Gerhard Giebisch and Erich Windhager
The kidneys help to maintain the body’s extracellular fluid (ECF) volume by regulating the amount of Na+ in the urine. Na+ is the most important contributor to the osmolality of the ECF; hence, where Na+goes, water follows. This chapter focuses on how the kidneys maintain the ECF volume by regulating Na+ and its most prevalent anion, Cl−.
The normal daily urinary excretion of Na+ is only a tiny fraction of the total Na+ filtered by the kidneys (Fig. 35-1). The filtered load of Na+ is the product of the glomerular filtration rate (GFR, ~180 L/day) and the plasma Na+concentration (~142 mM; see Table 5-2), or ~25,500 mmol/day. This amount is equivalent to the Na+ in ~1.5 kg of table salt, more than nine times the total quantity of Na+ present in the body fluids. With a typical Western diet containing ~120 mmol of Na+, the kidneys reabsorb ~99.6% of the filtered Na+ by the time the tubule fluid (TF) reaches the renal pelvis. Therefore, even minute variations in the fractional reabsorptive rate could lead to changes in total body Na+ that markedly alter ECF volume and, hence, body weight. Thus, it is not surprising that each nephron segment makes its own unique contribution to Na+ homeostasis.
Figure 35-1 Distribution and balance of Na+ throughout the body. The values in the boxes are approximations. ICF, intracellular fluid.
Na+ AND Cl− TRANSPORT BY DIFFERENT SEGMENTS OF THE NEPHRON
Na+ and Cl− reabsorption is largest in the proximal tubule, followed by Henle’s loop, the classic distal tubule, and the collecting tubules and ducts
Figure 35-2 summarizes the segmental distribution of Na+ reabsorption along the nephron. The proximal tubule reabsorbs the largest fraction of filtered Na+ (~67%). Because [Na+] in TF (or TFNa) remains the same as that in plasma(i.e., TFNa/PNa = 1.0; see Chapter 33) throughout the length of the proximal tubule, it follows that the [Na+] in the reabsorbate is the same as that in plasma. Because Na+ salts are the dominant osmotically active solutes in the filtrate, reabsorption must be a nearly isosmotic process. (See Note: Isosmotic Reabsorption by the Proximal Tubule; The Proximal Tubule Reabsorbate Is Slightly Hyperosmotic)
Figure 35-2 Estimates of renal handling of Na+ along the nephron. The numbered yellow boxes indicate the absolute amount of Na+—as well as the fraction of the filtered load—that various nephron segments reabsorb. The green boxes indicate the fraction of the filtered load that remains in the lumen at these sites. The values in the boxes are approximations. PNa, plasma Na+ concentration; UNa, urine Na+ concentration.
The loop of Henle reabsorbs a smaller but significant fraction of filtered Na+ (~25%). Because of the low water permeability of the thick ascending limb (TAL), this nephron segment reabsorbs Na+ faster than it reabsorbs water, so that [Na+] in the TF entering the distal convoluted tubule (DCT) has decreased substantially (TFNa/PNa ≅ 0.45).
The classic distal tubule (see Chapter 33) and collecting ducts reabsorb smaller fractions of filtered Na+ and water than do more proximal segments. The segments between the DCT and the cortical collecting tubule (CCT), inclusive, reabsorb ~5% of the filtered Na+ load. Finally, the medullary collecting duct reabsorbs ~3% of the filtered Na+ load. Although the distal nephron reabsorbs only small amounts of Na+, it can establish a steep transepithelial concentration gradient and can respond to several hormones, including mineralocorticoids and arginine vasopressin (AVP).
The tubule reabsorbs Na+ through both the transcellular and the paracellular pathways
The tubule can reabsorb Na+ and Cl− through both transcellular and paracellular pathways (Fig. 35-3A). In the transcellular pathway, Na+ and Cl− sequentially traverse the apical and basolateral membranes before entering the blood. In the paracellular pathway, these ions move entirely by an extracellular route, through the tight junctions between cells. In the transcellular pathway, transport rates depend on the electrochemical gradients, ion channels, and transporters at the apical and basolateral membranes. However, in the paracellular pathway, transepithelial electrochemical driving forces and permeability properties of the tight junctions govern ion movements.
Figure 35-3 A and B, Transcellular and paracellular mechanisms of Na+ and Cl− reabsorption. The example in B illustrates the electrochemical driving forces for Na+ in the early proximal tubule. The equivalent circuit demonstrates that the flow of positive charge across the apical membrane makes the apical membrane voltage more negative.
Transcellular Na+ Reabsorption The basic mechanism of transcellular Na+ reabsorption is similar in all nephron segments and is a variation on the classic two-membrane model of epithelial transport (see Chapter 5). The first stepis the passive entry of Na+ into the cell across the apical membrane. Because the intracellular Na+ concentration ([Na+]i) is low and the cell voltage is negative with respect to the lumen, the electrochemical gradient is favorable for passive Na+ entry across the apical membrane (Fig. 35-3B). However, different tubule segments use different mechanisms of passive Na+ entry across the apical membrane. The proximal tubule, the TAL, and the DCT all use a combination of Na+-coupled cotransporters and exchangers to move Na+ across the apical membrane; however, in the cortical and medullary collecting ducts, Na+ enters the cell through epithelial Na+ channels (ENaC).
The second step of transcellular Na+ reabsorption is the active extrusion of Na+ out of the cell across the basolateral membrane (Fig. 35-3B). This Na+ extrusion is mediated by the Na-K pump (see Chapter 5), which keeps [Na+]ilow (~15 mM) and [K+]i high (~120 mM). Because the basolateral membrane is primarily permeable to K+, it develops a voltage of ~70 mV, with the cell interior negative with respect to the interstitial space. Across the apical membrane, the cell is negative with respect to the lumen. The magnitude of the apical membrane voltage may be either lower or higher than that of the basolateral membrane, depending on the nephron segment and its transport activity.
Paracellular Na+ Reabsorption The basic mechanism of paracellular Na+ transport is similar among nephron segments: the transepithelial electrochemical gradient for Na+ drives transport. However, both the transepithelial voltage and the luminal [Na+] vary along the nephron (Table 35-1). As a result, the net driving force for Na+ is positive—favoring passive Na+ reabsorption—only in the S2 and S3 segment of the proximal tubule and in the TAL. In the other segments, the net driving force is negative—favoring passive Na+ diffusion from blood to lumen (“backleak”). In addition to the purely passive, paracellular reabsorption of Na+ in the S2 and S3 segments and TAL, Na+ can also move uphill from lumen to blood through solvent drag across the tight junctions. In this case, the movement of H2O—energized by the active transport of Na+ into the lateral intercellular space—from the lumen to the lateral intercellular space also sweeps Na+ and Cl− in the same direction.
Table 35-1 Transepithelial Driving Forces for Sodium
Nephron segments also vary in their leakiness to Na+ ions. This leakiness is largely a function of the varying ionic conductance of the paracellular pathway between cells across the tight junction. In general, the leakiness of the paracellular pathway decreases along the nephron from the proximal tubule (the most leaky) to the papillary collecting ducts. However, even the tightest renal epithelia have only what may be regarded as a moderate degree of tightness compared with truly “tight” epithelia, such as the skin, the gastric mucosa, and the urinary bladder (see Chapter 5).
The leakiness of an epithelium has serious repercussions for the steepness of the ion gradients that the epithelium can develop and maintain. For both Na+ and Cl−, the ability of specific nephron segments to establish large concentration gradients correlates with the degree of tightness, which limits the backflux of ions between cells. Thus, the luminal fluid in the distal nephron attains much lower concentrations of Na+ and Cl− than it does in the proximal tubule.
An important consequence of a highly leaky paracellular pathway is that it provides a mechanism by which the basolateral membrane voltage can generate a current that flows through the tight junctions and charges up the apical membrane, and vice versa (Fig. 35-3B). For example, hyperpolarization of the basolateral membrane leads to hyperpolarization of the apical membrane. A consequence of this paracellular electrical coupling is that the apical membrane of a leaky epithelium, such as the proximal tubule, has a membrane voltage that is negative (−67 mV in Fig. 35-3B) and close to that of the basolateral membrane (−70 mV in Fig. 35-3B), whereas one would expect that, based on the complement of channels and ion gradients at the apical membrane, the apical membrane would have a far less negative voltage. A practical benefit of this crosstalk is that it helps couple the activity of the basolateral electrogenic Na-K pump to the passive entry of Na+ across the apical membrane. If the Na-K pump rate increases, not only does [Na+]i decrease, enhancing the chemical Na+ gradient across the apical membrane, but also the basolateral membrane hyperpolarizes (i.e., the cell becomes more negative with respect to the blood). Electrical coupling translates this basolateral hyperpolarization to a concomitant apical hyperpolarization, thus also enhancing the electrical gradient favoring apical Na+ entry.
Na+, Cl−, AND WATER TRANSPORT AT THE CELLULAR AND MOLECULAR LEVEL
Na+ reabsorption involves apical transporters or ENaC and a basolateral Na-K pump
Proximal Tubule Along the first half of the tubule (Fig. 35-4A), a variety of cotransporters in the apical membrane couples the downhill uptake of Na+ to the uphill uptake of solutes such as glucose, amino acids, phosphate, sulfate, lactate, and other monocarboxylic and dicarboxylic acids. Many of these Na+-driven cotransporters are electrogenic, carrying net positive charge into the cell. Thus, both the low [Na+]i and the negative apical membrane voltage fuel the secondary active uptake of these other solutes, which we discuss in Chapter 35. In addition to the cotransporters, Na+ entry is also coupled to the extrusion of H+ through the electroneutral Na-H exchanger (NHE3). The role of NHE3 in renal acid secretion is discussed in Chapter 39.
Figure 35-4 A to D, Cell models of Na+ reabsorption. PCT, proximal convoluted tube.
Both cotransporters and exchangers exploit the downhill Na+ gradient across the apical cell membrane that is established by the Na-K pump in the basolateral membrane. The Na-K pump and, to a lesser extent, the electrogenic Na/HCO3 cotransporter (NBC) are also responsible for the second step in Na+ reabsorption, moving Na+ from cell to blood. The presence of K+ channels in the basolateral membrane is important for two reasons. First, these channels establish the negative voltage across the basolateral membrane and establish a similar negative voltage across the apical membrane through paracellular electrical coupling. Second, these channels permit the recycling of K+that had been transported into the cell by the Na-K pump.
Because of a lumen-negative transepithelial voltage in the early proximal tubule, as well as a paracellular pathway that is permeable to Na+, approximately one third of the Na+ that is transported from lumen to blood by the transcellular pathway diffuses back to the lumen by the paracellular pathway (“backleak”).
Thin Limbs of Henle’s Loop Na+ transport by the thin descending and thin ascending limbs of Henle’s loop is almost entirely passive and paracellular (see Chapter 38).
Thick Ascending Limb Two major pathways contribute to Na+ reabsorption in the TAL: transcellular and paracellular (Fig. 35-4B). The transcellular pathway includes two major mechanisms for taking up Na+across the apical membrane. The Na/K/Cl cotransporter (NKCC2) couples the inward movement of 1 Na+, 1 K+, and 2 Cl− ions in an electroneutral process driven by the downhill concentration gradients of Na+and Cl− (see Chapter 5). The second entry pathway for Na+ is an NHE3. As in the proximal tubule, the basolateral Na-K pump keeps [Na+]i low and moves Na+ to the blood.
Two features of the apical step of Na+ reabsorption in the TAL are noteworthy. First, the loop diuretics (e.g., furosemide and bumetanide) inhibit Na/K/Cl cotransport. Second, a large fraction of the K+ that the NKCC2 brings into the cell recycles to the lumen through apical K+ channels. These channels are essential for replenishing luminal K+ and thus for maintaining adequate Na/K/Cl cotransport.
A key aspect of the paracellular pathway for Na+ reabsorption in the TAL is lumen-positive voltage (Fig. 35-4B). Nearly all other epithelia have lumen-negative voltage because the apical membrane voltage is less negative than the basolateral membrane voltage (see Fig. 5-18D). The TAL is just the opposite. Because K+ channels dominate the apical membrane conductance, the voltage of the TAL apical membrane is more negative than that of the basolateral membrane, thereby resulting in lumen-positive transepithelial voltage. This lumen-positive voltage provides the driving force for the diffusion of Na+across the tight junctions, thus accounting for approximately half of the Na+ reabsorption by the TAL. The lumen-positive voltage also drives the passive reabsorption of K+ (see Chapter 37) and of Ca2+ and Mg2+ (see Chapter 36) through the paracellular pathway. Because the TAL has low water permeability, removing luminal NaCl leaves the remaining TF hypo-osmotic. Hence, the TAL is sometimes referred to as the diluting segment.
Distal Convoluted Tubule Na+ reabsorption in the DCT occurs almost exclusively by the transcellular route (Fig. 35-4C). The apical step of Na+ uptake is mediated by an electroneutral Na/Cl cotransporter(NCC; see Chapter 5) that belongs to the same family as the NKCC2 in the TAL. However, the NCC differs from the NKCC2 in being independent of K+ and highly sensitive to thiazide diuretics. Although the thiazides produce less diuresis than do the loop diuretics, the thiazides are nevertheless effective in removing excess Na+ from the body. The basolateral step of Na+ reabsorption, as in other cells, is mediated by the Na-K pump.
Initial and Cortical Collecting Tubules Na+ reabsorption in these nephron segments and in the connecting tubule is transcellular and is mediated by the majority cell type, the principal cell (Fig. 35-4D). The neighboring ß-intercalated cells are important for reabsorbing Cl−, as discussed later. Na+ crosses the apical membrane of the principal cell through the ENaCs, which are distinct from the voltage-gated Na+channels expressed by excitable tissues (see Chapter 7). ENaC is a heteromer comprising homologous α, ß, and γ subunits, each of which has two membrane-spanning segments. These ENaCs are unique in that low levels of the diuretic drug amiloride block them in a specific way. This compound is a relatively mild diuretic because Na+ reabsorption along the collecting duct is modest. The basolateral step of Na+reabsorption is mediated by the Na-K pump, which also provides the electrochemical driving force for the apical entry of Na+.
The unique transport properties of the apical and basolateral membranes of the principal cells are also the basis for the lumen-negative transepithelial potential difference of approximately –40 mV in the CCT (Table 35-1). In addition to ENaCs, the CCT has both apical and basolateral K+ channels, which play a key role in K+ transport (see Chapter 37). The apical entry of Na+ (which tends to make the lumen negative) and the basolateral exit of K+ (which tends to make the cell negative) are, in effect, two batteries of identical sign, arranged in series. In principle, these two batteries could add up to a transepithelial voltage of ~100 mV (lumen negative). However, under most conditions, K+ exit from cell to lumen partially opposes the lumen-negative potential generated by Na+ entry. The net effect of these three batteries is a transepithelial voltage of ~ −40 mV (lumen negative).
The transepithelial voltage of the CCT can fluctuate considerably, particularly because of changes in the apical Na+ battery owing to, for example, changes in luminal [Na+]. In addition, changing levels of aldosterone or AVP may modulate the number of ENaCs that are open in the apical membrane and may thus affect the relative contribution of this Na+ battery to apical membrane voltage.
Medullary Collecting Duct The inner and outer medullary collecting ducts reabsorb only a minute amount of Na+, ~3% of the filtered load (Fig. 35-2). It is likely that ENaCs mediate the apical entry of Na+ in these segments and that the Na-K pump extrudes Na+ from the cell across the basolateral membrane (Fig. 35-4D).
Cl− reabsorption involves both paracellular and transcellular pathways
Most of the filtered Na+ is reabsorbed with Cl−. However, the segmental handling of Cl− differs somewhat from that of Na+. Both transcellular and paracellular pathways participate in Cl− reabsorption.
Proximal Tubule The proximal tubule reabsorbs Cl− by both the transcellular and the paracellular routes, with the paracellular believed to be the dominant route in the early proximal tubule (Fig. 35-5A). The transcellular pathway is dominant in the late proximal tubule (Fig. 35-5B), where the energetically uphill influx of Cl− across the apical membrane occurs through an exchange of luminal Cl− for cellular anions (e.g., formate, oxalate, HCO−3, and OH−), mediated by CFEX (SLC26A6). Cl-base exchange is an example of tertiary active transport: the apical NHE3, itself a secondary active transporter (see Chapter 5), provides the H+ that neutralizes base in the lumen, thereby sustaining the gradient for Cl-anion exchange. The basolateral exit step for transcellular Cl− movement may occur in part through a Cl− channel that is analogous in function to the cystic fibrosis transmembrane conductance regulator (CFTR; see Chapter 43). In addition, the basolateral membrane of the proximal tubule may also have a K/Cl cotransporter(KCC), which is in the same family as NKCC2 and NCC.
Figure 35-5 A to F, Cell models of Cl− transport. In B, base may include formate, oxalate, HCO−3, and OH−. HBase represents the conjugate weak acid (e.g., formic acid).
Passive Cl− reabsorption through the paracellular pathway is driven by different electrochemical Cl− gradients in the early versus the late proximal tubule. The S1 segment initially has no Cl− concentration gradient between lumen and blood. However, the lumen-negative voltage (Table 35-1)—generated by electrogenic Na/glucose and Na/amino acid cotransport—establishes a favorable electrical gradient for passive Cl− reabsorption. Solvent drag also makes a contribution in the S1 segment. In the S2 and S3 segments, the lumen-positive voltage opposes paracellular Cl− absorption (Table 35-1). However, preferential HCO−3 reabsorption in the earlier portions of the proximal tubule leaves Cl− behind (see Chapter 39), so that [Cl−] in the lumen becomes higher than that in the blood. This favorable lumen-to-blood chemical gradient for Cl− overcomes the electrical gradient so that paracellular movement of Cl− in the late proximal tubule proceeds in the reabsorptive direction.
The upper part of Figure 35-6 shows the profile of the ratio of concentrations in TF versus plasma (TF/P) (see Chapter 33) for the major solutes in TF along the proximal tubule. Only minor changes occur in the TF/P for osmolality or Na+. Because the tubule does not reabsorb inulin, the substantial increase in TFIn/PIn indicates net fluid reabsorption. The fall in TFHCO−3/PHCO−3 mirrors an increase in TFCl/PCl because the tubule reabsorbs HCO−3 more rapidly than it does Cl−. The early proximal tubule avidly reabsorbs glucose and amino acids, leading to sharp decreases in the concentrations of these substances in the TF.
Figure 35-6 Changes in solute composition along the proximal tubule. In the upper panel, TF/P is the ratio of concentration or osmolality in TF versus blood plasma (P). Because we assume that the proximal tubule reabsorbs half of the filtered water, TFIn/PIn rises from 1 to 2. The lower panel shows the transition of transepithelial voltage from a negative to a positive value. In, inulin.
An important characteristic of the proximal tubule is that the transepithelial voltage reverses polarity between the S1 and the S2 segments (Fig. 35-6, lower panel). The early proximal tubule is lumen negative because it reabsorbs Na+ electrogenically, both through electrogenic apical Na+ transporters (e.g., Na/glucose cotransporter) and the basolateral Na-K pump. The late proximal tubule reabsorbs Na+ at a lower rate. Moreover, because TFCl is greater than PCl, the paracellular diffusion of Cl− from lumen to bath generates a lumen-positive potential that facilitates passive Na+ reabsorption by the same paracellular route.
Thick Ascending Limb Cl− reabsorption in the TAL takes place largely by Na/K/Cl cotransport across the apical membrane (Fig. 35-5C), as we already noted in our discussion of Na+ reabsorption (Fig. 35-4B). The exit of Cl−across the basolateral cell membrane through Cl− channels of the ClC family (see Table 6-2, number 15) overwhelms any entry of Cl− through the Cl-HCO3 exchanger. You may recall that only half of the Na+ reabsorption by the TAL is transcellular, whereas all Cl− reabsorption is transcellular. Overall, Na+ reabsorption and Cl− reabsorption are identical because the apical NKCC2 moves two Cl− for each Na+.
Distal Convoluted Tubule Cl− reabsorption by the DCT (Fig. 35-5D) occurs by a mechanism that is somewhat similar to that in the TAL, except the apical step occurs through the NCC, as discussed earlier in connection with Na+reabsorption by the DCT (Fig. 35-4C). Cl− channels that are probably similar to those in the TAL mediate the basolateral Cl− exit step.
Collecting Ducts The initial cortical tubule and the CCT reabsorb Cl− by two mechanisms. First, the principal cell generates a transepithelial voltage (~40 mV, lumen negative) that is favorable for paracellulardiffusion of Cl− (Fig. 35-5E). Second, the ß-type intercalated cells reabsorb Cl− using a transcellular process that involves Cl-HCO3 exchange across the apical membranes and Cl− channels in the basolateral membrane (Fig. 35-5F). Neither the α-type intercalated cells nor the principal cells are involved in transcellular Cl− reabsorption.
Water reabsorption is passive and secondary to solute transport
Proximal Tubule If water reabsorption by the proximal tubule were to follow solute reabsorption passively, then one would expect the osmolality inside the peritubular capillaries to be greater than that in the luminal fluid. Indeed, investigators have found that the lumen is slightly hypo-osmolar.
If, conversely, proximal tubule water reabsorption were active, one would expect it to be independent of Na+ reabsorption. Actually, the opposite was found by Windhager and colleagues in 1959. Using the stationary microperfusion technique, these investigators introduced solutions having various [Na+] values into the proximal tubule lumen, and they kept luminal osmolality constant by adding mannitol (which is poorly reabsorbed). Because these experiments were performed on amphibian proximal tubules, the maximal [Na+] was only 100 mM, the same as that in blood plasma. After a known time interval, these investigators measured the volume of fluid remaining in the tubule lumen and calculated the rate of fluid reabsorption (JV). They also measured [Na+] of the luminal fluid and calculated the rate of Na+reabsorption (JNa). These experiments showed that both JV and JNa are directly proportional to the initial luminal [Na+] (Fig. 35-7). Moreover, the ratio JNa/JV is constant and is equal to the osmolality of the lumen, findings indicating that Na+ reabsorption and secretion are approximately isosmotic. (See Note: Stationary Microperfusion; Isosmotic Reabsorption by the Proximal Tubule)
Figure 35-7 Isosmotic water reabsorption in the proximal tubule. JV and JNa are the rates of fluid (or volume) and Na+ reabsorption, respectively, in stationary microperfusion experiments.
Windhager and colleagues also found that at a luminal [Na+] of 100 mM, the JNa and JV were large and in the reabsorptive direction. When luminal [Na+] was only 65 mM, both JNa and JV were zero. At lower luminal [Na+] values, both JNa and JV reversed (i.e., the tubule secreted both Na+ and water). At these low luminal [Na+] values, active transcellular movement of Na+ from lumen to blood cannot overcome the increasing paracellular backleak of Na+, and the result is net secretion of an isosmotic NaCl solution. This experiment shows that Na+ can move uphill from lumen to blood, as long as the opposing Na+gradient is not too steep, and it also suggests that the movement of water is not active, but passively follows the reabsorption of Na+.
If water movement is passive, why is the difference in osmolality so small between the proximal tubule lumen and blood? The answer is that, because the water permeability of the proximal tubule epithelium to water is so high, the osmolality gradient needed to generate the observed passive reabsorption of water is only 2 to 3 mOsm. A somewhat greater osmolality gradient probably exists between the lumen and an inaccessible basolateral compartment comprising the lateral intercellular space and the microscopic unstirred layer that surrounds the highly folded basolateral membrane of the proximal tubule cell.
The pathway for water movement across the proximal tubule epithelium appears to be a combination of transcellular and paracellular transit, with the transcellular route dominating. The reason for the high rate of water movement through the proximal tubule cell is the presence of a high density of aquaporin 1 (AQP1) water channels in both the apical and basolateral membranes. Indeed, in the AQP1-null mouse, the fluid that the proximal tubule reabsorbs can be hyperosmotic to the luminal fluid.
Loop of Henle and Distal Nephron Two features distinguish water and Na+ transport in the distal nephron. First, the TAL and all downstream segments have relatively low water permeability in the absence of AVP (or antidiuretic hormone). The upregulation of this water permeability by AVP is discussed in Chapter 37. Second, the combination of NaCl reabsorption and low water permeability allows these nephron segments to generate a low luminal [Na+] and osmolality with respect to the surrounding interstitial fluid. Given this large osmotic gradient across the epithelium, the distal nephron is poised to reabsorb water passively from a hypo-osmotic luminal fluid into the isosmotic blood when AVP increases the water permeability.
Osmotic Diuresis
Normally, luminal [Na+] does not change along the proximal tubule. The only exception is osmotic diuresis, a state in which poorly permeable substances are present in the plasma and, therefore, in the glomerular filtrate. Examples are the infusion of sucrose and mannitol. Another is untreated diabetes mellitus (see Chapter 51 for the box on that topic), when the blood glucose level may become too high for the capacity of renal tubules to reabsorb the highly elevated glucose load. Glucose then acts as a poorly reabsorbed substance and as an osmotic diuretic. Because the proximal tubule must reabsorb isosmotic Na+ salts from a luminal mixture of Na+ salts and poorly reabsorbable solutes (e.g., mannitol), luminal [Na+] progressively falls and luminal [mannitol] rises, but luminal osmolality does not change (Fig. 35-8).
Figure 35-8 Osmotic diuresis and luminal [Na+] along the proximal tubule. In this example, blood and glomerular filtrate both contain 40 mM mannitol and have an osmolality of 300 mOsm. The isosmotic reabsorption of NaCl (but not mannitol) from proximal tubule lumen to blood causes luminal [mannitol] to rise and [NaCl] to fall, but causes no change in luminal osmolality. Once luminal [Na+] falls sufficiently, the Na+ backleak from peritubular capillaries balances active Na+ reabsorption, and net reabsorption of NaCl and water is zero. The absence of fluid absorption after this point causes osmotic diuresis. Viewed another way, the rising luminal [mannitol], with the osmotically obligated water the mannitol holds in the lumen, produces the diuresis. (TF/P)In, the ratio of inulin concentrations in TF/P.
Because, as we have seen, the rate of Na+ and fluid reabsorption falls as the luminal [Na+] falls (Fig. 35-7), the proximal tubule reabsorbs progressively less Na+ and fluid as the fluid travels along the tubule. This decrease in Na+ and water reabsorption leaves a larger volume of TF, thereby producing osmotic diuresis. Clinicians use osmotic diuretics in several settings. For example, mannitol osmotically draws water out of brain tissue into the vascular system, for ultimate excretion by the kidneys. For this reason, osmotic diuresis has proven useful in treating patients with acutely increased intracranial pressure from an expanding tumor or abscess, hematoma or hemorrhage, or edema. Mannitol also reduces the risk of radiocontrast-induced acute renal failure in susceptible patients (e.g., those with underlying renal disease, volume depletion, diabetes mellitus, or congestive heart failure) who must undergo radiological procedures that require a substantial dye load.
It is common to distinguish between two types of diuresis: solute and water diuresis. The foregoing example of solute or osmotic diuresis is characterized by excretion of a larger than normal volume of urine that is rich in solutes. In contrast, water diuresis leads to excretion of larger than normal volumes of urine that is dilute (i.e., poor in solutes).
The kidney’s high O2 consumption reflects a high level of active Na+ transport
Because virtually all Na+ transport ultimately depends on the activity of the ATP-driven Na-K pump and, therefore, on the generation of ATP by oxidative metabolism, it is not surprising that renal O2consumption is large and parallels Na+ reabsorption. Despite their low weight (<0.5% of body weight), the kidneys are responsible for 7% to 10% of total O2 consumption. Although it would seem that the high O2 consumption would necessitate a large arteriovenous difference in PO2, actually the renal blood flow is so large that the artery-vein PO2 difference is actually much smaller than that in cardiac muscle or brain.
If one varies Na+ reabsorption experimentally and measures renal O2 consumption, the result is a straight-line relationship (Fig. 35-9). However, the kidneys continue to consume a modest but significant amount of O2 even in the complete absence of net Na+ reabsorption. This transport-independent component reflects the basic metabolic needs for the maintenance of cell viability.
Figure 35-9 Dependence of O2 consumption on Na+ transport. Na+ reabsorption (JNa) was varied by changing GFR, administering diuretics, or imposing hypoxia. O2 consumption was computed from the arteriovenous PO2 difference.
REGULATION OF Na+ AND Cl− TRANSPORT
The body regulates Na+ excretion through the following three major mechanisms:
1. Changes in renal hemodynamics alter the Na+ load presented to the kidney and modulate the rate of NaCl reabsorption in the proximal tubule by a process known as glomerulotubular (GT) balance. We discuss these hemodynamic effects in the next three sections.
2. Three factors that regulate “effective circulating volume” (see Chapter 23) do so in part by increasing Na+ reabsorption (i.e., producing natriuresis). A fourth does so in part by decreasing Na+ reabsorption. We briefly introduce these four factors in the fourth section of this subchapter, and we discuss them more fully in Chapter 39.
3. In addition to the natriuretic factor alluded to in item 2, four other natriuretic humoral factors can decrease Na+ reabsorption, as we see in the final section of this chapter.
GT balance stabilizes fractional Na+ reabsorption by the proximal tubule in the presence of changes in the filtered Na+ load
When hemodynamic changes (e.g., caused by changes in intake of Na+ or protein or by extreme exercise, severe pain, or anesthesia) alter GFR and thus the Na+ load presented to the nephron, proximal tubules respond by reabsorbing a constant fraction of the Na+ load. This constancy of fractional Na+ reabsorption along the proximal tubule—GT balance—is independent of external neural and hormonal control and thus safeguards Na+ balance.
Figure 35-10 shows how the absolute reabsorption of Na+ increases proportionally with increases in the filtered Na+ load, achieved by varying GFRs at constant plasma [Na+]. The amount of luminal Na+remaining at the end of the proximal tubule also increases linearly with the filtered Na+ load. Later, we discuss the impact of such altered Na+ delivery to more distally located nephron segments.
Figure 35-10 Constancy of fractional Na+ reabsorption by the proximal tubule.
When excessive Na+ loss (e.g., sweating or diarrhea) contracts the ECF volume, the reduced renal perfusion pressure causes GFR to fall. In response, the proximal tubule excretes a constant fraction of Na+ and water, corresponding to a smaller absolute amount. Thus, GT balance helps to prevent additional Na+ and water loss. Conversely, when Na+ retention expands ECF (causing GFR to rise), the proximal tubule excretes a constant fraction of filtered Na+, corresponding to an increased absolute amount. This response tends to correct the volume expansion. At the level of the whole kidney, GT balance is not perfect, mainly because distal nephron Na+ absorption is under neural and hormonal control.
The proximal tubule achieves GT balance by both peritubular and luminal mechanisms
How do the proximal tubule cells sense that the GFR has changed? Both peritubular and luminal control mechanisms contribute, although no agreement exists concerning their relative roles.
Peritubular Factors in the Proximal Tubule As discussed in the previous chapter, Starling forces across the peritubular capillary walls determine the uptake of interstitial fluid and thus the net reabsorption of NaCl and fluid from the tubule lumen into the peritubular capillaries (see Chapter 34). These peritubular physical factors also play a role in GT balance. We can distinguish a sequence of three transport steps as reabsorbed fluid moves from the tubule lumen into the blood (Fig. 35-11A):
Step 1: Solutes and water enter a tubule cell across the apical membrane.
Step 2: Solutes and water from step 1 (i.e., “reabsorbate”) exit the tubule cell across the basolateral membrane and enter an intercellular compartment—the lateral interspace—that is bounded by the apical tight junction, the basolateral tubule cell membranes, and a basement membrane that does not discriminate between solutes and solvent. Steps 1 and 2 constitute the transcellular pathway
Step 3: The reabsorbate can either backleak into the lumen (step 3a) or move sequentially into the interstitial space and then into the blood (step 3b).
Figure 35-11 Peritubular mechanisms of GT balance. In A, PPC and PPC are, respectively, the hydrostatic and the oncotic pressures in the peritubular capillaries. At the level of the tubule, the net Na+ reabsorption is the difference between active transcellular transport and passive backleak through the paracellular pathway. At the level of the peritubular capillary, net fluid absorption is the difference between fluid absorption (driven by PPC) and fluid filtration (driven by PPC). In B, increasing the filtered fraction has effects on both the tubule and the peritubular capillaries. At the level of the tubule, the increased concentrations of Na+-coupled solutes (e.g., glucose) increase active transport. At the level of the capillaries, the lower PPC and higher protein concentration (PPC) pull more fluid from the interstitium. The net effect is reduced passive backleak.
At normal GFR (Fig. 35-11A), reabsorptive Starling forces—the low hydrostatic and high oncotic pressure in the capillaries—cause an extensive uptake of reabsorbate into the capillaries. Because of GT balance, alterations in GFR lead to changes in peritubular pressures (both hydrostatic and oncotic) that, in turn, modulate the forces that govern step 3 in the foregoing list. Peritubular mechanisms of GT balance come into play only when the changes in GFR are associated with alterations in filtration fraction (FF = GFR/RPF; see Chapter 34). Consider an example in which we increase GFR at constant glomerular plasma flow, thereby increasing FF (Fig. 35-11B). We can produce this effect by increasing efferent arteriolar resistance while concomitantly decreasing afferent arteriolar resistance (see Fig. 33-8A, lower panel). The result is an increase in glomerular capillary pressure (i.e., net filtration pressure) without a change in overall arteriolar resistance. These changes have two important consequences for peritubular capillaries. First, the increased GFR translates to less fluid remaining in the efferent arteriole, so that peritubular oncotic pressure (πPC) rises. Second, the increased GFR also translates to less blood flowing into the efferent arteriole, thereby decreasing hydrostatic pressure in the peritubular capillaries (PPC). As a consequence, the net driving force increases for the transport of fluid from the lateral interspace into the capillaries, thus leading to a more effective absorption of fluid and NaCl. The opposite sequence of events occurs during extracellular volume expansion.
Luminal Factors in the Proximal Tubule Luminal factors also contribute to GT balance, as evidenced by the observation that increased flow along the proximal tubule—without any peritubular effects—leads to an increase in fluid and NaCl reabsorption. The luminal concentrations of solutes such as glucose, amino acids, and HCO−3 fall along the length of the proximal tubule, as the tubule reabsorbs these solutes (Fig. 35-6). Increasing luminal flow would, for instance, cause luminal [glucose] to fall less steeply along the tubule length, and more glucose would be available for reabsorption at distal sites. The net effect is that, integrated over the entire proximal tubule, higher luminal flows increase the reabsorption of Na+, glucose, and other Na+-coupled solutes.
A second luminal mechanism may revolve around a flow sensor. It appears that increased flow causes increased bending of the central cilium on the apical membrane, an effect that may signal increased fluid reabsorption. Third, a hypothetical humoral factor may also contribute to GT balance. If one harvests TF and injects it into a single proximal tubule, Na+ reabsorption increases. Angiotensin II (ANG II)—a small peptide hormone that is both filtered in the glomeruli and secreted by proximal tubule cells—increases Na+ reabsorption in the proximal tubule.
The distal nephron also increases Na+ reabsorption in response to an increased Na+ load
The tubules of the distal nephron, like their proximal counterparts, also increase their absolute magnitude of Na+ reabsorption in response to increased flow. The principle is the same as that for glucose transport in the proximal tubule, except here it is luminal [Na+] that falls less steeply when flow increases (Fig. 35-12A). Because the transport mechanisms responsible for Na+ reabsorption by the distal nephron work faster at higher luminal [Na+] values, reabsorption at any site increases with flow (Fig. 35-12B). In contrast to GT balance in the proximal tubule, increasing flow by a factor of 4 in the distal nephron may cause cumulative Na+ reabsorption to rise by only a factor of 2 (Fig. 35-12C).
Figure 35-12 Flow dependence of Na+ transport in the distal nephron. A and B are idealized representations of the effect of increased flow (
) on luminal [Na+] and Na+ reabsorption rate (JNa) along the TAL. Limiting [Na] is the theoretical minimal value that the tubule could achieve at zero flow. Csummarizes the effect of flow on cumulative Na+ reabsorption.
The opposite changes in Na+ and fluid transport occur when GFR—and thus distal flow—acutely falls, as in circulatory shock. Because of GT balance, the proximal nephron reabsorbs a constant fraction of Na+, thereby delivering a lower absolute amount of Na+ to the DCT. The distal nephron—under the influence of neural and humoral factors discussed later—lowers luminal [Na+] even further so that the final urine may contain only traces of Na+.
Four parallel pathways that regulate effective circulating volume all modulate Na+ reabsorption
GT balance is only one element in a larger, complex system for controlling Na+ balance. As we see in Chapter 39, the control of effective circulating volume (i.e., Na+ content) is under the powerful control of four parallel effectors (see Chapter 40): the renin-angiotensin-aldosterone axis, the sympathetic nervous system, AVP, and atrial natriuretic peptide (ANP). In the previous chapter, we saw how these factors modulate renal blood flow and GFR (see Chapter 34). Here, we briefly discuss how these four effectors modulate Na+ reabsorption, a subject that we treat more comprehensively beginning in Chapter 40.
Renin-Angiotensin-Aldosterone Axis ANG II—the second element in the renin-angiotensin-aldosterone axis (see Chapter 40)—binds to AT1 receptors at the apical and basolateral membranes of proximal tubule cells and, predominantly through protein kinase C, stimulates apical NHE3s. ANG II also stimulates Na-H exchange in the TAL and stimulates apical Na+ channels in the initial collecting tubule. These effects promote Na+ reabsorption.
Aldosterone—the final element in the renin-angiotensin-aldosterone axis—stimulates Na+ reabsorption by the initial tubule and the CCT, and by medullary collecting ducts. Normally, only 2% to 3% of the filtered Na+ load is under humoral control by aldosterone. Nevertheless, the sustained loss of even such a small fraction would exceed the daily Na+ intake significantly. Accordingly, the lack of aldosterone that occurs in adrenal insufficiency (Addison disease) can lead to severe Na+ depletion, contraction of the ECF volume, and circulatory insufficiency. (See Note: Acute Effects of IV Aldosterone)
Aldosterone acts on the principal cells of the collecting ducts (Fig. 35-13A) by binding to cytoplasmic mineralocorticoid receptors (MRs) that then translocate to the nucleus and upregulate transcription (see Chapters 3 and 4). Thus, the effects of aldosterone require a few hours to manifest themselves because they depend on the increased production of aldosterone-induced proteins. One of these is SGK (serum- and glucocorticoid-regulated kinase), a key player in the early phase of aldosterone action. Early cellular actions of aldosterone action include upregulation of apical ENaCs, apical K+ channels, the basolateral Na-K pump, and mitochondrial metabolism. The effects on ENaC involve an increase in the product of channel number and open probability (NPo), and thus apical Na+ permeability. The simultaneous activation of apical Na+ entry and basolateral Na+ extrusion ensures that, even with very high levels of Na+ reabsorption, [Na+]i and cell volume are stable. Long-term exposure to aldosterone leads to the targeting of newly synthesized Na-K pumps to the basolateral membrane and to amplification of the basolateral membrane area.
Figure 35-13 Cellular actions of aldosterone. The inset in A shows the upregulation of ENaCs, based on patch-clamp data from the rat CCT. N is the number of channels in the patch, and PO is the open probability. In B, 11β-HSD2 prevents cortisol (a glucocorticoid), which is present at high plasma concentrations, from having mineralocorticoid effects in the target cell. In C, with the enzyme blocked, cortisol acts as a mineralocorticoid. GR, glucocorticoid receptor.
Because MRs distinguish poorly between glucocorticoids and mineralocorticoids, and because plasma concentrations of glucocorticoids greatly exceed those of aldosterone, one would expect glucocorticoids to exert a mineralocorticoid effect and cause Na+ retention. Under normal conditions, this does not happen because of the enzyme 11β-hydroxysteroid dehydrogenase 2 (11β-HSD2), which co-localizes with intracellular adrenal steroid receptors (Fig. 35-13B). This enzyme irreversibly converts cortisol into cortisone (see Fig. 50-2), an inactive metabolite with low affinity for MRs. In sharp contrast, the enzyme does not metabolize aldosterone. Thus, 11β-HSD2 enhances the apparent specificity of MRs by protecting them from illicit occupancy by cortisol. As may be expected, an 11β-HSD2 deficiency may mimic a mineralocorticoid excess. Carbenoxolone, a specific inhibitor of 11β-HSD2, prevents metabolism of cortisol in target cells, thus permitting abnormal activation of MRs by this glucocorticoid (Fig. 35-13C). Another inhibitor of 11β-HSD2 is glycyrrhetinic acid, a component of natural licorice. Thus, natural licorice can cause abnormal Na+ retention and hypertension, a condition known as mineralocorticoid excess syndrome (MES). (See Note: Reaction Catalyzed by 11β-HSD and Licorice as a Cause of Apparent Mineralocorticoid Excess)
Sympathetic Division of the Autonomic Nervous System Sympathetic nerve terminals in the kidney release norepinephrine, which has two major direct effects on Na+ reabsorption. First, high levels of sympathetic stimulation markedly reduce renal blood flow and therefore GFR (see Chapter 34). Both proximal GT balance (Fig. 35-10) and the flow response of the distal nephron (Fig. 35-12) cause Na+excretion to fall. Second, even low levels of sympathetic stimulation activate α-adrenergic receptors in proximal tubules. This activation stimulates both the apical NHE3 and basolateral Na-K pump (Fig. 35-4A), thereby increasing Na+ reabsorption, independent of any hemodynamic effects.
Arginine Vasopressin or Antidiuretic Hormone Released by the posterior pituitary, AVP binds to a V2 receptor at the basolateral membrane of target cells. Acting through Gs, the AVP increases [cAMP]i (see Chapter 3). As discussed in Chapter 39, the overall renal effect of AVP in humans is to produce urine with a high osmolality and thereby retain water (see Chapter 38). However, AVP also stimulates Na+reabsorption. In the TAL, AVP stimulates the apical NKCC2 and K+ channels (Fig. 35-4B). In principal cells of the initial collecting tubule and the CCT, AVP stimulates Na+ transport by increasing the number of open Na+ channels (NPo) in the apical membrane. (See Note: Effects of AVP on Na+ Channels)
Atrial Natriuretic Peptide Of the four parallel effectors (see Chapter 40) that control effective circulating volume, ANP is the only one that promotes natriuresis. A polypeptide released by atrial cardiomyocytes (see Chapter 19), ANP stimulates a receptor guanylyl cyclase to generate cGMP (see Chapter 3). The major effects of ANP are hemodynamic. It causes renal vasodilation, by massively increasing blood flow to both the cortex and the medulla. Increased blood flow to the cortex raises GFR and increases the Na+ load to the proximal tubule and to TAL (see Chapter 34). Increased blood flow to the medulla washes out the medullary interstitium, thus decreasing osmolality and ultimately reducing passive Na+ reabsorption in the thin ascending limb (see Chapter 38). The combined effect of increasing cortical and medullary blood flow is to increase the Na+ load to the distal nephron and thus to increase urinary Na+ excretion. In addition to its hemodynamic effects, ANP directly inhibits Na+ transport in the inner medullary collecting duct, perhaps by decreasing the activity of nonselective cation channels in the apical membrane. (See Note: Renal Actions of Atrial Natriuretic Peptide (ANP))
An endogenous steroid, prostaglandins, bradykinin, and dopamine all decrease Na+ reabsorption
Aside from ANP (see previous section), three humoral agents have significant natriuretic action, in part due to inhibition of Na+ reabsorption at the level of the tubule cell.
Endogenous Na-K Pump Inhibitor Human plasma contains an endogenous, ouabain-like steroid (see Chapter 5) that inhibits Na-K pumps in a wide variety of cells. This natural Na-K pump inhibitor increases with salt loading and is present in high levels in patients with hypertension. In response to Na+ loading, the body may increase levels of this inhibitor, which presumably would bind preferentially to Na-K pumps of collecting duct cells, thereby elevating [Na+]i and enhancing Na+ excretion.
Prostaglandins and Bradykinin Produced locally in the kidney, these agents act through protein kinase C (see Chapter 3) to inhibit Na+ reabsorption, probably by phosphorylating K+ or Na+ channels. In the TAL, prostaglandin E2(PGE2) inhibits the apical K+ channel, depolarizing the apical and basolateral membranes, and diminishing passive Cl− efflux across the basolateral membrane. [Cl−]i therefore rises, impeding the turnover of the apical NKCC2 (Fig. 35-4B). In addition, the transepithelial voltage becomes less lumen positive, thus decreasing the driving force for passive paracellular reabsorption of Na+ and other cations. In the CCT, both PGE2 and bradykinin inhibit ENaCs (Fig. 35-4D).
Dopamine The kidney forms dopamine locally from circulating L-dopa. Na+ loading increases the synthesis and urinary excretion rate of dopamine, whereas a low-Na+ diet has the opposite effect. As noted earlier, dopamine causes renal vasodilation (see Chapter 34), which increases Na+ excretion. Dopamine also directly inhibits Na+ reabsorption at the level of tubule cells. Indeed, D1 and D2 dopamine receptors are present in the renal cortex, where they both apparently lead to an increase in [cAMP]i. The result is an inhibition of both the apical NHE3 and the basolateral Na-K pump in proximal tubule and TAL cells. In humans, administering low doses of dopamine leads to natriuresis.
REFERENCES
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Quentin F, Chambrey R, Trinh-Trang-Tan MM, et al: The Cl−/HCO−3 exchanger pendrin in the rat kidney is regulated in response to chronic alterations in chloride balance. Am J Physiol 2004; 287:F1179-F1188.