The rapid upstroke of the cardiac action potential (phase 0) is caused by the flow of a large inward Na⁺ current (I_Na) (Box 5-2). I_Na is activated by depolarization of the sarcolemma to a threshold potential of −65 to −70 mV. I_Na activation, and hence the action potential, is an all-or-nothing response. Subthreshold depolarizations have only local effects on the membrane. After the threshold for activation of fast Na⁺ channels is exceeded, Na⁺ channels open (i.e., I_Na activates) and Na⁺ ions enter the cell down their electrochemical gradient. This results in displacement of the membrane potential toward the equilibrium potential for Na⁺ ions, around +50 mV. I_Na activation is transient, lasting at most 1 to 2 ms because, simultaneous with activation, a second, slightly slower conformational change in the channel molecule occurs (inactivation), which closes the ion pore in the face of continued membrane depolarization. The channel cannot open again until it has recovered from inactivation (i.e., regained its resting conformation), a process that requires repolarization to the resting potential for a defined period of time. The channels cycle through three states: resting (and available for activation), open, and inactivated. While the channel is inactivated, it is absolutely refractory to repeated stimulation.

BOX 5-2 Cardiac Action Potential

• Phase 0 (rapid upstroke): primarily Na⁺ channel opening

• Phase 1 (early rapid repolarization): inactivation of Na⁺ current, opening of K⁺ channels

• Phase 2 (plateau phase): balance between K⁺ currents and Ca²⁺ currents

• Phase 3 (final rapid repolarizations): activation of Ca²⁺ channels

• Phase 4 (diastolic depolarization): balance between Na⁺ and K⁺ currents

Molecular Biology of Ion Channels

The preceding sections have focused on the electrical events that underlie cardiac electrical excitability and on the identification of cardiac ionic currents on the basis of their biophysical properties. Subsequent sections examine the molecular physiology of these electrical phenomena. The first step in understanding the molecular physiology of cardiac electrical excitability is to identify the ion channel proteins responsible for the ionic currents.

Ion Channel Pore and Selectivity Filter

The presence of four homologous domains in voltage-gated Na⁺ and Ca²⁺ channels suggests that basic ion channel architecture consists of a transmembrane pore surrounded by the four homologous domains arranged symmetrically (Fig. 5-1).

Figure 5-1 Diagrams of ion channel molecular structure. A, Na⁺ channel. B, Ca²⁺ channel. C, K⁺ channels. ATP = adenosine triphosphate. For further discussion, see text.

Clinical Correlates

Ion Channels and Antiarrhythmic Drugs

The prototype antiarrhythmic agents, such as disopyramide and quinidine, have diverse effects on cardiac excitability, and these, along with agents introduced more recently, frequently exhibit significant proarrhythmia with potentially fatal consequences. In the Cardiac Arrhythmia Suppression Trial (CAST), mortality among asymptomatic post–myocardial infarction patients was approximately doubled by treatment with the potent Na⁺ channel-blocking agents encainide and flecainide, an effect that is likely attributable to slowing of conduction velocity with a consequent increase in fatal reentrant arrhythmias.^2,³ The only drugs currently available that definitely prolong life by reducing fatal arrhythmias are β-blockers, and these agents have no channel-blocking effects.

Ion Channels in Disease

Elucidation of the molecular mechanisms of the cardiac action potential is beginning to directly affect patient management, particularly in patients with inherited genetic abnormalities of ion channels leading to cardiac sudden death. Two groups of diseases serve to illustrate this point—the LQT syndrome and the Brugada syndrome. An understanding of the molecular mechanism of cardiac electrical excitability is also leading to the emergence of gene therapies and stem cell therapies that may in the future allow manipulation of cardiac rhythm and function.^4,⁵

CONTROLLING CARDIAC FUNCTIONING: RECEPTORS

Receptors are membrane proteins that transduce signals from the outside to the inside of the cell. When a ligand—a hormone carried in blood, a neurotransmitter released from a nerve ending, or a local messenger released from neighboring cells—binds to the receptor, it induces a conformational change in the receptor molecule. The configuration of the intracellular segment of the receptor changes and results in activation of intracellular systems, with a variety of effects, ranging from enhanced phosphorylation and changes in intracellular (second) messenger concentrations to activation of ion channels.

Receptors

Receptors are grouped into several broad classes, the protein tyrosine kinase receptors and the G protein–coupled receptors (GPCRs) being the most important ones. The protein tyrosine kinase receptors are large molecular complexes. Ligand binding induces activation of a phosphorylating enzyme activity in the intracellular segment of the molecule. Because phosphorylation is one of the major mechanisms of cellular regulation, such receptors can have a variety of cellular effects (Box 5-3). In contrast, GPCRs are much smaller. Ligand binding results in activation of an associated protein (G protein) that subsequently influences cellular processes.

The heart and blood vessels express a variety of GPCRs. The β-adrenergic and muscarinic acetylcholine receptors are those most important for regulation of cardiac functioning, but a number of others play relevant modulatory roles. These include the α-adrenergic, adenosine A₁, adenosine triphosphate (ATP), histamine-2 (H₂), vasoactive intestinal peptide (VIP), and angiotensin II receptors (Fig. 5-2).

Figure 5-2 Model of G protein–coupled receptor. A, Linear model. Seven hydrophobic stretches of approximately 20 amino acids are present, presumably forming α helices that pass through the cell membrane, thus forming seven-transmembrane domains (t1 through t7). Extracellularly the N terminus (N) and three outside loops (o1 through o3) are found; intracellularly there are similarly three loops (i1 through i3) and the C terminus (C). B, Top-down view. Although in A the molecule is pictured as a linear complex, the transmembrane domains are thought to be in close proximity, forming an ellipse with a central ligand-binding cavity (indicated by a dashed circle). Asp and Tyr refer to two amino acids important for ligand interaction. G protein binding takes place at the i3 loop and the C terminus.

Adrenergic Receptors and Signaling Pathways

Adrenergic Receptors

Main control over cardiac contractility is provided by the β-adrenergic signaling pathways, which can be activated by circulating catecholamines or those released locally from adrenergic nerve endings on the myocardium.

The two main subtypes of β-adrenergic receptors are the β₁ and β₂ subclasses. A β₃ subtype exists as well, but its role in the cardiovascular system is unclear; its most important role is in fat cells. Both β₁ and β₂ receptors are present in heart, and both contribute to the increased contractility induced by catecholamine stimulation (this is different from the situation in vascular muscle, where β-adrenergic stimulation induces relaxation). Under normal conditions, the relative ratio of β₁ to β₂ receptors in heart is approximately 70:30, but, as discussed later, this ratio can be changed dramatically by cardiac disease.

Structurally, as well as functionally, the various β-adrenergic receptors are closely related. Both couple to G_s proteins and thereby activate adenylate cyclase, leading to increased intracellular levels of cyclic adenosine monophosphate (cAMP). There may, however, be differences in some details of their intracellular signaling. For example, it has been suggested that β₂ receptors couple more effectively than β₁ receptors and induce greater changes in cAMP levels. In addition to their effect on cAMP signaling, β-adrenergic receptors may couple to myocardial Ca²⁺ channels.

The inotropic and electrophysiologic effects of β-adrenergic signaling are an indirect result of increases in intracellular cAMP levels. cAMP activates a specific protein kinase (PKA) that in turn is able to phosphorylate a number of important cardiac ion channels (including L-type Ca²⁺ channels, Na⁺ channels, voltage-dependent K⁺ channels, and Cl⁻ channels). Phosphorylation alters channel functioning, and it is these changes in membrane electrophysiologic events that modify myocardial behavior.

The α-adrenergic receptors, like their β-receptor counterparts, can be divided into several molecular groups: the α₁– and α₂-receptors. Both of these groups consist of several closely related subtypes, with different tissue distributions and functions that are as yet not very well differentiated. In general, α₁-receptors couple to G_q proteins, thereby activating phospholipase C, which increases intracellular Ca²⁺ concentrations. α₂-Receptors couple to G_i, which inhibits adenylate cyclase, thereby lowering intracellular cAMP concentrations.

Regulation of β-Receptor Functioning

Although β-receptor stimulation allows the dramatic increases in cardiac output of which the human heart is capable, it is clearly intended to be a temporary measure. Prolonged adrenergic stimulation has highly detrimental effects on the myocardium: The pronounced increases in cAMP levels are followed by increases in intracellular Ca²⁺ concentration, reductions in RNA and protein synthesis, and finally cell death. Thus, β-receptor modulation is best viewed as part of the “fight or flight” response—beneficial in the short term but detrimental if depended on too long. Cardiac failure, in particular, has been shown to be associated with prolonged increases in adrenergic stimulation, even to the extent that norepinephrine “spillover” from cardiac nerve endings can be detected in the blood of patients in heart failure.⁶

One mechanism for decreasing β-receptor functioning is the downregulation (i.e., decrease in density) of receptors. In cardiac failure, receptor levels are reduced up to 50%. β₁-Receptors downregulate more than do β₂-receptors, resulting in a change in the β₁:β₂ ratio. As mentioned earlier, the normal ratio is approximately 70:30; in the failing heart, it is approximately 3:2. Various molecular mechanisms exist for this downregulation. Some of them, particularly in the longer term, are degradation and permanent removal of receptors from the cell surface. In the short term, receptors can be temporarily removed from the cell membrane and “stored” in intracellular vesicles, where they are not accessible to agonists. These receptors are, however, fully functional and can be recycled to the membrane when adrenergic overstimulation has ceased.⁷

Muscarinic Receptors and Signaling Pathways

Muscarinic Acetylcholine Receptors

The second major receptor type involved in cardiac regulation is the muscarinic receptor. Although five subtypes of muscarinic receptors exist, only one of these (m2) is present in cardiac tissue. Most of these muscarinic receptors are present on the atria. Indeed, it was thought until recently that there was no vagal innervation of the ventricles, but this view turns out to be incorrect. The ventricles are innervated by the vagus, and muscarinic receptors are, in fact, present in the ventricles, albeit at lower concentrations than in the atria; the amount of muscarinic receptor protein in atrium is approximately twofold greater than in ventricle (200 to 250 vs. 70 to 100 fmol/mg protein). Thus, although the primary function of cardiac muscarinic signaling is heart rate control through actions at the atrial level, vagal stimulation is, in fact, able to directly influence ventricular functioning.⁸

Clinical Correlate: Adenosine Signaling and Cardiac Function

Understanding of the role of adenosine in cardiac regulation has expanded significantly over the past years. Its established use as an antiarrhythmic compound and its probable role in cardiac preconditioning are two examples of clinical advances resulting from this increase in understanding. Adenosine acts through a GPCR, activating several intracellular signaling systems.⁹^–¹¹

Adenosine Signaling

Although adenosine can be generated by several pathways, in the heart it is usually found as a dephosphorylation product of AMP.¹² Because AMP accumulation is a sign of a low cellular energy charge, an increased adenosine concentration is a marker of unbalanced energy demand and supply; thus, ischemia, hypoxemia, and increased catecholamine concentrations are all associated with increased adenosine release. Adenosine is rapidly degraded by various pathways, both intracellularly and extracellularly. As a result, its half-life is extremely short, on the order of 1 second. Therefore, it is not only a marker of a cardiac “energy crisis” but its concentrations will fluctuate virtually instantly with the energy balance of the heart; it provides a real-time indication of the cellular energy situation.

Adenosine signals through GPCRs of the purinergic receptor family. Two subclasses of purinoceptors exist: P₁ (high affinity for adenosine and AMP) and P₂ (high affinity for ATP and ADP). The P₁-receptor class can be divided into (at least) two receptor subtypes: A₁ and A₂. A₁-receptors are present mostly in the heart, and, when activated, inhibit adenylate cyclase; A₂-receptors are present in the vasculature and, when activated, stimulate adenylate cyclase. The A₂-receptors mediate the vasodilatory actions of adenosine. The A₁-receptors mediate its complex cardiac effects.

Antiarrhythmic Actions of Adenosine

From these molecular actions of adenosine, its clinical effects can easily be deduced. The antiarrhythmic actions are largely a result of its activation of K_ACh. Remembering the tissue distribution of K_ACh, it could be anticipated that adenosine will be much more effective in the treatment of supraventricular arrhythmias than ventricular arrhythmias, and such is indeed the case. Because of its negative chronotropic effects on the atrial conduction system, the compound is most effective in treating supraventricular tachycardias that contain a reentrant pathway involving the atrioventricular node. The efficacy of adenosine in terminating such tachycardias has been reported as greater than 90%. In contrast, it is consistently ineffective in tachycardias not involving the atrioventricular node.^13,¹⁴

ANESTHETIC ACTIONS

Interactions with Channels: Ca²⁺ Channels

Of the variety of ion channels present in the heart, those most likely to be significantly affected by anesthetics in the clinical setting are the voltage-gated Ca²⁺ channels.

Anesthetic actions on cardiac Ca²⁺ channels have been studied in a variety of models. The original observations that halothane blocked Ca²⁺ flux into heart cells date back to the 1970s, and much specific information has been gained since then.¹⁵

Almost all volatile anesthetics inhibit L-type Ca²⁺ channels.¹⁶ Inhibition is modest (25% to 30% at 1 MAC anesthetic) but certainly sufficient to account for the physiologic changes induced by the anesthetics. Volatile anesthetics decrease peak current and, in addition, tend to increase the rate of inactivation. Hence, maximal Ca²⁺ current is depressed and duration of Ca²⁺ current is shortened. Together, these actions significantly limit the Ca²⁺ influx into the cardiac myocyte.

Not only volatile, but also injected, anesthetics have been reported to inhibit cardiac L-type Ca²⁺ channels in some models. However, the concentrations used generally exceed those used in clinical practice. Thiopental and methohexital block L-type Ca²⁺ currents. Similarly, propofol has been reported to inhibit these channels, but at concentrations well beyond the clinical range.¹⁷