Cardiac Potassium Channels: Physiological Insights for Targeted Therapy

Abstract

The development of novel drugs specifically directed at the ion channels underlying particular features of cardiac action potential (AP) initiation, recovery, and refractoriness would contribute to an optimized approach to antiarrhythmic therapy that minimizes potential cardiac and extracardiac toxicity. Of these, K⁺ channels contribute numerous and diverse currents with specific actions on different phases in the time course of AP repolarization. These features and their site-specific distribution make particular K⁺ channel types attractive therapeutic targets for the development of pharmacological agents attempting antiarrhythmic therapy in conditions such as atrial fibrillation. However, progress in the development of such temporally and spatially selective antiarrhythmic drugs against particular ion channels has been relatively limited, particularly in view of our incomplete understanding of the complex physiological roles and interactions of the various ionic currents. This review summarizes the physiological properties of the main cardiac potassium channels and the way in which they modulate cardiac electrical activity and then critiques a number of available potential antiarrhythmic drugs directed at them.

Introduction

Orderly propagation of cardiac electrophysiological excitation and recovery depends on a normal sequence of cardiac action potential (AP) generation through its component myocytes. The depolarization and repolarization of AP is mediated by multiple, interacting, inward and outward currents mediated by different ion charge carriers dependent on the action of specific membrane ion channels (Figure 1). The initial depolarization phase takes the form of a rapid upstroke and is mainly driven by inward Na⁺ current (I_Na) through voltage-gated sodium channels (Na_v1.5). The succeeding plateau phase is dominated by inward Ca²⁺ current (I_Ca). The resulting entry of extracellular Ca²⁺ induces release of sarcoplasmic reticular Ca²⁺ stores, thereby activating excitation–contraction coupling. Repolarization, ultimately returning the membrane to the resting potential, is principally driven by outward current through voltage-gated K⁺ channels (K_v).¹ K⁺ channel activity is thus a principal determinant of AP duration (APD) as it limits the depolarization duration and therefore both the time course of the Ca²⁺-mediated contraction and the refractory period. There are numerous and diverse K⁺ channels types, each with particular kinetic and voltage-dependent properties. These result in numerous and diverse current contributions, each with specific roles at different phases of repolarization. Together these determine the relatively prolonged but finely tuned repolarization time course and the repolarization reserve following recovery of the resting membrane potential. The repolarization reserve refers to the partly overlapping function of these currents, namely, I_Kr, I_Ks, and I_K1, that gives a limited level of redundancy to the system.² The kinetics of repolarization varies greatly with cardiac region and species. This reflects variations in the occurrence and density of the different K⁺ channel subtypes. All these characteristics suggest that explorations of K⁺ channels may yield a useful group of pharmacological targets for arrhythmic conditions.

Figure 1.

The ventricular action potential as a paradigm for cardiac electrophysiological activity. In the resting state, the voltage of the cell intracellular space is negative to the external environment. This reflects its higher K⁺ but lower Na⁺ and Ca²⁺ concentrations and its lower membrane permeability to Na⁺ and Ca²⁺ in comparison to K⁺. K⁺ efflux from the cell is then controlled by the inward rectifier K⁺ channel (I_K1). When excitation threshold is reached, a large Na⁺ influx (I_Na) into the cell through Na⁺ channels produces phase 0 depolarization. This is followed by activation of fast and slow transient outward K⁺ currents (I_tof and I_tos, respectively) mediating a K⁺ efflux driving a rapid phase 1 repolarization. There is also an activation of a depolarizing inward Ca²⁺ current through L-type Ca²⁺ channels (I_Ca), which initiates excitation contraction coupling. The reduced membrane K⁺ permeability due to I_K1 rectification combined with I_Ca maintains the action potential phase 2 plateau phase. Phase 3 repolarization is driven by K⁺ efflux through the rapid and slow delayed rectifier K⁺ channels (I_Kr and I_Ks, respectively), as well as I_K1. At the end of phase 3, the Na⁺ and Ca²⁺ that have accumulated in the cells are removed by the Na⁺, K⁺ pump, and the Na⁺, Ca²⁺ exchanger (NCX). The atrial action potential shows greater contributions to recovery from the ultrarapid delayed rectifier outward currents (I_Kur) and acetylcholine-activated inward rectifying K⁺ channel (I_KACH). Adapted with permission from Huang.¹

Potassium Channels

K⁺ channels represent the most functionally diverse cardiac ion channel type.^3-6 Together, they tightly regulate cardiac repolarization, thus ensuring stable and consistent AP signaling. The different K⁺ channel types have overlapping functions,^2,7 resulting in some degree of functional redundancy,² which in turn contributes to repolarization reserve. Table 1 summarizes their encoding genes with their chromosomal locations and the structural properties of their pore-forming α- and accessory β-subunits. The α-subunit of different K⁺ channel types all possess a conserved pore-forming region allowing K⁺ movement across the plasma membrane down an electrochemical gradient possessing a selective permeability to K⁺ attributable to a specific structural motif. They may also exhibit gating mechanisms responsive to membrane depolarization and ligand-binding sites whose occupancy could alter channel conformation. Finally, individual monomeric α-subunits may assemble into functional dimers or tetramers due to the presence of one or more subunit-assembly domains.^6,8-10 K⁺ channel α-subunits fall into 3 structural types based on subunit topology (Figure 2). The first has 1 pore-forming region with 6 or 7 transmembrane regions (Figure 2A), the second has 1 pore-forming region and 2 transmembrane regions (Figure 2B), and the third has 2 pore-forming and 4 transmembrane regions (Figure 2C).^5,6,10

Table 1.

Molecular Details and Activation Mechanisms of the Cardiac Potassium Channels.²

Current	Gene	Chromosomal Location	Associated Protein	Type of Subunit
I tof	KCND3	1p13.2	Kv4.3	α
	KCNIP2	10q24.32	KChIP2	β
	KCNE3	11q13.4	MiRP2	β
I tos	KCNA4	11p14.1	Kv1.4	α
I Ks	KCNQ1	11p15.5-p15.4	Kv1.7.1/KvLQT1	α
	KCNE1	21q22.12	minK	β
	AKAP9	7q21.2	AKAP-9	β
I Kr	KCNH2	7q36.1	Kv11.1/hERG	α
	KCNE2	21q22.11	MiRP1	β
I K1	KCNJ2	17q24.3	Kir2.1/IRK1	α
	KCNJ12	17p11.2	Kir2.2/IRK2	α
I KATP	KCNJ8	12p12.1	Kir6.1	α
	KCNJ11	11p15.1	Kir6.2	α
	ABCC9	12p12.1	SUR2A/SUR2Ba	β
I Kur	KCNA5	12p12.32	Kv1.5	α
	KCNAB1-B3	N/A	Kvβ1-3	β
I KAch	KCNJ3	2q24.1	Kir3.1/GIRK1	α
	KCNJ5	11q24.3	Kir3.4/GIRK4	α

Abbreviations: I_K1, inward rectifier K⁺ current; I_KACH, acetylcholine-activated inward-rectifier K⁺ current; I_KATP, ATP-sensitive K⁺ current; I_Kr, rapid component of the delayed rectifier K⁺ current; I_Ks, slow component of the delayed rectifier K⁺ current; I_Kur, ultrarapid component of the delayed rectifier K⁺ current; I_tof, fast transient outward K⁺ current; I_tos, slow transient outward K⁺ current.

^aSUR2A and SUR2B are splice variant of ABCC9 and considered as cardiac (SUR2A) and vascular (SUR2B) isoforms.

Figure 2.

Structure of different cardiac potassium channel species: Schematic representation of selected potassium channel α-subunits. A, The 6-transmembrane 1-pore-region voltage-dependent K⁺ channel (K_v) α-subunits mediating I_Kur, I_to, I_Ks, I_Kr, and I_f. B, The 2-transmembrane 1-pore-region inward rectifying K⁺ channel (K_ir) α-subunits mediating I_K1, I_KATP, and I_KAch. C, The 4-transmembrane 2-pore-region K⁺ channel (K_2P) mediating “leak” K⁺ currents. The arrows indicate the location of the pore-forming region(s). HCN indicates hyperpolarization-activated cyclic nucleotide-gated channel; I_f, inward rectifier mixed Na⁺ and K⁺ “funny” current; I_K1, inward rectifier K⁺ current; I_KACH, acetylcholine-activated inward rectifier K⁺ current; I_KATP, ATP-sensitive K⁺ current; I_Kr, rapid component of the delayed rectifier K⁺ current; I_Ks, slow component of the delayed rectifier K⁺ current; I_Kur, ultrarapid component of the delayed rectifier K⁺ current; I_to, transient outward K⁺ current. Reprinted with permission from Giudicessi and Ackerman. Macmillan Publishers Ltd, copyright 2012.³

K⁺ channel β-subunits encompass many molecular groups, such as adenosine triphosphate (ATP)–binding cassette transport-related proteins (eg, sulfonylurea receptors) for inward rectifiers, cytoplasmic proteins (KChIP, KChAP, and K_vβ1-3), and single transmembrane spanning proteins (minK).¹⁰ These β-subunits form complexes with the α-subunits and can modify the channel’s functional properties. For example, K_vβ subunits can alter channel trafficking and the kinetics of current activation and inactivation when interacting with K_v1.5.¹¹ More specifically, K_vβ2.1 and K_vβ4.1 behave as chaperone proteins.¹² Furthermore, the N-terminus of K_vβ1.2 and K_vβ1.3 has an inactivation domain resembling the inactivation particle of the α-subunit, allowing it to modulate 2channel inactivation.^12-14

Cardiac Potassium Currents

Cardiac K⁺ channels vary in their permeability properties, membrane potential dependence, and their opening or closing activation and inactivation kinetics. The major currents are classified into the transient outward currents, delayed rectifier outward currents, and the inward rectifiers (Figure 3). Advances in electrophysiological and molecular biology techniques have demonstrated additional currents that may fall outside this basic classification. Some brief notes on the major cardiac K⁺ currents, their role in the cardiac AP, and their functional importance follow.

Figure 3.

Classification of K⁺ currents: General classification of the main cardiac K⁺ currents. The relatively new additions to the K⁺ channel family (Ca²⁺-activated K⁺ current [I_KCa] and 2-pore domain K⁺ current [I_K2p]) have not been grouped under this scheme. Most K⁺ currents are grouped according to the direction of their overall rectification property. In some instances, this may vary. With the inward rectifying K⁺ channels, the name refers to the unusual characteristic whereby net potassium flow is into the cell at potentials lower than the reversal potential where channel conductance is high. As the channel potential becomes more positive, channel conductance decreases. Net ion flow direction reverses at the reversal potential, meaning that net potassium flow is outward at potentials more positive than this. Therefore, at depolarized potentials, potassium loss from the cell is low as conductance through the channel is low.

Transient outward K⁺ (I_to1) currents

When first described, the transient outward currents (I_to) were attributed to 2 distinct channels, one blocked by 4-aminopyridine (4-AP) and unaffected by extracellular Ca²⁺ (I_to1) and the other not blocked by 4-AP but sensitive to Ca²⁺ (I_to2).⁶ I_to drives the initial rapid repolarization phase of the AP. Regions with shorter APDs, such as the epicardium, right ventricle, and septum, have higher I_to expression. It was later discovered that I_to2 is a Cl^- rather than a K⁺ current.¹⁵ Further characterizations subdivided I_to1 into fast (I_tof) and slow (I_tos) currents (Figure 3). I_tof predominates in the atria, whereas both I_tof and I_tos occur in the ventricles.¹⁶ While I_tos requires longer recovery times, its classification as “slow” is relative only to I_tof. Thus, both I_tof and I_tos channels activate and inactivate rapidly in comparison to the corresponding processes in other K⁺ channels.¹⁵ Due to differences in the biophysical properties of I_tof and I_tos, the existence of molecular heterogeneity between these 2 channels has been previously suggested.¹⁵

Ultrarapid delayed rectifier currents (I_Kur)

In addition to I_to, the ultrarapid delayed rectified K⁺ current (I_Kur) plays a role in the initial rapid phase 1 AP repolarization. I_Kur activates rapidly in under 10 milliseconds at voltages in the plateau range and deactivates slowly over the course of the AP.^17-19 I_Kur is the predominant delayed rectifier current for the atria and thus results in the shorter APD seen in the atria compared to the ventricles.^10,16,17,19 Where I_Kur is present, its channels are not evenly distributed over the myocyte surface but instead found at high densities in the intercalated disk.⁶ This pattern of distribution is often disrupted after cardiac ischemic damage.¹⁰ The selective presence of I_Kur in the atria makes it an interesting target for atria selective therapy, whereby inhibition of I_Kur would prolong the APD in the atria but not the ventricles.⁴

Rapid delayed rectifier K⁺ currents (I_Kr)

The voltage-gated rapid delayed rectifier outward K⁺ current (I_Kr) is critical to phase 3 repolarization. It shows a relatively rapid activation with depolarization. However, its inactivation rate is around 10 times faster than its activation rate due to voltage-dependent C-type inactivation. This renders it relatively nonconducting in phases 1 and 2 of the cardiac AP.^20-23 Thus, although termed a delayed rectifier current, it also shows an inward rectification property at positive potentials.^22,24 However, with the end of phases 1 and 2, as the membrane potential becomes negative to 0 mV, I_Kr becomes activated once again, but the deactivation during this phase is much slower. This results in a large outward K⁺ efflux during phase 3 repolarization.^2,10 I_Kr is found in both human atria and ventricles but is differentially expressed with higher levels in the left atrium and ventricular endocardium.¹⁶

Slowly activating delayed rectifier K⁺ current (IKs)

Cardiac repolarization is also influenced by a third, slowly activating delayed rectifier K⁺ current (I_Ks). I_Ks slowly activates at potentials positive to −20 mV. Unlike I_Kr, I_Ks barely inactivates^25,26 and consequently accumulates over phase 2 repolarization, significantly influencing phase 3 repolarization.² This feature of I_Ks is particularly important during atrial and ventricular APs of long duration. It is also involved in APD shortening during physiological increases in heart rate. An increase in heart rate thus reduces the time required for I_Ks inactivation. In consequence, more I_Ks accumulates, leading to a steeper drop in the repolarization rate.^27,28 Blocking I_Ks results in an APD prolongation at increased heart rates.^21,28 Inhibition of I_Ks will increase the vulnerable window for reactivation of voltage-gated Ca²⁺ channels, thereby increasing the risk of arrhythmic trigger events.² I_Ks is found in all cardiac cell types, but its expression is significantly reduced in the mid-myocardial wall; this accounts for the long APD seen in this region.¹⁶

Inward rectifier K⁺ current (I_K1) 

The inward rectifier K⁺ current (I_K1) functions over a narrow membrane potential range. Its rectifying property results in a marked reduction in I_K1 conductance at positive, depolarized, membrane potentials and an increase in I_K1 at negative membrane potentials, with the effect of stabilizing the membrane resting potential close to the K⁺ equilibrium potential (E_K).¹⁰ The channel mediating I_K1 does not show voltage-dependent gating and does not possess a voltage sensor. Nevertheless, I_K1 modulation associated with movement of Mg²⁺ and polyamines results in an indirect sensitivity to voltage.^29-32 Between phase 0 and phase 2 of the AP, the membrane potential is more positive than −20 mV, and at this potential, there is no conductance of I_K1 as the channel is inhibited by Mg²⁺ and polyamines. The resulting marked inward rectification property limits the outward current at these positive potentials. This in turn minimizes the inward depolarizing current, which confers energetic efficiency for AP generation as it minimizes changes to ionic gradients that would need to be restored.¹⁶ As the potential returns to more negative values (typically around −40 mV), the inhibition by Mg²⁺ and polyamines is reversed. I_K1 conductance then resumes and this contributes to phase 3 cardiac repolarization.³¹ I_K1 occurs in both atria and ventricles and is thereby involved in setting their resting membrane potentials. Channels conducting I_K1 are expressed in greater density in the ventricles, making the ventricles less susceptible to pacemaker influence.¹⁶

Acetylcholine-activated K⁺ current (I_KACH)

The inwardly rectifying acetylcholine (ACh)-activated K⁺ current (I_KACH) is regulated by G proteins rather than voltage gating. Cardiac parasympathetic nerve endings release ACh, thereby activating M2 muscarinic receptors. This reduces the depolarizing effect of the pacemaker current (I_f), reducing firing rates of pacemaker cells and in turn reducing heart rate.⁶ Acetylcholine also opens muscarinic-sensitive I_KACH channels allowing the inward rectification of K⁺. The inward rectifying current shortens the AP and hyperpolarizes the membrane potential.¹⁶ Membrane hyperpolarization reduces the rate at which the sinoatrial and atrioventricular (AV) nodes drive pacemaker depolarization in addition to reducing AV conduction velocity.^6,33 I_KACH is thought to be specific to the atria,² but there has been a suggestion that it may exist both in the atria and ventricle,¹⁶ but with densities 6 times greater in the atria than the ventricles.³⁴

ATP-activated K⁺ current (I_KATP)

The ATP-activated K⁺ current (I_KATP) occurs at both the sarcolemmal (sarc-K_ATP) and mitochondrial inner membrane (mito-K_ATP) of cardiomyocytes. The sarc-K_ATP channels are highly expressed in cardiomyocytes and are composed of K_ir6.2 and SUR2A subunits. There may also be contributions from K_ir6.1 and SUR1.³⁵ In contrast, although the subunits of mito-K_ATP channels have been difficult to identify due to the challenge of isolating pure mitochondrial membrane fractions, ROMK2 pore-forming subunits and SUR2 regulatory subunits have been suggested to contribute.^36,37

Both channels are controlled by ATP and are thus directly responsive to the cell’s metabolic status, thereby influencing cell membrane potential.⁶ I_KATP is inhibited by physiological intracellular ATP levels, but this reverses with ATP depletion. Thus, under normal energetic circumstances, there is limited I_KATP current. However, under both physiological and pathological conditions that reduce ATP, there is increased I_KATP current that is essential for adaptation to stress. For example, compared to wild-type controls, mice lacking K_ir6.2-containing K_ATP channels perform less well in acute treadmill exercise testing.³⁸ The increased I_KATP has a cardioprotective role in ischemia by shortening the cardiac AP, thus limiting calcium influx into the cytosol.^39-41 Specifically, studies have suggested that mito-K_ATP rather than sarc-K_ATP channel opening has an energy-modulating property that confers cardioprotection in ischemic hearts.^42,43

In some situations, the I_KATP-mediated APD shortening and corresponding heterogeneities in repolarization can create a substrate for cardiac reentry arrhythmia. In other situations, K_ATP channel openers have been described to have antiarrhythmic effects,^44-48 and evidence suggests that activation and block of K_ATP can be pro- or antiarrhythmic depending on the arrhythmogenic mechanism in different animal models.⁴⁹ For example, selective sarcolemma K_ATP channel blockers, such as HMR 1883, confer antiarrhythmic effects in the short term,⁵⁰ although this could be metabolically disadvantageous in the long term due to the abolished adaptive response to stresses. Finally, it is important to note that the channel involved in the conductance of I_KATP is also thought to be involved in the regulation of smooth muscle tone and insulin secretion in pancreatic β-cells.⁶

Other K⁺ channel family: Ca²⁺-activated K⁺ current (I_KCa), 2-pore domain K⁺ current (I_K2p), and hyperpolarization-activated cyclic nucleotide-gated channels

Recently, several further currents have been characterized. The Ca²⁺-activated K⁺ current, also known as the small conductance Ca²⁺-activated K⁺ (SK) current (I_KCa), and the 2-pore domain K⁺ current (I_K2p) have attracted considerable physiological and pharmacological interest. I_KCa was initially thought to not exist in the human heart.⁵¹ However, subsequent studies demonstrated the presence of I_KCa, with a higher density in the atria than the ventricle. Various subtypes of Ca²⁺-activated K⁺ channels exist in different tissues; the channel subtype conducting the cardiac I_KCa is the SK channel.^51,52 In neuronal cell, SK channels that are involved in modulating the tonic firing frequency and activation of these channels cause membrane hyperpolarization, thus limiting neuronal AP firing frequency.⁵¹ In contrast, cardiac SK channels and consequently I_KCa are involved in late AP repolarization, controlling the resting membrane potential in human atria.⁵² I_KCa appears to not play physiologically significant roles in the ventricle.⁵² I_KCa is accordingly of particular pharmacological interest for atrial fibrillation (AF) therapy. Thus, I_KCa occurs during late repolarization, when the atrial AP is susceptible to irregular or abnormal excitation such as that resulting from early after-depolarizations (EADs).⁵¹

I_K2p contributes to the background current, the resting membrane potential, and cellular excitability. The channel involved in the conductance of this current has no voltage dependence, but its activity is modulated by lipids, particularly fatty acids, pH, drugs, particularly local and inhalation anesthetics, and membrane stretch.^53,54 These mediators act upon the channel via secondary messenger phosphorylation.⁵⁵ I_K2p is a background current that persists through all phases of the cardiac AP. It thus stabilizes the membrane potential toward E_m. I_K2p may also prevent the occurrence of EADs, and it may be involved in fine-tuning of Na⁺ channel availability for phase 0 depolarization.⁵⁶ The current has been found to occur selectively in the atria and AV node, thereby making it a target for drug development.^57-59 Although not entirely new but only recently well characterized, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel is instrumental in conducting the inward funny current (I_f) in the heart. The channel is activated by the hyperpolarization of the membrane and is additionally stimulated by intracellular cyclic nucleotides.^60,61 The generation of I_f is attributable to the inward permeability of both Na⁺ and K⁺ and occurs at threshold close to the resting membrane potential.⁶² Although the HCN channel under physiological circumstances conducts both Na⁺ and K⁺, the primary sequence of the HCN pore region suggests that it is primarily related to a selective potassium channel.⁶³ In certain pathological conditions such as AF and myocardial infarction, I_f is increased unusually outside the pacemaker cells, leading to increased propensity to arrhythmia. Thus, targeting the I_f in such pathological conditions has proven to be therapeutically advantageous.⁶⁴

Cardiac K⁺ Channel as Targets for Drug Development

Although there have been significant recent advances in the development and use of cardiological devices and procedures directed at arrhythmic conditions, antiarrhythmic drugs continue to be important whether by themselves or as adjunct therapy to such interventions. These include situations involving acute management of potentially fatal arrhythmic events, particularly where such procedures are contraindicated. Yet progress in antiarrhythmic drug development has been relatively limited. This likely reflects a lack of understanding of cardiac arrhythmic mechanisms. However, recent developments of our understanding of the role of the ion channels in normal AP generation have led to a specific interest into ion channels and their associated currents whose abnormal activity potentially leads to arrhythmia. This would encourage interest in the development of cardiac ion channel activator or blockers directed at modulating the cardiac AP or its refractory period. Introduction of drugs acting specifically on ion channels would optimize the efficacy of therapeutic actions on arrhythmogenic tendency, while minimizing problems arising from potential cardiac and extracardiac toxicity. K⁺ channels play a vital role in cardiac AP repolarization and thus naturally form potential targets for the development of ion channel-specific antiarrhythmic therapy, such as for AF. However, a limitation of this approach is that arrhythmic conditions, such as AF, are heterogeneous and the efficacy of targeting ion channels varies according to the cause and extent of the arrhythmia.

This is complicated by the fact that in various physiological and pathological conditions, remodeling of K⁺ channel expression can occur, which can alter the AP and increase the risk of sudden cardiac death.⁶⁵ For example, AF is maintained and progressed partly due to electrical remodeling, mediating APD shortening.⁶⁶ Thus, in chronic AF, there is upregulation of I_K1, I_Ks, and I_K2P3.1, which offsets the possible downregulation of I_Kur and I_to.^67-69 Nevertheless, the experimental evidence for the reduction in I_Kur during remodeling is conflicting, as some reports suggest reduced I_Kur density^70,71 and others suggest no change.^72,73 It has been suggested that receptor-activated I_KACH (rI_KACH) mediates AF induced by vagal stimulation, while constitutive I_KACH (c I_KACH) develops in the time course of AF remodeling.^67,74

In physiological cardiac hypertrophy, induced by chronic exercise, for example, there is an increase in I_K density.⁷⁵ This contrasts with pathological cardiac hypertrophy caused by pressure overload where a reduced I_K density is noted that was attributable to cellular hypertrophy rather than gene expression changes in I_tof and I_K1.⁷⁶ In heart failure, AP prolongation is associated with downregulation of several genes, leading to reduced I_tof, I_Ks, I_Kr, and I_K1.^65,77,78 Considering the changes in K⁺ channel expression in remodeling is clinically important as the sensitivity and efficacy of blocking these channels will change.

Table 2 outlines selected drugs that have been experimentally proven to target different K⁺ channels, using either native cardiac myocytes or human cell line expression systems. Some of these drugs presently in clinical use have been primarily developed for other ion channels such as the Na⁺ or Ca²⁺ cardiac ion channel but have corresponding effects on K⁺ channels. Several drugs have been proposed to be selective to specific K⁺ channels, such as A935142, XEN-D0103, and XEN-D0101. However, despite promising experimental findings, many of these drugs have not progressed to clinical use. This may be attributable to limitations associated with experimental studies. Expression systems can often produce off-target effects or nonspecific interactions which may mask the true effect of these drugs. Additionally, expression systems may run the risk of either overexpressing or underexpressing the channel of interest. On the other hand, native cardiac myocytes, while more physiologically representative, may not provide the right platform for the study of specific targets. Additionally, acquisition of viable native cardiac myocytes from a minimally heterogeneous population remains a challenge, and it is widely accepted that channel functions can differ by gender and age. Consequently, while experimental studies may suggest potentially promising options to selectively target K⁺ channels, the translational capacity of such studies remains limited.

Table 2.

Selected Pharmacological Agents Affecting the Human K⁺ Channels.

Current	Pharmacological Agent (Expression System), Reference
Activators
I Kr	A-935142 (HEK),79 ICA-105574 (HEK),80 NS1643 (HEK),81 PD-118057 (HEK)82
I Ks	Ephedrine (HEK),83 Tanshinone IIA (HEK)84
I Kca	NS1619 (HEK)85
Blockers
I to	Chromanol 293B (nHVM),86 Flecainidea (nHAM)87
I Kur	Amiodaronea (HEK),88 Bepridil (HEK),88 DPO-1 (nHAM),89 MK-0448 (nHAM),90 NIP-142 (HEK),91 Papaverine (nHAM),92 Pimozide (HEK),93 Sertindole (HEK),94 XEN-D0103 (nHAM)95
I Kr	Cocaine (HEK),96 Fluvoxamine (HEK),97 Ketoconazole (HEK),98 Ketanserin (HEK),99 Ziprasidone (HEK)100
I Ks	HMR 1556 (HEK),101 SKF-96365 (HEK)102
I KACH	NIP-151 (HEK),103 U73122/U73343 (HEK)104
I KATP	5-Hydroxydecanoate (HEK),105 HMR1098 (HEK)105
Ito, IKur	AVE-0118 (nHAM),106 Acacetin (nHAM),107 Ambasilide (nHAM),108,109 4-aminopyridine (nHAM),87 Diltiazemb (nHAM),110 Docosahexaenoic acid (nHAM),111 Eicosapentaenoic acid (nHAM),111 Nifedipineb (nHAM),110 Quinidinea (nHAM),87 Raloxifene (nHAM),112 U50488 H (nHAM),113 XEN-D0101 (nHAM),114 Vernakalant (RSD1235) (HEK)115
Ito, IKur, IK1	Propafenonea (nHAM)116
Ito, IKur, IKr, IKs	Clotrimizole (nHAM)117
Ito, IKur, IKr, IKs, IK1	Azimilide (nHAM)118
IKur, IKr	Cisapride (HEK),119 Verapamilb (nHAM, HEK)120,121
IKr, IKs	Sotalol (nHAM)21,108

Abbreviations: nHAM, native human atrial myocyte; HEK, human embryonic kidney; nHVM, native human ventricular myocyte; I_K1, inward rectifier K⁺ current; I_Kr, rapid component of the delayed rectifier K⁺ current; I_KACH, acetylcholine-activated inward rectifier K⁺ current; I_KATP, ATP-sensitive K⁺ current;; I_Ks, slow component of the delayed rectifier K⁺ current; I_Kur, ultrarapid component of the delayed rectifier K⁺ current; I_to, transient outward K⁺ current; I_KCa, small conductance Ca²⁺-activated K⁺ (SK) current.   

^aPrimary Na⁺ channel blocker.

^bPrimary Ca²⁺ channel blocker. 

Furthermore, these activators and blockers often target more than 1 K⁺ channel species and thus are not entirely specific.^10,122 However, a large proportion of these drugs also typically target I_Kr (known to be present in all cardiac regions) and as such do not constitute ideal candidates for targeted therapy. Nevertheless, mechanisms of cardiac arrhythmia are likely to be region dependent. Drugs that may be antiarrhythmic in some cardiac regions may potentially be pro-arrhythmic in others. Thus, the presence of atrial-specific K⁺ channels has provided focus on developing drugs that could specifically increase refractory periods, thus preventing atrial reentry arrhythmia, which is the most common mechanism for AF.¹²³

Of ion channels specific to the atrium that might offer specific therapeutic targets, the channel conducting I_Kur tends to prolong repolarization and effective refractory period (ERP) without altering QT intervals.^124,125 The experimental drugs AVE0118 and XEN-D101 are thought to be I_Kur selective blockers with both prolonging APD in atrial tissue from patients with permanent AF in common with the known I_Kur blocker 4-AP.^106-129 However, a subsequent “first-in-human” study using the highly selective I_Kur blocker MK-0448 (N-{6-[(1S)-1-(4-fluorophenyl)-2,2-di (pyridine-3-1) ethyl] pyridine2yl} methane sulfonamide) did not reveal any increase in atrial ERP. This led to the conclusion that selective blocking of I_Kur may have limited clinical value.¹³⁰ I_KACH channels are also atrium specific or at least predominantly occur in the atria and have minimal physiological function in the ventricle.¹²⁹ Opening of the I_KACH channel will lead to shortening of atrial APD and thus increase the likelihood of AF. Therefore, blocking the opening of I_KACH channels will prevent such shortening of APD with minimal effect on ventricular APD, in turn reducing the chances of AF. Several drugs block I_KACH, but have limited specificity. Nevertheless, selective blocking of I_KACH has been experimentally achieved using the compound NTC-801. The compound was found to have selective antifibrillatory properties, achieved by prolonging the atrial ERP.¹³¹ Another potential atrial-specific therapeutic target of interest is the I_KCa current conducted by SK channels. The selective presence of this current in the atria has recently led to several investigative drugs beings explored. NS8593 is a selective SK channel inhibitor demonstrating significant atrial antiarrhythmic effects in canine and equine experimental models. Experiments using human atrial cardiac myocytes from patients with normal sinus rhythm demonstrated reduction in K⁺ currents and prolongation in APD. No such changes were observed in intraventricular myocytes.⁵²

Conclusion

There is currently an incomplete understanding of the cellular physiological role of the various cardiac potassium currents and their interacting effects and how dysregulation of their function and expression can provide arrhythmogenic mechanisms. It is thought that the site-specific distribution of some K⁺ channels could allow targeted therapy to be more spatially selective. However, complex electrical remodeling events that occur in disease states may change channel expression levels to the extent that the selectivity of the drug is hindered, making even this potential therapeutic strategy challenging. Although targeting ion channels responsible for discrete parts of the cardiac AP to modulate the system towards a more physiological state has therapeutic appeal, there are inherent difficulties in developing successful drugs. This is because the ion channels targeted are functionally complex and are interdependent, thus adding a dynamic situation in which function and expression are altered depending on the cell environment. Furthermore, pathophysiological processes of arrhythmic disease may involve functional alterations in 1 or more ion channels. Such single or multiple ion channel functional abnormalities may therefore warrant corresponding use of a single or multichannel activator/blocker approach. However, this approach will only be possible if we are able to identify the specific pathophysiological process affecting individual patients (ie, is this arrhythmic disease related to a single or multichannel abnormality). Thus, although we may be able to develop single or multichannel activators/blockers, actual clinical use will be dependent on a detailed understanding of the exact arrhythmogenic mechanisms affecting individual patients, which thus far is limited. Presently, decisions to use single or multichannel activators/blockers are largely dependent on resolution of clinical signs or the actual arrhythmia rather than a therapeutic approach targeting ion channel functional abnormality. Furthermore, the availability of truly specific ion channel activators/blockers is limited as these agents tend to have off-target actions with corresponding side effects, and this limits the clinical use of selective agents. Focusing on understanding the system at a cellular physiological level through further experimental and computational modeling is needed to enable development of novel insights at a pharmacological level.