Pharmacological Regulation of Cardiac Rate + Rhythm 2024-notes.pdf

Pharmacological Regulation of Cardiac Rate & Rhythm Rodrigo Andrade, 3108 Scott Hall. October 2022 Objectives To develop a broad understanding of the ion channels important for cardiac physiology and pharmacology. To understand the mechanisms by which parasympathetic a...

Pharmacological Regulation of Cardiac Rate & Rhythm Rodrigo Andrade, 3108 Scott Hall. October 2022 Objectives To develop a broad understanding of the ion channels important for cardiac physiology and pharmacology. To understand the mechanisms by which parasympathetic and sympathetic activation regulates heart rate. To develop an understanding of adrenergic receptors, their coupling mechanisms and pharmacology. To understand the physiology of sodium channels and prototypical sodium channel blockers/antiarrhythmics. To understand the physiology of the potassium channels responsible for the repolarization of the action potential and their pharmacological regulation. Lecture Outline (lecture timing marks) Introduction (0:00) I. Ion channels in cardiac Physiology (03:02) II. Physiological and pharmacological mechanisms underlying the parasympathetic slowing of heart rate (16:16) III. Physiological and pharmacological mechanisms underlying the sympathetic acceleration of heart rate (20:00) IV. Sodium channels, calcium channels and the cardiac action potential (31:00) V. Potassium channels and the cardiac action potential (35:55) VI. Pharmacological regulation of Na+ Channels (38:00) VII. Potassium channel blockers (49:02) VIII. End (55:39) Introduction Previous lectures have introduced you to cardiac electrophysiology. In this lecture I would like to expand on this material to address the pharmacological avenues available for regulating the electrical properties of the heart. These drugs are mostly used in the treatment of arrhythmias, pathological abnormalities in the rhythmic firing of the heart. We can classify arrhythmias into two main classes, one class involving abnormalities in impulse initiation and a second class involving abnormalities of impulse conduction (Fig. 1). These two classes include mechanistically diverse disorders but their underlying pathophysiology reflects disturbances in the electrical activity of the heart. This offers the possibility of treating these conditions using drugs that target the ion channels responsible for the electrical activity of the heart. Figure 1. Two main classes of arrhythmias I. Ion channels in cardiac physiology Cardiac physiology is traditionally approached in terms of action potentials and the underlying ionic currents (e.g. IK1, ICa,L, etc, Fig. 2). The elucidation of mammalian genomes has allowed us to link specific ion channel genes and proteins to these physiologically defined macroscopic currents. This is a useful complementary approach since it allows for a mechanistic understanding of the functional properties of the currents, their regulation and pharmacology. Equally important, they allow us to link mutations in specific genes and the resulting cardiac pathophysiology.  Figure 2. The cardiac action potential and underlying ion currents Ion channels are intrinsic membrane proteins that can be thought as membrane pores that allow for the fluxes of ions across the membrane. Ion channels exhibit two important properties. First, they can assume either an open, conducting, state or a closed, non- conducting, state, and the transition between these two states can be regulated. Thus, for example, some ion channels open (or close) in response to changes in membrane voltage while others open (or close) in response to changes in the concentration of intracellular or extracellular ligands, or even membrane distortion (Fig. 3). Ion channels can also exhibit ionic selectivity, they may allow the selective permeability of some ions while excluding others. For example, potassium channels will allow permeation of potassium ions while overwhelmingly excluding sodium and other cations and anions. Cation nonselective channels may allow only monovalent cations while excluding calcium and magnesium as well as anions (Fig. 3). Ion channels are highly diverse at the molecular level. Most plasma membrane channels important to cardiac electrophysiology belong to the voltage gated superfamily of ion channels (even as some channels in this family are not voltage-dependent! (Fig. 3). Thanks to the work by Rod McKinnon and others (2003 Nobel Prize) we know that Ion channels in this family are tetramers comprised by modular subunits defined by the presence of two transmembrane helices that form the walls of the Figure 3. Ion channel can open and close and exhibit channel. These two helices are inked by a protein selective ionic permeability loop that combine to create the selectivity filter, a narrowing of the channel that that allows for the selective permeation of sodium, potassium, or calcium. The four pairs of helices form an v “ ” apex in the closed state of the channel. The channel opens when these helices splay thus allowing for ions to permeate through the pore. As you will see, this structure provides a broad canvas for mutations and disease states. Figure 4. The voltage-dependent cation channel superfamily The mammalian genome contains many dozens of genes encoding for these subunits (or in the case of sodium and calcium channels, concatemers of these subunits). These pore forming subunits are known as alpha subunits, and often associate with auxiliary subunits which fine tune the functional properties of the fully assembled channel. The simplest alpha subunits in this ion channel superfamily are made of two transmembrane helices connected by the selectivity filter loop (Fig. 4). Such subunits assemble into potassium channels known as Kir that ’. Additionally, many alpha subunits in this superfamily are made up of 2+4 helices, the two helices forming the walls of the channel plus 4 additional helices that form a voltage sensor. As illustrated in figure 5, these four additional helices form a voltage sensor protein module whose position responds to the electric field across the membrane. This allows the channels to open in response to changes in the voltage across the membrane, in other words to become voltage-sensitive (voltage- dependent). Many of these channels and their regulatory mechanisms “ ” represent important pharmacotherapeutic targets for the treatment of cardiovascular diseases. Figure 5 Membrane topology and gating mechanism of voltage-dependent cation channels. A. membrane topology of a cation channel subunit. B. Sodium and calcium channels are made up of four concatenated subunits (domains I-IV). II. Physiological and pharmacological mechanisms underlying the parasympathetic slowing of heart rate The ion channels assembled from “ w domains (i.e., the Kir channels) play an important role in cardiac physiology. These channels are tetramers assembled from over a dozen subunits and fall into four main functional classes, three of which are expressed in the heart and help to establish the cardiac myocites membrane potential (Fig. 6). All these channels allow for the selective permeation of potassium across the membrane and exhibit the unusual property of allowing potassium to cross the membrane in the inward direction more readily than in the outward direction (Fig. 6). This property is known as inward rectification and results from channel permeation being blocked from the inside by intracellular magnesium or polyamines. Inward rectification is a very useful property in the heart since it results in a reduction in potassium conductance during the comparatively long cardiac action potential.  v Figure 6. inwardly rectifying potassium channels. You have already heard about some of the functions carried through by different Kir channels in the heart. For example, Kir2.1 appears to be indispensable for the formation of the ion channels carrying IK1 as deletion of the corresponding gene (KCNJ2) leads to the complete loss of this current. Thus Kir2.1 is an important contributor to the resting membrane permeability responsible for maintaining a negative membrane potential in cardiac myocites between beats. At a clinical level, mutations in this gene results in Andersen-Tawil syndrome (type 1) a rare (1 in 1 million) disorder characterized by episodic paralysis, arrhythmias, and developmental abnormalities. Most importantly for the purposes of this class, Kir3 channels are the final effector in the signaling mechanism that underlies the ability of acetylcholine to slow the heart during parasympathetic stimulation. The heart, and especially the SA node and atria, receive a strong parasympathetic innervation through the vagus nerve which releases acetylcholine to activate muscarinic receptors of the M2 subtype (Fig. 7). These receptors couple to G proteins of the Gi subtype and, when activated, trigger the dissociation of the heterotrimeric G protein. This results in the freeing of G beta-gamma which then binds and activates the Kir3 channels to produce an increase in potassium conductance. The resulting current, generally referred to as IKAch, contributes to the slowing of the heart rate and shortens the duration of the cardiac action potential in the atria. This mechanism is antagonized by muscarinic receptor blockers such as atropine or scopolamine and is facilitated by cholinesterase inhibitors such as physostigmine. This mechanism also form the basis for the cardiac adverse effects of many drugs exhibiting antimuscarinic activity (e.g. ampicillin, clozapine).  Figure 7. Parasympathetic regulation of Kir3 potassium channels in the heart. This basic mechanism can be exploited for therapeutics. The heart also expresses A1 adenosine receptors that, like muscarinic M2 receptors, couple to heterotrimeric G proteins of the Gi subtype and activate Kir3 channels. This forms the basis for the use of adenosine for the acute termination of reentrant supraventricular arrhythmias in the Emergency Room. In this case, intravenous injection of adenosine produces a brief (seconds) asystole thus terminating the fibrillation. This action rapidly subsides through the dilution, uptake and elimination of the adenosine. III. Physiological and pharmacological mechanisms underlying the Sympathetic acceleration of heart rate In the healthy heart action potentials are initiated in the sinoatrial (SA) node and propagate to the atria and the ventricle. Consequently, the beating of the heart is controlled by the activity of the SA node that acts as a beating pacemaker. The autonomous firing of SA node cells is supported by the presence (and absence vis a vis other cardiac myocytes) of a set of voltage- Figure 8. HCN channels carry If, the pacemaker current in the SA node dependent ion channels whose combined activity generates the autonomous pacemaker of the SA node. Prominent among these are HCN (Hyperpolarization-activated Cation Nonselective) channels that carry a current known in the heart as If “ ” HCN channels are encoded by 4 different genes which code for ion channel subunits conferring slightly different functional properties (activation kinetics, voltage dependence, etc). The human heart expresses predominantly HCN4. HCN channels are cation non-selective channels (i.e. they permeate sodium and potassium) that are closed near rest and activate slowly upon hyperpolarization (Fig. 8). Consequently, they are activated during the hyperpolarization following each action potential in the SA node. The resulting inward current (sodium influx>>potassium efflux) is a major contributor to the slow diastolic depolarization. Sympathetic activation increases heart rate (chronotropic effect) and the force of contraction (inotropic effect) through a variety of interacting mechanisms (see below). The chronotropic effect is mediated at least in part by an effect on HCN channels. To understand how this happens mechanistically it is helpful to step back and consider sympathetic signaling in a broader context. As we saw during the autonomic pharmacology lectures in the foundations module, sympathetic stimulation is mediated by the release of norepinephrine and epinephrine that act on three receptor subtypes Alpha1 (α1 , 2 α2 β ptors which couple to different heterotrimeric G protein classes to activate different intracellular signaling cascades. Specifically, the three β -adrenergic receptor subtypes all couple to Gs to activate adenylate cyclase leading to a rise in intracellular cAMP which in turn activates protein kinase A (Fig. 9). Protein kinase A phosphorylates a variety of target and is the principal intracellular effector of this signaling cascade. Figure 9. β-adrenergic receptor signaling The heart expresses 1- and 2- adrenergic receptors with the 1 receptors being overall several fold more abundant than the 2 receptors. Sympathetic activation of these receptors leads to an increase in the activity of adenylate cyclase and a rise in intracellular cAMP. HCN channel subunits contain a cyclic nucleotide binding domain that, when bound to cAMP, undergoes a rightward shift in their voltage dependence. This results in a larger inward (depolarizing) current at any given voltage and hence an increase in the slope of the diastolic depolarization and an increase in heart rate (Fig. 10). This process is also thought to work in reverse. Parasympathetic stimulation releases acetylcholine that binds M2 muscarinic receptors and triggers the activation of the heterotrimeric G protein Gi. The resulting inhibition of adenylate cyclase elicits a reduction of intracellular cAMP, which in turn causes a leftward shift in the voltage-dependence of If. The combined effect of M2 receptors triggering an increase in IAch and the shift in the voltage dependence of If are thought to be the main determinants of the parasympathetic slowing of heart rate. The sympathetic regulation of the heart offers important avenues for the regulation of cardiovascular function. Nonselective -adrenergic agonists such as isoproterenol (isoprenaline) will result in an increase in heart rate and, as we will see during our next class, an increase in the strength of contraction. Thus, administration of beta agonists can be used to increase the work of the heart. An important limitation of such an approach is that - adrenergic receptors are widely distributed in the body, which increases the chances of the drug eliciting adverse effects. A better approach to accomplish the same therapeutic goals would be “ v ” agonist such as the preferential 1 adrenergic selective Figure 10. Sympathetic regulation of HCN channels and heart rate. agonist dobutamine, a drug that is used in the treatment of acute heart failure. Figure 11.Prototypical adrenergic drugs In terms of therapeutics, even more important than beta agonists are the -adrenergic receptor antagonists (aka “ blockers” , which exert their therapeutic effects by antagonizing the sympathetic tone on the heart. These drugs reduce heart rate and the strength of contraction and thus reduce cardiac output. Consequently, they are widely used in the treatment of a variety of cardiovascular conditions including the treatment of exertional angina, cardiac heart disease, and hypertension. They are also used to treat supraventricular tachycardias, including sinus tachycardia, AV nodal reentry, atrial fibrillation, and atrial flutter. The classic beta blocker is propranolol, which antagonizes both 1 and 2 receptors. Other nonselective beta blocker included carvedilol, labetalol and timolol. However, for the control of heart function, selective 1 blockers such as atenolol, metoprolol or the short acting esmolol are preferred (Fig. 11). As we saw in the foundation lectures, an alternative approach to reducing ongoing sympathetic stimulation of the heart would be to simply block the release of epinephrine/norepinephrine. Drugs such as Reserpine (Fig. 12) and guanethidine interfere with the storage of catecholamines and thus reduce or eliminate their release. Reserpine inhibits VMAT2, the vesicular Figure 12. Mechanism of action of reserpine monoamine transporter essential for the storing of and guanethidine monoamines into synaptic vesicles. Guanethidine has a more complex mechanism of action but essentially displaces and replaces catecholamines within synaptic vesicles. The net result is that these drugs reduce the catecholamine content within synaptic vesicles and hence decrease the ability of sympathetic neurons to secrete them. These drugs are seldom used in the clinic nowadays but are worth knowing for their unique mechanism of action. Drugs targeting the sympathetic system to treat arrhythmias are referred to as Class II antiarrhythmics. Finally, given the fundamental role of If in the regulation of heart rate HCN channels would appear to offer a potential avenue for the selective control of heart rate. Work along these lines led to the development of ivabradine, a selective inhibitor of HCN channels that can selectively inhibit heart rate (Fig. 13). Ivabradine is currently approved in the US for use in patients with stable heart failure and in normal sinus rhythm (with a HR at least 70 bpm) to reduce heart rate and thus myocardial oxygen consumption. Figure 13.Ivabradine blocks HCN channels and slows heart rate. IV. Sodium Channels, calcium channels and the cardiac action potential Sodium and calcium channels play essential roles in the generation of the cardiac action potential. These channels are unique among voltage activated channels in that they are not assembled from four separate subunits but rather the 4 subunits that would normally co- assemble to form the channel have become fused into a single gene (and protein) coding for the entire channel (Fig 14). Mammals express 9-10 different voltage -dependent sodium channels (NaVs), mostly in neurons on the central and peripheral nervous system. The voltage- dependent sodium current in the heart is carried principally by NaV1.5. Mammals also express 10 different voltage-dependent calcium channels (Fig 14). Calcium channels fall into two main classes, high-voltage activated (HVA) and low voltage activated L , “ ” e) depending on their voltage sensitivity. HVA calcium channels fall into two main subclasses, the L type calcium channels that are sensitive to dihydropyridines, and the N-P/Q type channels. In terms of cardiac function, we will be primarily concerned with L type calcium channels and secondarily with T type calcium channels. Figure 14. Voltage-dependent sodium and calcium channels The ionic mechanisms underlying action potentials differ in the different regions of the heart. The most significant difference is between action potentials generated by the cells in the core regions of the SA and AV nodes, that are mediated solely by calcium channels, and in the rest of the heart, that involve both sodium channels and calcium channels. In the SA node, as the membrane depolarizes during the diastolic depolarization (as a result of If), calcium channels of the LVA type (T Type) begin to open. This results in a continuation of the diastolic depolarization even as If begins to close. The membrane potential eventually reaches the range of activation for the abundant L type calcium channels which begin to open and produce the upstroke of the SA (and AV) node action potential (Fig. 15). As we move from the core of the SA (and AV) node towards non-nodal tissue, the myocytes begin to express NaV1.5 sodium channels. Current flow from the SA nodes leads to depolarization of the periphery of the SA and AV nodes and the opening of the NaV1.5 sodium channels assumes the principal role carrying the inward current responsible for the upstroke of the action potential (phase 0). The large, fast currents carried by NaV channels allow for the fast rise of the atrial and ventricular action potential and contribute to their robust spread. Importantly, while L type calcium channels now begin to play a less important role in the rise of the action potential (phase 0) they nevertheless open during the action potential and are the primary contributors to the inward current responsible for phase 2. As we will see next class, calcium influx through L type calcium channels during this phase plays an essential role in excitation contraction coupling and hence the beating of the heart. Drugs targeting calcium channels (which we will cover next class) are known as Class IV antiarrhythmics. Figure 15. Contribution of calcium and sodium currents to the generation of the action potential. V. Potassium channels shape the cardiac action potential L type calcium channels, which are principally responsible for Phase 2 of the action potential, show limited inactivation. Thus, potassium channels play an important role in the repolarization of the cardiac action potential. Several voltage-dependent K channels (from multiple families) contribute to the cardiac action potential, with their quantitative roles showing considerable regional variability. The initial phase of repolarization (phase 1) results from the inactivation of NaV channels and the activation repolarization of the of transient (i.e. inactivating) potassium currents, mostly IKto. The second phase of repolarization, that terminates the action potential plateau (phase 2) results from the activation of the slower, non-inactivating potassium currents, mostly IKr and IKs. These currents are carried by channels made by Kv11 (hERG) and Kv7 (KvLQT  “M” channels) (Fig. 16). This is important because mutations in each of these genes give rise to specific pathologies while the differences in the channel proteins account for the ability of pharmacological agents to differentially target these currents. Figure 16. Main potassium currents contributing to action potential repolarization in the heart. VI. Pharmacological regulation of Na+ Channels A variety of drugs block sodium channels in the heart. These drugs are often used to treat arrhythmias and thus are often referred to as Class I antiarrhythmics. By reducing the sodium current these drugs slow conduction in the heart and can change the duration of the action potential and hence the effective refractory period. A key feature of NaV sodium channels is that they open in a voltage dependent manner and then inactivate (Fig. 17). In the inactive state the channels do not permeate sodium and hence they cannot contribute to the generation of action potentials. This accounts for the refractory period. The inactive state of the sodium channel is different from the closed state. The sodium channel opens when the helices 5 and 6 of the tandem domain (I-IV) spay in response to a membrane depolarization. This creates a path for sodium permeation through the channel. The splaying of the helices however exposes a set of binding sites for the intracellular look linking tandem domains III and IV w , , “ ” channel. “ ” v Thus, while the inactivated NaV channel is technically speaking in the active state conformation the channel is nevertheless non-conducting.  Figure 17. Sodium channel inactivation This is important because NaV channel blocker antiarrhythmics such as such as lidocaine, procainamide, disopyramide, quinidine and flecainide inhibit sodium channels by binding to the open or the inactivated state of the receptor to block sodium fluxes through the channel. These drugs differ however in their NaV binding properties. Based upon these differential effects and how they affect the cardiac action potential, class I antiarrhythmics are subdivided into three subclasses (Class IA, IB, and IC; Fig 18). Figure 18. Effects of antiarrhythmics on the cardiac action potential Class IA drugs such as quinidine, procainamide and disopyramide bind relatively tightly to (hence dissociate relatively slowly from) the sodium channel and also inhibit potassium channels. Consequently, they reduce the rate of rise of the action potential and increase its duration (Fig 18). These drugs are used to treat re-entry arrhythmias, such as supraventricular and ventricular tachycardias and atrial & ventricular premature beats Class IB drugs such as lidocaine and mexiletine are use-dependent sodium channels blockers that reduce the rate of rise of the action potential that generally reduce the duration of the action potential. As a result of their rapid kinetics of binding/unbinding they preferentially target depolarized or ischemic tissue where sodium channels can be expected to spend longer in the open or inactivated state. These drugs are used in the treatment of ventricular fibrillation or pulseless ventricular tachycardia. Finally, Class IC drugs such as flecainamide and propafenone binds relatively tightly (dissociate slowly) to the sodium channel and inhibit the rate of rise of the action potential, but do not change its duration. These drugs are used for the restoration and maintenance of sinus rhythm in patients with structurally normal hearts who have atrial fibrillation. Flecainamide is a selective inhibitor of NaV1.5. Sodium channels inactivate rapidly and thus do not contribute to the duration of the cardiac action potential. However, there is a small residual sodium current that persists during the action potential, in part due to incomplete sodium channel inactivation (Fig 19). This current i w “ ” , , contributes significantly to the sodium loading (and indirectly calcium loading, see below) of the myocytes and thus also to the development of afterdepolarizations. Ranolazine and amiodarone are preferential inhibitors of the “ ” Ranolazine has been approved by the FDA for the treatment of angina. Amiodarone has a variety of targets besides the persistent sodium channel and is used in the treatment Figure 19. The late sodium current of arrhythmias. VII. Potassium channel blockers As outlined above voltage-dependent potassium channels play a key role in the repolarization of the cardiac action potential. Thus, potassium channels are an effective pharmacological target (Class III antiarrhythmics). Potassium channel blockade also plays a significant role mediating adverse effect of a variety of therapeutic compounds and underlies the pathophysiology of several channelopathies (pathological conditions emerging from mutations in ion channels) (Fig 20). Figure 20. Potassium channels and the duration of the action potential. Potassium channel blockers like ibutilide and dofetilide which block hERG channels (IKr), and amiodorone, that blocks several channels including Kv7.1/KCNE1 channels (IKs) increase the duration of the action potential and hence the effective refractory period. This feature makes these drugs useful in the treatment of arrhythmias including reentrant supraventricular tachycardia and ventricular tachycardia. A limitation of these drugs is that the elongation of the action potential is itself pro-arrhythmic since it can lead to the generation of early afterdepolarizations (EADs) and result in ventricular tachycardia/fibrillations (torsades de pointes) (Fig 21). The lengthening of the action potential, through mutations in either hERG, Kv7.1 or NaV5, also underlies the pathophysiology of genetic Long QT syndromes. The hERG v q “v ”w susceptible to pharmacological blockade. This may account for the QT prolonging effect of many drugs (>200 as per most recent survey) including terfenadine (an antihistamine), grepofloxacin (an antibiotic), cisapride (a GI prokinetic agent) and several antipsychotic. Figure 21 Delayed afterdepolarization and early afterdepolarization