospholipids to PGs (see also Chapter 11 ). Recently, an additional splice variant of the COX-1 gene, COX-3, isoforms was identified.[182] COX-1 and COX-2 catalyze the synthesis of PGH2, which then converted into the various prostanoids.[183] COX-1 is constitutively expressed in many cell types, with abundant expresson in renal cells where high immunoreactive levels are found, especially in the collecting duct and medullary interstitial cells of most species.[184] In contrast, the expression of COX-2 is inducible and cell-type specific, with prominent renal expression levels varying among species. [185] [186] Published studies in dog, rat, and rabbit revealed COX-2 expression in medullary interstitial cells, cells of the TALH, and cells of the macula densa, where expression has been shown to be regulated in response to varying salt intake. [187] [188] [189] Lower levels of COX-2 were detected in the tubular epithelial cells of the collecting duct. [186] [190] In human and monkey, COX-2 is expressed in the glomerular podocytes and blood vessels.[186] However, a more recent study in humans older than 60 years detected COX-2 in the macula densa and medullary interstitial cells.[189] Furthermore, the profile of sensitivity to pharmacologic inhibitors differs between the two isoforms. [191] [192] The principal eicosanoid metabolites of cyclooxygenase in the kidney are PGI2 (human) and PGE2 (rat), with smaller amounts of PGF2α, PGD2, and thromboxane A2 (TXA2).[184] Metabolism of arachidonic acid by other pathways (lipoxygenase, epoxygenase) leads to other products of importance in the modulation of nephron function.[184] The major sites for PG production (and hence for local actions) are the renal arteries and arterioles and glomeruli in the cortex and the renal medullary interstitial cells in the medulla, with additional contributions from epithelial cells of the cortical and medullary collecting tubules.[184] [186] [193] Studies have revealed that PGI2 and PGE2 are the prominent products in the cortex of normal kidney, with PGE2 predominating in the medulla.[184]
TABLE 12-3 -- Major Renal Biologic Effects of Prostaglandins and Thromboxane
|
Agent |
Target Structure |
Mode of Action |
Direct Consequences |
|
PGE2, PGI2 |
Intrarenal arterioles |
Vasodilation |
Increased renal perfusion (more pronounced in inner cortical and medullary regions) |
|
PGI2 |
Glomeruli |
Vasodilation |
Increased filtration rate |
|
PGE2, PGI2 |
Efferent arterioles |
Vasodilation |
Increased Na† excretion through increased postglomerular perfusion |
|
PGE2, PGI2, PGF2α |
Distal tubules |
Decreased transport |
Increased Na† excretion, decreased maximum medullary hypertonicity |
|
PGE2, PGI2, PGF2α |
Distal tubules |
Inhibition of cAMP synthesis |
Interference with ADH action |
|
PGE2, PGI2 |
Juxtaglomerular apparatus |
cAMP stimulation (?) |
Increased rennin release |
|
TxA2 |
Intrarenal arterioles |
Vasoconstriction |
Decreased renal perfusion |
|
ADH, antidiuretic hormone; cAMP, cyclic adenosine monophosphate; PGE, prostaglandin E; PGI, prostaglandin I; TxA2, thromboxane A2. |
The two major roles for the contribution of PGs to volume homeostasis are related to their effect on RBF, on one hand, and on their effect on tubular handling of salt and water, on the other. Table 12-3 lists target structures, mode of action, and major biologic effects of the renal active PGs and TXA2. Some of the information provided by this table is still subject to active research. In balance, it appears that PGI2 and PGE2 have a predominantly vasodilating and natriuretic activity; they also interfere with action of ADH and tend to stimulate renin secretion. TxA2 has been shown to cause vasoconstriction; the importance of the physiologic effects of TxA2 on the kidney is still controversial. The end result of the stimulation of PG secretion in the kidney eventually leads to vasodilation, increased renal perfusion, natriuresis, and facilitation of water excretion.
The role of PGs acting as vasodilators in the glomerular microcirculation has been extensively characterized and well established. The cellular targets for vasoactive hormones in the glomerular microcirculation are vascular smooth muscle of the afferent and efferent arteriole and mesangial cells within the glomerulus. Action at these sites governs renal vascular resistance (RVR), glomerular function, and downstream microcirculatory function in peritubular capillaries and vasa recta. In vivo studies showed that intrarenal infusions of PGE2 and PGI2 cause vasodilation and increased RPF.[184] In agreement with these findings, in vitro experiments with isolated renal microvessels showed that both PGE2 and PGE1 attenuate AII-induced afferent arteriolar vasoconstriction and PGI2 antagonizes AII-induced efferent arteriolar vasoconstriction.[194] Similarly, PGE2 has been shown to counteract AII-induced contraction of isolated glomeruli and glomerular mesangial cells in culture, and conversely, cyclooxygenase inhibition augments these contractile re-sponses. An inhibitory counterregulatory role of PGs with respect to renal nerve stimulation has been demonstrated in micropuncture studies.[195] Therefore, the elimination of the vasorelaxant action of PGE2 and PGI2 at these target sites by treatment with selective and nonselective cyclooxygenase inhibitors is believed to result in an augmented fall in glomerular blood flow. However, this occurs mainly in the setting of heightened vasoconstrictor input, such as occurs during states of real or perceived volume depletion. [184] [186] [192] These conditions, which include overt dehydration, CHF, liver cirrhosis, nephrotic syndrome, and adults older than 60 years, are invariably associated with activation of pressor mechanisms RAAS, CNS, and ADH.[196] The renal vasoconstrictive influences of NE and AII are mitigated by their simultaneous stimulation of vasodilatory renal prostaglandins. [196] [197] RBF and GFR are thus maintained, averting prerenal azotemia or even ischemic damage to the renal parenchyma. When this PG-mediated counterregulatory mechanism is suppressed by drugs that inhibit cyclooxygenase (e.g., nonsteroidal anti-inflammatory drugs [NSAIDs]), an impairment of renal hemodynamics develops, thereby leading to rapid deterioration in renal function. Although the introduction of selective COX-2 inhibitors has been associated with clear-cut decrease in gastrointestinal bleeding, it is becoming increasingly apparent that COX-2 inhibitors can cause a spectrum of renal effects nearly identical to those observed with the classic, nonselective NSAIDs. [198] [199] These adverse effects are not surprising in light of the recent laboratory observations indicating that COX-2 is constitutively expressed in the kidney and plays a critical role in regulating renal hemodynamics, excretory function, and renin secretion. [198] [199] COX-2–derived prostanoids are required for preservation of RPF and GFR, especially in states of ECF volume deficit, and also promote natriuresis and stimulate renin secretion during low Na+ intake or the use of loop diuretics. [186] [200] Selective COX-2 inhibitors decrease GFR, and renal perfusion and may cause acute renal injury. Moreover, COX-2 inhibitors such as celecoxib or rofecoxib caused Na+ and K+ retention, edema formation, CHF, and hypertension similar to the nonselective COX inhibitors diclofenac and naproxen. [186] [192] Thus, acute Na+ retention by NSAIDs in volume-depleted healthy adults is extensively mediated by inhibition of COX-2.
Whereas the role of PGs in modulating glomerular vasoreactivity in states of varying salt balance is firmly established, the effects of PGs on salt excretion per se are less well established. Certainly, the aforementioned vascular effects of PGs can be expected to have secondary effects on tubule function through the various physical factors described previously in this chapter. One particular consequence of PG-induced renal vasodilatation may be medullary interstitial solute washout. Such a change in medullary interstitial composition could potentially account for the observed increase in urinary Na+ excretion with intrarenal infusion of PGE2. [184] [201] Studies by Haas and colleagues[202] showed that the natriuretic response to PGE2 may be attenuated by preventing an increase in renal interstitial hydraulic pressure, even in the presence of a persistent increase in RBF. The same group demonstrated in rats that the natriuresis usually accompanying direct expansion of renal interstitial volume can be significantly attenuated by inhibition of PGs synthesis. These findings are consistent with the proposal that changes in PGs have a significant effect on renal Na+ excretion. A number of micropuncture and microcatheterization studies in vivo suggested effects of PGs on urinary Na+ excretion independent of hemodynamic changes.[184] Motivated by such findings, investigators sought direct effects of PGs on epithelial transport in individual isolated perfused tubule preparations, taken from various nephron segments. These studies showed that the effects of PGE2 on transport processes vary considerably in different nephron segments.[203] In the medullary TALH and the collecting tubule, PGE2 has been reported to cause a decrease in the reabsorption of water, Na+, and chloride.[203] This inhibition of Na+ reabsorption in the medullary TALH and in the cortical collecting duct is correlated with reduced Na+,K+-ATPase activity. In contrast, in the distal convoluted tubule, PGE2 caused increased Na+,K+-ATPase activity.[203] Most likely, the net effects of locally produced PGs on tubular Na+ handling is inhibitory because complete blockade of PGs synthesis by indomethacin in rats receiving a normal or salt-loaded diet increased fractional Na+ reabsorption and enhanced the activity of the renal medullary Na+,K+-ATPase.[204] In addition, PGs diminish the renal response to ADH. [194] [205] Several studies revealed that PGE2 inhibits ADH-stimulated Na+ chloride reabsorption in the medullary TALH and ADH-stimulated water reabsorption at the collecting duct. [194] [206] Both of these effects would tend to antagonize the overall hydroosmotic response to ADH. However, because no such effect is seen in the cortical TALH, which is capable of augmenting Na+ chloride reabsorption in response to an increased delivered load, and the effects of PGs on solute transport in the collecting tubule remain controversial, no conclusions can be reached with respect to the contribution of direct epithelial effects of PGs to overall Na+ excretion.[194]
Similarly, it is not surprising that whole animal and clinical balance studies that have examined the effect of PG infusion or prostaglandin synthesis inhibition on urinary Na+ excretion, or that have attempted to correlate changes in urinary PG excretion with changes in salt balance, also yielded conflicting and inconclusive results. One complicating feature stems from the fact that PG excretion rates vary with urine flow rates. Nevertheless, one conclusion can be stated with confidence: PGs have an important influence on urinary Na+ excretion, precisely in the settings in which they are important in preserving GFR, namely, states of vasoconstrictor hormone activation (e.g., Na+-depletion states). A particularly striking illustration of this role emerged from studies using HWI by Epstein and co-workers.[207] In these studies, the natriuretic response to HWI was accompanied by an increase in urinary PG excretion. However, inhibition of cyclooxygenase did not blunt the natriuretic response to this central volume-expanding maneuver in salt-replete subjects but did blunt the natriuretic response in salt-depleted individuals.
The influence of changes in Na+ intake on renal COX-1 and COX-2 expressions has been studied intensively in the last few years. The expression of COX-2 in the macula densa and TALH is increased by low-salt diet. Similar alteration in COX-2 was reported by inhibition of renin angiotensin aldosterone and by renal hypoperfusion. [185] [189] In contrast, a high-salt diet has been reported to decrease COX-2 expression in the renal cortex. [185] [189] None of these changes on Na+ intake affected the expression of COX-1 in the cortex of the kidney. In the renal medulla, whereas low-salt diet down-regulated both COX-1 and COX-2, high-salt diet enhanced the expression of these cyclooxygenase isoforms.[189] In vitro studies showed that high osmolarity of the medium of cultured IMCD cells induces the expression of COX-2.[186] Infusion of nimesulide (a selective COX-2 inhibitor) into anesthetized dogs on normal Na+ diet reduced urinary Na+ excretion and urine flow rate, despite the lack of effect on renal hemodynamics or systemic blood pressure.[186] Collectively, these findings suggest that COX-2 is distinctly regulated in the renal cortex and medulla and that its expression is altered by Na+ intake on the one hand and that COX-2 inhibition hampers the urinary Na+ excretion, on the other hand.
Finally, it should be recalled that in addition to the hemodynamically mediated and potential direct epithelial effects of PGs already enumerated, PGs may mediate observed physiologic responses to other hormonal agents. The intermediacy of PGs in renin release responses has already been cited. As another example, some, but not all, of the known physiologic effects of BK and other products of the kallikrein-kinin system are mediated through BK-stimulated PG production (e.g., inhibition of ADH-stimulated osmotic water permeability in the cortical collecting tubule).[194] In addition, some of the renal and systemic actions of AII are mediated via various PG production by both COX-1 and COX-2. For instance, COX-2 inhibitors or COX-2 knockout dramatically augment the pressor effects of AII and reduced medullary blood flow of this hormone.[208] In contrast, in COX-1–deficient mice, AII did not reduce the medullary blood flow, suggesting synthesis of COX-2–dependent vasodilators. Moreover, the diuretic and natriuretic effects of AII were absent in COX-2 deficiency. The authors concluded that COX-1 and COX-2 exert opposite effects on systemic and renal function: COX-2 mediates the vasodilatory and natriuretic effects of AII, whereas COX-2 mediates the pressor effect of AII.[208]
The Natriuretic Peptide Family
Major advances have taken place in our understanding of the physiologic and pathophysiologic roles of the NP family in the regulation of Na+ and water balance since the discovery of ANP by de Bold and colleagues.[209] ANP is an endogenous 28–amino acid peptide secreted mainly by the right atrium. Besides ANP, the NP family contains at least two other members, BNP and CNP.[27] Although encoded by different genes, these peptides share a high similarity in chemical structure, gene regulation, and degradation pathways, yielding a unique hormonal system that exerts various biologic actions on the renal, cardiac, and blood vessel tissues. [210] [211] ANP plays an important role in blood pressure and volume homeostasis owing to its ability to induce natriuretic/diuretic and vasodilatory responses. [212] [213] [214] BNP has an amino acid sequence similar to that of ANP, with an extended NH2-terminus. In humans, BNP is produced from proBNP, which contains 108 amino acids and, following a proteolytic process, releases a mature 32–amino acid molecule and N-terminal fragment into the circulation. Although BNP was originally cloned from the brain, it is now considered a circulating hormone produced mainly in the cardiac ventricles.[215] CNP, which is produced mostly by endothelial cells, shares the ring structure common to all NP members; however, it lacks the C-terminal tail ( Fig. 12-5 A).
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FIGURE 12-5 A, Amino acid sequences and structures of the three mature members of the natriuretic peptide family: atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). B, Schematic model of the structures of the different types of the natriuretic peptide receptors. cGMP, cyclic guanosine monophosphate; GTP, guanine triphosphate; NPR-A, ANP receptor; NPR-B, BNP receptor; NPR-C, CNP receptor. (From Abassi Z, Karram T, Ellaham S, et al: Implications of the natriuretic peptide system in the pathogenesis of heart failure: Diagnostic and therapeutic importance. Pharmacol Ther 102:223-241, 2004.) |
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The biologic effects of the NPs are mediated by binding the peptide to specific membrane receptors localized to numerous tissues, including vasculature, renal artery, glomerular mesangial and epithelial cells, collecting duct, adrenal zona glomerulosa, and CNS.[210] At least three different subtypes of NP receptors have been identified: NP-A, NP-B, and NP-C (see Fig. 12-5B ).[216] NP-A and NP-B, single transmembrane proteins with a molecular weight (MW) of ≈120 to 140 kDa, mediate most of the biologic effects of NPs. Both are coupled to GC, which contains the protein kinase and GC domains in their intracellular portions. [211] [217] [218] After binding to their receptors, all NPs isoforms (ANP, BNP, and CNP) markedly increase cGMP in target tissues and in plasma. Therefore, analogs of cGMP or inhibition of degradation of this second messenger mimic the vasorelaxant and renal effects of these peptides. The third class of NP-binding sites, NP-C (MW of 60-70 kDa), are believed to serve as clearance receptors because they are not coupled to any known second messenger system.[219] ANP-C receptors are the most abundant type of NP receptors in many key target organs of these peptides.[219] Additional routes for the removal of NPs includes enzymatic degradation by neutral endopeptidase 24.11 (NEP), a metalloproteinase located mainly in the lung and the kidney.[220]
Atrial Natriuretic Peptide.
Both in vivo and in vitro studies, in humans as well as in experimental animals, established the role of ANP in the regulation of ECF volume and the control of blood pressure by acting on all organs/tissues involved in the homeostasis of Na+ and blood pressure [212] [221] [222] ( Table 12-4 ).
TABLE 12-4 -- Physiologic Actions of the Natriuretic Peptides
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Target organ |
Biologic Effects |
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Kidney |
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Cardiac |
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Hemodynamic |
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Endocrine |
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Mitogenesis |
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Therefore, it is not surprising that ANP and NH2-terminal ANP levels are increased in (1) conditions associated with enhanced atrial pressure, (2) systolic or diastolic cardiac dysfunction, (3) cardiac hypertrophy/remodeling, and (4) severe myocardial infarction (MI). [222] [223] [224] [225] In the kidney, ANP exerts hemodynamic/glomerular effects that increase Na+ and water delivery to the tubule, in combination with inhibitory effects on tubular Na+ and water reabsorption, leading to remarkable diuresis and natriuresis. [222] [226] In addition to its powerful diuretic and natriuretic activities, ANP also relaxes vascular smooth muscle, thus acting as antagonist to vasoconstriction. The vasodilatory actions of ANP appear to be most evident in the context of antagonizing the concomitant action of such vasoconstrictive influences as AII, endothelin (ET), ADH, and a1-adrenergic input. [27] [221] [226] In addition, ANP reduces cardiac output by shifting fluid from the intravascular to the extravascular compartment, an effect medicated by increased capillary hydraulic conductivity to water.[212] In this context, ANP provokes vasodilation, which leads to reduced preload and subsequently to a fall in cardiac output. [27] [226] [227] ANP inhibits the activity of vasoconstrictor systems, such as the RAAS, the SNS, ADH, and ET system, and acts on the CNS to modulate vasomotor tone, thirst, and ADH release. [27] [226] ANP has also been shown to exert antiproliferative, growth-regulatory properties in cultured glomerular mesangial cells, vascular smooth muscle cells (VSMC), and endothelial cells. [27] [226]Within the kidney, ANP causes afferent vasodilation and efferent vasoconstriction, thus leading to a rise in glomerular capillary pressure, GFR, and filtration fraction (FF). [228] [229] In combination with increased medullary blood flow, these hemodynamic effects enhance diuresis and natriuresis. However, the overall natriuretic effect of ANP infusion does not require these changes in glomerular function (except to larger doses of the peptide). At the tubular level, ANP inhibits the stimulatory effect AII on Na+,H+ exchanger localized to the luminal side of the proximal tubule. [230] [231] Likewise, ANP, acting via cGMP, inhibits thiazide-sensitive Na+,Cl- cotransporter in the distal tubule and Na+ channels in the collecting duct, along with inhibition of ADH-induced AQP II incorporation in the apical membrane of these segments of the nephron [228] [232] [233] [234] (see Table 12-4 ).
Brain Natriuretic Peptide.
Administration of BNP to human subjects induces natriuretic, endocrine, and hemodynamic responses similar to those induced by ANP.[235] It is well established that BNP is produced and secreted in small amounts by the atrium, compared with the ventricles, which are the major sites of its production.[215] Increased volume or pressure overload states such as CHF and hypertension enhance the secretion of BNP from the ventricles. Despite the comparable elevation in plasma levels of ANP and BNP in patients with CHF and other chronic volume-expanded conditions, acute intravenous saline loading or infusion of pressor doses of AII yields different patterns of ANP and BNP secretion. [224] [236] Whereas plasma levels of ANP increase rapidly, the changes in plasma BNP of atrial origin are negligible, supporting the fact that the atrium contains tiny amounts of BNP, in contrast to the high abundance of ANP.[236] Moreover, plasma levels of BNP rise with age. Whereas circulating BNP levels are 26±2 pg/mL in subjects aged 55 to 64 years, they increase to 31±2 and 64±6 pg/mL in patients aged 65 to 74 years and 75 years or older, respectively.[237]
Animal and human studies demonstrated the natriuretic effects of pharmacologic doses of BNP. When administered to normal volunteers and hypertensive subjects at low doses, BNP induces a significant increase in urinary Na+excretion and to a lesser extent in urinary flow.[236] Significant natriuresis and diuresis were observed following the infusion of either ANP or BNP to normal subjects. The combination of ANP and BNP did not produce a synergistic renal effect, suggesting that these peptides share similar mechanisms of action.[236] Moreover, in similarity to ANP, BNP exerts a hypotensive effect in both animals and human subjects. For instance, transgenic mice that overexpress the BNP gene exhibit significant and lifelong hypotension to the same extent as transgenic mice that overexpress the ANP gene.[27] Therefore, it is clear that BNP induces its biologic actions through mechanisms similar to those of ANP. [27] [236] This notion is supported by several findings: (1) Both ANP and BNP act via the same receptors, and both induce similar renal, cardiovascular, and endocrine actions in association with an increase in cGMP production[238] (see Table 12-4 ); (2) BNP suppresses the ACTH-induced aldosterone generation by cultured human adrenal cells.[236] Similar results were observed when BNP was infused in vivo.[239] The latter may be attributed to the inhibitory effects of BNP on renin secretion, as was shown in dogs.[238] In contrast, when BNP was given to humans, no significant change in plasma renin activity (PRA) was obtained. Similar to ANP, the hemodynamic effects of BNP vary according to the dose range and the species. When injected as a bolus at high doses, BNP caused a profound fall in systolic blood pressure; however, when infused at low doses, this peptide failed to change blood pressure or heart rate.[27] The effects of BNP have been utilized in the clinical setting in the treatment of the volume overload state of CHF, though recent studies showed a potentially deleterious effect of such therapy on renal function, as is outlined later in the section on CHF.
C-type Natriuretic Peptide.
Although CNP is considered a neurotransmitter in the CNS, most recently it has been shown that endothelial cells produce considerable amounts of this NP, where it plays a role in the local regulation of vascular tone.[240] Smaller amounts of CNP are produced in kidney, heart ventricles, and intestine. [241] [242] In addition, CNP has been found in human plasma, which could be of endothelial or cardiac origin. The physiologic stimuli for CNP production have not been identified, although enhanced CNP mRNA expression has been reported after volume overload.[27] Intravenous infusion of CNP decreases blood pressure, cardiac output, urinary volume, and Na+ excretion. Furthermore, the hypotensive effects of CNP are less than those of ANP and BNP, but strongly stimulate cGMP production and inhibit VSMC proliferation.[27] Although all three NP forms inhibit the RAAS, CNP (unlike ANP and BNP) failed to induce significant hemodynamic changes in sheep, such as depression of cardiac output, reduction in blood pressure, and plasma volume contraction, [27] [243] supporting the widely accepted concept that ANP and BNP are the major circulating NPs, whereas CNP is largely considered a local regulator of vascular structure/tone. Although all forms of NPs exist in the brain, their role and significance in the regulation of salt and water balance are not understood.
Taken together, the various biologic actions of NPs lead to reduction of effective volume, an expected response to perceived overfilling of the central intrathoracic circulation. Furthermore, all NPs counteract the adverse effects of RAAS, suggesting that the two systems are acting oppositely in the regulation of body fluid and cardiovascular homeostasis. Collectively, NPs are believed to participate in the regulation of Na+ and water balance and blood pressure.
Endothelium-Derived Factors
The endothelium has been recognized as a major source of active substances that regulate the vascular tone in health and disease. [244] [245] [246] [247] [248] The most known representatives of these substances are: ET, NO (or as originally termed, endothelium-derived relaxing factor [EDRF]), and PGI2. It is now well established that these vasoconstricting and vasodilating factors regulate the perfusion pressure of multiple organ systems that are strongly involved in water and Na+ balance, such as the kidney, heart, and vasculature.
This section summarizes some of the concepts regard-ing actions of ET and EDRF/NO relevant to volume homeostasis.
Endothelin.
The ET system consists of three vasoactive peptides, namely endothelin 1 (ET-1), endothelin 2 (ET-2), and endothelin 3 (ET-3). These peptides are synthesized and released mainly by endothelial cells and act in a paracrine/autocrine mode of action. [249] [250] [251] ET-1, the major representative of the ET family, is the most potent vasoconstrictor known at present.[252] All ETs are synthesized by proteolytic cleavage from specific preproETs that are further cleaved to form a 37-39–amino acid precursor, called big ET. Big ET is then converted into the biologically active, 21–amino acid peptide by a highly specific endothelin-converting-enzyme (ECE), a phosphoramidon-sensitive membrane-bound metalloprotease. To date, two isoforms of ECE have been identified: ECE-1 and ECE-2.[253] Four differentially spliced isoforms of ECE-1—ECE-1α, ECE-1β, ECE-1c, ECE-1d, are expressed in a variety of tissues including endothelial cells, and process big ET both intracellularly and on the cell surface. ECE-2 is localized mainly to VSMC, and is most likely an intracellular enzyme.[253] In ECE-1 knockout mice, tissue levels of ET-1 are reduced by about one third, suggesting that ECE independent pathways are involved in the synthesis of this peptide.[254] Recently, Wypij and co-workers[255] reported that chymase generates ET-11-21 as well as ET-11-31 peptide. The ETs bind to two distinct receptors, designated ETA and ETB. [251] [253] The ETA receptor shows a higher affinity for ET-1 than ET-2 and ET-3. The ETB receptor shows equal affinity for each of the three ETs. Both receptors are expressed in a variety of tissues, including blood vessels, kidney, myocardium, lung, and brain. [250] [251] [252] [253] The vasoconstrictor response to ET is induced by a ETA receptor-mediated increase in cytosolic Ca2+. The endothelium-dependent relaxation is mediated by the ETB receptors via an NO-coupled mechanism. ET is detectable in the plasma of human subjects and many experimental animals and therefore may also act as a circulating vasoactive hormone. [249] [252] The best known action of ET-1 is vasoconstriction, and its role in vascular homeostasis has been established. [251] [253] In addition, accumulating evidence indicates that it has a variety of effects on the kidney.[250] The kidney is both a source of ET production (mainly the inner medulla) and an important target organ of the peptide. ET-1 is synthesized by the endothelial cells of the renal vessels, and ET-1 and ET-3 are produced by various cell types of the nephron.[256] Three major aspects of renal function are affected by ET: (1) renal vascular and mesangial cell tone, (2) renal tubular transport of salt and water, and (3) proliferation and mitogenesis of glomerular mesangial cells. Both ETA and ETB receptors are present in the glomerulus, renal vessels, and tubular epithelial cells, but the vast majority of the ETB subtype are found within the medulla.[250] The renal vasculature appears to be most sensitive to the vasoconstrictor action of ET-1 as compared with other vascular beds. Infusion of ET-1 into the renal artery of anesthetized rabbits decreases RPF, GFR, natriuresis, and urine volume.[257]Micropuncture studies demonstrated that ET-1 increases afferent and efferent arteriolar resistance (afferent more than efferent), resulting in a reduction in glomerular plasma flow rate. In addition, the ultrafiltration coefficient Kf is reduced owing to contraction of the mesangial cells, resulting in a diminished SNGFR. The profound reduction of RPF and concomitant lesser reduction in GFR should result in a rise in FF, but the effect of ET-1 on the FF appears to be variable, with some groups using low doses in a canine model reporting a rise[258] and others reporting no significant effect.[259] Infusion of ET-1 for 8 days into conscious dogs increased plasma levels of ET by two- to threefold and resulted in increased RVR and decreased GFR and RPF.[260] Interestingly, the effect of ET on regional intrarenal blood flow is not homogenous. By using laser-doppler flowmetry, Gurbanov and colleagues[261] reported that administration of ET-1 in control rats produced a sustained cortical vasoconstriction and a transient medullary vasodilatory response. These results are in line with previous studies reporting that the medulla predominantly expresses ETB receptors, whereas the cortical vasculature contains a high density of ETA-binding sites.[250]
The effect of ET on Na+ and water excretion varies and depends on the dose and the source of ET. Systemic infusion of ET in high doses results in a profound antinatriuretic and antidiuretic effect, apparently secondary to the decrease in GFR and RBF.[262] However, in low doses or when produced locally in tubular epithelial cells, ET has been claimed to decrease the reabsorption of salt and water, suggesting the presence of ET-1 target sites on renal tubules.[263] Also, administration of the ET precursor, big ET, has been shown to cause natriuresis, supporting the notion of a direct inhibitory autocrine action of ET on tubular salt reabsorption. Hoffman and colleagues[264]observed that the natriuretic and diuretic actions of big ET-1 can be significantly reduced by ETB-specific blocker, A-192621. Similar results were reported by Pollock[265] when the same ETB antagonist was given chronically via osmotic minipump. Furthermore, Gariepy and co-workers[266] recently demonstrated that ETB knockout rats have salt-sensitive hypertension and that the luminal ENaC blocker amiloride restores normal arterial pressure in these rats, suggesting that in vivo ETB in the collecting duct tonically inhibits ENaC activity, the final regulator of Na+ balance. Similarly, mice with collecting duct-specific knockout of the ET-1 gene have impaired Na+ excretion in response to Na+ load and develop hypertension with high salt intake.[250] The mice also have heightened sensitivity to ADH and reduced ability to excrete acute water load. These findings are in line with in vitro observations that ETB mediates the inhibitory effects of ET-1 on ion and water transport in various medullary tubular segments. [250] [267] For instance ET-1 in vitro can inhibit Na+ or water transport in the collecting duct and TALH.[250] Thus, if vascular and mesangial ET exerts a greater physiologic effect than tubule-derived ET, then RBF is diminished and net fluid retention occurs, whereas if the tubule-derived ET effect predominates, salt and water excretion is increased.
The ability of ET-1 to inhibit the hydro-osmotic effect of ADH is firmly established. Oishi and co-workers[268] examined the response of the isolated perfused IMCD to ET-1 and showed that ADH-stimulated water permeability was reversibly inhibited. The precise mechanism remains to be determined. However, ET-1 reduces ADH-stimulated cAMP accumulation and water permeability in the IMCD.[250] In addition, ET-1 mitigates the hydro-osmotic effect of ADH in the cortical collecting duct and the OMCD. Moreover, studies in rabbit cortical collecting duct indicate that ET-1 may inhibit luminal amiloride-sensitive Na+ channels by a Ca2+-dependent effect. Taking into account that the medulla contains ETB receptors and the highest ET concentrations in the body and that ETs also inhibit Na+,K+-ATPase in IMCD,[250] collectively, these effects may contribute to the diuretic and natriuretic actions of locally produced ET-1. This may also explain the natriuretic effect of ET-1 reported by some investigators, despite the reduction in RBF and GFR.[267]
Renal ET production is modulated differently than that in the vasculature. Whereas the vascular (and mesangial) ET generation is controlled by thrombin, AII, and transforming growth factor-β, the renal tubule ET production is under unique control. The nature of such regulation was initially derived from studies examining urinary ET excretion, which was entirely of renal origin.[269] Volume expansion in humans increased urinary ET excretion, suggesting an inhibitory action of renal ET on water reabsorption, particularly in collecting duct.[250] Today, it is believed that water balance regulates nephron ET production, but it is uncertain whether Na+ balance has a direct impact. However, most recently, Sasser and colleagues[270] and Pollock and Pollock[271] provided evidence that ET-1 plays an important role in the response to high salt and that urinary ET-1 excretion is elevated in rats on a high-salt diet. Water balance may modulate collecting duct fluid reabsorption by altering the medullary tonicity. For instance, increasing the media tonicity with NaCl, raffinose, or mannitol decreased ET-1 synthesis by rat IMCD, Madin-Darby Canine Kidney cell line (MDCK), and M1 cortical collecting duct cells.[250] Moreover, inducing medullary hypotonicity, as occurred in water load, is most likely associated with augmented synthesis/release of medullary ET, thus provoking water loss. In contrast, medullary hypertonicity during dehydration probably reduces the generation of ET by the collecting duct and thus enhances fluid retention. Although not well established, the existence of such a system, in which collecting duct ET participates in the renal regulation of salt and water transport, is a tempting hypothesis.
Nitric Oxide.
NO, originally described as the EDRF, is a diffusable gaseous molecule produced in endothelial cells of the renal vasculature as well as in tubular epithelial and mesangial cells from its precursor L-arginine by the enzyme NO synthase (NOS), which exists in three distinct isoforms: NOS 1 (bNOS), NOS 2 (iNOS), and NOS 3 (eNOS).[272] The use of selective NOS inhibitors and knockout mice has improved the ability to investigate the individual role of the NOS isoforms in the regulation of renal function.[273] However, it is difficult to identify the role of NO produced by a given isoform in a given cell type. Therefore, we refer to the renal effects of NO regardless of its enzymatic isoform source.
In the past decade, evidence has been provided regarding the importance of locally produced NO in the regulation of renal function, including RPF, salt excretion, and renin re-lease as well as the long-term control of blood pressure.[273] [274] [275] NO has been shown to exert a tonic vasodilatory action on af-ferent arterioles and to mediate the renal vasorelaxant action of acetylcholine and BK. [273] [275] The action of NO is mediated by activation of a soluble GC in adjacent VSMCs, thereby increasing intracellular levels of its second messenger, cGMP. [276] [277] Evidence now indicates that all NOS isoforms are present in the human and other mammalian kidney. [273] [276] [277] [278]Recent studies have shown that renal NOS activity is regulated by several humoral factors such as aldosterone and salt intake (see later).[273]
NO plays an important role in the regulation of renal hemodynamics and excretory function,[279] best evidenced by the fact that inhibiting of intrarenal NO production results in increased blood pressure and kindey function. [280] [281] Infusion of NOS inhbitor, Ng monomethyl-l-arginine (l-NMMA), into one kidney of anesthetized dogs resulted in a dose-dependent decrease in urinary cGMP levels; decreases in RPF and GFR, antinatriuresis, and antidiuresis; and a decline in fractional Na+ excretion in that kidney compared with the one on the contralateral side.[281] In addition, acute NO blockade amplifies the renal vasoconstriction action of AII in isolated micoperfused rabbit afferent arterioles[282] and conscious rats,[283] suggesting that NO and AII interact in the control of renal vasculature. This notion is supported by the findings that L-NMMA-induced vasoconstriction, decreased RBF, and reduced hydraulic coefficient, Kf, were prevented when the RAAS was blocked, suggesting that some of the major effects of NO are to counterbalance the vasoconstrictive action of AII. In addition, it is evident that NO plays a significant role in regulating TGF and in modulating renin secretion by the juxtaglomerular apparatus. [273] [284] Inhibition of the NO system by nonselective blockers of NOS results in attenuation of the activity of TGF, augmentation of both its vasoconstriction and its vasodilator capacities, and stimulation of renin secretion. [273] [284] Most likely, bNOS, which localizes to the macula densa, is the major NOS isoform responsible for TGF behavior.[279]
The involvement of the NO system in the regulation of Na+ balance is well described. In a study by Salazar and colleagues,[285] conscious dogs were utilized to examine the role of NO in mediating the arterial pressure and renal excretory response to a prolonged increase in Na+ intake. These investigators demonstrated that, with a normal Na+ diet, NO inhibition induced a significant decrease in natriuresis and diuresis without a change in pressure. In dogs receiving a high-Na+ diet and treated with an NO inhibitor, both arterial pressure and cumulative Na+ balance were higher than in dogs receiving a comparable diet but untreated with NO inhibitors. Shultz and Tolins[286]demonstrated that exposure of rats to high-salt intake (1% NaCl drinking water) for 2 weeks induced increased serum concentration and urinary excretion of the NO metabolic products, NO2±NO3. Urinary NO2 and NO3 and Na+excretion are significantly correlated. The increase in urinary NO metabolites is attributed to the enhanced expression of all three NOS isoforms in the renal medulla by high-salt intake.[273] These findings suggest that NO may have a role in promoting diuresis and natriuresis in both normal and increased salt intake/volume-expanded states. [286] [287] In line with this notion, using micropuncture technique, Eitle and co-workers[288] showed that NO, like ANP, was able to inhibit proximal tubular fluid absorption via a cGMP-mediated mechanism. As mentioned earlier, L-nitroarginine methyl ester (l-NAME) infused directly into the renal medullary interstitium of anesthetized rats reduced the papillary blood flow, in association with decreased Na+ and water excretion, indicating that NO exerts a tonic influence on renal medullary circulation and Na+ excretion.[289] It should emphasized that high levels of eNOS in the renal medulla, on one hand, and with the inhibitory effect of NO on Na+,K+-ATPase in the collecting duct, on the other.[290] The renal NO system interacts with the local ET system at different levels.[267] The inhibition of NOS by L-NAME or of ETB receptor by A-192621 (highly selective ETB antagonist) abolished the diuretic and natriuretic effects of big ET-1 in the kidneys of anesthetized rats. [264] [291] These findings indicate that NO mediates the diuretic and natriuretic action of locally produced ET-1 in the renal inner medulla. Likewise, substantial evidence indicates that NO inhibits the ADH-enhanced Na+ reabsorption and hydro-osmotic water permeability of the cortical collecting duct.[292] Additional support for the involvement of the NO system in Na+ homeostasis is derived from several studies that examined the mechanism of salt-sensitive hypertension. According to these studies, NOS activity, mainly of neural-type (bNOS), is significantly lower in salt-sensitive rats that were maintained on a high-salt diet than in salt-resistant animals. [293] [294] In another study, the impaired activity of NOS in salt-sensitive rats was evidenced by a decreased urinary nitrate plus nitrite excretion. [293] [295] Intravenous administration of L-arginine increased NO production and prevented the development of salt-induced hypertension in Dahl-sensitive rats.[295]These findings suggest that bNOS plays an important role in Na+ handling and that decreases in bNOS activity may in part be involved in the mechanism of salt hypertension. The involvement of NO in the abnormality of Na+handling in this disease state could emerge from an inadequate direct effect on tubular pumps responsible for Na+ reabsorption in proximal and distal segments. However, it may also be influenced by attenuated inhibitory actions of NO on renin secretion and TGF. In this context, recent studies concluded that NO of macula densa origin blunts the TGF vasoconstriction during high-salt intake in salt-resistant rats, whereas in salt-sensitive rats, this response is lost and thus may contribute to salt retention and subsequently to hypertension.[296]
Renal Nerves
Extensive autonomic innervation of the kidney makes an important contribution to the physiologic regulation of all aspects of renal function.[195] Sympathetic nerves, predominantly adrenergic, have been observed at all segments of the renal vasculature and tubule.[297] Adrenergic nerve endings reach VSMCs and mesangial cells, cells of the juxtaglomerular apparatus, and all segments of the tubule: proximal, loop of Henle, and distal. Only the basolateral membrane separates the nerve endings from the tubular cells.[298] Initial studies determined that the greatest innervation was found in the renal vasculature, mostly at the level of the afferent arterioles followed by the efferent arterioles and outer medullary descending vasa recta.[299] However, high-density tubular innervation was found in the ascending limb of the loop of Henle and the lowest density was observed in the collecting duct, inner medullary vascular elements, and papilla. [298] [300] It is inferred that the magnitude of the tubular response to renal nerve activation may be proportional to the differential density of innervation.
Consistent with these anatomic observations, stimulation of the renal nerve results in vasoconstriction of afferent and efferent arterioles. [195] [300] Pharmacologic evidence obtained in a variety of experimental animals indicates that the renal vasoconstriction generated by the renal nerves is mediated by the activation of postjunctional α1-adrenoreceptors.[301] The presence of high-affinity adrenergic receptors in the nephron also supports a significant role of the renal nerves in tubule function. The α1-adrenergic and most of the α2-adrenergic receptors are found in the proximal tubule and have been localized in the basolateral membranes.[302] In the rat, β-adrenoreceptors have been found in the cortical TALH and have been subtyped as β1-adrenoceptors.[303] The predominant neurotransmitters in renal sympathetic nerves are noradrenaline and, to a lesser extent. dopamine and acetylcholine.[300]
It is widely believed that changes in the activity of the renal sympathetic nerve play an important role in controlling body fluid homeostasis. [195] [297] [304] Renal sympathetic nerve activity can influence renal function and Na+excretion through several mechanisms: (1) changes in renal and glomerular hemodynamics, (2) effect on renin release from juxtaglomerular cells with increased formation of AII, and (3) direct effect on renal tubular fluid and electrolyte reabsorption.[195] Whether renal hemodynamics is influenced by changes in renal nerve activity within the physiologic range is a matter of debate.[197] Application of graded direct electrical renal nerve stimulation produces frequency-dependent changes in RBF and GFR, renal tubule Na+ and water reabsorption, and renin secretion. [297] [305] The lowest frequency (0.5-1.0 Hz) stimulates renin secretion, followed by increases in renal tubule Na+ and water reabsorption at frequencies of 1.0 to 2.5 Hz. Increasing the frequency of stimulation to 2.5 Hz and higher results in decreases in RBF and GFR. [195] [305] The decrease in SNGFR in response to enhanced renal nerve activity has been attributed to a combination of increases in both afferent and efferent glomerular resistance and decreases in glomerular capillary hydrostatic pressure (ΔP) and glomerular ultrfiltation coefficient. [195] [197] [297] [306]Micropuncture experiments before and after renal nerve stimulation at different frequencies in Munich-Wistar rats revealed that the effector loci for vasomotor control by renal nerves localize to the afferent and efferent arteriole. In addition, although urine flow and Na+ excretion declined with renal nerve stimulation, there was no change in absolute proximal fluid reabsorption rate, suggesting that increased reabsorption occurs in the more distal segments of the nephron. However, earlier studies in the rat found alterations in proximal fluid reabsorption in response to renal nerve stimulation or acute denervation.
Studies of the response of the kidney to reflex activation of renal nerves are also supportive of a role for the SNS in regulating renal hemodynamic function and Na+ excretion.[298] DiBona and colleagues[195] measured renal nerve activity in rats receiving different Na+ diets in response to isotonic saline volume expansion and furosemide-induced volume contraction. A low-Na+ diet resulted in a reduction in right atrial pressure and an increase in renal nerve activity. The high-Na+ diet resulted in opposite changes, that is, an increase in right atrial pressure and a reduction in renal nerve activity. Thus, the relationship between atrial pressure and renal sympathetic nerve activity is both linear and bidirectional, with a gain of approximately -20%/mm Hg rise in atrial pressure. [195] [307] Other studies in conscious animals, utilizing maneuvers such as HWI and left atrial balloon inflation,[308] support the importance of reflex regulation of renal nerve activity. Collectively, these studies demonstrate the reciprocal relationship between ECF volume and renal nerve activity, consistent with the role of central cardiopulmonary mechanoreceptors governing renal nerve activity. These authors also demonstrated that the contribution of efferent renal nerve activity is of greater significance during conditions of dietary Na+ restriction when the need for renal Na+ conservation is maximal. When this linkage between the renal SNS and the excretory kidney function is defected, abnormalities in the regulation of ECF volume and blood pressure may develop. [300] [309]
Several studies that have examined the response of denervated kidneys to various physiologic maneuvers also indicated a role for renal nerves in regulating renal hemodynamic function and Na+ excretion. Early studies showed that acute denervation of the kidney is associated with increased urine flow and Na+ excretion.[195] Micropuncture techniques showed that, in euvolemic animals, elimination of renal innervation does not alter any of the determinants of SNGFR, indicating that renal nerves contribute little to the vasomotor tone of normal animals under baseline physiologic conditions. Yet, absolute proximal reabsorption was significantly reduced, in the absence of changes in peritubular capillary oncotic pressure, hydraulic pressure, and renal interstitial pressure.[195] Other studies showed that the decrease in tubular electrolyte and water reabsorption following renal denervation is not limited to the proximal nephron, but occurs also in the loop of Henle and the distal nephron segments. [195] [310] In another micropuncture study, measurements obtained before and after denervation in control rats and rats with experimental CHF or acute volume depletion demonstrated that denervation resulted in diuresis and natriuresis in normal rats but failed to alter any of the parameters of renal cortical microcirculation.[311] In contrast, in rats with CHF, denervation caused an amelioration of renal vasoconstriction, by decreasing afferent and efferent arteriolar resistance, and again a natriuresis. This study indicates that in situations in which efferent neural tone is heightened above baseline level, renal nerve activity may profoundly influence renal circulatory dynamics. However, although the basal level of renal nerve activity in normal rats or conscious animal is apparently insufficient to influence renal hemodynamics, it is sufficient to exert a tonic stimulation on renal tubular epithelial Na+ reabsorption and renin release.[195]
Clinical studies, in which guanethidine was given to achieve autonomic blockade or in patients with idiopathic autonomic insufficiency, revealed that intact adrenergic innervation is required for the normal renal adaptive response to dietary Na+ restriction.[312] More direct examination of efferent renal sympathetic nerve activity in humans has been made possible by the measurement of renal NE spillover methodology to elucidate the kinetics of NE release. A study by Friberg and associates[313] determined that, in normal subjects, a low-Na+ diet resulted in a fall in urinary Na+ excretion and an increase in NE spillover, with no change in cardiac NE uptake, which supports the concept of a true increase of efferent renal nerve activity secondary to Na+ restriction. Further evidence that the SNS plays a role in Na+ balance in humans comes from a study by McMurray and co-workers,[314] who demonstrated that low-dose infusion of NE to normal salt-replete volunteers resulted in a physiologic plasma increment of this neurotransmitter in asscociation with antinatriuretic. This reduction in Na+ excretion occurred without any change in GFR but was associated with a significant decline in Li+ clearance, an indication of reduced proximal tubule reabsorption.
The cellular mechanisms mediating the tubular actions of NE are believed to include stimulation of Na+,K+-ATPase activity and Na+/H+ exchange in the proximal tubular epithelial cells.[195] It is assumed that α1-adrenoreceptor stimulation, acting via phospholipase C, causes an increase in intra-cellular Ca2+ that activates Ca2+/calmodulin-dependent calcineurin (phosphatase). Calcineurin dephosphorylates Na+-K+-ATPase from its inactive phosphorylated form to its active dephosphorylated form.[315] The stimulatory effect of renal nerve on Na+/H+ antiport is mediated through stimulation of α2-adrenoreceptor.[195]
In addition to its direct action on epithelial cell transport and renal hemodynamics, interactions of renal nerve input with other effector mechanisms may contribute to the regulation of renal handling of Na+. Efferent sympathetic nerve activity influences the rate of renin secretion from the kidney by a variety of mechanisms, directly or by interacting with the renal tubule macula densa and vascular baroreceptor mechanisms for renin secretion.[195] The increase in renin secretion is mediated primarily by direct stimulation of β1-adrenergic receptors located on juxtaglomerular granular cells.[195] Sympathetic activation of renin release is augmented during RPP reduction.[195] Studies in the isolated perfused rat kidney suggest that intrarenal generation of AII has an important prejunctional action on renal sympathetic nerve terminal to facilitate NE release during renal nerve stimulation.[195] However, the physiologic significance of this facilitatory interaction on tubular Na+ reabsorption remains controversial. Thus, administration of an ACE inhibitor or an AII receptor antagonist attenuated the antinatriuretic response to electrical renal nerve stimulation in anesthetized rats.[195] In contrast, when nonhypotensive hemorrhage was used to produce reflex increase in renal sympathetic activity in conscious dogs, the associated antinatriuresis was unaffected by ACE inhibition or AII receptor blockade.[316]
Sympathetic activity is also a stimulus for the production and release of renal PGs, coupled in series to the adrenergic-mediated renal vasoconstriction.[195] Evidence indicates that renal vasodilatory PGs attenuate the renal hemodynamic vasoconstrictive response to activation of the renal adrenergic system in vivo and on isolated renal arterioles.[195] Micropuncture experiments in Munich-Wistar rats provided evidence that the primary factor responsible for the reduction in the glomerular ultrafiltration coefficient during renal nerve stimulation may be AII rather than NE and that endogenously produced PGs neutralize the vasoconstrictive effects of renal nerve stimulation at an intraglomerular locus rather than at the arteriolar level.
Another interaction examined is that between the renal SNS and ADH. Studies in conscious animals showed that ADH exerted a dose-related effect on arterial baroreflex, such that low doses of ADH may sensitize the central baroreflex neurons to afferent input, whereas higher doses caused direct excitations of these neurons, resulting in a reduction in sympathetic outflow.[195] Nishida and colleagues[317] demonstrated that ADH suppresses renal sympathetic outflow and determined that this response depends on the number of afferent inputs from baroreceptors. Simon and associates[318] examined the plasma ADH response to renal nerve stimulation in conscious, baroreceptor-intact, Wistar rats. Renal nerve stimulation resulted in an elevated plasma concentration of ADH and a rise in arterial pressure.
Many studies demonstrated, in both normal and pathologic situations, that increased renal nerve sympathetic activity can antagonize the natriuretic/diuretic response to ANP and that removal of the influence of sympathetic activity enhances the natriuretic action of the peptide. [228] [319] [320] Awazu and co-workers[321] noted that, in Wistar rats, renal denervation increased ANP receptors and cGMP generation in glomeruli, resulting in an increase in ultrafiltration coefficient after ANP infusion.
In summary, evidence indicates that the renal sympathetic nerves can regulate urinary Na+ and water excretion by changing RVR, by influencing renin release from the juxtaglomerular granular cells, and through a direct effect on tubular epithelial cells. These effects may be modulated via interactions with various other hormonal systems including ANP, PGs and ADH.
Other Factors
Kinins.
The kallikrein-kinin system (KKS) is a complex cascade responsible for the generation and release of vasoactive kinins, that is, BK and related peptides.[322] This endogenous metabolic system includes precursors of kinins, known as kininogen, and tissue and circulatory kallikreins. Kinins are produced by many cell types in the body and can be detected in secretory products such as urine, saliva, and sweat, interstitial fluid, and rarely, venous blood. That renal KKS can produce local concentrations of BK much higher than those present in blood is well known.[323] Kinins play an important role in hemodynamic and excretory processes through their receptors that include BK-B1, and BK-B2. The BK-B2 receptors mediate most of the actions of kinins and are located mainly in kidney, although they are also detectable in heart, lung, brain, uterus, and testes. Activa-tion of BK-B2 receptors results in vasodilation most likely via an NO- or arachidonic acid metabolites-dependent mechanism. [322] [324] [325] BK is known for its multiple effects on the cardiovascular system, particularly vasodilation and plasma extravasation.[322]
Besides the vasculature, the kidney is an important target organ of kinins, where they induce diuresis and natriuresis via activation of BK-B2 receptors. These effects are attributaed to an increase in RBF and to inhibition of Na+ and water reabsorption in the distal nephron. [323] [326] The latter effect is secondary to the observed action of kinins in reducing vascular resistance. Unlike many vasodilators, BK increases RBF without significantly affecting GFR or Na+ reabsorption at the proximal tubule level, but with a marked decrease in the water and salt reabsorption in the distal portions of the nephron, thus contributing to increased urine volume and Na+ excretion. Several studies that utilized transgenic animals enriched our understanding of the physiologic role of the kinins and the interaction between the KKS and the RAAS.[323] For instance, in the kidney, AII acting via AT2 receptor stimulates a vasodilator cascade of BK, NO, and cGMP, which is tonically activated only during conditions of increased AII, such as Na+ depletion.[327] In the absence of the AT2 receptor, pressor and antinatriuretic hypersensitivity to AII is associated with BK and NO deficiency.[326] Furthermore, the involvement of the renal kinins in pressure natriuresis phenomenon has been documented.[98] The heptapeptide angiotensin (Ang)-(1-7) is currently considered one of the biologically active end products of the RAAS.[323] BK mediates the biologic actions of Ang-(1-7), because rats transgenic to kallikrein gene display significant augmentation in the diuretic and natriuretic actions of Ang-(1-7).[323] Because ACE is involved in the degradation of kinins, ACE inhibitors not only attenuate the formation of AII but also may lead to the accumulation of kinins. Therefore, the latter are believed to be responsible in part for the beneficial effects of ACE inhibitors in patients with CHF.[328] Based on that, the KKS is believed to play a pivotal role in the regulation of fluid and electrolyte balance, mostly through its renal actions.
Adrenomedullin.
Human adrenomedullin (AM) is a 52–amino acid peptide that was discovered over a decade ago by Kitamura and associates[329] in extracts of human pheochromocytoma. AM shares structural homology with calcitonin gene-related peptide and amylin. [329] [330] Like the calcitonin gene, the AM gene is situated in a single locus of chromosome 11. Besides human AM, the amino acid sequence of AM has been determined in many species including rat, canine, mouse, porcine, and bovine. AM is produced from a 185 a.a. preprohormone that also contains a unique 20 a.a. sequence in the NH2-terminus and termed proadrenomedulin NH2-terminal 20 peptide (PAMP). PAMP exists in vivo and has biologic activity similar to that of AM. AM-mRNA is expressed in several tissues including atrium, ventricles, vascular tissue, lung, kidney, pancreas, ventricle, smooth muscle cells, small intestine, and brain. The synthesis and secretion of AM are stimulated by chemical factors and physical stress. Among these stimulants are cytokines, corticosteroids, thyroid hormones, AII, NE, ET, BK, and shear stress.[331] AM immunoreactivity has been localized in most of the body tissues. [330] [332] For instance, high concentrations of AM are present in pheochromocytoma, adrenal medulla, cardiac atria, pituitary gland, and at lower levels in cardiac ventricles, VSMC, endothelial cells, renal distal and collecting tubules, digestive, respiratory, reproductive, and endocrine systems. [331] [332] Interestingly, endothelial cells produce and secrete AM in amounts comparable to that of ET.[330] In contrast to the other tissues, the AM synthesized in the adrenal medulla is stored in granules and secreted in a controlled pathway.[331] AM acts through a membrane receptor that consists of 395 a.a. that structurally resembles G-protein-linked receptor, containing seven transmembrane domains. AM receptors comprise the calcitonin receptor-like receptor and a family of receptor-activity-modifying protein (RAMPs 1-3).[333] Activation of these receptors increases intracellular cAMP, which most likely serves as a second messenger for the peptide. [330] [334] Since its discovery, AM has undergone intensive investigation in regard to possible participation in the regulation of cardiovascular and volume homeostatsis. [335] [336] Multiple biologic actions of AM have been reported. The most impressive biologic effect of AM is long-lasting and dose-dependent vasodilation of the vascular system including coronary arteries. [330] [334] [335] Injection of AM into anesthetized rats, cats, or conscious sheep, induced a potent and long-lasting hypotensive response associated with reduction in vascular resistance in the kidney, brain, lung, hind limbs, and mesentery.[331]The hypotensive action of AM is accompanied by increases in heart rate and cardiac output owing to positive inotropic effects.[331] The vasodilating effect of AM can be blocked by inhibiting NOS, suggesting that NO partly mediates the decrease in systemic vascular resistance.[330] Besides its hypotensive action, AM increases RBF via preglomerular and postglomerular arteriolar vasodilation. [334] [337] The AM-induced hyperperfusion is associated with a dose-dependent diuresis and natriuresis. [331] [334] These effects result from a decrease in tubular Na+ reabsorption despite the AM-induced hyperfilteration.[337] Similar to natriuretic peptides, AM suppresses aldosterone secretion in response to AII and high potassium.[330] Furthermore, in cultured VSMC, AM inhibits ET production induced by various stimuli.[331] AM acts in the CNS to inhibit both water and salt intake.[338] In the hypothalamus, AM inhibits the secretion of ADH, an effect that may contribute to its diuretic and natriuretic actions.[338] Taken together, these findings show that AM is a vasoactive peptide of potential importance that may be involved in the physiologic control of renal, adrenal, vascular, and cardiac function. Furthermore, the existence of AM-like immunoreactivity in the glomerulus and in the distal tubule, in association with detectable amounts of AM mRNA in the kidney, suggests that AM plays a renal paracrine role.[339]
Urotensin.
Urotensin-II (U-II) is a cyclic peptide originally isolated from the caudal neurosecretory organ of teleost fish. [340] [341] The human isoform was cloned in 1999 and has been identified as the natural ligand for the orphan G-protein-coupled receptor GPR-14. [342] [343] [344] [345] The U-II/GPR-14 system is ex-pressed in the CNS, the cardiovascular system, and the kidney of various mammalian species, including humans. [345] [346] [347] [348] [349] In the human kidney, immunoreactive staining for U-II was detected in the epithelial cells of the tubules, mostly in the distal tubule, with moderate staining in the endothelial cells of the renal capillaries.[350] In addition, in fish, some evidence indicates that U-II modulates transepithelial ion (Na+/Cl-) transport. [351] [352] Human U-II (hU-II) possesses potent vasoactive properties, although these effects are largely dependent on the species and the vascular bed examined.[342] [353]
In the original study by Ames and colleagues,[342] hU-II induced a potent vasoconstrictor effect on isolated arteries from nonhuman primates that was an order of magnitude greater than that of ET-1. Since then, hU-II has been considered the most potent mammalian vasoconstrictor identified so far. [342] [353] [354] However, careful analysis of the literature reveals that this general notion may be unjustified and that U-II may exert both vasoconstrictor and vasodilatory effects. These actions depend largely on the animal species as well as on the vascular bed examined. [353] [354] In the rat, the predominant cardiovascular actions of U-II are hyperemic vasodilatation in the mesenteric and hindquarter vascular beds, associated with hypotension and dose-dependent tachycardia.[355] Gibson[356] showed that U-II at low concentrations caused relaxation of noradrenaline-precontracted aortic stripes of rats in an endothelium-dependent manner. Evidence also indicates that hU-II may act as a potent vasodilator of human small pulmonary arteries and abdominal resistance arteries.[357] Likewise, in the isolated perfused rat heart, U-II elicited a sustained coronary vasodilatation through factors such as COX products and NO.[358] These findings may suggest that, although the direct effect of U-II on large vessels is contraction, U-II also relaxes blood vessels by the release of vasodilators from endothelium.[359]
The involvement of the U-II system in the regulation of renal function in mammals has not been thoroughly investigated. Recently, Zhang and asociates[360] demonstrated that hU-II is an NO-dependent renal vasodilator and acts a natriuretic peptide in the rat kidney. Also, recent evidence by Clozel and co-workers[361] suggests that the UT-II system may be involved in the pathogenesis of the no-reflow phenomenon of renal ischemia induced by clamping of the renal artery.
Digitalis-like Factor.
Hamlyn and co-workers [362] [363] [364] identified an endogenous ouabain-like compound in human and other mammalian plasma that interacts with the cardenolide receptor on the Na+,K+-ATPase pump and whose mechanism of inhibition was strikingly similar to that of the digitalis glycosides used in the treatment of CHF and certain cardiac arrhythmias. This substance, which was later termed endogenous digitalis-like factor (EDLF), is secreted by the adrenal cortex. The physiologic role of the EDLF has not yet been fully elucidated. However, initial studies hypothesized that natriuretic hormone and the vascular Na+,K+-ATPase inhibitor are the same factor and, furthermore, that this factor played a causative role in the pathophysiology of certain types of hypertension. Indeed, prolonged elevation of circulating EDLF in the rat produces sustained hypertension.[365] Similarly, among white patients with essential hypertension, a large fraction has high circulating concentrations of EDLF.[365] Moreover, owing to its inhibitory action on the Na+ pump, EDLF increases cytosolic stores of Ca2+ in many types of cells, including VSMC, leading to an increase in vascular resistance.[366] In newborns, the inhibition of renal Na+,K+-ATPase may enhance elimination of surplus Na+.[367]
Neuropeptide Y.
Neuropeptide Y (NPY), a 36-residue peptide, is a sympathetic co-transmitter stored and released together with noradrenaline by adrenergic nerve terminals of the SNS. Structurally, NPY shares high homology with two other members of the pancreatic polypeptide family, peptide YY (PYY) and pancreatic polypeptide (PP). These two closely related peptides are produced and released by the intestinal endocrine cells and pancreatic islet cells, respectively, and act as hormones. [368] [369] Although NPY was originally isolated from the brain and is highly expressed in the CNS, it has been clearly demonstrated that NPY exhibits a wide spectrum of biologic activities in peripheral organs such as the cardiovascular system, the GIT, and the kidney. [370] [371] [372] Numerous studies utilizing both in vivo and in vitro techniques demonstrated the capacity of the NPY to reduce RBF and increase RVR in various species including rat, rabbit, pig, and humans.[372] Despite of the potent vasoconstrictor effect of this peptide on renal vasculature, this effect does not appear to be associated with a similar reduction in GFR. Indeed, most of the studies in which this parameter was evaluated show only minor or no alterations in GFR in response to NPY administration. Considering the potent renal vasoconstrictor action of NPY, a decrease in electrolyte and water excretion could be expected following the administration of the peptide.[372] However, the available data at present suggest that NPY may exert either a natriuretic[373] or an antinatriuretic[374] action, depending on the experimental conditions and the species utilized.
Collectively, numerous studies using physiologic and pharmacologic approaches indicated that this peptide has the capacity to alter renal function. In particular, these studies suggest that NPY may exert renal vasoconstrictor and tubular actions that are species dependent and may also influence renin secretion by the kidney. The question of whether NPY plays an important role in the physiologic regulation of renal hemodymaics and electrolyte excretion remains largely unanswered at present.
PATHOPHYSIOLOGY OF EDEMA FORMATION
Generalized edema formation, the clinical hallmark of ECF volume expansion, represents the accumulation of excessive volumes of fluid in the interstitial compartment and is invariably associated with renal Na+ retention. It occurs most commonly in response to CHF, cirrhosis with ascites, and the nephrotic syndrome. In CHF and cirrhosis with ascites, the primary disturbance leading to Na+ retention does not originate within the kidney. Instead, renal Na+retention is the response to a disturbance of the circulation induced by disease of the heart or liver. In the nephrotic syndrome, glomerular injury accompanied by heavy proteinuria is associated with Na+ retention and leads to a profound disturbance in circulatory homeostasis. In each of these conditions, the renal effector mechanisms that normally operate to conserve Na+ and protect against an Na+ deficit are exaggerated and continue despite subtle or overt expansion of ECF volume.
Local Mechanisms in Interstitial Fluid Accumulation
Transcapillary fluid and solute transport can be viewed as consisting of two types of flow, convective and diffusive. Bulk water movement occurs via convective transport induced by hydraulic and osmotic pressure gradients.[375]Capillary hydraulic pressure is under the influence of a number of factors, including systemic arterial and venous blood pressures, local blood flow, and the resistances imposed by the pre- and postcapillary sphincters. Systemic arterial blood pressure, in turn, is determined by cardiac output, intravascular volume, and SVR; systemic venous pressure is determined by right atrial pressure, intravascular volume, and venous capacitance. Na+ balance is a key determinant of these latter hemodynamic parameters. It should also be noted that, conversely, the massive accumulation of fluid in the peripheral interstitial compartment (anasarca) can itself diminish venous compliance and, hence, alter overall cardiovascular performance.[376]
The balance of Starling forces prevailing at the arteriolar end of the capillary (ΔP>Δπ) favors the net filtration of fluid into the interstitium. Net outward movement of fluid along the length of the capillary is associated with an axial decrease in the capillary hydraulic pressure and an increase in the plasma COP. Nevertheless, the local transcapillary hydraulic pressure gradient continues to exceed the opposing COP gradient throughout the length of the capillary bed in several tissues, such that filtration occurs along its entire length.[377] In such capillary beds, a substantial volume of filtered fluid must, therefore, return to the circulation via lymphatics. Given this importance of lymphatic drainage, the ability of lymphatics to expand and proliferate and the ability of lymphatic flow to increase in response to increased interstitial fluid formation provide protective mechanisms for minimizing edema formation.
Other mechanisms for minimizing edema formation have also been identified. Precapillary vasoconstriction tends to lower capillary hydraulic pressure and diminish the filtering surface area in a given capillary bed. Indeed, excessive precapillary vasodilatation in the absence of appropriate microcirculatory myogenic reflex regulation appears to account for lower extremity interstitial edema associated with Ca2+ entry blocker vasodilator therapy.[378] Increased net filtration itself is associated with dissipation of capillary hydraulic pressure, dilution of interstitial fluid protein concentration, and a corresponding rise in intracapillary plasma protein concentration. The resulting change in the profile of Starling forces associated with increased filtration, therefore, tends to mitigate against further interstitial fluid accumulation. [379] [380] Interstitial fluid pressure is normally subatmospheric. Furthermore, even small increases in interstitial fluid volume tend to augment tissue hydraulic pressure, again opposing further transudation of fluid into the interstitial space.[381]
The appearance of generalized edema, therefore, implies one or more disturbances in microcirculatory hemodynamics associated with expansion of the ECF volume: increased venous pressure transmitted to the capillary, unfavorable adjustments in pre- and postcapillary resistances, or lymphatic flow inadequate to drain the interstitial compartment and replenish the intravascular compartment. Insofar as the continued net accumulation of interstitial fluid without renal Na+ retention might result in prohibitive intravascular volume contraction and cessation of interstitial fluid formation, generalized edema, therefore, implies substantial renal Na+ retention. Indeed, the volume of accumulated interstitial fluid required for clinical detection of generalized edema (>2-3 L) necessitates that all states of generalized edema are associated with expansion of ECF volume and, hence, body exchangable Na+ content. In conclusion,t all states of generalized edema reflect past or ongoing renal Na+ retention.
Renal Sodium Retention and Edema Formation in Congestive Heart Failure
CHF is a clinical syndrome in which the heart is unable to satisfy the requirements of peripheral tissues for oxygen and other nutrients. This happens most commonly in the setting of a decrease in cardiac output (low-output CHF), but may occur as well when cardiac output is increased, for example, in patients with AV fistula, hyperthyroidism, and beriberi (high-output CHF). In both situations, the kidney responds in a similar manner, that is, by avidly retaining Na+ and water despite expansion of the ECF volume.
The syndrome of CHF encompasses pathophysiologic alterations related to a reduction of the distending pressure within the arterial circuit and those that are related to increases in the volume of blood and the filling pressures in the atrium and great veins, behind the failing ventricle. In response to these changes, a series of adjustments occur that result from the operation of circulatory and neurohumoral compensatory mechanisms. These adjustments may be viewed teleologically as tending to support arterial pressure and maintain perfusion to critical organs such as the heart and brain. As long as these adaptations are able to maintain their compensatory role, they may prove to be beneficial (compensated CHF). However, with the development of CHF, excessive activation of these systems may become detrimental by further promoting peripheral vasoconstriction and increasing the abnormal loading conditions in the failing heart. At this turning point, a vicious circle is created (decompensated CHF) in which the “compensatory” mechanisms themselves contribute to further deterioration of the cardiovascular system.
From the standpoint of ECF volume homeostasis, two key abnormalities occur in CHF: (1) The perception of an inadequate circulating volume by various sensors within the circulation. (2) A disturbance in the effector arm of volume control, with excessive activation of antinatriuretic vasoconstrictor systems and failure of natriuretic vasodilatory mechanisms, shifting the balance between these systems toward Na+ and water retention by the kidney.
Afferent Limb of Volume Homeostasis in Congestive Heart Failure: Abnormalities in Sensing Mechanisms
What constitutes the afferent signal for the continued retention of Na+ and water by the kidney in CHF has been the focus of interest and debate for many years. [382] [383] [384] [385] The observation that the kidney is intrinsically normal in CHF but continues to retain Na+ and water avidly, despite expansion of the extracellular volume, indicated that it must be responding to “inadequate” signals from the volume regulatory system. This suggests either that a critical sensing area in the vascular tree is “underfilled” or that some sensing mechanisms of body fluid volume fail to detect appropriately the elevated circulating volume. Compelling evidence suggests that both mechanisms may contribute to development of salt and water retention and edema formation in CHF.
The recognition of the important role of arterial underfilling in mediating renal salt and water retention dates back to the concepts of “backward failure” and “forward failure” formulated by Starling,[386] Harrison,[387] and Stead and Ebert[388] as well as the concept of “effective circulating volume” suggested in 1948 by Peters.[389] According to the theory of “backward failure,” accumulation of blood behind the failing myocardium results in venous congestion with increased capillary pressure, leading to transudation of fluid into the interstitium with edema formation and depletion of plasma volume. The decrease in plasma volume then initiates renal Na+ and water retention. In contrast, the concept of “forward failure” emphasized the importance of the failure of the heart as a pump in supplying adequate blood flow to the tissues, similar to the mechanism of acute circulatory failure (shock), such that the kidneys are no longer able to excrete salt in a normal manner. Evidently, both mechanisms contain elements of “underfilling” of the arterial circulation.
Many early studies showed that cardiac output was reduced in CHF, in agreement with both the “backward” and “forward” concepts. Thus, the notion that a decrease in cardiac output might be the signal dictating renal Na+ retention in CHF found increasing support over the years. [390] [391] However, it was soon recognized that situations associated with high cardiac output, such as AV fistula, may be associated with renal salt and water retention and identical neurohormonal responses to those observed in low-output CHF.[41]
Schrier [383] [384] [385] [392] refined the concept of arterial underfilling in a unifying hypothesis of body fluid volume regulation to explain the continued renal Na+ and water retention in various edematous disorders. According to this view, the relative fullness of the arterial circulation, as determined by the relation between cardiac output and peripheral arterial resistance, constitutes the primary afferent signal for renal retention of salt and water. A decrease in cardiac output is the most obvious reason for arterial underfilling. However, a decrease in peripheral arterial resistance, as a result of diversion of blood flow from the arterial to venous circuit, may provide another afferent signal for arterial underfilling, which causes retention of salt and water by the kidney. Thus, arterial underfilling, caused by either an absolute decrease in cardiac output (low-output CHF) or diversion of blood flow through anatomic or physiologic AV shunt (high-output CHF), initiates the sequence of adaptive neurohormonal and renal hemodynamic responses that result in enhanced Na+ and water reabsorption by the kidney. In that respect, activation of neurohormonal and hemodynamic compensatory mechanisms in CHF is not different from those occurring in true hypovolemia. It is important to note, however, that in contrast to true volume-depletion states, CHF is associated with a rise in intracardiac pressures, which, in theory, should promote natriuresis by activating cadiopulmonary reflexes and the release of ANP. In that respect, the increase in intracardiac pressures in CHF may be substantially higher than in other edema-forming states. Given the potency of these important volume-regulatory cardiopulmonary reflexes, it is conceivable that the blunted natriuresis associated with CHF reflects a disturbance in the afferent signaling mechanisms emanating from these volume-sensing sites.
As previously discussed, the sensory information that initiates the neurohumoral responses to changes in volume homeostasis originates from mechanosensitive nerve endings located in the cardiac atria, ventricles, and pulmonary circulation (cardiopulmonary receptors) and the arterial baroreceptors located in the aortic arch and carotid sinus. Information from these nerve endings is carried by the vagal and glossopharyngeal nerves to centers in the medulla and brainstem. In the normal situation, the prevailing discharge from these receptors exerts a tonic restraining effect on the heart and circulation by inhibiting the sympathetic outflow and augmenting parasympathetic activity. In addition, changes in transmural pressure across the atria and great vessels also influence the secretion of ADH and renin and the release of ANP.
In CHF, it is widely accepted that both the cardiopulmonary reflexes and the arterial baroreflexes are blunted, such that they can no longer exert an adequate tonic inhibitory effect on sympathetic outflow. [393] [394] As a result of the diminished inhibitory input from these receptors sites, the SNS is activated, and the secretion of ADH and renin may be augmented, thus promoting Na+ and water retention by the kidney despite a high circulating volume. Gabrielsen and co-workers[395] demonstrated that neuroendocrine link between volume sensing and renal Na+ excretion is preserved in compensated CHF. However, the natriuretic response to volume expansion is modulated by the prevailing AII and aldosterone concentrations. Inhibition of AII formation by an ACE inhibitor increased Na+ excretion to the same extent found in control subjects. In contrast, renal free water clearance is attenuated in response to volume expansion in compensated CHF despite normal plasma levels of ADH.
Greenberg and colleagues[396] were the first to report that the firing of atrial receptors in response to saline infusion was markedly attenuated over a wide range of central venous pressures in dogs with CHF induced by pulmonic valve stenosis and tricupid regurgitation. These findings were confirmed and extended in an aortocaval fistula canine model of CHF by Zucker and co-workers[397] who demonstrated a reduced firing of type B left atrial receptors in response to dextran volume expansion in the dogs with CHF. In addition, sonomicrometry of the left atrial appendage demonstrated reduced atrial compliance and microscopy indicated loss of nerve ending arborization.
Such an attenuated sensitivity of cardiopulmonary reflexes may explain the clinical observation indicating a sustained activation of the SNS in CHF patients with venous congestion,[398] a situation that would normally result in suppression of NE release. Likewise, studies using maneuvers that selectively altered central cardiac filling pressures (i.e., head-up tilt or LBNP) showed that patients with CHF, in contrast to normal subjects, usually do not demonstrate significant alterations in limb blood flow, circulating catecholamines, ADH, or renin activity in response to postural stimuli. [399] [400] This diminished reflex responsiveness may be most impaired in patients with the greatest ventricular dysfunction.
In addition to the dysfunction of the cardiopulmonary reflexes, abnormalities in the arterial baroreflex control of the cardiovascular system also exist in CHF. [393] [394] [401] Ferguson and associates[402] demonstrated a high baseline muscle sympathetic activity in patients with CHF who failed to respond to activation and deactivation of arterial baroreceptors by infusion of phenylephrine and Na+ nitroprusside, respectively. Depressed function of carotid and aortic baroreceptors were also reported in experimental models of cardiac failure. [403] [404] These changes were associated with a resetting of receptor threshold to higher levels and a reduced range of pressures over which the receptors function.
Multiple abnormalities have been described in cardiopulmonary and arterial baroreceptor control of renal sympathetic activity in CHF. DiBona and co-workers[405] demonstrated in rats with coronary ligation an increased basal level of efferent renal sympathetic activity that failed to suppress normally during volume expansion. Similar observations were reported by Dibner-Dunlap and Thames[406] in sinoaortic denervated dogs with pacing-induced CHF. In this experimental model, cardiopulmonary receptors were stimulated by volume expansion, and left atrial baroreceptors were stimulated by inflating small balloons at the left atrial-pulmonary vein junctions. With both stimuli, a marked attenuation of the cardiopulmonary baroreflex control of the efferent renal sympathetic activity was found. In a more recent study, DiBona & Sawin[407] performed simultaneous recordings of efferent renal sympathetic activity with either single aortic or single vagal nerve units in a rat model of CHF. This study demonstrated that the abnormal regulation of efferent renal sympathetic activity was due to impaired function of both the aortic and the cardiopulmonary baroreflexes. These investigators reported later that the defect in cardiopulmonary baroreceptor was functionally more important than that in arterial baroreceptors in mediating the augmented efferent renal sympathetic activity.[408]
Several mechanisms have been implicated in the pathogenesis of the abnormalities in cardiopulmonary and arterial baroreflexes in CHF. Zucker and co-workers[397] suggested that loss of compliance in the dilated hearts as well as gross changes in the morphology of the receptors themselves were the mechanisms underlying the depressed atrial receptor discharge in dogs with aortocaval fistula. Additional studies in dogs with pacing-induced CHF raised the possibility that the decrease in carotid sinus baroreceptors sensitivity might be related to augmented Na+,K+-ATPase activity in the baroreceptor membranes. [403] [409] Local perfusion of the carotid sinus with the cardiac glycoside ouabain led to a significant improvement of baroreceptor function.[403] Recent studies also demonstrated a role for AII in modulating baroreflex function, suggesting that increased activity of this peptide could be involved in the abnormal reflex regulation in CHF. Specifically, intracerebral or systemic administration of the AT1 receptor antagonist losartan to rats with CHF significantly improved arterial baroreflex control of renal sympathetic activity.[394]Similarly, Murakami and colleagues[410] demonstrated that intravenous infusion of another AT1 receptor antagonist, L-158,809, resulted in a significant enhancement of baroreflex control of heart rate in conscious rabbits with CHF. In addition, Dibner-Dunlap and co-workers[411] reported that treatment with the ACE inhibitor enalaprilat augmented arterial and cardiopulmonary baroreflex control of sympathetic nerve activity in patients with CHF. Taken together, these data support the possibility that high endogenous levels of AII in CHF may contribute to the depressed baroreflex sensitivity observed in CHF.
Although most of the studies indicated that the defects in the baroreflex function in CHF reside primarily in the afferent limb of the reflex arch, presumably at the receptor level, it has been suggested that alterations in more central sites may also be involved. As noted earlier, intracerebroventricular administration of an AT1 antagonist improved baroreflex sensitivity in rats with CHF, suggesting that AII may also act on a central component of the reflex arch.[394] Indeed, in a study in which AII was injected into the vertebral artery of normal rabbits, a significant attenuation in arterial baroreflex function was observed.[412] Furthermore, this effect of AII could be blocked by prazosin, suggesting that the modulation of baroreflex function was mediated via a central α1-adrenoreceptor.
As pointed out earlier, the blunted cardiopulmonary and arterial baroreceptor sensitivity in CHF may lead to an increase not only in total sympathetic outflow but also in ADH release and renin secretion. However, compared with the influence on sympathetic outflow, less information links the abnormalities in cardiopulmonary and arterial baroreflexes with enhanced ADH release and renin secretion in CHF.
The discovery that ANP is localized at some of the critical volume-sensing sites in the heart raised the possibility that alterations in secretory capacity of this hormone may exist in CHF. Thus, it was suggested that plasma ANP-atrial stretch relationship could be altered in CHF owing to limited reserve of the hormone in atrial storage as a result of a tonically increased stimulus for release of the hormone.[413] However, it is unlikely that such a defect could contribute significantly to salt and water retention in CHF for the following reasons. (1) Numerous studies in patients and animal models with CHF consistently demonstrated that circulating levels of the hormone are not depressed but rather elevated in CHF in proportion to the severity of cardiac dysfunction. [414] [415] [416] [417] (2) It appears that cardiac ventricles become a major source of peptide secretion in CHF, as evidenced by increased tissue immunoreactive ANP and ANP mRNA in ventricles of patients and experimental models of CHF. [418] [419] [420] (3) Na+ retention of CHF is not reversed when plasma ANP levels are further increased by exogenous administration of the peptide. The failure of ANP infusion to induce appropriate natriuretic and diuretic responses in patients[421] and experimental models of CHF [422] [423] indicates that the main abnormality in CHF is the development of “resistance” to ANP rather than impaired secretion of the peptide.
The disturbances in the sensing mechanisms that initiate and maintain renal Na+ retention in CHF are summarized in Figure 12-6 . As indicated, a decrease in cardiac output or a diversion of systemic blood flow (anatomic or physiologic) diminishes the blood flow to the critical sites of the arterial circuit with pressure- and flow-sensing capabilities. The perception of diminished blood flow culminates in renal Na+ retention, mediated by effector mechanisms to be described. An increase in systemic venous pressure promotes the transudation of fluid from the intravascular to the interstitial compartment by increasing the peripheral transcapillary hydraulic pressure gradient. These processes augment the perceived loss of volume and flow in the arterial circuit. In addition, distortion of the pressure-volume relationships as a result of chronic dilatation in the cardiac atria attenuates the normal natriuretic response to central venous congestion. This attenuation is manifested predominantly as diminished neural suppressive response to atrial stretch, which results in increased sympathetic nerve activity and augmented release of renin and ADH.
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FIGURE 12-6 Sensing mechanisms that initiate and maintain renal Na+ retention in CHF. (Adapted from Skorecki KL, Brenner BM: Body fluid homeostasis in congestive heart failure and cirrhosis with ascites. Am J Med 72:323-338, 1982.) |
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Efferent Limb of Volume Homeostasis in Congestive Heart Failure: Abnormalities in Effector Mechanisms
CHF is also characterized by a series of adaptive changes in the efferent limb of volume control. In many respects, these effector mechanisms for Na+ retention are similar to those that govern renal function in states of true Na+depletion. These include adjustments in glomerular hemodynamics and tubule transport, which, in turn, are brought about by alterations in neural, humoral, and paracrine systems. However, in contrast to true volume depletion, CHF is also associated with activation of vasodilatory natriuretic agents, which tend to oppose the effects of the vasoconstrictor antinatriuretic systems. The final effect on urinary Na+ excretion is determined by the balance between these antagonistic effector systems, which, in turn, may shift during the evolution of cardiac failure toward a dominance of Na+-retaining systems. The abnormal regulation of the efferent limb of volume control reflects not only the exaggerated activity of the antinatriuretic systems but also the failure of natriuretic vasodilatory systems that are activated in the course of the deterioration in cardiac function.
Alterations in Glomerular Hemodynamics
CHF in patients and experimental models is characterized by significant alterations in renal hemodynamics that include an increase in RVR, reduced GFR, but an even more marked reduction of RPF, so that the FF is increased. [424] [425] [426] [427] At the single-nephron level in rats with CHF induced by coronary ligation, Ichikawa and colleagues[428] demonstrated that SNGFR was lower than in control rats, but glomerular plasma flow was disproportionately reduced such that single-nephron filtration fraction (SNFF) was markedly elevated. Ultrafiltration coefficient was diminished, and both afferent and efferent arteriolar resistances were elevated, accounting for the diminished single-nephron glomerular plasma flow. The rise in SNFF was due to a disproportionate increase in efferent arteriolar resistance. Similar alterations in glomerular hemodynamics have been reported by Nishikimi and Frolich[429] in rats with aortocaval fistula, a high output failure model. In Figure 12-7 , a comparison of the glomerular capillary hemodynamic profile in the normal (left) versus the CHF state (right) is illustrated on the left graph of each panel. First, the transmural hydraulic pressure gradient ΔP declines along the distance of the glomerular capillary in both the normal and the CHF states, but compared with the normal state, ΔP in CHF is much higher because of the increased efferent arteriolar resistance. Second, the transmural plasma COP gradient Δπ increases over the length of the glomerular capillary in both states as fluid is filtered in the Bowman space, but it increases to a greater extent in CHF because of the increased filtration fraction. It is evident that a major component of the glomerular hemodynamic alterations in CHF emanates from the disproportionate increase in efferent compared with afferent arteriolar resistance. As outlined in a previous section of this chapter, this alteration is mediated principally by the action of AII. The preferential efferent vasoconstriction induced by AII is considered to be an important adjustment in glomerular hemodynamics to preserve GFR in the presence of reduced RPF. [430] [431] [432] A study by Cody and associates[433] emphasized the importance of this mechanism in the regulation of glomerular filtration in patients with chronic CHF. In these patients, failure to maintain GFR was correlated with a diminished RPF as well as an impaired ability to maintain an adequately high FF. Thus, individuals with the greatest impairment of GFR had the greatest increase in overall RVR and the lowest FF. Moreover, because of the dependency of GFR on AII-induced efferent arteriolar vasoconstriction in CHF, removal of the influence of AII, for example, by ACE inhibitors, may result in a marked decline in renal function, particularly in patients with preexisting renal failure, massive diuretic treatment, and limited cardiac reserve. [431] [434]
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FIGURE 12-7 Peritubular control of proximal tubule fluid reabsorption. Current concept of the role of peritubular capillary physical forces in the regulation of proximal tubule. Fluid reabsorption for the normal state (left) and in patients with CHF (right) is depicted. ΔP and Δπ are the transcapillary hydraulic and oncotic pressure differences across the peritubular capillary, respectively. The increase in filtration fraction causes Δπ to rise in CHF. The increase in renal vascular resistance in CHF is believed to reduce ΔP. Both the increase in Δπ and the fall in ΔP enhance peritubular capillary uptake of proximal reabsorbate and thus increase absolute Na+ reabsorption by the proximal tubule. (From Humes HD, Gottlieb M, Brenner BM: The Kidney in Congestive Heart Failure: Contemporary Issues in Nephrology, Vol 1. New York, Churchill Livingstone, 1978, pp 51-72.) |
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Alterations in Tubular Reabsorption
A direct consequence of the glomerular hemodynamic alterations that have been outlined is an increase in the fractional reabsorption of filtered Na+ at the level of the proximal tubule. In Figure 12-2 , a comparison of the peritubular capillary hemodynamic profile between the normal state (left) and the CHF state (right) is shown on the right graph of each panel. Compared with the normal state, in CHF, the average value of Δπ along the peritubular capillary is increased and that of ΔP is decreased. This favors fluid movement into the capillary and may also reduce back-leakage of fluid into the tubule via paracellular pathways, promoting overall net reabsorption. The peritubular control of proximal fluid reabsorption in normal and CHF states is illustrated schematically in Figure 12-7 .
The contribution of enhanced fractional proximal Na+ reabsorption in CHF and its dependence on abnormal glomerular hemodynamics have been demonstrated in a number of early experimental and clinical studies. Studies using mannitol infusion in conjunction with clearance techniques,[435] pharmacologic blockade of distal nephron transport,[436] and mineralocorticoid escape with deoxycorticosterone acetate (DOCA)[437] all provided indirect evidence for enhanced proximal Na+ reabsorption and a consequent decrease in delivery of Na+ to more distal sites. Evidence for the dependence of enhanced proximal fractional Na+ reabsorption on altered glomerular hemodynamics in CHF was likewise obtained in the coronary ligation model of MI in rats by Ichikawa and co-workers.[428] When the increased SNFF was restored toward normal (with the use of an ACE inhibitor), there was a normalization of proximal peritubular capillary Starling forces and Na+ reabsorption.
Notwithstanding the importance of physical factors in determining the increase in proximal reabsorption, the contribution of other factors, such as the direct actions of the renal nerve and of AII on proximal Na+ transport, should not be underestimated. Thus, AII may act by modulating physical factors through its effect on efferent resistance, as well as by augmenting directly proximal epithelial transport, thereby amplifying the overall increase proximal Na+reabsorption.
Distal nephron sites also participate in the enhanced tubule Na+ reabsorption in experimental models of CHF. Micropuncture studies in dogs and in rats with AV fistulas [438] [439] and in dogs with pericardial constriction[440] or chronic partial thoracic vena caval obstruction[441] demonstrated enhanced distal nephron Na+ reabsorption. Levy[442] showed that the inability of dogs with chronic vena cava obstruction to excrete an Na+ load is a consequence of enhanced reabsorption of Na+ at the loop of Henle. Furthermore, the mechanism leading to the augmented reabsorption of Na+ by the loop of Henle in dogs with constriction of the vena cava seems to involve physical factors determined by renal hemodynamics, much as in the case of the proximal tubule.[443] Specifically, renal vasodilatation and elevation of RPP in dogs with vena cava constriction served to prevent the enhanced reabsorption of filtrate by the loop of Henle, thereby permitting a normal natriuretic response to saline loading.
Humoral Mechanisms
The homeostatic responses to myocardial failure include activation of vasoconstrictive/antinatriuretic systems, such as RAAS, SNS, ADH, and ETs, which increase vascular resistance and enhance renal water and salt reabsorption. In addition, several vasodilatory/natriuretic substances, such as NPS, NO, prostaglandins PGs, AM, and U-II, are also activated. It is recognized that salt and water homeostasis is largely determined by the fine balance between these vasoconstrictive/antinatriuretic and vasodilator/natriuretic systems, and that the development of positive Na+ balance and edema formation in CHF represents a turning point at which the balance is in favor of the former ( Fig. 12-8 ). This aspect of CHF became of special interest in the last few years, since a large-scale study on 1906 patients with CHF revealed that impaired renal function is a stronger predictor of mortality than impaired cardiac function.[444]These and other reports confirm that, in CHF, the activation of neurohormonal systems in association with renal dysfunction is strictly related to long-term mortality.[445]
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FIGURE 12-8 Efferent limb of extracellular fluid (ECF) volume control in CHF. Volume homeostasis in CHF is determined by the balance between the natriuretic and the antinatriuretic forces. In decompensated CHF, enhanced activities of the Na+-retaining systems overwhelm the effects of the vasodilatory/natriuretic systems, leading to a net reduction in Na+ excretion and an increase in ECF volume. (Adapted from Winaver J, Hoffman A, Abassi Z, et al: Does the heart's hormone, ANP, help in congestive heart failure? News Physiol Sci 10:247-253, 1995.) |
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Vasoconstrictive/Antinatriuretic Systems
Renin-Angiotensin-Aldosterone System.
The activity of the RAAS is enhanced in most patients with CHF in correlation with the severity of cardiac dysfunction.[446] Therefore, the activity of this system provides a prognostic index in the CHF patients. It has become increasingly apparent that, despite providing initial benefits in hemodynamic support, continued activation of RAAS contributes to the progression and worsening of CHF. [153] [447] RAAS activation induces direct systemic vasoconstriction and activates other neurohormonal systems such as ADH, which contribute to maintaining adequate intravascular volume.[448] However, numerous studies in patients and in experimental models of CHF established the deleterious role of the RAAS in the progression of cardiovascular and renal dysfunction in CHF. [153] [449] In particular, the kidney is highly sensitive to the action of the vasoconstrictor agents, especially AII, and a decrease in RPF is one of the most common pathophysiologic alterations in clinical and experimental CHF. Micropuncture techniques demonstrated that rats with chronic stable CHF display depressed glomerular plasma flow rates and SNGFR, as well as elevated efferent arteriolar resistance and FF. Direct renal administration of an ACE inhibitor did not affect renal function in sham-operated control rats but it did normalized it in the rats with experimental CHF. Utilizing a different model of CHF, induced by surgical creation of an aortocaval fistula, our group[450] showed that only a certain percentage of animals with AV fistula developed Na+ retention, whereas the rest maintained Na+ balance. The former subgroup is characterized by a marked increase in PRA and plasma aldosterone levels. In contrast, PRA and aldosterone levels in compensated animals were not different compared with those in sham-operated controls. Treatment with the ACE inhibitor enalapril resulted in a dramatic natriuretic response in rats with Na+ retention. The finding that animals with AV fistula either develop Na+ retention or maintain normal Na+ balance was previously demonstrated in dogs with CHF due to AV fistula.[451] A similar trend was also observed in patients with CHF. Whereas most CHF patients maintain normal Na+ balance when placed on a low-salt diet, about 50% of patients develop positive Na+ balance when fed a normal-salt diet. A common feature of both animals and patients with Na+ retention was the activation of the RAAS. In dogs with experimental high-output CHF, the initial period of Na+retention was associated with a profound activation of the RAAS, and the return to normal Na+ balance was accompanied by a progressive fall in PRA. In sum, these findings clearly demonstrate that activation of the RAAS contributes to the pathogenesis of Na+ and water retention in CHF. The deleterious effects of the RAAS on renal function are not surprising in light of the previously mentioned actions of AII and aldosterone on kidney hemodynamics and excretory function. Activation of AII in response to the decreased pumping capacity of the failing myocardium promotes systemic vasoconstriction in association with the preferential renal vasoconstrictive action on the efferent and afferent arteries and glomerular mesangial cells. [113] [153] [452] In addition, AII exerts both a negative influence on renal cortical circulation in rats with CHF and increases tubular Na+ reabsorption directly and indirectly by augmenting aldosterone release.[452] In combination, these hemodynamic and tubular actions lead to avid Na+ and water retention, thus promoting circulatory congestion and edema formation.
Whereas most studies related renal Na+ retention in CHF to elevated levels of renin, AII, or aldosterone,[453] other studies found no consistent relationship between RAAS and positive Na+ balance.[454] For instance, in dogs with pulmonary artery or thoracic inferior vena cava constriction, the RAAS was activated to a striking degree during the early phase of constriction and was necessary for the support of systemic blood pressure.[455] Administration of the ACE inhibitor captopril resulted in systemic hypotension. Over subsequent days, Na+ retention and ECF volume expansion were pronounced and inhibition of converting-enzyme activity was no longer accompanied by significant hypotension.[455] However, animals with severe impairment of cardiac output remained sensitive to the hypotensive effects of ACE inhibition. Similarly, among patients with CHF, PRA and levels of vasoconstrictor hormones were most elevated in patients with acute, severe, and poorly compensated CHF.[455] Levels declined when CHF became stable in the chronic stage. The foregoing experimental and clinical data therefore indicate that the influence of the RAAS in maintaining circulatory homeostasis may depend on the stage of CHF, being most pronounced in acute and decompensated CHF, and least pronounced in chronic stable CHF. However, even though the circulating RAAS is not activated in chronic stable CHF, alterations in renal function can still be corrected by ACE inhibition.[456] Therefore, it has been hypothesized that activation of local RAAS in certain tissues including heart, vasculature, kidney and brain may occur in the absence of alterations in the circulating hormone. Schunkert and associates[457] studied the relative status of the circulating and intrarenal RAAS by examining the intrarenal expressions of renin and angiotensinogen mRNA in rats with stable compensated CHF 12 weeks after experimental MI induced by coronary artery ligation. Compared with sham-operated control rats, the chronic CHF rats demonstrated no significant difference in the components of the circulating RAAS. However, there was a significant increase in the renal angiotensinogen mRNA level in the CHF rats and a parallel increase in renal AII concentrations, directly correlated with infarct size, suggesting that the magnitude of activation of the tissue RAAS is influenced by the degree of CHF. Long-term ACE inhibition in CHF rats increased renal renin mRNA and enzyme levels but normalized renal angiotensinogen mRNA levels.[457] In this context, although originally the RAAS was viewed solely as an endocrine system, increasing evidence suggests that all its components reside within several individual organs, such as kidney, lung, heart, and VSMCs. [118] [153] Moreover, several studies suggested that, in addition to the mechanical stress exerted on the myocardium due to AII-mediated increased afterload, activation of the local RAAS in these tissues may play a crucial role in the pathogenesis of CHF. [117] [458] In turn, pressure overload activates the production of locally AII, perhaps more than circulating AII, owing to up-regulation of angiotensinogen and tissue ACE.[153] Local AII acts via AT-1 in a functionally independent paracrine/autocrine fashion, where it is believed to play a significant role in the development of cardiac hypertrophy (owing to its growth properties), remodeling and fibrosis, and in the reduced coronary flow, hallmarks of severe CHF. [459] [460] In support of these observations are the well-established beneficial effects of ACE inhibitors and AT1 blockers (ARBs) in humans and animals with CHF, that is, improved cardiac function, prolonged survival, prevention of end-organ damage, and prevention or regression of cardiac hypertrophy. [114] [153] In addition, ACE inhibitors and ARBs may offer benefits with respect to endothelial dysfunction, vascular remodeling, and potentiation of the vasodilatory effects of the KKS. [113] [114] [461] [462] A significant component of these salutary effects emanate from the blockade of the local RAAS rather than the circulating system. [114] [153] Similar to AII, the other component of the RAAS, namely, aldosterone seems also to act directly on the myocardium.[463] The role of aldosterone in cardiac remodeling has emerged in the last few years. It is widely accepted that structural remodeling of the interstitial collagen matrix is regulated by both AII and aldosterone. Moreover, cardiac aldosterone production is increased in patients with CHF, especially when caused by systolic dysfunction. Convincing evidence for the local production of aldosterone was provided by the finding that CYP11β2 mRNA (aldosterone synthase) is expressed in cultured neonatal rat cardiac myocytes. The adverse contribution of aldosterone to the functional and structural alterations of the failing heart was elegantly proved by the use of eplerenone, a specific aldosterone antagonist, in which it prevented progressive left ventricular systolic and diastolic dysfunction in association with reducing interstitial fibrosis, cardiomyocyte hypertrophy, and left ventricular chamber sphericity in dogs with CHF. Similarly, Delyani and co-workers[464] reported that eplerenone attenuated the development of ventricular remodeling and reactive but not reparative fibrosis after MI in rats. These findings are in agreement with the results observed in clinical trials. The Randomized Aldactone Evaluation Study (RALES) showed that therapy with spironolactone reduced overall mortality in patients with advanced CHF by 30% compared with placebo.[465] Recently, the EPHESUS study showed that addition of eplerenone to optimal medical therapy reduces morbidity and mortality among patients with acute MI complicated by left ventricular dysfunction and CHF.[466]
Based on the maladaptive actions of locally produced or circulatory AII, one may envision that blocking the formation of this peptide may improve cardiorenal functions in CHF. Indeed, many studies indicate that ACE inhibition improves renal function in patients with CHF and is responsible for the improved cardiac performance and increasing life expectancy of these patients, [467] [468] [469] whereas others report that renal functional deterioration is a frequent complication.[434] The latter may stem from the beneficial effect of AII in helping to maintain glomerular capillary pressure, and thus the GFR, by its preferential constricting action on the efferent arterioles.[434] Studies of experimental CHF in animals demonstrated similar variability.[470] Some of these discrepancies might be attributable to differences in study design, specific drug use, titration of dose, and hypotensive response. In an elegant study examining this issue,[471] detailed analysis of renal function (using inulin and p-aminohippurate clearance) was performed for patients with CHF in New York Heart Association (NYHA) functional classes II and III. After ACE inhibition, a small but insignificant decrease in GFR occurred and a concomitant but not statistically significant increase in RPF with no change in plasma creatinine. Because patients with CHF are unable to escape from the Na+-retaining action of aldosterone and continue to retain Na+ in response to aldosterone, blockade of the latter by spironolactone has substantial natriuresis in these patients.[446] In recent years and with the development of selective nonpeptide orally active ARBs, several studies indicated beneficial effects of these drugs on cardiac performance, comparable with those of ACE inhibition. [114] [472] At the renal level, losartan was able to induce a significant natriuretic response in rats with decompensated CHF, induced by the placement of an aortocaval fistula.[473] In a model of ovine heart failure, acute administration of losartan was able to maintain GFR and urinary Na+ excretion despite a fall in RPP.[474] Likewise, in dogs with CHF due to rapid atrial pacing, chronic administration of TVC-116, another AII antagonist, prevented the decrease in GFR, RPF, and Na+ excretion.[475] Recent clinical studies found no differences between the efficacies of captopril and losartan on cardiac and renal functions in patients with CHF. [114] [446] [476] Overall, the effect of AII receptor blockade or ACE inhibition on renal function in CHF depends on a multiplicity of interacting factors. On the one hand, RBF may improve as a result of lower efferent arteriolar resistance. Systemic vasodilatation may be associated with a rise in cardiac output. Under such circumstances, reversal of hemodynamically mediated effects of AII on Na+ reabsorption would promote natriuresis. On the other hand, the aim of AII-induced elevation of the SNFF is to preserve GFR in the presence of diminished RPF. In patients with precarious renal hemodynamics, a fall in systemic arterial pressure below the autoregulatory range combined with removal of the AII effect on glomerular hemodynamics may cause severe deterioration of renal function. The net result depends on the integrated sum of these physiologic effects, which, in turn, depends on the severity and stage of heart disease ( Table 12-5 ).
TABLE 12-5 -- Converting-Enzyme Inhibition in Congestive Heart Failure
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As noted previously, in addition to its renal and cardiovascular hemodynamic effects, the RAAS is involved directly in the exaggerated Na+ reabsorption by the tubule in CHF. The most active component of this system, that is, AII, has a dose-dependent direct epithelial effect on the proximal tubule that favors active Na+ reabsorption. [143] [144] [477] The predominant effect of the RAAS on distal nephron function is mediated by the action of the second active component, that is, aldosterone, which acts on cortical and medullary portions of the collecting duct to enhance Na+ reabsorption, as outlined in a previous section. Numerous studies reported elevated plasma aldosterone concentration or urinary aldosterone secretion or natriuretic effects of pharmacologic aldosterone antagonists in animal models and human subjects with CHF, despite further activation of other antinatriuretic systems, supporting the pivotal role of this steroid hormone in the mediation of Na+ retention in CHF.[478] Variabilities in the relative importance of mineralocorticoid action in the Na+ retention of CHF emerging from these reports should be interpreted in light of the same considerations regarding stage and severity of disease that were noted with respect to the hemodynamic actions of AII.
Further evidence about the involvement of the RAAS in the development of positive Na+ balance can be gleaned from studies showing that the renal and hemodynamic response to ANP is impaired in CHF.[450] And administration of either losartan or ACE inhibitor restored the blunted response to ANP (for further details, see section on The Natriuretic Peptides).[450] Recent studies have demonstrated that omapatrilat, a mixed inhibitor of ACE and NEP, has hemodynamic and clinical benefits in patients with CHF, compared with ACE inhibitors. [213] [223] Interestingly, the rate of renal dysfunction was significantly less in those on omapatrilat.[223] This is of potential beneficial value because renal function frequently deteriorates during the progression of chronic CHF, and renal impairment is one of the most powerful predictors of prognosis in patients with CHF. [223] [444] Although patients with CHF have low serum osmolarity, they display increased thirst, most likely owing to the high concentrations of AII, which stimulate thirst center cells in the hypothalamus.[446] This behavior may contribute to the positive water balance and hyponatremia in these patients.
Sympathetic Nervous System.
Patients with CHF experience progressive activation of the SNS with progressive decline of cardiac function.[479] Elevated plasma NE levels are frequently observed in CHF and a strong consensus exists as to the adverse influence of sympathetic overactivity on the progression and outcome of patients with CHF.[480] Thus, sympathetic neural activity is significantly correlated to intracardiac pressures, cardiac hypertrophy, and left ventricular ejection fraction (LVEF).[479] Direct intraneural recordings in patients with CHF also showed increased neural traffic, which correlated with the increased plasma NE levels.[481] Activation of the SNS not only precedes the appearance of congestive symptoms but also is preferentially directed toward the heart and kidney. Clinical investigations revealed that patients with mild CHF have higher plasma NE in the coronary sinus than in the renal vein.[482] At the early stages, increased activity of SNS in CHF restores the hemodynamic abnormalities including hyoperfusion, diminished plasma volume, and impaired cardic function by producing vasoconstriction and avid Na+ reabsorption. [459] [479]However, chronic exposure to this system induces several long-term adverse myocardial effects including induction of apoptosis and hypertrophy, with an overall reduction in cardiac function, which reduces contractility. Some of these effects may be mediated, in turn, by activation of the RAAS. [446] [459]
Measurements using catecholamine spillover techniques revealed that the basal sympathetic outflow to the kidney is significantly increased in patients with CHF. [479] [480] The activation of the SNS and increased efferent renal sympathetic activity may be involved in the alterations in renal function in CHF. For example, exaggerated renal sympathetic nerve activity contributes to the increased renal vasoconstriction, avid Na+ and water retention, renin secretion, and attenuation of the renal actions of ANP.[228] Experimental studies demonstrated that renal denervation of rats with experimental CHF due to coronary artery ligation resulted in an increase in RPF and SNGFR and a decrease in afferent and efferent arteriolar resistance.[311] Similarly, in dogs with low cardiac output induced by vena cava constriction, administration of a ganglionic blocker resulted in a marked increase in Na+ excretion.[480] In rats with CHF induced by coronary ligation, the decrease in renal sympathetic nerve activity in response to an acute saline load was less than that of control rats.[307] Bilateral renal denervation restored the natriuretic response to volume expansion, implicating increased renal sympathetic nerve activity in the Na+ avidity characteristic of CHF.[480] Studies in dogs with high-output CHF induced by aortocaval fistula demonstrated that total postprandial urinary Na+ excretion was approximately twofold higher in renal-denervated dogs compared with control dogs with intact nerves.[483] In line with these observations, clinical investigation showed that administration of α-adrenoreceptor blocker dibenamine to patients with CHF caused an increase in fractional Na+ excretion, without a change in RPF or GFR. Treatment with ibopamine, an oral dopamine analog, resulted in vasodilation and positive inotropic and diuretic effects in patients with CHF.[484] Marenzi and associates[445] found that, for a given degree of cardiac dysfunction, the concentration of NE is significantly higher in patients with concomitant abnormal renal function than in patients with preserved renal function. These findings are similar to those observed by Hillege and colleagues[444] and suggest that the association between renal function and prognosis in patients with CHF is linked by neurohormonal activation, including CNS.
An additional mechanism by which renal sympathetic activity may affect renal hemodynamics and Na+ excretion in CHF is through its antagonistic interaction with ANP. ANP has sympathoinhibitory effects.[228] In contrast, CNS that retains water and salt in CHF plays a role in reducing renal responsiveness to ANP. For example, it has been demonstrated that the blunted diuretic/natriuretic response to ANP in rats with CHF could be restored by prior renal denervation,[485] or administration of clonidine,[486] a centrally acting α2-adrenoreceptor agonist, which decreases renal sympathetic nerve activity in CHF. These experimental and clinical data indicate that the SNS may play a role in the regulation of Na+ excretion and glomerular hemodynamics in CHF, either by a direct renal action or by attenuating the action of ANP. However, other studies failed to show an ameliorative effect of renal denervation on renal hemodynamics and Na+ excretion in CHF. Thus, in a study by Mizelle and colleagues,[487] no differences in renal hemodynamics or electrolyte excretion between innervated and denervated kidneys occurred following chronic unilateral denervation in conscious dogs with CHF induced by rapid ventricular pacing. Similarly, in dogs with reduced cardiac output due to pulmonary constriction, no significant differences in renal hemodynamics or Na+excretion occurred between the denervated and the intact kidney.[488] These discrepant results are probably due to species differences, the presence or absence of anesthesia, and the method of inducing CHF. It is also possible that high circulating catecholamines could interfere with the effects of renal denervation.
In summary, the perturbation in the efferent limb of volume homeostasis in CHF is a result of a complex interplay of the SNS and several other neurohormonal mechanisms on the glomeruli and the renal tubules.
Antidiuretic Hormone.
Since the early 1980s, numerous studies demonstrated that plasma levels of ADH are elevated in patients with CHF, mostly in advanced CHF with hyponatremia, but also in asymptomatic patients with left ventricular dysfunction.[489] [490] [491] [492] Potentially, these high circulating levels of ADH could adversely affect the kidney and the cardiovascular system in CHF. The mechanisms underlying the enhanced secretion of ADH in CHF are related to non-osmotic factors such as attenuated compliance of the left atrium, hypotension, and activation of the RAAS. [384] [493] In a study by Pruszczynski and colleagues[494] in patients with CHF, baseline plasma ADH levels were higher and were not suppressed after administration of an oral water load, although marked hypo-osmolality occurred. Although impairment of the baroreflex control mechanism for ADH release could be involved in this phenomenon, a study in humans with CHF by Manthey and co-workers[495] found an intact reflex response of ADH to baroreceptor unloading. An early study suggested that AII may stimulate the release of ADH,[496] implicating an additional mechanism for increased ADH in CHF; however, a later study indicated that AII does not release ADH.[497] Bichet and colleagues[498] noted that treatment with captopril or with prazosin resulted in suppression of ADH and improved water excretion in response to water loading in patients with CHF. It is likely that improved cardiac function in response to afterload reduction was responsible for removal of the nonosmotic stimulus to ADH release. It is noteworthy that the observed decline in MAP was considered too small to have an effect on the hormone. This suggests that hemodynamic variables other than MAP alone (e.g., pulse pressure, stroke volume) that improve with afterload reduction therapy may have been sufficient to abrogate the nonosmotic stimulus for ADH release.
The most recognized renal effect of ADH in CHF is the development of hyponatremia, which usually occurs in advanced stages of the disease and may occur at concentrations much lower than those required for vasoconstriction.[499] This phenomenon has been attributed to water retention by the kidney owing to sustained release of ADH, irrespective of plasma osmolality. This notion has been demonstrated in both animal and human studies. In a study by Pruszczynski and colleagues,[494] free water clearance and minimal urine osmolality were markedly impaired in patients with CHF compared with control subjects, and only in control subjects was the plasma ADH level correlated with plasma osmolality. Because many patients with CHF have positive water balance that results in hyponatremia, it is reasonable to attribute the hyponatremia to the elevated plasma levels of this neuropeptide.[500] Mulinari and associates[501] demonstrated that administration of an ADH antagonist with dual V1/V2 antagonism to rats with ischemic CHF induced by left coronary ligation resulted in a rise in cardiac output, a decline in PVR, and an increase in urine output of 4- to 10-fold over baseline, confirming the role of ADH in the water retention and the increased vascular resistance of CHF. Recent reports provided further insights into the mechanisms of ADH-mediated water retention in experimental CHF. These studies in animal models of CHF also demonstrated an increased renal expression of AQP II in these animals, suggesting that this may also contribute to the enhanced water reabsorption in the collecting duct.[502] It is, therefore, not surprising that initial studies showed that administration of V2 vasopressin receptor antagonists of peptidic and nonpeptidic nature to rats with inferior vena cava constriction,[503] dogs with CHF induced by rapid pacing,[504] and rats with CHF induced by coronary ligation[499] resulted in correction of the impaired urinary dilution in response to acute water load. The mechanism underlying these expected findings relies on the fact AQP II are expressed in the collecting duct and mediate the antidiuretic action of ADH.[505] Furthermore, the expression of AQP II and its immunoreactive levels has been reported to be elevated in the kidney of rats with experimental CHF induced by coronary artery ligation. [506] [507] Oral treatment of these rats with V2 antagonist (OPC31260) induced significant diuresis, a decrease in urinary osmolarity, and increased plasma osmolarity, which were associated with down-regulation of renal AQP II.[506] These findings indicate a major role for ADH in the up-regulation of AQP II water channels and subsequently enhanced water retention in experimental CHF. In agreement with the studies in experimental animals, several recent clinical studies demonstrated that chronic treatment with selective V2 and dual V1α/V2 antagonists may be beneficial in the correction of hyponatremia in CHF. [500] [508] [509]For instance, administration of the oral selective V2 receptor antagonist VPA-985 to patients with CHF for 7 days at incremental doses induced significant diuretic response accompanied by increase in plasma Na+ concentration and decreased urine osmolarity.[214] Similarly, when YM087, an orally V1/V2 antagonist was given orally to patients with CHF, it increased plasma Na+, reduced osmolarity of the urine, and increased urine output. [491] [500]Interestingly, Eisenman and colleagues[510] demonstrated that low doses of ADH can restore urinary flow in patients with end-stage CHF. This effect may be due to activation of V1 receptor subtype secondary to ANP release. It should be emphasized that, despite the promising therapeutic potential of the nonpeptidic ADH antagonist, care must be taken to avoid excessive or too rapid an aquaretic response, because this may predispose the patients to very serious CNS or even hemodynamic complications, as outlined in Chapter 13 .
However, in addition to hyponatremia, CHF is characterized by other alterations in renal function. These include a decrease in RBF in particular to the renal cortex, a decrease in GFR, and Na+ retention by the kidney. To what extent, if at all, enhanced levels of ADH are involved in these renal manifestations remains largely unknown.
In addition, ADH can impair cardiac function indirectly through its effect on SVR (increased cardiac afterload) as well as by V2-receptor-mediated water retention leading to systemic and pulmonary congestion (increased preload). In addition, ADH, through a direct endocrine action on cardiomycytes, could contribute to cardiac remodeling, dilatation, and hypertrophy, that might be further exacerbated by the aforementioned abnormalities in preload and afterload.
The notion that ADH may potentially contribute to the alterations in renal and cardiac function in CHF through the mechanisms previously discussed is indeed supported by several in vitro and in vivo studies.[492] Yet, given that other vasoconstrictor systems may share similar actions in CHF, the key question about the relative role of the increased endogenous ADH, compared with other systems, remains largely unanswered. To answer this question, efficient tools to block the biologic activities of ADH are required.
In the past, a number of cyclic and linear derivatives of the natural hormone were designed in an attempt to create effective antagonists of the ADH receptors.[511] Although these compounds provided valuable tools for the classification and for mapping the distribution of the ADH receptors, they were largely inefficient as blockers because of their short half-life and the fact that they had agonistic effects as well, especially in humans. In the 1990s, significant progress was made in the development and synthesis of highly selective and potent antagonists for the V1A, V2, and most recently, for the V1β receptor subtypes.[512] Likewise, mixed V1A/V2 receptor antagonists are now available. These compounds are small nonpeptide molecules, are orally active, lack agonist effects, and display high affinity and specificity to their corresponding receptors. [512] [513] The term Vaptan has been coined to name the members of this new class of drugs. Several of these compounds have been utilized in experimental models of CHF and were found to produce hemodynamic improvement with transient decrease in SVR, increase cardiac output, and improve water diuresis. [504] [514] [515] [516]
However, these studies examined primarily the acute effects of the drugs, and only limited and incomplete data are available at present on the long-term effects of the drugs in experimental CHF. [517] [518] Similarly, in patients with CHF, there are only initial reports dealing mainly with the improvement in hyponatremia induced by the drugs. [508] [509] One additional study reported a beneficial action of a dual V1A/V2 antagonist on right atrial pressure and pulmonary wedge pressure, but no change in cardiac output or SVR.[519]
In summary, from reviewing the current literature, it is clear that additional data are necessary to clarify the role of ADH in CHF as well as the efficacy of these drugs as a novel treatment in CHF. The question not only is of academic interest but also has important therapeutic implications, given that mortality in CHF remains high despite the effective use of ACE inhibitors.
Collectively, these data suggest that ADH is involved in the pathogenesis of water retention and hyponatremia that characterize CHF and that vasopressin receptor antagonist results in remarkable diuresis in both experimental and clinical CHF.
Endothelin.
Recent evidence implicated ET-1 in the devel-opment and progression of CHF. Furthermore, this peptide is probably involved in the reduced renal function that characterizes the cardiorenal state, by inducing renal modeling, interstitial fibrosis, glomerulosclerosis, hypoperfusion/hypofiltration, and positive salt and water balance. [251] [256] The pathophysiologic role of ET-1 in CHF is supported by two major lines of evidence: (1) Several studies demonstrated that the ET system is activated in CHF. [247] [262] [520] (2) Some clinical and experimental studies showed that ET-1 receptor antagonists modify this pathophysiologic process. The first line of evidence is based on the demonstration that plasma ET-1 and big ET-1 concentrations in both clinical CHF and experimental models of CHF are elevated and correlate with hemodynamic severity and symptoms. [521] [522] Cavero and co-workers[523]reported that plasma immunoreactive ET-1 levels are elevated two- to three-fold above normal in dogs with CHF induced by rapid ventricular pacing. Elevated circulating ET-1 levels have also been reported in patients with CHF.[251] [521] A negative correlation between plasma ET-1 concentration and LVEF has been reported.[524] In another study, the degree of pulmonary hypertension was the strongest predictor of plasma ET-1 level in patients with CHF.[525] [526] Moreover, the 1-year mortality rate among patients who have had an MI strongly correlates with plasma levels of ET measured 3 days after the infarction.[527] These prognostic reports are in line with the observation that plasma ET-1 is elevated only in patients with moderate and severe CHF, but not in patients with asymptomatic CHF. The mechanisms underlying the increased plasma levels of ET-1 have not been clarified, although this may be due to either enhanced synthesis of the peptide in the lungs, heart, and circulation by several stimuli such as AII and thrombin or decreased clearance of the peptide by the pulmonary system. [251] [522] [528] Parallel to ET-1, ETA receptors are up-regulated, whereas ETB are down-regulated in the failing human heart. [528] [529] Whether the activation of ET-1 system in CHF has any pathophysiologic significance is another area of debate. However, increasing plasma ET-1 levels in normal animals to concentrations found in CHF is associated with significant reduction in RBF and increased vascular resistance.[530] Bearing in mind that CHF is characterized by reduced RBF associated with increased vascular resistance along elevated levels of ET-1, it is appealing that a cause-and-effect relationship exists between these hemodynamic abnormalities and ET-1 in this disease state. This notion became evident with the development of numerous selective and highly specific ET receptor antagonists. [531] [532] [533] [534] Experimental studies demonstrated that acute administration of bosentan, a mixed ETA/ETB receptor antagonist, significantly improved renal cortical perfusion in rats with severe decompensated CHF induced by aortocaval fistula formation.[535] Similarly, tezosentan, a dual parenteral ET receptor antagonist, reversed the profoundly increased RVR and improved RBF and Na+ excretion in rats with CHF induced by MI.[536] This conclusion gained further support from several studies that showed that chronic blockade of ETA by selective antagonists [537] [538] or by dual ETA/ETB receptor antagonists[539] attenuates the magnitude of Na+ retention and prevents the decline in GFR in experimental CHF. These effects are in line with recent observations that infusion of ET-1 to normal rats produced a sustained cortical vasoconstrictor and a transient medullary vasodilatory response. [261] [494] In contrast, rats with decompensated CHF displayed severely blunted cortical vasoconstriction, but significantly prolonged and preserved medullary vasodilation.[290] The significance of these attenuated renovascular effects of ET-1 and big ET in CHF experimental animals is uncertain, but the effect could probably result from activation of vasodilatory systems such as PGs and NO. Indeed, the medullary tissue of rats with decompensated CHF contains higher eNOS immunoreactive levels compared with that in sham controls.[290] These findings indicate that ET may be involved in the altered renal hemodynamics and the pathogenesis of cortical vasoconstriction in CHF.
Initial clinical studies showed that acute ET antagonism by bosentan decreased vascular resistance and increased cardiac index and cardiac output in patients with CHF, suggesting that ET-1 plays a role in the pathogenesis of CHF by increasing SVR.[540] However, in contrast to early studies, recent comprehensive clinical trials demonstrated no benefits from treating CHF patients with bosentan, which actually increased hepatic transaminases and mortality rate.[541] Unfortunately, none of these studies examined whether these antagonists have any beneficial effects on renal function. However, given the marked vasoconstrictor and mitogenic properties of ET-1 and the increased local cardiac-pulmonary-renal production of this peptide in CHF, it is appealing to assume that ET-1 contributes directly and indirectly to the enhanced Na+ retention and edema formation by aggravating kidney and heart functions, respectively. [529] [539] [542] [543] [544] However, establishing the importance of ET in the renal hemodynamic and excretory dysfunction in CHF requires further study.
Vasodilatory/Natriuretic Systems
The Natriuretic Peptide System.
Renal Na+ and water retention in decompensated cardiac failure occurs despite expansion of the ECF volume and in the face of activiation of the NP system. Actually, CHF is the most prominent example of a clinical condition that involves abnormalities in the NP system. Several clinical and experimental studies implicated both ANP and BNP in the pathophysiology of the deranged cardiorenal axis in CHF.
Atrial Natriuretic Peptide
Despite the high levels of this potent natriuretic and diuretic agent, patients and experimental animals with CHF retain salt and water owing to attenuated renal responsiveness to NPs. Infusion of pharmacologic doses of synthetic ANP to experimental animals[548] and to patients with CHF [421] [549] also consistently demonstrated an attenuated renal response compared with normal control subjects. However, other beneficial effects accompany the infusion of ANP to patients with CHF, such as hemodynamic improvement and inhibition of activated neurohumoral systems. Hirsch and co-workers[550] showed in patients with CHF and Kanamori and associates[551] showed in dogs with CHF that ANP is a weak counterregulatory hormone, insufficient to overcome the substantial vasoconstriction mediated by the SNS, the RAAS, and ADH. However, despite the blunted renal response to ANP in CHF, elimination of the source of production of this peptide by surgical means aggravates the activation of these vasoconstrictive hormones in this disease state. For instance, Lohmeier and colleagues[552] demonstrated that atrial appendectomy to eliminate the source of ANP production in dogs with CHF due to rapid pacing resulted in substantial increments in PRA and plasma NE as well as marked Na+ and water retention—suggesting that ANP plays a critical role as a suppressor of Na+-retaining systems. Therefore, the increase in circulating NPs is still considered an important adaptive or compensatory mechanism aimed at reducing PVR and effective blood volume. Actually, the Na+ balance in the initial compensated phase of CHF has been attributed in part to the elevated levels of ANP and BNP.[213] This notion is supported by the findings that inhibition of NP receptors in experimental CHF induces Na+ retention. [515] [553] Furthermore, NPs inhibit the systemic vasoconstrictive effect of AII,[554] AII-stimulated proximal tubule Na+ reabsorption,[230] AII-enhanced secretion of aldosterone,[554] and the secretion of ET.[555] Therefore, NPs in CHF are an ideal counterregulatory hormone, influencing RPF and Na+ excretion either through their direct renal actions or through inhibition of release or action of other vasoconstrictive agents. Moreover, besides these cardiovascular and endocrine effects, NPs likely play an important role in promoting salt and water excretion by the kidney in the face of myocardial failure. Indeed, studies in an experimental model of CHF demonstrated that inhibition of the NPs by either specific antibodies to their receptors or the ANP receptor antagonist HS-142-1 causes further impairment in renal function, as expressed by increased RVR and decreased GFR, RBF, urine flow, Na+ excretion, and activation of the RAAS. [228] [555]
A key question, then, related to why salt and water retention occurs in overt CHF, despite the remarkable activation of the NP system?
Several mechanisms have been suggested to explain this apparent discrepancy ( Table 12-6 ):
|
1. |
Appearance of abnormal circulating peptides such as β-ANP and inadequate secretory reserves compared with the degree of CHF. However, the fact that circulating levels of native biologically active NPs are clearly elevated in CHF indicates that these putative factors cannot account for the exaggerated salt and water retention. |
|||||||||||||
|
2. |
Decreased availability of NPs. Because NPs are removed from the circulation by two means, that is, NEP and clearance receptors, increased activity of these routes may theoretically contribute to the decreased effects of these peptides.[228] So far, no convincing evidence suggests that up-regulation of clearance receptors exist in the renal tissue of CHF animals or patients, although increased abundance of clearance receptors for NPs in platelets of patients with advanced CHF has been reported.[556] In contrast, several studies demonstrated that expression and activity of NEP are enhanced in experimental CHF. [557] [558] This may contribute to increased elimination of NPs, thus leading to reduced availability of these peptides and consequently to renal resistance to these hormones. Further support for this notion comes from numerous reports wherein NEP inhibition by pharmacologic means has been shown to improve the vascular and renal response to NPs in CHF (see later). |
|||||||||||||
|
3. |
Activation of antinatriuretic systems. The ability of NPs to antagonize the renal effects of AII may be limited in the presence of markedly impaired RPF such as in CHF.[560] Abassi and co-workers[451] de-monstrated that chronic blockade of the RAAS by enalapril partially but significantly improved the natriuretic response to endogenous and exogenous ANP in rats with CHF induced by aortocaval fistula. The improvement in renal response to ANP was more evident in rats with decompensated CHF than in rats with compensated CHF. It should be emphasized that decompensated CHF is characterized by profound activation of RAAS. These findings are in line with the fact that activation of RAAS in CHF largely contributes to Na+ and water retention by antagonizing the renal actions of ANP. Actually, AII, the main active component of the RAAS, counteracts the natriuretic effects of ANP even under normal conditions.[561] Potential mechanisms of this phenomenon may include AII-induced afferent and efferent vasoconstriction, mesangial cell contraction, activation of cGMP phosphodiesterases that attenuate the accumulation of the second messenger of NPs in target organs, and finally, stimulation of Na+,H+-exchanger and Na+ channels in the proximal tubule and collecting duct.[451] The mechanisms underlying the attenuated renal effects of ANP in CHF are not completely understood.
|
TABLE 12-6 -- Possible Mechanisms Underlying the Renal Resistance to Natriuretic Peptides in Congestive Heart Failure
|
Release of less active forms of ANP, such as β-ANP and proANP |
|
Down-regulation of natriuretic peptide receptors coupled to guanylate cyclase |
|
Decreased renal perfusion pressure |
|
Increased degradation of natriuretic peptides by neutral endopeptidase and of its second messenger, cGMP, by specific phosphodiesterases |
|
Activation of antagonizing hormonal systems, such as reninangiotensin-aldosterone system, sympathetic nervous system, ET, and ADH |
|
ANP, antinatriuretic peptide; cGMP, cyclic guanosine monophosphate; ET, endothelin. |
Brain Natriuretic Peptide
As noted previously,BNP (32 amino acids in human)is structurally similar to ANP, but is produced mainly by the ventricles in response to ventricular stretch and pressure overload. [236] [604] [605] [606] Similar to ANP, plasma levels of BNP are elevated in patients with CHF in proportion to the severity of myocardial systolic and diastolic dysfunction. [559] [607] [608] [609] [610] [611] [612] [613] [614] Wei and co-workers[615] reported that plasma levels of BNP are elevated only in patients with severe CHF,whereas the circulating concentrations of ANP are high in mild and severe cases. Similar results were obtained by Rademaker and colleagues,[616] who demonstrated that acute rapid atrial pacing in conscious sheep increased the secretion of ANP and BNP by 8.6- and 3.6-fold, respectively; whereas chronic rapid pacing elevated plasma levels of ANP and BNP by 7.8- and 9-fold, respectively. The extreme elevation of plasma BNP in severe CHF probably stems from the increased synthesis of BNP, predominantly by the hypertrophied ventricular tissue, although the contribution of the atria cannot be understated.[617] Asymptomatic experimental left ventricular dysfunction in dogs was not associated with enhanced expression of BNP in the ventricles, although the atrial tissue significantly increased the expression of this peptide in both mild and overt CHF.[617]
Plasma levels of ANP and NH2-terminal ANP increase early in the course of CHF.[618] Lerman and co-workers[619] and Hall and associates[620] demonstrated that left ventricular dysfunction is associated with increased plasma levels of NT-proANP (1-98 NH2-terminal). In this context, several studies showed that plasma ANP levels correlate with the severity of symptomatic CHF,[621] suggesting that the concentration of circulating ANP may serve as a diagnostic tool in the determination of cardiac dysfunction and as a prognostic marker in the prediction of survival of patients with CHF with a sensitivity and specificity of more than 90%.[611] Although echocardiography remains the gold standard for the evaluation of left ventricular dysfunction, numerous studies introduced plasma levels of BNP as a reliable marker for the diagnosis and management of CHF. Actually, in the past few years, the superiority of BNP to ANP as a diagnostic and prognostic factor in CHF has been supported by numerous clinical studies. [612] [622] [623] [624] These and other studies showed that systemic BNP concentrations are significantly elevated in overt CHF, and these concentrations reflect left ventricular function with fidelity. [607] [625] Patients diagnosed with CHF have a mean BNP value of 1076±138 pg/mL compared with 38±4 pg/mL in non-CHF subjects presenting to the urgent care unit. [237] [624] Moreover, plasma levels of BNP correlate with NYHA class number, in which circulating levels of BNP range between ∼200 pg/mL in class I and ∼1000 pg/mL in class IV CHF patients.[237] Luchner and associates,[626] in their study on NT-proBNP after MI, observed an increase in NT-proBNP in subjects with MI. This increase was particularly pronounced in the presence of significant left ventricular dysfunction and renal dysfunction. Patients with an EF of less than 35% were detected by NT-proBNP with sensitivity, specificity, and negative predictive value of 75%, 62%, and 99%, respectively, at an optimal cutoff of 44 pmol/L. Patients with concomitant left ventricular hypertrophy were detected with sensitivity, specificity, and negative predictive value of 90%, 80%, and 99.9%, respectively, at a cutoff of 76 pmol/L. Similar results were obtained for patients with concomitant renal dysfunction at a cutoff of 162 pmol/L. These authors concluded that NT-proBNP is a marker of integrated cardiorenal function and a potential diagnostic tool for the detection and exclusion of impaired let ventricular function, particularly in the presence of concomitant left ventricular hypertrophy or renal dysfunction.[626] Similarly, de Lemos and associates[627] demonstrated the ability of circulating BNP, measured within a few days of acute coronary syndromes, to predict risk of mortality, clinical CHF, and new MI, suggesting that activation of this neurohormonal axis may be a common feature among patients at high risk for death after acute MI. Most recently, Richards and co-workers[628] showed that plasma BNP (or NH2-terminal BNP) and LVEF are complementary independent predictors of major adverse events on follow-up after MI. For example, elevated BNP predicted new MI only in patients with LVEF<40%. LVEF<40% coupled to elevated NT-BNP over the group median conferred a substantially 3-year increased risk of death, CHF, and new MI of 37%, 18%, and 26%, respectively. These findings indicate that combined measurement of these two parameters provides risk stratification substantially better than that provided by either alone. The plasma level of BNP is a powerful marker for prognosis and risk stratification in the setting of CHF. [629] [630] According to Harrison and colleagues,[631] BNP levels greater than 240 pg/ml are associated with high relative risk of 6 months' death in CHF dyspneic patients. Similarly, Berger and co-workers[632] found that BNP levels were the only independent predictor of sudden death in arrhythmic CHF patients with an ejection fraction of less than 35%. According to this study, the cut-off value was 130 mg/mL, which is comparable with those suggested by others ∼80 to 100 pg/mL. [629] [630] [633]
As many as 40% to 50% of patients with a diagnosis of CHF have normal systolic function, which implicates diastolic dysfunction as the most likely potential abnormality responsible for this disorder. Diastolic heart failure cannot be distinguished from systolic heart failure on the basis of history, physical examination, chest x-ray, and electrocardiogram alone. As a result, indirect and noninvasive assessments of left ventricular filling dynamics have been used to characterize diastolic properties, especially echocardiographic Doppler transmitral velocity measurements. There are four distinct echocardiographic patterns: normal, delayed relaxation, pseudonormal, and restrictive. BNP release appears to be directly proportional to ventricular volume expansion and pressure overload, and elevated BNP levels in patients with normal systolic function correlate with diastolic abnormalities on Doppler studies. Thus, BNP represents a circulating plasma marker providing positive evidence of the presence of diastolic dysfunction, even in asymptomatic patients. Conversely, a reduction in BNP levels with treatment are associated with a reduction in left ventricular filling pressures, a lower readmission rate, and a better prognosis, such that monitoring of BNP levels may provide valuable informa-tion regarding treatment efficacy and expected patient outcomes. [237] [634]
In addition, plasma levels of BNP are useful in distinguishing dyspnea caused by CHF or disorders other than CHF, such as pulmonary causes. [629] [630] [633] [635] Dao and colleagues[633] reported that patients presenting to urgent care units owing to CHF have BNP plasma levels 28-fold those obtained in a non-CHF group. BNP at the cut-off point of 80 pg/mL was highly selective and sensitive for the diagnosis of CHF. According to this study, which included 250 patients, BNP values lower than 80 pg/mL have a negative predictive value of 98% for CHF diagnosis. Plasma levels of BNP in patients with dyspnea owing to CHF were sixfold those obtained in patients without CHF (675 compared with 110 pg/mL). Moderate values of BNP were observed in patients with mild left ventricular dysfunction (346 pg/mL). It is widely believed that a BNP level below 50 pg/mL has strong negative predictive value (96%) in the assessment of patients with dyspnea caused by a disorder of noncardiac origin. In line with this conclusion, the diagnostic accuracy, sensitivity, specificity of BNP at a cut-off of 100 pg/mL were 83.4%, 90%, and 74%, respectively. [629] [630] According to this and other studies,[610] the predictive accuracy of circulating BNP for the diagnosis of CHF equals and even exceeds the accuracy of classic examinations such as x-ray and physical examination. In a large-scale study, Maisel and colleagues [629] [630] reported that a single determination of circulating BNP level was more accurate than both the National Health and Nutrition Examination score and Framingham clinical parameters (the most established criteria in use for the diagnosis of CHF) in differentiating dyspnea of cardiac versus noncardiac origin. This conclusion was supported by a recent prospective, randomized, and controlled study of 452 patients who presented to the emergency department with acute dyspnea. Half of the patients were randomly assigned to a diagnostic strategy involving the measurement of BNP levels with the use of a rapid bedside assay (BNP group), and half of the patients were assessed in a conventional manner (control group). In addition, the median time of discharge and cost of treatment were significantly higher in the former group compared with the latter. Again, measurement of BNP levels for the diagnosis of acute dyspnea of cardiac etiology should be assessed in conjunction with other conventional clinical parameters and not alone.
In addition to diagnostic and prognostic applications, circulating BNP and its NT-proBNP have been used as a guide in determining the therapeutic efficacy of typically prescribed drugs for CHF patients, including ACE inhibitors, diuretics, digitalis, and β-blockers. [587] [588] Kawai and associates[636] reported that plasma BNP correlates with left ventricular end-diastolic dimension, LVEF, and left ventricular mass in patients with idiopathic dilated cardiomyopathy and that administration of carvedilol to these patients for 6 months improved these parameters in most patients in association with decreased BNP levels in responders. Similarly, Motwani and colleagues[637] found that BNP, but not ANP, accurately reflects the improvement in the ejection fraction of patients treated with ACE inhibitor following MI. In treated as well as untreated patients with CHF, high levels of BNP are an independent predictor of mortality. Maeda and co-workers[638] demonstrated that plasma levels of BNP and interleukin-6 are independent risk factors for morbidity and mortality in patients with CHF after 3 months of optimized treatment.
Taken together, these and other findings suggest that a simple and rapid determination of plasma levels of BNP in patients with CHF can be used to assess cardiac dysfunction and serve as a diagnostic and prognostic marker. In addition, measurements of plasma BNP may be useful in titrating relevant therapy. In this context, Troughton and associates [587] [588] reported that the first cardiovascular event after 6 months of therapy was less frequent in CHF patients whose plasma BNP levels decreased in response to medical treatment.
However, it should be emphasized that measurement of plasma levels of either ANP and BNP outside of the broader clinical context is of limited diagnostic value, because the concentrations of these peptides in the circulation are affected by several factors, including age, salt intake, gender, and hemodynamic status. Therefore, a combination of conventional parameters such as clinical and echocardiographic measures taken together with plasma levels of BNP yield better clinical guidelines in patients with CHF than utilizing each tool alone.[639] This approach gained further support from a recently published study by Tang and co-workers,[640] who reported that in the ambulatory care setting, patients with asymptomatic and symptomatic chronic stable systolic heart failure present a wide range of plasma BNP levels. Nevertheless, still 21% of symptomatic patients display BNP plasma levels below 100 pg/mL.
In light of the reports that BNP is less susceptible to degradation by NEP 24.11 compared with ANP, [641] [642] it is not surprising that, on a mole-to-mole basis, BNP is a more biologically potent natriuretic agent than the latter.[641]With this in mind, the efficacy of exogenous human BNP was examined in patients with decompensated CHF.[643] Bolus or sustained infusion of BNP (nesiritide) for short (minutes to hours) and long (hours to days) periods to patients with decompensated CHF (mostly NYHA classes III and IV) resulted in substantial beneficial hemodynamic changes. These changes included reductions in elevated right atrial pressure, pulmonary artery pressure (PAP), PCWP, MAP, and SVR, in association with increased cardiac index, urinary flow rate, and Na+ excretion without activation of neurohumoral systems. [563] [644] [645] [646] [647] [648] The hemodynamic and natriuretic effects of exogenous BNP administration were significantly greater than those obtained following the use of similar doses of ANP in patients with CHF.[644] These effects of BNP were associated with enhanced release of cGMP. Comparable results were reported by Abraham and colleagues,[649] who found that BNP infusion to patients with CHF improved cardiac performance and suppressed plasma levels of NE and aldosterone, but only one third of the patients showed increased Na+ excretion. The attenuated natriuretic response to BNP in these patients is not surprising in light of the structural similarity between ANP and BNP and the fact that both peptides share the same mechanism of action. Similar attenuated renal responsiveness to BNP, despite elevated plasma levels of this peptide, was reported by Hoffman and co-workers[650] in rats with CHF induced by the placement of aortocaval fistula. When BNP was given at low and high subcutaneous doses for 10 days to dogs with experimental CHF, cardiac filling pressure was reduced in association with increased urinary Na+ excretion, urine flow, and RPF.[651] After 10 days of treatment, cardiac output was increased and RVR and PCWP decreased, suggesting that chronic administration of BNP via a subcutaneous route may be used as a novel strategy for the treatment of CHF. It should be noted, however, that one of the most adverse effects of recombinant BNP (nesiritide) is dose-related hypotension.[564] The later adverse effect may impose serious problem when nesiritide is given with other vasodilators, such as ACE-I.[618] Nevertheless, when acutely infused into patients with decompensated CHF, neseritide was less tachycardic or arrhythmogenic than dobutamine.[632]
When comparing intravenous nesiritide with nitroglycerin in treating patients with CHF, nesiritide displayed more prominent hemodynamic effects, such as reduction in PCWP, compared to standard care plus nitroglycerin or placebo, and these effects were sustained for at least 24 hours. Symptomatic effects, such as improvement in dyspnea, were observed with both drugs, although more pronounced following administration of nesiritide. The hemodynamic and symptomatic improvement with nesiritide, coupled with a safety profile similar to that of nitroglycerin, suggests that nesiritide, along with diuretics, is a useful addition to the initial therapy of patients hospitalized with acutely decompensated CHF.[652]
C-type Natriuretic Peptide
Although CNP is synthesized mainly by endothelial cells, small amounts are also produced by cardiac tissue.[653] In contrast to other NPs, CNP is predominantly a vasodilator and has little effect on urinary flow and Na+ excretion, and in some cases, even reduces these parameters. [654] [655] [656] However, the production of CNP by the endothelium in proximity to its receptors in VSMC suggests that this peptide may play a role in the control of vascular tone and growth.[657] In contrast to ANP and BNP, plasma levels of CNP are not increased in CHF; however, local concentrations of CNP are elevated in the myocardium in this disease state.[615] In a recent large study (n=305), Wright and assoicates[658] demonstrated that plasma levels of CNP are elevated in patients with symptomatic CHF and that the use of BNP as a predictor for CHF shows a significant relation to concurrent plasma CNP. These findings suggest a possible peripheral vascular compensatory response to CHF by overexpression of this vasodilatory peptide. Most recently,[653] it has been demonstrated that CNP possesses an inhibitory effect on cultured cardiac myocyte hypertrophy, suggesting that overexpression of CNP in the myocardium during CHF may be involved in counteracting cardiac remodeling.
Nitric Oxide.
Recent studies showed that endothelial dysfunction has a fundamental impact on the development of impaired cardiac performance with all the concomitant adverse systemic consequences. It is widely believed that endothelial dysfunction contributes to the increase in vascular resistance in CHF [245] [247] [659] and to the impaired endothelium-dependent vascular responses in correlation to the clinical severity of cardiac dysfunction. [247] [659] [660] [661]Thus, the response to acetylcholine, an endothelium-dependent vasodilator that acts by releasing NO, was found to be markedly attenuated in patients and experimental animals with CHF. Similar observations were reported in isolated vessels from animals with CHF examined in vitro. The mechanisms mediating the impaired activity of the NO system in CHF are largely unknown. Several potential mechanisms have been offered as an explanation. These include a reduction in shear stress associated with the decreased cardiac output,[247] down-regulation of NOS, decreased availability of the NO precursor L-arginine, [660] [662] increased levels of dimethyl arginine and overriding activity of counterregulatory vasoconstrictor systems such as the RAAS. [660] [663]
In view of the importance of NO in regulating RBF, it is possible that altered activity of the NO system may be involved in the pathogenesis of the renal hypoperfusion in CHF. The latter possibility is supported by our findings that rats with CHF induced by aortocaval fistula have attenuated NO-mediated renal vasodilation.[663] Moreover, this impairment could be reversed by pretreatment with an AT1 receptor antagonist, suggesting that AII may be involved in mediating the impaired NO-dependent renal vasodilatation.[663] The resulting imbalance between NO and excessive activation of the RAAS and ET systems explains some of the beneficial effects of ACE-inhibitors, ARBs, and aldosterone antagonists.[664] A blunted response to endothelium-dependent vasodilators has been generally equated with a decrease in NOS activity and NO generation. However, several studies demonstrated that patients with CHF have higher plasma levels of NO2±NO3 and exhibit augmented responsivness to inhibitors of NOS, suggesting that NO generation and release are enhanced in CHF. [660] [665] [666] [667] [668] According to these studies, the NO system in CHF represents another failing counterregulatory mechanisms in the face of the activated vsoconstrictors. In line with this concept, most recently, Abassi and co-workers[290] demonstrated that rats with experimental CHF induced by aortocaval fistula express higher abundance of eNOS-mRNA and protein immunoreactivity in the kidney, particularly in the renal medulla. It was speculated that the overexpression of eNOS in the renal medulla may play an important role in the preservation of intact medullary perfusion. In addition, the increased eNOS levels in the cortex, although to a lesser extent than in the medulla, may serve a compensatory mechanism in ameliorating the severe cortical vasoconstriction.
An additional issue worthy of consideration is the fact that the myocardium contains all the three isoforms of NOS, and the locally generated NO is believed to play a modulatory role on cardiac function. [278] [446] Thus, it might be that alterations in the cardiac NO system in CHF contribute to the pathogenesis of cardiac dysfunction and, therefore, indirectly contribute to the impaired renal function.[669] Indeed, increasing evidence indicates that iNOS and eNOS overexpression occurs in failing myocardium. The deleterious actions of the increased NO levels on ventricle contractility is well documented.[278] Based on the foregoing, achieving NO balance by either NO donors or selective NOS inhibitors has emerged as one of the most important therapeutic concepts in addressing and correcting the pathophysiology of CHF.[670] Although early clinical trials have yielded encouraging initial results, extensive efforts remain to be investigated in order to verify whether this treatment option is feasible/beneficial.
In summary, the endothelium-dependent vasodilatation is attenuated in various vascular beds in CHF. This attenuation may occur in the presence of increased NO production, suggesting that the vascular NO may be another example of failed vasodilator system in CHF.
Prostaglandins.
Although the PGs have little contribution to kidney function in euvolemic and unstressed states, they play an important role in maintaining renal function during setting of pathophysiologic compromise, including CHF. As previously noted, when RBF is impaired, hypoperfusion of the kidney or activation of the RAAS stimulates the release of PGs that exert a vasodilator effect predominantly at the level of the afferent arteriole and promote Na+ excretion by inhibiting Na+ transport in TALH and medullary collecting duct. [671] [672] [673] Two previous observations suggested a compensatory role of PGs in experimental and clinical CHF: First, plasma levels of PGE2, PGE2 metabolites, and 6-keto-PGF1 were elevated in CHF patients compared with normal subjects.[674] Moreover, studies in experimental and human CHF demonstrated a direct linear relationship between the PRA and AII concentrations and levels of circulating and urinary PGE2 and PGI2 metabolites.[675] This correlation probably reflects both stimulation of PG synthesis by AII and increased release of renin induced by PGs. A similar counterregulatory role of PGs with respect to the other vasoconstrictors (catecholamines and ADH) may also be inferred. An inverse correlation between serum Na+ concentrations and plasma levels of PGE2 metabolites has been demonstrated. The second approach that established the protective role of renal and vascular prostaglandins in CHF was derived from studies of nonsteroidal anti-inflammaatory drugs (NSAIDs), which inhibit the synthesis of PGs. In various experimental models of CHF, inhibition of PG synthesis was associated with adverse renal hemodynamic consequences. [470] [495] [674] In one of these studies,[470] using a rapid ventricular pacing canine model of CHF, the induction of CHF was associated with an elevation in urinary excretion of PGE2. Administration of indomethacin was associated with a significant increase in body weight, serum creatinine, and urea and a significant decline in urine flow rate. In another study of experimental chronic moderate CHF,[676] increased urinary excretion of PGE2 was obtained. Administration of indomethacin was associated with a profound increase in RVR and a resultant decrease in RBF, mainly related to afferent arteriolar constriction. On the basis of these observations, it is not surprising to find that patients with hyponatremia accompanied by the most striking activation of the SNS and the RAAS were most susceptible to adverse glomerular hemodynamic consequences after the administration of indomethacin.[674] In this regard, Townend and colleagues[677] demonstrated that administration of indomethacin to patients with chronic CHF resulted in a significant decrease in RBF and GFR in association with reduced urinary Na+ excretion. These effects were prevented by intravenous infusion of PGE2. In the same study, the authors showed that pretreatment of these patients with indomethacin prior to captopril administration attenuated the captopril-induced increase in RBF. These results suggest that PGs have a significant role in the regulation of renal function in patients with CHF. In addition, these results indicate that captopril-induced improvement in renal hemodynamics is mediated in part by an increase in PG synthesis. In this context, renal PGs also play an important role in mediating the natriuretic effects of ANP in dogs with experimental CHF induced by an AV fistula.[678] According to this study indomethacin reduced ANP-induced Na+ excretion and creatinine clearance in these dogs by 75% and 35%, respectively, suggesting a substantial role of PGs in determining their nartiuretic responsiveness to this hormone. Collectively, both human and animal studies indicate that CHF is a “prostaglandin-dependent” state, in which elevated AII and enhanced renal sympathetic nerve activity stimulate renal synthesis of PGE2 and PGI2 that would counteract the vasoconstrictor effects of these stimuli to maintain GFR and RBF. Therefore, administration of NSAIDs to patients or animals with CHF would leave these vasoconstrictor systems unopposed, leading to hypoperfusion/hypofiltration and subsequently to Na+ and water retention.[671]
In recent years, several studies reported a close relationship between the consumption of NSAIDs and a significant worsening of chronic CHF, especially in elderly patients taking diuretics. [679] [680] The exacerbation of this condition was reported in patients taking either nonselective COX inhibitors or selective COX-2 inhibitors. The deleterious effects of the latter on cardiac and renal functions are in line with the relatively high abundance of COX-2 in renal tissue and to a lesser extent in the myocardium and with the observation that the renal immunoreactive levels of this isoform are enhanced in experimental CHF.[190] Moreover, the significant increase in the risk of MI with COX-2 inhibitors—rofecoxib and celecoxib—raised serious safety problems in the use of these drugs and led to their withdrawal from the market. [681] [682]
In summary, patients with preexisting CHF or hypertension are at high risk to develop volume overload, edema formation, and deterioration of cardiac function following the use of COX-2 inhibitors to the same or even higher frequencies than observed with use of conventional NSAIDs.
Adrenomedullin.
Evidence suggests that AM plays a role in the pathophysiology of CHF. Compared with healthy subjects, in CHF patients, plasma levels of the mature form of AM as well as the glycine extended AM (AM-gly) are elevated in proportion to the severity of cardiac and hemodynamic impairment. [331] [338] [683] For instance, plasma levels of AM in subjects with severe CHF are fivefold higher than in controls, and plasma levels of the peptide fell with effective anti-CHF treatment, such as with carvedilol. [336] [684] [685] [686] The origin of the increased circulating AM appears to be the failing myocardium itself including both the ventricles and, to a lesser extent, the atria.[335] In addition, plasma AM levels correlate with plasma concentrations of NE, ANP, BNP, PAP, PCWP, and PRA in these patients, indicating that the more severe the disease the higher the plasma AM levels measured. [687] [688] Moreover, Jougasaki and associates [686] [689] reported that ventricular and renal tissue AM were significantly increased in dogs with CHF induced by rapid ventricular pacing, compared with normals.
The years following the discovery of AM witnessed intensive investigation in regard to its involvement in the pathophysiology of positive salt and water balance characterizing CHF. Both experimental and clinical studies showed that infusion of AM produced beneficial renal effects in states of volume overload of cardiac origin. For example, brief administration (90 min) of AM into sheep with CHF due to rapid pacing produced a threefold increase in sodium excretion with maintenance of urine output and a rise in creatinine clearance compared with baseline levels in normal sheep.[690] Chronic administration of AM for 4 days in sheep with CHF produced a significant and sustained increase in cardiac output in association with enhanced urine volume.[335] In light of the positive results in experimental CHF, the effects of acute infusion of AM into patients with CHF have been examined. However, the results obtained were less encouraging as compared with normal subjects. For example, acute administration of AM to patients with CHF resulted in increased forearm blood flow but to a lesser extent than in normal subjects, suggesting that the AM vascular effects are significantly attenuated in CHF.[691] In addition, Lainchbury and colleagues[692] demonstrated that AM has no significant effect on urine volume and Na+ excretion in patients with CHF, but remarkably reduced plasma aldosterone levels. Nagaya and co-workers[693] extended this study and found that intravenous infusion of human AM into patients with CHF predominantly improved cardiac function as expressed by increased cardiac stroke index, dilatation of the resistant arteries, and urinary Na+ excretion. The improvement in cardiac function following AM infusion is not surprising in light of its beneficial effects on pre- and afterload and cardiac contractility.[331] Collectively, the vasodilatory and natriuretic activities of AM, and its origin from the failing heart, suggest that AM acts as a compensatory agent to balance the elevation in SVR and volume expansion in this disease state. However, most recently, the complementary interactions between AM and other vasoactive substances such as ET, NPs and NO in myocardial dysfunction were assessed. Indeed, AM in combination with other therapies such as ACE and NEP inhibitors resulted in hemodynamic and renal benefits greater than those achieved by the agents administered separately.[335]
In summary, the alterations in the efferent limb of volume regulation in CHF include enhanced activities of vasoconstrictor/Na+-retaining systems as well as activation of counterregulatory vasodilatory/natriuretic systems. The magnitude of Na+ excretion by the kidney and, therefore, the disturbance in volume homeostasis in CHF are largely determined by the balance between these antagonistic systems. In the early stages of CHF, the balancing effect of the vasodilatory/natriuretic systems is of importance in the maintenance of circulatory and renal function. However, with the progression of CHF, there is a shift of this balance, with dysfunction of the vasodilatory/natriuretic systems and marked activation of the vasoconstrictor/antinatriuretic systems. These disturbances are translated at the renal circulatory and tubular level to alterations that result in avid retention of salt and water, thereby leading to edema formation.
Urotensin.
A role of the U-II/GPR-14 in the pathogenesis of CHF has been suggested, based on the following findings: First, some but not all studies reported that plasma levels of U-II are elevated in patients with CHF, correlating with other markers, such as NH2-terminal BNP and ET-1. [340] [694] [695] In addition, strong expression of U-II was demonstrated in the myocardium of patients with end-stage CHF, in correlation with the impairment of cardiac function.[696]This suggests that up-regulation of the U-II/GPR-14 system could play a part in the cardiac dysfunction associated with CHF.
In view of the documented vasoactive and natriuretic properties of U-II and the finding that the U-II/GPR-14 system may be up-regulated in CHF, several studies examined a possible role of U-II in the regulation of renal function in CHF. A recent set of studies in rats with aortocaval fistula as an experimental model of CHF showed that hU-II acts primarily as a renal vasodilator.[697] Moreover, the renal vasodilatory properties of the peptide are augmented in rats with experimental CHF, apparently by an NO-dependent mechanism. Finally, hU-II increased GFR only in rats with CHF, but did not alter urinary Na+ excretion, in either control or CHF rats. However, in contrast to the negligible renal vasodilatory effect in control rats, the peptide produced a prominent and prolonged decrease in RVR associated with a significant increase in RPF and in GFR in rats with experimental CHF. Thus, under these conditions of increased baseline renal vascular tone, hU-II has the capacity to act as a potent vasodilator in the kidney. Furthermore, our findings suggest that this increase in renal perfusion is dependent in part on NO production. This is consistent with the finding of Zhang and co-workers[360] regarding the importance of NO in the mediation of hU-II-induced renal vasodilatation.
In summary, several reports suggest that the U-II/GPR-14 system may participate in the control of renal hemodynamics in the rat. This regulation may be significantly altered in rats with experimental CHF, which could contribute to the adaptive changes in renal function and renal hemodynamic responses in CHF.
Neuropeptide Y and Heart Failure.
Because many neurohormonal mediators have been implicated in the pathogenesis of CHF, it is of no surprise that NPY has also been a subject of investigation in this condition, especially because NPY colocalization and release with the adrenergic neurotransmitters suggested excessive corelease with NE from the activated peripheral sympathetic system.[698] Indeed, numerous reports demonstrated elevated plasma levels of NPY of patients with CHF, regardless of the etiology of the disease. [699] [700] [701] This increase correlates with the severity the disease, suggesting that NPY might serve as an independent prognostic factor for CHF severity and outcome.[702]
Although circulating levels of NPY are elevated in patients with CHF, little information is available concerning the local concentrations of NPY in the myocardium. It appears, however, that NPY levels are not elevated and, in fact, might be rather lower than normal, as is also the case with NE, suggesting that NPY depletion might follow the state of the SNS in general.[703] The functional significance of elevated local or systemic levels of NPY in the circulation of patients with CHF is not entirely clear and can only be speculated upon at this time. In light of the complex physiologic actions of NPY, it may be involved in the regulation of cardiac actions. Recent studies by groups of Haramati and Zukowska brought new insights into the role of NPY receptors in chronic CHF. By utilizing an AV fistula to induce CHF in rats, the investigators found that cardiac Y1 receptor gene expression decreases in proportion to the severity of cardiac hypertrophy and decompensation.[704] Interestingly, at the same time, Y2 receptor expression was shown to increase markedly in failing hearts.[704] Similar patterns of receptor expression change were observed in the kidneys and were also proportional to the degree of renal failure.[704] Because Y1 receptor appears to mediate known NPY growth-promoting activities in blood vessels [705] [706] and myocytes,[707] this receptor may play a pathogenic role in development of cardiac hypertrophy in the failing heart. Y2 receptor activation is also strongly implicated in the angiogenic activity of NPY,[708] suggesting that up-regulation of this receptor may play an important compensatory role aimed at improving angiogenesis in the ischemic heart.
Furthermore, NPY was shown in experimental models of CHF to exert diuretic and natriuretic properties,[709] most likely owing to increasing the release of ANP and inhibiting the RAAS,[710] thereby facilitating water and electrolyte clearance and reducing congestion. Because the RAAS and the SNS play an important role in the progression of CHF, the higher circulating levels of NPY could be considered as a counteracting mechanism to potentially reduce the progression of CHF. However, NPY acting via Y1 receptors is also a potent mediator of vascular constriction, which could contribute to increases in vascular resistance, including coronary vessel constriction with compromise of cardiac blood flow and blood flow to other essential organs.
In summary, a potential role of NPY in the pathogenesis of CHF progression, cardiac, and salt and water homeostasis via actions on vascular, cardiomyocyte, and kidney functions exerted via multiple receptors (Y1, Y2, and Y5) requires further investigation.
Renal Sodium Retention and Edema Formation in Cirrhosis with Ascites
Afferent Limb of Volume Homeostasis in Cirrhosis
Abnormalities in renal Na+ and water excretion are commonly found in cirrhosis, in humans as well as in experimental animal models. [711] [712] Avid Na+ and water retention may lead eventually to ascites, a common complication of cirrhosis and a major cause of morbidity and mortality, with the occurrence of spontaneous bacterial peritonitis, variceal bleeding, and development of the hepatorenal syndrome. [713] [714] In similarity to CHF, the pathogenesis of the renal water and Na+ retention in cirrhosis is related not to an intrinsic abnormality of the kidney but to extrarenal mechanisms that regulate renal Na+ and water handling. Indeed, when kidneys from cirrhotic patients are transplanted into recipients with normal liver function, renal Na+ and water retention no longer occurs.
Several formulations have been proposed over the years to explain the mechanism(s) by which patients with cirrhosis develop positive Na+ balance and ascites formation. Two major theories put forward to explain the mechanisms of Na+ and water retention in cirrhosis are the “overflow” and the “underfilling” theories of ascites formation. Whereas the occurrence of primary renal Na+ and water retention and plasma volume expansion prior to ascites formation was favored by the “overflow” hypothesis, the classic “underfilling” theory posits that ascites formation causes hypovolemia that further initiates secondary renal Na+ and water retention. In 1988, Schrier and co-workers[715] proposed the “peripheral arterial vasodilatation hypothesis” as a mechanism that could explain the retention of Na+ and water in cirrhosis. This concept was promoted in the 1990s as a unifying hypothesis of the disturbance in body fluid volume homeostatsis to explain the mechanism of renal Na+ and water retention also in diverse states of edema formation, in addition to cirrhosis, including pregnancy. [383] [716] [717] At the same time, the importance of NO as a cardinal player in the hemodynamic abnormalities that mediate salt and water retention in cirrhosis became increasingly evident. [718] [719] The contribution of this molecule, as well as other vasodilatory mechanisms, to generation of the “hyperdynamic” circulation in cirrhosis was further supported by numerous other investigators (see Iwakiri and Groszmann[720]).
In the next sections, these theories are briefly presented, followed by a description of the efferent limb of the volume control system in the regulation of renal handling of Na+ in cirrhosis.
The “Overflow” and “Underfilling” Concepts: Role in Disturbed Volume Regulation and Ascites Formation in Cirrhosis
Based on studies in patients with cirrhosis, Lieberman and co-workers[721] postulated that non-volume-dependent renal Na+ retention is the primary disturbance in Na+ homeostasis in cirrhosis. In their view, this renal Na+ retention leads to total plasma volume expansion, including its nonsplanchnic component. The predilection of the renal salt and water retention to cause ascites was explained by the local alteration of Starling forces in the portosplanchnic bed (“overflow” concept).
Strong support for the overflow theory came from the extensive and carefully designed experiments by Levy and co-workers in dogs with experimental cirrhosis (see review by Levy[722] and references therein). They studied sequentially the events that led to Na+ retention and ascites after the institution of dimethylnitrosamine cirrhosis in the dog. They were also able to measure directly the changes in volume of the vascular compartments after salt retention. These studies indicated that renal Na+ retention and volume expansion may precede formation of ascites by 10 days. The Na+ retention was reported to occur independent of changes in cardiac output, MAP, splanchnic blood volume, hepatic arterial blood flow, GFR, RPF, aldosterone, and increased renal sympathetic nerve activity.[723] Also, elimination of ascites in these cirrhotic dogs with the LeVeen shunt did not prevent salt retention during liberal salt intake. Taken together, these studies supported the view that the initiating event in the renal Na+ retention of cirrhosis is not related to “underfilling.” Rather, primary renal Na+ retention has been suggested as a cause.
In a series of additional studies in dogs with experimental cirrhosis, intrahepatic hypertension, secondary to hepatic venous outflow obstruction, was suggested to be of primary importance in the induction of salt retention by the kidney.[724] In dogs with cirrhosis due to common bile duct ligation and portocaval anastomosis, Levy's group[724] demonstrated that Na+ retention and ascites formation occurred only in dogs with partially or fully occluded portocaval fistulae, but not in dogs with patent portocaval anastomosis and normal intahepatic pressure.
For intrahepatic hypertension to act as a primary stimulus for renal Na+ retention, without the intermediary of underfilling, it is necessary to invoke the hepatic volume-sensing mechanisms mentioned earlier in this chapter. In particular, a sensing mechanism that specifically responds to elevated hepatic venous pressure with increased hepatic afferent nerve activity could be a candidate. The relays for these impulses consist of two hepatic autonomic nerve plexuses, one surrounding the hepatic artery and the other the portal vein. Kostreva and co-workers[725] delineated a neural reflex pathway composed of these elements that connects hepatic venous congestion to enhanced renal and cardiopulmonary sympathetic activity. Occlusion of the inferior vena cava at the diaphragm was associated with a rise in hepatic, portal, and renal venous pressures and resulted in markedly increased hepatic afferent nerve traffic and renal and cardiopulmonary sympathetic efferent nerve activity. Section of the anterior hepatic nerves eliminated the reflex increase in renal efferent nerve activity.[725] Similarly, Levy and Wexler[61] showed that denervation of the liver of dogs with vena cava constriction increases urinary Na+ excretion. Such neural networks, or alternatively other, as yet undefined humoral pathways, could provide an anatomic or physiologic basis for the primary effects of alterations in intrahepatic hemodynamics on renal function. This mechanism implies that renal Na+ retention could be a consequence of disturbed hepatic function, independent of input from extrahepatic volume sensors. As a result, renal Na+ retention would occur without reduction and possibly in the face of expansion of all vascular compartments.
In contrast, the classic “underfilling” theory suggested that, during the development of cirrhosis, true hypovolemia may occur as a result of transudation of fluid and its accumulation in the peritoneal cavity, mostly in the form of ascites. As a result, true intravascular hypovolemia develops, which, in turn, is sensed by the various components of the afferent volume control system described in previous sections of this chapter. The kidney then responds normally to the perceived hypovolemia by increasing Na+ and water reabsorption along the nephron, through activation of the efferent limb of the volume control system. This response includes activation of the RAAS and the SNS, as well as the nonosmotic release of ADH. This sequence of events results in enhanced renal water and Na+ retention, failure to escape from the Na+-retaining effect of aldosterone, and an impaired capacity to excrete solute-free water. However, such a mechanism would eventually result in further accumulation of Na+ and water and the development of positive Na+ balance.
Several mechanisms were offered to account for the development of the hypovolemia. One such mechanism arose as a consequence of the disruption in the normal Starling relationships that govern fluid movement in the hepatic sinusoids. These, unlike capillaries elsewhere in the body, are highly permeable to plasma proteins. As a result, partitioning of ECF between the intravascular (intrasinusoidal) and the interstitial (space of Disse and lymphatic) compartments of the liver is determined predominantly by the hydraulic pressure gradient along the length of the hepatic sinusoids. Obstruction to hepatic venous outflow promotes enhanced efflux of a protein-rich filtrate into the space of Disse and results in augmented hepatic lymph formation. Such augmented hepatic lymph flow, the main source of ascites formation, has been observed in human subjects with cirrhosis as well as in experimental models of liver disease. Vastly increased hepatic lymph formation is accompanied by increased flow through the thoracic duct. When the rate of enhanced hepatic lymph formation exceeds the capacity for return to the intravascular compartment via the thoracic duct, hepatic lymph accumulates in the form of ascites and the intravascular compartment is further compromised. As liver disease progresses, a fibrotic process surrounds the Kupffer cells lining the sinusoids, rendering the sinusoids less permeable to serum proteins. Under such circumstances, termed capillarization of sinusoids, a decrease in oncotic pressure also promotes transudation of ECF within the hepatic lymph space, much as it does in other vascular beds.
Additional consequences of intrahepatic hypertension have also been postulated to contribute to perceived volume contraction. Among these, transmission of elevated intrasinusoidal pressures to the portal vein leads to expansion of the splanchnic venous system, collateral vein formation, and portosystemic shunting. This results in increased vascular capacitance and diversion of blood flow from the arterial circuit.[726] Vasodilatation was believed to occur not only in the splanchnic circulation but in the systemic circulation as well and was attributed to refractoriness to the pressor effects of vasoconstrictor hormones such as AII and catecholamines, probably due to an as-yet-undefined uncoupling effect of bile salts.[727]
Along with diminished hepatic reticuloendothelial cell function, portosystemic shunting allows various products of intestinal metabolism and absorption to bypass the liver and escape hepatic elimination. Among these, endotoxins have been considered to contribute to perturbations in renal function in cirrhosis, possibly secondary to the hemodynamic consequences of endotoxemia or through direct renal effects.[728]
Elevated levels of conjugated bilirubin and bile acids may result from intrahepatic cholestasis or extrahepatic biliary obstruction. In experimental studies of bile duct ligation, it is difficult to distinguish the effects on renal function of jaundice itself from the effects of cirrhosis that ensue after the bile duct ligation. However, it has been shown that bile acids actually decrease proximal tubular reabsorption of Na+, a direct renal action that would tend to promote natriuresis.[729] Nevertheless, the diuretic-like effect of bile salts may also contribute to the underfilling state in cirrhotic patients. [729] [730]
Hypoalbuminemia was proposed as another factor that could contribute to the development of hypovolemia, by diminishing the colloid osmotic forces in the systemic capillaries and hepatic sinusoids. Hypoalbuminemia was believed to occur as a result of decreased synthesis of albumin by the liver as well as dilution caused by ECF volume expansion. The development of hypoalbuminemia is a relatively late event in the course of chronic liver disease.
Likewise, a relative impairment of cardiac function could contribute to diminished arterial blood pressure in some cirrhotic patients. [730] [731] [732] Thus, in some patients, tense ascites reduce venous return (preload) to the heart. Other factors in patients with chronic liver disease may adversely affect cardiac performance, and the concomitant cardiac dysfunction has been termed cirrhotic cardiomyopathy, although the mechanisms behind the cardiac abnormalities are only partly understood. [731] [732]
Finally, volume depletion in cirrhotic patients may be aggravated by vomiting, occult variceal bleeding, and excessive use of diuretics. It is not surprising, therefore, that patients with cirrhosis tolerate hemorrhage or fluid loss very poorly, and they are prone to suffer cardiovascular collapse in the setting of hemodynamic disturbances.
Table 12-7 summarizes the various etiologic factors contributing to underfilling of the circulation in patients with advanced liver disease.
TABLE 12-7 -- Possible Etiologic Factors Causing “Underfilling” of the Circulation in Patients With Cirrhosis
|
Peripheral vasodilatation and blunted vasoconstrictor response to reflex, chemical, and hormonal influences |
|
Opening arteriovenous shunts, particularly in the portal circulation |
|
Increase in the vascular capacity of the portal as well as the nonportal circulation |
|
Hypoalbuminemia |
|
Impaired left ventricular performance, “cirrhotic cardiomyopathy” |
|
Diminished venous return secondary to advanced tense ascites |
|
Occult gastrointestinal bleeding from ulcers, gastritis, or varices |
|
Volume losses due to vomiting and excessive use of diuretics |
Two major arguments have been provided in support of the traditional underfilling theory. First, the progression of cirrhosis is characterized by increased neurohumoral activity with stimulation of the RAAS, increased sympathetic activity, and elevated plasma ADH levels. These classic markers of hypovolemia could not be explained by the overflow hypothesis. Second, a salutary improvement in volume homeostasis was observed after volume replenishment in these patients. For example, volume expansion could suppress the RAAS, increase the GFR, and cause a natriuresis and a negative salt balance in patients with cirrhosis. Indeed, several maneuvers of volume expansion, such as reinfusion of ascitic fluid, placement of peritoneojugular LeVeen shunt, HWI, were found to cause a brisk diuretic/natriuretic response in patients with cirrhosis, thus supporting the underfilling concept. Conversely, the main argument against the underfilling theory was that actual measurements of volume content in body fluid compartments failed to show true hypovolemia in most patients with compensated cirrhosis. In fact, when plasma volume was measured in patients with cirrhosis, it was found to be increased, and this increase in many circumstances antedated the formation of ascites.[716] In addition, although volume repletion by diverse measures, as described previously, could result in a dramatic improvement and natriuresis, such an improvement is at best temporary and occurs only in a subset of affected patients. Only 30% to 50% of cirrhotic patients exhibit natriuresis during volume expansion or HWI, although the latter procedure effectively suppresses the RAAS. Some of the variability could be due to the fact that the degree of volume replenishment achieved was inadequate in those who failed to respond. Nevertheless, it appears that underfilling cannot be the entire explanation for the renal Na+ and water retention that characterizes the cirrhotic patient. Moreover, it may occur only in a limited proportion of patients with cirrhosis, perhaps at a specific stage of their disease.
“Peripheral Arterial Vasodilatation Hypothesis” and the Hyperdynamic Circulation of Cirrhosis
In 1988, Schrier and associates[715] proposed that primary peripheral vasodilatation, initially in the splanchnic vascular bed and later in the systemic circulation, leads to a “relative underfilling” of the arterial circulation. As a result of the discrepancy between the blood volume and the capacitance of the arterial circulation, this perceived underfilling unloads the arterial high-pressure baroreceptors as well as other volume receptors, which, in turn, stimulate a compensatory neurohumoral response. The latter includes activation of the RAAS and the SNS, as well as the nonosmotic release of ADH. [383] [715] [717] [733] The “relative” rather than the “absolute” underfilling leads to the apparent decrease in the effective arterial blood volume (EABV) and initiates the compensatory neurohumoral response. According to this theory, the main mechanism initiating the abnormal Na+ and water retention and ascites formation is splanchnic vasodilatation.[715] Thus, increased hepatic resistance to portal flow causes a gradual development of portal hypertension, collateral vein formation, and shunting of blood to the systemic circulation. As portal hypertension develops, local production of vasodilators, mainly NO, increases, leading to splanchnic vasodilatation. [715] [719] In the early stages of cirrhosis, arterial pressure is maintained through increases in plasma volume and cardiac output, in the form of a “hyperdynamic” circulation. However, as the disease progresses, vasodilatation in the splanchnic vascular bed, and presumably in other vascular beds, is so pronounced that EABV decreases markedly, leading to a sustained neurohumoral activation that further results in Na+ and fluid retention. [715] [734] This hypothesis could, therefore, potentially explain the increased cardiac output and the enhanced neurohumoral changes over the entire spectrum of cirrhosis.[716] Moreover, it is now believed that the vasodilatation in cirrhotic patients is not confined only to the splanchnic circulation but may occur in other vascular beds, such as the peripheral systemic and pulmonary circulations as well.[720] Thus, decreases in SVR associated with low arterial blood pressure and high cardiac output are clinical manifestations of the hyperdynamic circulation that are commonly seen in patients with cirrhosis. Indeed, the combination of “warm extremities, cutaneous vascular spiders, wide pulse pressure, and capillary pulsations in the nail bed” has been known in cirrhotic patients from the early 1950s. [718] [720]
Pulmonary vasodilatation, associated with the hepatopulmonary syndrome, one of the most severe complications of chronic liver disease, may also be a considered an example of the hyperdynanic circulation caused by increased production of NO (and possibly also carbon monoxide [CO]) in the lung. [720] [735] It has been also suggested that the hepatorenal syndrome may develop when the heart is not able to compensate any longer for the progressive decrease in peripheral resistance.[736] Thus, the hyperdynamic syndrome of chronic liver disease should be considered as a “progressive vasodilatory syndrome” that finally leads to multiorgan involvement, as suggested recently by Iwasaki and Groszmann.[720] As pointed out earlier, increased production of NO in the splanchnic vasculature plays a cardinal role in initiating this process.
Role of Nitric Oxide in the Hyperdynamic Circulation of Cirrhosis
Considerable evidence now indicates that aberrations in the endothelial vasodilator NO system are involved in the pathogenesis of the hyperdynamic circulation and Na+ and water retention in cirrhosis. [716] [719] [720] [737] NO is produced in excess by the vasculature of different animal models of portal hypertension, when measured by various methods, [738] [739] [740] as well as in cirrhotic patients. [741] [742] [743] In the carbon tetrachloride rat model of cirrhosis, this increased production of NO can be detected early in the course of the disease. Niederberger and colleagues[740] demonstrated that cGMP, the intracellular messenger of NO, was already increased in the aorta of cirrhotic rats without ascites when the cirrhotic rats begin to retain Na+. This increased vascular NO production is supported by in vitro or combined in vivo and in vitro studies that have demonstrated an increased production of NO by the vessels of cirrhotic animals and a role for NO in the impaired vascular responsiveness to vasoconstrictors. [744] [745] Moreover, removal of the vascular endothelial layer has been demonstrated to abolish the difference in vascular reactivity between cirrhotic and control vessels.
Inhibition of NOS has beneficial effects in experimental models of cirrhosis. Niederberger and colleagues[746] reversed the high NO production in cirrhotic rats with ascites to normal control levels, by using 7 days of low-dose L-NAME treatment. This normalization of NO production corrected the hyperdynamic circulation. Further studies confirmed that normalization of NO production was accompanied by a marked increase in urinary Na+ and water excretion and a concomitant decrease of ascites in cirrhotic rats.[747] These effects of NO inhibition and reversal of the hyperdynamic circulation were associated with a decrease in PRA and in the concentrations of aldosterone and vasopressin.
In patients with cirrhosis, the vascular hyporesponsiveness of the forearm circulation to noradrenaline has been shown to be reversed by the administration of the NOS inhibitor, L-NMMA, further supporting the increased vascular syn-thesis of NO in cirrhosis.[748] Inhibition of NO production also corrected the hypotension of cirrhosis. Similar observa-tions, namely, correction of the hyperdynamic circulatory syndrome by inhibition of NOS activity, were reported in a more recent study in patients with compensated cirrhosis.[749] An improvement in renal function and Na+ excretion was also observed in these patients, as well as a decrease in plasma NE levels.
The main source of the increased systemic vascular NO generation in cirrhosis has been demonstrated to be eNOS in the arterial and splanchnic circulations. [719] [750] The up-reguation of eNOS appears to be, at least in part, caused by increased shear stress as a result of portal venous hypertension with increased flow in the splanchnic circulation. [719] [720] [737] However, in the rat with portal vein ligation, eNOS up-regulation and increased NO release in the superior mesenteric arteries were found to precede the development of the hyperdynamic splanchnic circulation. [751] [752]
Interestingly, in contrast to the increased NO generation in the splanchnic and systemic circulation, there is also evidence for impaired NO production and “endothelial dysfunction” in the intrahepatic microcirculation in cirrhotic rats. [737] [753] [754] The mechanisms of this paradoxical behavior of the intrahepatic vascular bed is unknown. However, it has been speculated that this “intrahepatic endothelial dysfunction” and NO deficiency may play a significant role in the pathogenesis of the increased hepatic vascular resistance, as well as in the increased intrahepatic thrombosis and collagen synthesis in cirrhosis (for review, see Wiest and Groszman[737]). Indeed, it is currently believed that the increase in intrahepatic vascular resistance is not merely due to mechanical distortion of the vasculature by fibrosis. Rather, a dynamic process, due to contraction of myofibroblasts and stellate cells, is believed to determine the degree of intrahepatic vascular resistance. [712] [737] The decrease in NO production due to endothelial dysfunction may shift the balance in favor of vasocostictors (ET, leukotrienes, thromboxane A2, AII, etc.), thus causing an increase in intrahepatic vascular resistance.[737] Indeed, studies utilizing in vivo gene transfer techniques, for delivery of either eNOS or neuronal NOS (nNOS), to livers of rats with experimental cirrhosis, showed that this maneuver is associated with a decrease in portal hyprtension. [755] [756]
It has been clearly shown that eNOS protein is increased in animal models of portal hypertension and that this increase is already detectable in cirrhotic rats without ascites. [737] [757] However, Iwakiri and co-workers[758]demonstrated that mice with targeted deletion of eNOS alone, or with combined deletion of eNOS and iNOS, may develop a hyperdynamic circulation associated with portal hypertension. The latter finding suggests that other vasodilatory agents may be activated in these mice. Indeed, some evidence indicates that PGI2,[759] endothelium-derived hyperpolarizing factor (EDHF),[760] CO,[761] AM,[762] and other vasodilators may participate in the pathogenesis of the hyperdynamic circulation in experimental cirrhosis (see Iwakiri and Groszmann[720]).
Some evidence also suggests that another isoform of NOS, nNOS, may be involved in the generation of the hyperdynamic circulation and fluid retention in experimental cirrhosis.[763] Increased expression of nNOS has recently been suggested to partially compensate for the endothelial isoform deficiency in the eNOS knockout mice.[764] In contrast, the role of iNOS remains controversial. Vallance and Moncada[765] postulated that endotoxin-mediated induction of iNOS might play a role in the pathogenesis of the arterial vasodilation in cirrhosis. However, in more recent studies, the results were inconclusive, with some groups showing an increased iNOS in arteries of animals with experimental biliary cirrhosis[766] but not in other forms of experimental cirrhosis. [750] [757] Although nonspecific inhibition of NOS may correct the hyperdynamic circulation, use of drugs that preferentially inhibit iNOS and cytokine production has shown varying results, ranging from an amelioration[767] to no effect.[744]
Several cellular mechanisms have been implicated in the up-regulation of eNOS activity in experimental cirrhosis. Elevated shear stress due to the hyperdynamic circulation and portal hypertension may be involved, because this is a well-documented mechanism that up-regulates transcription of the eNOS gene. However, it is believed that additional factors related to the hepatic dysfunction could further stimulate this up-regulation. For example, the activity of eNOS may be regulated at a post-transcripional level by tetrahydrobiopterin (BH4).[768] It has been shown in rats with experimental cirrhosis that circulating endotoxins may increase the enzymatic production of BH4, thereby enhancing the activity of eNOS in the mesenteric vascular bed.[769] Evidence also indicates that the activity of eNOS in experimental cirrhosis may be modulated by protein-protein interactions, for example, caveolin[754] and heat-shock protein 90 (HSP90),[770] as well as by direct phosphorylation of eNOS protein.[771] However, the relative contribution of the latter mechanisms to the activation of eNOS in cirrhosis remains to be determined.
Efferent Limb of Volume Homeostasis in Cirrhosis: Abnormalities in Effector Humoral Mechanisms
The efferent limb of the volume regulation in cirrhosis consists of factors similar to those described in CHF. Neurohumoral activation and alterations in circulating levels of vasoactive substances are believed to play a major role in promoting the enhanced Na+ and water reabsorption by the kidney. [711] [712] [716] [717] [718] [772] [773] The RAAS and the SNS, together with ANP, are considered to be the main endogenous neurohumoral systems involved in Na+and volume homeostasis in this disease state. There is, however, evidence that other systems, such as renal PGs and ET, might contribute as well.
Vasoconstrictive and Antinatriuretic Systems
Renin-Angiotensin-Aldosterone System.
The renal and extrarenal sites and actions of AII and aldosterone that promote renal Na+ retention were considered in previous sections of this chapter. Extrarenal vascular, glomerular microcirculatory, and direct tubular actions are all involved and are mutually interdependent. Both clinical and experimental evidence suggest that the RAAS contributes to Na+ and fluid retention in cirrhosis. Indeed, elevated PRA and aldosterone levels were noted in parallel with the progressive severity of cirrhosis and the increase in Na+ retention. In humans, activation of the RAAS is more commonly noted in patients with ascites than in preascitic patients. It was, therefore, assumed that activation of the RAAS occurs in a relatively advanced stage of the disease. Studies in animal models of cirrhosis tended, in general, to support this notion by showing the temporal relationship between Na+ retention and activation of RAAS.[774] Whereas there is no doubt that the RAAS plays a dominant role in the mechanism of Na+ retention in patients with cirrhosis and ascites, evidence suggests that increased activity of this system may also contribute earlier, in the preascitic phase of the disease.[775] At this early phase, patients with cirrhosis may develop positive Na+ balance, but their PRA and aldosterone levels are maintained within normal range or even depressed. This finding was believed for years to support the role of the overflow theory in the mechanism of ascites formation. However, Bernardi and co-workers[776] found an elevated aldosterone level in preascitic cirrhotic patients that was inversely correlated with renal Na+ excretion, particularly when the patients were standing. This suggested that aldosterone-dependent, Na+ retention can develop in preascitic cirrhosis during standing and that posture-induced activation of the RAAS could already exist at this stage of the disease. Another study demonstrated that renal Na+ retention induced by LBNP was associated with a prominent increase in renal renin and AII excretion.[777] Moreover, the same group reported that treatment with the AII receptor antagonist losartan at low dose that did not affect systemic and renal hemodynamics or glomerular filtration was associated with a significant natriuretic response.[778] The mechanism by which losartan induced natriuresis in the face of PRA within the normal range was attributed the action of losartan on the local intrarenal RAAS. [778] [779] Indeed, it has been demonstrated in rats with chronic bile duct ligation that activation of the intrarenal RAAS may occur prior to the circulating system.[780] In addition, losartan has been shown to cause a decrease in portal pressure in cirrhotic patients with portal hypertension.[781]
The mechanisms of the postural-induced activation of RAAS at this early stage of the disease as well as the beneficial effects of low-dose losartan treatment in these patients require further investigation.
In contrast, in advanced cirrhosis with ascites, attempts to inhibit AII in Na+-retaining cirrhotic patients yielded variable results. Administration of captopril to cirrhotic patients with ascites resulted in a decrease in both GFR and urinary Na+ excretion, even when given in low doses.[782] At this stage of the disease, activation of the RAAS serves to support arterial pressure and maintain adequate circulation. Removal of these actions of the RAAS, either by blocking AII formation or by blocking of its receptor, may lead to deterioration with a profound decrease in RPP. This formulation might be important in the pathogenesis of the hepatorenal syndrome, which is regularly preceded by a state of Na+ retention and may be precipitated by a hypovolemic insult. Abnormalities of the renal circulation characteristic of this syndrome include marked diminution of RPF with renal cortical ischemia and increased RVR, abnormalities consistent with the known actions of AII on the renal microcirculation. It is not surprising, then, that several groups correlated activation of the RAAS with worsening hepatic hemodynamics and decreased survival in patients with cirrhosis. For this reason, ACE inhibitors and ARBs are best avoided in patients with cirrhosis and ascites.
Sympathetic Nervous System.
Activation of the SNS is a common feature in patients with chronic liver disease and cirrhosis with ascites.[783] Circulating NE levels, as well as urinary excretion of catecholamines and their metabolites, are elevated in patients with cirrhosis and usually correlate with the severity of the disease. Moreover, plasma NE in patients with decompensated cirrhosis may reach levels found in ischemic heart disease and is considered to be a grave prognostic sign associated with a high degree of mortality.[783] The source of the increased NE levels is enhanced SNS activity, rather than reduced disposal, with nerve terminal spillover from the liver, heart, kidney, muscle, and cutaneous innervation. [783] [784] [785] A causal relationship between the elevated SNS activity and the impaired Na+ and water was suggested by Bichet and associates.[786] In this study, it was demonstrated that increased sympathetic activity, as assessed by plasma levels of NE, correlates closely with Na+ and water retention in cirrhotic patients and thus may be of pathogenetic importance. Evidence also suggests the existence of an increase in efferent renal sympathetic tone in cirrhosis, based on direct recordings in experimental animals, as shown by DiBona and co-workers.[787] The same group also demonstrated a defect in the arterial and cardiopulmonary baroreflex control of renal sympathetic nerve activity, in rats with experimental cirrhosis.[788] This can explain why volume expansion fails to suppress the enhanced renal sympathetic activity in cirrhosis.
Concomitant with the increase in NE release, cardiovascular responsiveness to reflex autonomic stimulation may be impaired in patients with cirrhosis.[789] This includes impaired vasoconstrictor response to a variety of stimuli, such as mental arithmetic, LBNP, and the Valsalva maneuver. This interference in the peripheral and central autonomic nervous system in cirrhosis could be explained partially by increased occupancy of endogenous catecholamine receptors, by down-regulation of the adrenergic receptors, or by a defect at the level of postreceptor signaling. [773] [783] It is also possible that the excessive NO-dependent vasodilatation found in cirrhosis could account for the vascular hyporesponsiveness. This assumption is supported by the finding that the hyporesponsiveness to pressor agents is not limited to NE but may be observed in response to AII in patients and experimental animals. [727] [790]
It has been also suggested that metabolic derangements due to hepatic dysfunction may be an additional cause for sympathetic overactivity in cirrhosis.[783] In particular, alterations in glucose metabolism, hypoglycemia, and hyperinsulinemia are known to stimulate the activity of the SNS. However, overt hypoglycemia is seldom observed in patients with compensated cirrhosis. Hypoxia is an additional potential factor that may stimulate the SNS in patients with cirrhosis. A negative correlation was found between circulating NE levels and arterial oxygen tension in patients with cirrhosis.[791] Moreover, inhalation of oxygen significantly reduced the circulating levels of NE, suggesting a causal relationship between hypoxia and increased SNS activity in these patients.
The increase in renal sympathetic tone and plasma NE levels could contribute to the antinatriuresis of cirrhosis by decreasing total RBF, or its intrarenal distribution, or by acting directly at the tubular epithelial level to enhance Na+reabsorption. It is indeed known that patients with compensated cirrhosis may have a decreased RBF even in early stages, and during the progression of the disease, RBF tends to decline further concomitant with the increase in sympathetic activity.[783]
In parallel with the increase in sympathetic activity, patients with progressive cirrhosis show also an increase in the activities of two other important pressor systems, namely, the RAAS and ADH. [716] [773] The marked neurohumoral activation that occurs at relatively advanced stages of cirrhosis probably represent a shift toward a decompensation, characterized by a severe decrease in effective blood volume and perhaps true volume depletion. In this setting, activation of the three pressor systems represents an attempt to support mean arterial blood pressure. A correlation also exists between plasma NE and ADH levels, suggesting that the increased activity of the SNS may stimulate the release of ADH. [716] [786] A direct relationship also exists between plasma NE and the activity of the RAAS, which may imply that the three systems are activated by the same mechanisms and operate in concert to counteract the low arterial blood pressure and the decrease in effective blood volume. [383] [715] [716] [783]
Antidiuretic Hormone.
Patients with advanced hepatic cirrhosis frequently demonstrate a disturbed capacity to regulate water excretion by the kidney and, consequently, develop water retention with hyponatremia. [716] [792] Nonosmotic release of ADH is believed to play a dominant role in the mechanism of water retention and the development of hyponatremia in these patients. [715] [717] [792]
Bichet and co-workers[793] subjected patients with cirrhosis to a standard oral water load and demonstrated that those who were unable to normally excrete the water load had high immunoreactive levels of ADH compared with those patients who exhibited a normal response. These patients also had higher plasma renin and aldosterone levels and lower urinary Na+ excretion, suggesting that the inability to suppress vasopressin was secondary to a decrease in effective arterial blood volume.[793]
Elevated plasma levels of ADH in association with overexpression of hypothalamic ADH mRNA were found in rats with experimental cirrhosis, together with a diminished pituitary ADH content.[794] This suggests an increased synthesis and release of ADH in this experimental model of cirrhosis. In addition, Fujita and colleagues[795] reported that the expression of AQP II, the ADH-regulated water channel in the collecting duct, was significantly increased in rats with CCl4-induced cirrhosis. This finding seems to be a consequence of the increased ADH secretion, because blocking of ADH action by an ADH receptor antagonist, OPC-31260, significantly diminished AQP II expression. It is, therefore, possible that up-regulation of AQP II plays an important role in water retention associated with hepatic cirrhosis as well as in other pathologic states.[795]
As noted in a previous section of this chapter, the biologic actions of ADH are mediated through at least two G-protein-coupled receptors. It is believed that ADH supports arterial blood pressure through its action on the V1 receptors found on the VSMCs, whereas the V2 receptor is responsible for water transport in the collecting duct. [716] [717] The development of selective blockers of these receptors provided clear evidence for the role of ADH in the induction of water abnormalities in cirrhosis.[796] Indeed, compelling evidence now indicates that the administration of a V2 receptor antagonist to cirrhotic patients increased urine volume and decreased urine osmolality (for review, see Ferguson and colleagues[796]). Serum Na+ concentrations can also be corrected in hyponatremic cirrhotic patients by V2 receptor antagonists. [214] [796] Similarly, in rats with experimental cirrhosis, treatment with selective nonpeptide V2 receptor antagonists proved to exert beneficial effects by increasing urine flow and decreasing urine osmolality. [797] [798] It appears that this new class of aquaretic agents may have important clinical applications in the future in the treatment of patients with liver cirrhosis.
The role of the V1 receptor was studied by Claria and co-workers[799] in rats with cirrhosis and ascites. They administered a selective antagonist of the vascular effect of vasopressin after blocking the actions of AII with saralasin and demonstrated a pronounced fall in arterial blood pressure. This suggests that ADH, acting via its V1 receptor, is important for the maintenance of arterial pressure and circulatory integrity in cirrhosis.[799] Moreover, it argues against the use of nonselective ADH receptor antagonists that block both receptor subtypes in patients with cirrhosis.
ADH increases the synthesis of the vasodilatory PGs (PGE2 and PGI2) in several vascular beds, including the kidney. This, in turn, may inhibit the vasoconstrictor action as well as the hydro-osmotic effect of ADH. The modulation of ADH effects by PGE2 is of particular importance in pathophysiologic situations, including cirrhosis. It is known that many patients with cirrhosis and ascites are able to generate positive free water clearance (CH2O) and dilute the urine after a water load, despite an impaired ability to suppress ADH.[792] Perez-Ayuso and associates[800] offered an explanation for the latter finding by showing that urinary PGE2 was markedly increased in cirrhotic patients with positive free water clearance. Because PGE2 inhibits the hydro-osmotic effect of ADH, it was suggested that, in cirrhotic patients with ascites and positive free water clearance, urinary diluting capacity is enhanced after a water load by increased synthesis of PGE2 in the collecting duct. [792] [800]
Endothelins.
Plasma levels of immunoreactive ET (irET) are markedly elevated in patients with cirrhosis and ascites and in the hepatorenal syndrome [801] [802] [803] [804] [805] However, the role of ET in the pathogenesis of fluid and Na+ retention in cirrhosis is controversial. Specifically, the causal relationship between an increased level of serum ET and the hemodynamic disturbances in cirrhotic patients (characterized by systemic and splanchnic vasodilation, salt and water retention, and renal vasoconstriction) is still under debate.
Although ETs function as autocrine or paracrine agents by interacting with specific receptors located at or near the site of synthesis, a fraction may be released to the general circulation (spillover), where it can have systemic effects. Indeed, a number of studies have shown that there is a net hepatosplanchnic release of ETs in cirrhosis that correlates positively with portal pressure and cardiac output and inversely with central blood volume. [802] [803] [806]Increased local intrahepatic production of ET in the liver is also believed to contribute to the development of portal hypertension, probably through contraction of the stellate cells and a concomitant decrease in sinusoidal blood flow.[806]
In an attempt to provide further insight into the pathogenic significance of ET-1 in cirrhosis, Martinet and co-workers[807] measured ET-1 and its precursor, big ET-1, in the systemic circulation as well as in the splanchnic and renal venous beds of patients with cirrhosis and refractory ascites before and after transjugular intrahepatic portosystemic shunt (TIPS) to provide relief of portal hypertension. They found that the blood levels of both peptides were higher in the vena cava, hepatic vein, portal vein, and renal vein of cirrhotic patients compared with normal controls. One to 2 months after the TIPS procedure, creatinine clearance and urinary Na+ excretion increased, accompanied by a significant reduction of ET-1 and big ET-1 in portal and renal veins. The authors suggested that splanchnic and renal hemodynamic changes occurring in patients with cirrhosis and refractory ascites could be related to the production of ET-1 by the splanchnic and renal vascular beds. However, because the status of other hormones (e.g., renin, aldosterone) was altered as well, it is hard to attribute the change in hemodynamic variables after TIPS to an isolated change in the level of ETs. An opposite effect, namely, an increase in plasma ET-1, was recently reported in response to acute temporary occlusion of TIPS by angioplasty balloon, with a transient increase in portal pressure.[808]Interestingly, this was associated with a marked reduction of RPF and increased generation of ET-1 by the kidney.
Because the kidney is uniquely sensitive to the vasoconstrictor effect of ET-1, it was suggested that ET-1 may play an important role in the pathogenesis of the hepatorenal syndrome. [803] [804] This is supported by the finding that the high plasma ET-1 levels in patients with the hepatorenal syndrome decreased within 1 week after successful orthotopic liver transplantation and that this decrease in circulating ET-1 was accompanied by an improvement in renal function.[809] Recently, the importance of the intrarenal ET system was demonstrated in a rat model of acute liver failure induced by galactosamine, in which renal failure also develops.[810] This experimental model shares some of the hallmarks of the hepatorenal syndrome in humans, in particular the marked reduction in renal function with normal renal histology. Plasma concentrations of ET-1 were increased twofold following the onset of liver and renal failure, and there was significant up-regulation of the ETA receptor in the renal cortex. Administration of bosentan, a nonselective ET receptor antagonist, prevented the development of renal failure when given before or 24 hours after the onset of liver injury.[810] It is possible that activation of the intrarenal ET system may play a role in the pathogenesis of the hepatorenal syndrome. From that point of view, ET antagonists may represent a potentially beneficial therapy for hepatorenal syndrome.[714] However, at present, not enough clinical evidence exists to support this view.
Vasodilators and Natriuretic Systems
Prostaglandins.
The important contribution of PGs to the maintenance of renal function in cirrhosis was noted in previous sections of this chapter, with regard to their modulating effects on the renal hydro-osmotic effect of ADH. Patients with decompensated cirrhosis with ascites, but without renal failure, excrete greater amounts of vasodilatory PGs than do healthy subjects, suggesting that renal production of PGs is increased. [196] [811] Likewise, in experimental animal models of cirrhosis, mostly rats with CCl4-induced cirrhosis, there is evidence for increased synthesis and activity of renal and vascular PGs. [811] [812]
One may conceive of renal PGs as constituting critical modulators of renal function during disease states involving volume contraction. In the setting of decompensated cirrhosis (presence of ascites and increased activity of endogenous vasoconstrictor systems), the ability to enhance PG synthesis constitutes a compensatory or adaptive response to incipient renal ischemia. The corollary of this formulation is that administration of agents that impair such an adaptation by inhibiting PG synthesis might result in a clinically important deterioration of renal function, primarily in patients with low effective blood volume who are retaining Na+ avidly. Indeed, in several investigations, administration of nonselective in-hibitors of COX, such as the NSAIDs indomethacin and ibuprofen, resulted in a significant decrement in GFR and RPF in patients with cirrhosis and ascites, in contrast to healthy subjects. The decrement in renal hemodynamics varies directly with the degree of Na+ retention and the extent of neurohumoral activation, so that patients with high plasma renin and NE levels are particularly sensitive to these adverse effects.[811] [813] However, the deleterious effects of NSAIDs on renal function were also observed in cirrhotic patients without ascites. [196] [814]
In contrast to nonazotemic patients with cirrhosis and ascites, it has been suggested that patients with hepatorenal syndrome have reduced renal synthesis of vasodilatory PGs.[815] This renal PG “deficiency” may be an important factor in the pathogenesis of hepatorenal syndrome that exacerbates renal vasoconstriction and Na+ and fluid retention.[811] Yet, an attempt to improve renal function in these patients by treatment with intravenous infusion of PGE2or its oral analog, misoprostol, was unsuccessful.[816]
It is now accepted that renal production of PGs is mediated by two isoforms of COX (i.e., COX-1 and COX-2). In contrast to other organs, both isoforms of COX are constitutively expressed in the kidney, but in different locations (see Chapter 11 ). It was recently demonstrated, by Western blotting analysis, that the COX-2 isoform is strongly up-regulated in kidneys from rats with CCl4-induced cirrhosis with ascites.[811] The mechanism(s) responsible for this finding is not clear, although it should be mentioned that up-regulation of COX-2 in the kidney was observed in other situations associated with a decrease in effective arterial blood volume, such as low-salt diet and high-output CHF. Nevertheless, despite the increase in COX-2 expression, it is beleived that maintenance of renal function in cirrhosis is dependent primarily on COX-1-derived PGs.[811] This assumption is based on the finding that administration of SC-236, a selective COX-2 antagonist, spared renal function in cirrhotic rats with ascites, whereas nonselective inhibition of COX led to deterioration in renal function. [811] [817] Further support for this notion was obtained recently in cirrhotic patients with ascites treated for a short duration with celecoxib, a selective COX-2 antagonist.[818] It was shown that short-term administration of celecoxib did not impair renal function in nonazotemic patients with cirrhosis and ascites, as opposed to the nonselective COX antagonist naproxen.[818] It should be emphasized that, in these studies, in both patients and experimental animals, administration of the selective COX-2 inhibitor was carried out on a short-term basis. Additional, long-term, studies are required in order to establish the safety of these drugs in patients with advanced cirrhosis.
Natriuretic Peptides.
Plasma levels of ANP are elevated in patients with cirrhosis, despite the reduction in effective circulating volume in the late stages of the disease. [819] [820] In the preascitic stage of cirrhosis, the increase in plasma ANP may be important for the maintenance of Na+ homeostasis, but with the progression of the disease, the patients develop resistance to the natriuretic action of the peptide. [819] [820] Although it was suggested that reduced clearance may contribute to the increased levels of ANP in cirrhosis,[821] it appears that the high levels of ANP reflect mostly an increased cardiac release rather than impaired clearance of the peptide. In particular, intra-atrial processing of pro-ANP was found to be normal in cirrhotic dogs. Cardiac ANP mRNA levels were found to be increased by 2.8 to 4.1 times in cirrhotic rats compared with controls.[822]
The stimulus for increased cardiac ANP synthesis and release in cirrhosis has not been fully clarified. Overfilling of the circulation in early cirrhosis, secondary to intrahepatic hypertension-related renal Na+ retention, could be responsible for the increased plasma ANP concentrations at these early stages. Indeed, some studies measured increased left atrial size, in association with increased intervascular volume and plasma ANP concentration, in both ascitic and nonascitic alcoholic cirrhosis patients.[823] Wong and co-workers[824] measured central blood volume (CBV), that is, the volume of the cardiac chambers, pulmonary circulation, and thoracic vessels, by radionuclide angiography in patients with cirrhosis. Interestingly, the preascitic patients had a significantly elevated CBV with higher left and right pulmonary volumes, despite having normal blood pressure, and normal renin, aldosterone, and NE levels.[824] Such an increase in CBV may trigger the release of ANP, as shown by numerous HWI studies in the past.[823] More recently, Wong and colleagues[825] examined the status of Na+ homeostasis in preascitic cirrhosis by investigating renal Na+ handling in patients with proven cirrhosis without ascites who submitted to a high-Na+ diet for 5 weeks. The authors demonstrated that high Na+ intake results in weight gain and positive Na+ balance for 3 weeks, returning to a complete Na+ balance thereafter. Thus, despite continued high Na+ intake, preascitic patients reach a new steady state of Na+ balance, thereby preventing fluid retention and the development of ascites. Interestingly, the RAAS and the SNS were suppressed, whereas the ANP concentration was elevated, suggesting that ANP plays an important role in preventing the transition of these patients from the preascitic stage to ascites.[825]
The factors responsible for maintaining relatively high levels of ANP during the later stages of cirrhosis, associated with arterial underfilling, have not been determined. However, ANP levels do not increase further as patients proceed from early compensated to late decompensated stages of cirrhosis.
As pointed out earlier, with the progression of the disease, many patients with cirrhosis and ascites lose their ability to respond normally to exogenous administration of ANP or to the high endogenous levels of the peptide. [819] [820]An extensive series of investigations have been done in many laboratories to document and determine the potential basis for this apparent resistance to ANP. Documentation of ANP resistance was obtained in a study by Skorecki and colleagues,[826] who used HWI in a series of patients with cirrhosis. All study subjects experienced with HWI an increase in ANP and in plasma and urinary cGMP, the second messenger for ANP action, but not all subjects responded with a natriuresis. Those who developed natriuresis were termed “responders” as opposed to “nonresponders,” who failed to increase urinary Na+ excretion. No difference in the cGMP response was observed between the responders and the nonresponders. In subsequent human and animal studies that examined the renal response to volume expansion or to infusion of ANP, a similar heterogeneity of response was observed, with nonresponders experiencing equivalent increases in ANP and cGMP but also tending to have more severe and advanced disease. [827] [828] [829] The potential mechanisms responsible for the diminished natriuretic response in this subgroup of patients are discussed later. Nevertheless, the findings suggest that the interference with the natriuretic action of ANP occurs at a late stage of cellular signaling, beyond cGMP production, because both ANP release and cGMP generation in response to HWI remained intact in the nonresponders.
A number of experimental interventions were shown to ameliorate ANP resistance in cirrhosis. These included infusion of endopeptidase inhibitors, BK, kininase II inhibitors, mannitol; renal sympathetic denervation; peritoneovenous shunting; and orthotopic liver transplantation. [830] [831] [832] [833] [834] Analysis of these and other studies suggests that antinatriuretic factors counterbalance and overcome the natriuretic effect of ANP in later stages of cirrhosis.[828] In particular, the two best-studied antinatriuretic systems in cirrohsis are the SNS and RAAS. As discussed previously, the activation of the SNS in cirrhosis is characterized by an increase in circulating NE and increased efferent renal nerve sympathetic activity. When excessive, both may lead to a decrease in RPF and excessive proximal reabsorption of Na+. Indeed, Koepke and associates[835] demonstrated that renal denervation reversed the blunted diuretic and natriuretic responses to ANP in cirrhotic rats. With respect to the RAAS, excessive activation of the system and failure to suppress the RAAS with HWI or ANP infusion was clearly associated with resistance to the natriuretic effects of ANP.[826] Furthermore, infusion of AII mimicked the nonresponder state by causing patients in the early stages of cirrhosis who still responded to ANP to become unresponsive[836] ( Fig. 12-9 ). This effect of AII infusion was reversible and occurred at both proximal (decreased distal delivery of Na+) and distal nephron sites to abrogate ANP-induced natriuresis. The importance of distal Na+ delivery was further confirmed in other studies, which showed that the administration of mannitol to increase distal delivery (as measured by lithium clearance) resulted in an improved natriuretic response to ANP. [832] [837]
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FIGURE 12-9 Effect of antiotensin II (AII) infusion in atrial natriuretic peptide (ANP)–induced natriuresis, showing Na+ excretion during the four experimental protocols. Response was defined by a natriuresis greater than 0.83 mmol/hr (20 mmol/day). Note that urinary sodium excretion dropped to almost baseline with combined ANP/AII infusion and returned to ANP levels when AII was discontinued. *P<.05 from previous phase of experiment. ANP/AII, infusion of ANP and AII combined; ANPI, ANP infusion alone; ANP2, ANP alone; BL, baseline. (Adapted from Tobe SW, Blendis LM, Morali GA, et al: Angiotensin II modulates ANP induced natriuresis in cirrhosis with ascites. Am J Kidney Dis 21:472-479, 1993.) |
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Altogether, five possible factors in ANP resistance were postulated by Warner and colleagues[828]: (1) impaired delivery of salt and water to distal nephron sites that are normally responsive to ANP; (2) the presence of antinatriuretic forces favoring Na+ retention that override the natriuretic effects of ANP and its distal site of action in the medullary collecting duct; (3) down-regulation of a population of ANP receptors at a distal nephron site, not reflected in plasma or urinary cGMP concentrations (which are elevated in concert with the elevated ANP); (4) biochemical abnormalities in the biologic responsiveness to ANP at a site parallel to or beyond the level of cGMP production (e.g., enhanced degradation); and (5) decreased delivery or effect of permissive cofactors that allow appropriate ANP action at its distal nephron site (e.g., PGs and kinins; salutary effect of endopeptidase and kininase inhibitors). Most of the current evidence does not favor an abnormality in ANP receptor number or action, but rather favors a combined effect of decreased delivery of Na+ to ANP-responsive distal nephron sites (glomerulotubular imbalance due to abnormal systemic hemodynamics and activation of the RAAS) together with an effective antinatriuretic factor overcoming the natriuretic action of ANP at its site of action in the medullary collecting tubule. Therefore, when mannitol was coadministered with ANP to responder patients with cirrhosis, a marked natriuresis was observed, suggesting that increased distal tubular Na+ delivery is essential for the expression of the renal actions of ANP.[837]Nonresponder cirrhotic patients did not show increased Na+ delivery to the distal tubule in response to a similar maneuver. There was no difference in the increase of urinary cGMP excretion between responders and nonresponders, indicating that the ANP receptors in the collecting duct are not defective.
An overall formulation for the role of ANP in cirrhosis is summarized in Figure 12-10 . Na+ retention is initiated early in cirrhosis as a result of hepatic venous outflow block or peripheral vasodilatation. In early disease, this results in intravascular volume expansion and a subsequent rise in plasma ANP. This increase is sufficient to counterbalance the antinatriuretic influences, resulting in Na+ balance, albeit at the expense of an expanded intravascular volume. Secondary to peripheral vasodilatation, the circulation may become progressively more underfilled at later stages of the disease, thereby activating antinatriuretic factors. With further progression of cirrhosis, a disruption of intrasinusoidal Starling forces occurs, increasing the potential for volume loss into the peritoneal compartment and causing ascites. The underfilling of the circulation may attenuate further increases in ANP levels and promote the activation of antinatriuretic factors. At this later stage of disease, increased levels of ANP may not be sufficient to counterbalance the antinatriuretic influences. ANP resistance ensues, leading to a state of persistently positive Na+balance and clinical decompensation.
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FIGURE 12-10 Working formulation for the role of atrial natriuretic peptide (ANP) in the renal Na+ retention of cirrhosis. The primary hepatic abnormality that is necessary and sufficient for renal Na+ retention is hepatic venous outflow blockade. In early disease, this signals renal Na+retention with consequent intravascular volume expansion and a compensatory rise in plasma ANP levels. At this stage of disease, the rise in ANP is sufficient to counterbalance the primary antinatriuretic or renal Na+-retaining influences; however, it does so at the expense of an expanded intravascular volume with the potential for overflow ascites. With progression of disease, disruption of intrasinusoidal Starling forces and loss of volume from the vascular compartment into the peritoneal compartment occur. This underfilling of the circulation may attenuate further increases in ANP levels and promote the activation of antinatriuretic factors. Whether the antinatriuretic factors activated by underfilling are the same as or different from those that promote primary renal Na+ retention in early disease remains to be determined. At this later stage of disease, increased levels of ANP may not be sufficient to counterbalance antinatriuretic forces. (From Warner LC, Leung WM, Campbell P, et al: The role of resistance to atrial natriuretic peptide in the pathogenesis of sodium retention in hepatic cirrhosis. In American Society of Hypertension Series, Vol 3: Advances in Atrial Peptide Research. New York, Raven Press, 1989, pp 185-204.) |
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BNP levels have also been found to be elevated in patients with cirrhosis and ascites, and similar to ANP, its natriuretic effect is also blunted in cirrhotic patients with Na+ retention ascites.[838] Recent findings in patients with nonalcoholic cirrhosis suggest that plasma BNP levels may correlate with the severity of the disease and that BNP might be of prognostic value in the progression of cirrhosis.[839] However, additional studies are required in order to establish the prognostic value of BNP in cirrhosis, as already recognized for CHF.
In summary, two general explanations of Na+ reten-tion complicating cirrhosis have been offered. The over-flow mechanism of ascites formation in cirrhosis originally offered by Lieberman and Reynolds[721] envisions a volume-independent stimulus for renal Na+ retention. Possible mediators include adrenergic reflexes activated by hepatic sinusoidal hypertension and increased systemic concentrations of an unidentified antinatriuretic factor as a result of impaired liver metabolism. The underfilling theory, in contrast, postulates “effective” vascular volume depletion. According to the peripheral arterial vasodilation hypothesis, reduced SVR lowers blood pressure and activates arterial baroreceptors, initiating Na+ retention. The retained fluid extravasates from the hypertensive splanchnic circulation, preventing arterial repletion, and Na+ retention and ascites formation continues.
It is quite obvious that neither the underfilling nor the overflow theory can account exclusively for all the observed derangements in volume regulation in cirrhosis. Rather, it is possible that elements of the two concepts may occur simultaneously or sequentially in cirrhosis patients (see Fig. 12-10 ). Thus, sufficient evidence suggests that, early in cirrhosis, intrahepatic hypertension due to hepatic venous outflow block signals primary renal Na+ retention with consequent intravascular volume expansion. Whether, at this stage, underfilling of the arterial circuit consequent to vasodilatation also applies remains to be determined. Owing to expansion of the intrathoracic venous compartment at this stage, plasma ANF levels rise. The rise in ANP levels is sufficient to counterbalance the renal Na+ retaining forces; however, it does so at the expense of an expanded intravascular volume, with the potential for overflow ascites. The propensity for the accumulation of volume in the peritoneal compartment and the splanchnic bed results from altered intrahepatic hemodynamics. With progression of disease, there are disruptions of intrasinusoidal Starling forces and loss of volume from the vascular compartment into the peritoneal compartment. These events coupled with other factors such as portosystemic shunting, hypoalbuminemia, and vascular refractoriness to pressor hormones, lead to underfilling of the arterial circuit, without the necessity for measurable underfilling of the venous compartment.
This underfilling of the circulation may attenuate further increases in ANP levels and promote the activation of antinatriuretic factors. Whether these antinatriuretic factors activated by underfilling are the same as or different from those that promote primary renal Na+ retention in early disease remains to be determined. At this later stage of disease, elevated levels of ANP may not be sufficient to counterbalance antinatriuretic influences.
It should be noted that, in early cirrhosis, salt retention is isotonic and accompanied by ECF expansion and normonatremia. However, with advancing cirrhosis, defective water excretion supervenes, resulting in hyponatremia, reflecting combined ECF and ICF space expansion. However, it is worth emphasizing that impaired water excretion and hyponatremia in cirrhotic patients with ascites is a marker of the severity of the same accompanying hemodynamic abnormalities that initiate Na+ retention and eventuate in hepatorenal failure. The pathogenesis is primarily related to nonosmotic stimuli for release of vasopressin acting together with additional factors such as impaired distal Na+ delivery. It was demonstrated that certain models of hepatic cirrhosis are associated with increased expression of both AQP II mRNA and AQP II immunoreactivity in the collecting duct. These changes in AQP II density may contribute to the positive water balance and hyponatremia in cirrhosis, although additional studies are needed to fully clarify this matter. The development of new aquaretic drugs that are very effective and the correction of the increased production of NO could provide new perspectives in the treatment of renal Na+ and water retention in cirrhosis.
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