Sergio G. Thal and Patrick J. Tchou
The aim of this chapter is to cover the main aspects of basic cardiac electrophysiology, developed in a review fashion for the Cardiovascular Medicine Board Examination. The information is organized as follows:
1. Basic action potential (AP) and ion channel implications
2. Electrical activity coupling mechanisms
3. Conduction system anatomy
4. Local electrophysiology characteristics of various conduction system components
MEMBRANE ACTION POTENTIAL
The initiation of the cell membrane AP is the first event in a process that ends with a cardiac contraction. Grossly, myocardial cells can be divided into those dependent on sodium ions (Na+) or calcium ions (Ca2+) to drive AP depolarization.
Sodium-Dependent Cells
Each AP starts with net movement of ions across the cell membrane. In a steady state, the membrane is polarized near -90 mV. The transmembrane ionic current is the result of the balance between many inward and outward ionic currents. The sodium channel is voltage sensitive. This means that the probability of the channel being open for transport of the sodium ion increases with increase of transmembrane voltage (toward zero). When a cell receives depolarizing current, sodium channels open and increase the inward current. When the inward current exceeds the total outward current, a rapid opening of sodium channels occurs that overwhelms any outward current, resulting in the rapid upstroke portion of the AP termed Phase 0 (Figs. 5.1 and 5.2).
FIGURE 5.1 Action potential (sodium channel tissue). AP model of a sodium tissue. Numbers 0, 1, 2, 3, 4 delineate the different phases of the AP.
FIGURE 5.2 Main ion channel activities in AP phases. Na, sodium; K, potassium; Cl, chloride; Ca, calcium.
Sodium channels are characterized by a protein that works as a voltage-gated system. The active portion of this channel is the α subunit, which consists of a 2,000–amino acid glycoprotein. Properties of these channels include the following:
Selective permeation
Gating (activation and inactivation)
Drug binding (local anesthetics)
Susceptibility to many different neurotoxins
Levels of Na+ Channel Activity Regulation
1. Transcriptional regulation of Na+ channel proteins is a mechanism to control Na+ channel expression at a genomic level. This pattern of regulation can be influenced by feedback originating in the tissue electrical activity. The exact mechanism of this gene regulation remains incompletely understood.
2. Phosphorylation/dephosphorylation of the α subunit
3. Glycosylation. The regulation mechanism affects all channel subunits.
Abnormalities of the sodium channel can result in both the long QT syndrome and the Brugada syndrome.
Phase 1 (see Fig. 5.2) starts with the opening of a rapid outward potassium (K+) current called Ito. This determines a fast early repolarization with a prominent notch shape that approximates the membrane potential to 0 mV. These channels are characterized by outward movement of K+ ions, which constitutes the principal source of membrane repolarization early during the AP. The channels inactivate soon after activation, although not as rapidly as the sodium current does. A dynamic interaction of four α subunits and an apparatus composed of a cytoskeleton and signaling complexes mainly form the K+ pore. During the AP, these K+ channels activate in response to membrane depolarization and inactivate in a timed manner. The channels are regulated by:
Angiotensin II, which reduces Ito fast velocity
α-Adrenergic stimulation, which reduces Ito fast velocity
Hyperthyroidism, which increases Ito current density
Aldosterone, which mediates a receptor-specific downregulation of Ito
In human pathophysiology, these channels provide an early repolarization current that can drive the transmembrane voltage toward resting membrane potential when the sodium current is dysfunctional. Thus, in Brugada syndrome, in which there is an abnormality in the sodium current that results in depressed sodium conductance, the Ito current may cause full repolarization in a portion of the myocardium early during the AP, resulting in a large voltage gradient between the repolarized part and parts that have more normal AP. Such gradients have been demonstrated in isolated tissue preparations to be capable of initiating reentrant wavefronts. These reentrant wavefronts can initiate polymorphic ventricular tachycardia or ventricular fibrillation.
The next portion of the AP, termed Phase 2 or the plateau phase (see Fig. 5.2), is the result of the balance of two different ion currents. During Phase 0, at the level of -40 mV, Ca2+ channels open, creating an inward Ca2+ current. This current, acting as an antagonist to the outward K+ current, exerts its action by stabilizing transmembrane potential during the plateau phase. This phase concludes as the Ca2+ current declines by inactivation of L-type Ca2+channels. These channels are also the critical initiators of cardiac excitation–contraction coupling through the initial increase in intracellular Ca2+, which triggers the release of Ca2+ from the sarcoplasmic reticulum, which in turn provides a contraction signal to the cellular contractile elements. At a level of -40 mV of membrane potential, these channels rapidly activate, reaching a peak in approximately 2 to 7 milliseconds. Inactivation of the channel depends on time, membrane potential, and Ca2+ concentration.
Phase 3 (see Fig. 5.2), the repolarization phase, is dominated by the outward current of K+ through the so-called “delayed rectifier” K+ channels, which are responsible for the return of the cell membrane to its resting polarized state. Two types of delayed rectifiers are important in the repolarization of human ventricular myocardium, a rapidly activating IKr and a more slowly activating IKs that peaks late in the AP, during Phase 3. Abnormalities in either of these two types of delayed rectifier K+ channels can cause the long-QT syndrome.
Phase 4 (see Fig. 5.2) constitutes a stable polarized membrane. This stabilization of membrane AP after the descending Phase 3 is achieved mainly by the action of the voltage-regulated inward rectifiers (IK1). These channels behave differently than the delayed rectifiers, which open in response to depolarization. The inward rectifier K+ channels are opened at near-resting membrane potential, stabilizing the resting membrane potential near the K+equilibrium potential, but close in response to depolarization, facilitating the AP, hence the description of “inward rectifying.”
Myocardial Tissues That Have Calcium-Dependent AP versus Tissues with Sodium Channels
The main differences between these two types of myocardial tissue can be found in Phases 4 and 0 of the AP. Calciumdependent tissues are the principal cellular component of the specialized conduction system and the sinus node. These cells have the ability to generate a spontaneous AP based on the differential characteristic of Phase 4. This difference is produced by ion currents that affect Na+ and K+concentrations, called If, which activate at membrane potentials below -40 mV, and the K rectifier currents. These currents confer an unstable electrical property, causing these cells to develop spontaneous diastolic depolarization and automatic onset of APs in a rhythmic fashion. Once spontaneous diastolic activity raises the membrane potential to a value of -40 mV, opening of the slow Ca2+ channels results in an inward Ca2+ current (L-type Ca) that generates the slow AP upstroke (Phase 0). Na+ channels possess a small, if any, role in the AP generation in these particular cells (Fig. 5.3).
FIGURE 5.3 Action potential (calcium channel tissue). AP model of a calcium tissue. Numbers 0, 1, 2, 3, 4 delineate the different phases of the AP.
ELECTRICAL COUPLING CELLS (GAP JUNCTION)
GAP junction channels are the functional units that produce direct ionic communication between cardiac cells and play a major role in the propagation of the AP from one myocardial cell to the next. The molecular unit of the GAP junction is a protein called connexin. The oligomerization of six connexins forms a connexon, and two connexons form the final channel called the GAP junction. Among the 20 different subtypes of connexins identified in the human genome, connexin 43 is the most abundant in myocardial cells. Besides the genomic regulation of channel expression, they are also affected by the activity of protein kinase activity, low intracellular pH, and dephosphorylation.
GAP junction channels are not uniformly distributed within cardiac tissues. They are almost absent in the sinus node, are found in low concentration in various areas of the atrioventricular node (AVN), and demonstrate significant expression in the faster conducting atrial and ventricular muscle as well as His–Purkinje fibers. In these cells, the distribution of the connexons is not uniform. They are more concentrated along the ends of the myocytes than along the sides of the cell, thus giving a directional propensity for AP propagation. This gives rise to anisotropic propagation of depolarization, with faster conduction velocities along the muscle fiber orientation compared to the slower transverse fiber–orientated velocities.
CONDUCTION SYSTEM ANATOMY AND PHYSIOLOGY
The sinus node is located beneath the epicardial surface of the crista terminalis, at its junction with the high right atrium. It possesses a spindle-shaped structure and measures an average 10 to 20 mm in the long axis and 2 to 3 mm in the transverse axis. It is composed of a cumulus of small cells called P cells; the main component of the natural pacemaker. They are grouped in elongated clusters and are centrally located within the sinus node. Transitional cells called T cells surround the P cells and transmit the impulse generated by the P cells to the surrounding atrium. The final synchronized activity of the sinus node is achieved via the presence of GAP junctional channels that electrically couple the depolarization of P cells. At the periphery of the node, strands of nodal cells interdigitate with atrial cells, forming lateral connections and transferring the pacemaker impulse to the atrial cells. This organization is believed to be important to impulse propagation from a small source (the nodal cells) to a large reservoir (the atrial myocardium), preventing excessive dampening of the pacemaker current within the nodal cells by the large reservoir of atrial myocardium.
Cells within the sinus node demonstrate spontaneous diastolic depolarization, initiating APs in a repetitive fashion. These APs are calcium channel dependent and possess a similar morphology to those of the atrioventricular nodal cell, characterized by a slow Phase 0 upstroke velocity. Sinus node cells do not possess the GAP junction protein connexin 43. Therefore, electrical coupling at the node center is poor, reflected as a low measured conduction velocity. The periphery is associated with an increase in conduction velocity. This characteristic is most likely important in isolating the sinus node from the potential suppressive hyperpolarizing influence of the atrial myocardium.
Normal sinus node function is affected by age. In the young, the intrinsic heart rate is faster, but vagal tone predominates at rest, causing slowing of the heart rate. In the elderly, resting autonomic tone tends to shift away from vagal predominance to sympathetic outflow. Thus, the extrinsic sinus rate at rest, the rate as modified by autonomic tone, tends to be similar within the ages of adulthood.
The impulse generated at the sinoatrial node is next transmitted to the AVN through atrial myocytes. There is anatomic evidence for the presence of three atrionodal pathways traversing the right atrium. A fourth pathway, called Bachmann bundle, derived from the anterior atrionodal pathway, directs impulse propagation to the left atrium via the interatrial septum. Anatomically, these so-called pathways do not demonstrate any specialized conduction tissue. Rapid conduction along these intra- and interarterial paths appears to be correlated with fiber size and orientation rather than the presence of specialized conduction tissue.
The AVN is a fusiform structure located subendocardially along the annular regions of the interatrial septum, with its distal end, the compact node, at the superior corner of the triangle of Koch. The triangle of Koch is defined by the insertion of the septal leaflet of the tricuspid valve, the tendon of Todaro, and the line that connect the os of the coronary sinus and the tricuspid annulus. The body and the proximal end (the tail) of the AVN are directed posteriorly along the tricuspid annulus. A second tail extends from the body of the AVN along the mitral annulus. The so-called slow pathway of the AVN corresponds to the tail of the AVN, whereas the fast pathway involves atrial inputs into the distal compact node. Similar to the sinus node, transitional cells surround this structure. Circulation to the AVN is provided in nearly 90% of individuals by branches of the right coronary artery extending superiorly from the crux into the trigone area along the AV annulus.
The His bundle is the anatomic structure that connects the compact AVN to the bundle branches. At the junction between the distal AVN and the proximal His bundle, the cells undergo a gradual change from possessing node-like APs to having His–Purkinje APs. That is the APs change from having slow upstrokes dependent on Ca2+ current to fast upstrokes dependent on Na+ current. The branches from the anterior and posterior descending arteries provide circulation to this portion of the conduction system and confer a better security margin for ischemic damage. The His bundle penetrates the AV ring at the central fibrous body and then arches anteriorly and inferiorly along the crest of the septal myocardium that forms the lower edge of the membranous ventricular septum. As it courses along the crest, left-sided fibers in the His bundle drop over the crest into the left ventricle, forming the posterior, septal, and anterior fascicles of the left bundle branch. The His bundle then continues its course over the right ventricular septum as the right bundle branch. The right bundle brunch adopts a subendocardial trajectory over the right side of the interventricular septum and transmits the cardiac impulse to the Purkinje fibers located at the apical portions of the right ventricle. The bundle branches spread into a smaller Purkinje bundle and then into finer fibers that terminate at the myocardium. This branching structure of the His–Purkinje system facilitates a near-synchronous arrival of the sinus impulse at the myocardial endocardial surface.
Electrophysiologically, the AVN can be differentiated into three portions: atrionodal, compact node, and nodo-His. The compact node area presents a response characterized by an AP with a slow rate of rise during its upstroke and a low amplitude. The other two zones have transitional characteristics between the compact node zone and the atrial and His bundle potentials, respectively.
Calcium-type APs characterize the main AVN cellular type. Differentiating the AVN from the sinus node, GAP junctions play an important role in AVN conduction. Connexin 45 is present in this portion of the conduction system, though at a low level. It has also been demonstrated that the expression of connexin 45 constitutes the molecular basis of AVN dual pathways.
SUGGESTED READINGS
Beardslee MA, Tadros PN, Laing JG, et al. Dephosphorylation and intracellular redistribution of ventricular connexin43 during electrical uncoupling induced by ischemia. Circ Res. 2000;87(8):656–662.
Boyett MR, Kodama I. The sinoatrial node, a heterogeneous pacemaker structure. Cardiovasc Res. 2000;47(4):658–687.
Coppen SR. Diversity of connexin expression patterns in the atrioventricular node: vestigial consequence or functional specialization? J Cardiovasc Electrophysiol. 2002;13(6):625–626.
de Carvalho A, de Almeida D. Spread of activity through the atrioventricular node. Circ Res. 1960;8:801–809.
DiFrancesco D, Mazzanti M, Tromba C. Properties of the hyperpolarizing-activated current (if) in cells isolated from the rabbit sino-atrial node. J Physiol. 1986;377:61–88.
Douglas P, Zipes MJJ. Cardiac Electrophysiology. From Cell to Bedside. 4th ed. Philadelphia: WB Saunders; 2004.
Gourdie RG, Green CR, Rothery S, et al. The spatial distribution and relative abundance of gap-junctional connexin40 and connexin43 correlate to functional properties of components of the cardiac atrioventricular conduction system. J Cell Sci. 1993;105(4):985–991.
Irisawa H. Cardiac electrophysiology: past, present and future. Part II. Membrane currents in cardiac pacemaker tissue. Experientia. 1987;43:1131–1135.
Kwak BR, De Jonge HR, Lohmann SM, et al. Differential regulation of distinct types of gap junction channels by similar phosphorylating conditions. Mol Biol Cell. 1995;6(12):1707–1719.
Makowski L, Phillips WC, Goodenough DA. Gap junction structures. II. Analysis of the x-ray diffraction data. J Cell Biol. 1977;74(2):629–645.
Morley GE, Delmar M. Intramolecular interactions mediate pH regulation of connexin43 channels. Biophys J. 1996;70(3):1294–1302.
Nikolski VP, Lancaster MK, Boyett MR, et al. Cx43 and dual-pathway electrophysiology of the atrioventricular node and atrioventricular nodal reentry. Circ Res. 2003;92(4):469–475.
Trabka-Janik E, Lemanski LF, Delmar M, et al. Immunohistochemical localization of gap junction protein channels in hamster sinoatrial node in correlation with electrophysiologic mapping of the pacemaker region. J Cardiovasc Electrophysiol. 1994;5(2): 125–137.
QUESTIONS AND ANSWERS
Questions
1. Which of the following is not a characteristic of sodium channels?
a. Selective permeation
b. Pump electrolytes exchange mechanism
c. Gating
d. Drug binding
e. Susceptibility to many different neurotoxins
2. Which of the following is not a characteristic of Phase 1 of the action potential (AP)?
a. Phase 1 starts with the opening of a rapid outward K+ ion current called Ito.
b. These K+ channels activate in response to membrane depolarization.
c. These K+ channels inactivate in a time-dependent manner.
d. α-Adrenergic stimulation increases Ito maximum current.
e. Aldosterone mediates a receptor-specific downregulation of Ito.
3. Which of the following is not a characteristic of calcium channels?
a. During Phase 0, at the level of -40 mV, Ca2+ channels open, creating an inward Ca2+ current.
b. This current acts as an agonist to the outward K+ current.
c. Phase 2 concludes as Ca2+ current declines by inactivation of L-type Ca2+ channels.
d. Inactivation of the channel depends on time, membrane potential, and Ca concentration.
e. Intracellular Ca2+ concentration acts as a critical initiator of cardiac excitation–contraction coupling.
4. Which of the following is the main mechanism by which resting membrane potential (Phase 4 of the AP) is maintained?
a. Delayed rectifier K+ channels
b. Voltage-regulated inward rectifiers
c. These channels open in response to depolarization.
d. These channels open after reaching the resting membrane potential.
e. These potassium channels stabilize the resting membrane potential near the sodium equilibrium potential.
5. Which of the following statements about GAP junctions is wrong?
a. GAP junction channels are the functional units that allow direct ionic communication between cardiac cells.
b. GAP junctions play a major role in the propagation of the AP.
c. The molecular unit of the GAP junction is a protein called connexin.
d. Connexin 43 is the most abundant in cardiac conduction system cells.
e. Connexin 43 may be affected by the activity of protein kinase, low intracellular pH, and dephosphorylation.
Answers
1. Answer B: Sodium channels are characterized by a protein that works as a voltage-gated sodium channel. The active portion of this channel is the α subunit, which consists of a 2,000–amino acid glycoprotein. The other choices are all properties of these channels.
2. Answer D: Phase 1 starts with the opening of a rapid outward K+ ion current called Ito. This determines a fast early repolarization. These K+ channels activate in response to membrane depolarization and inactivate in a time-dependent manner. These channels may be regulated by the following means:
Angiotensin II reduces Ito maximum velocity.
α-Adrenergic stimulation reduces Ito fast velocity.
Hyperthyroidism increase Ito current density.
Aldosterone mediates a receptor-specific downregulation of Ito.
3. Answer B: During Phase 0, at the level of -40 mV, Ca2+ channels open, creating an inward Ca2+ current. This current act as an antagonist to the outward K+ current. Phase 2 concludes as Ca2+ current declines by inactivation of L-type Ca2+ channels, letting the plateau phase subside. These channels are also the critical initiators of cardiac excitation–contraction coupling through the initial increase in intracellular Ca2+ concentration that triggers the release of Ca2+ from the sarcoplasmic reticulum, which in turn provides a contraction signal to the contractile elements of the cell. Inactivation of the channel depends on time, membrane potential, and Ca concentration.
4. Answer B: The stabilization of resting membrane potential after the descending Phase 3 of the AP is achieved mainly by the action of the voltage-regulated inward rectifiers (IK1). These channels behave differently than the delayed rectifiers, which open in response to depolarization. The inward rectifier K+ channels are opened near resting membrane potential, stabilizing the resting membrane potential near the K+ equilibrium potential, but close in response to depolarization, facilitating the AP, hence the description as "inward rectifying."
5. Answer D: GAP junction channels are the functional units that allow direct ionic communication between cardiac cells and play a major role in the propagation of the AP from one cell to the next. The molecular unit of the GAP junction is a protein called Connexin. The oligomerization of six connexins forms a connexon, and two connexons form the final channel called the GAP junction. Among the 20 different subtypes of connexins identified in the human genome, connexin 43 is the most abundant in myocardial cells. Besides the obvious genomic regulation of the expression of these channels, they may also be affected by the activity of protein kinase activity, low intracellular pH, and dephosphorylation.