Voltage-Regulated Potassium Channels

Introduction

In atrial and ventricular myocytes, the action potential upstroke, attributed to inward currents through voltage-gated Na ⁺ (Na _V ) channels, is rapid ( Fig. 3.1 ). In nodal tissues, which lack functional Na _V channels, the upstroke of the action potential is substantially slower and is dominated by Ca ²⁺ influx through voltage-gated Ca ²⁺ (Ca _V ) channels. There are also marked differences in action potential amplitudes and durations, as well as in the time courses of action potential repolarization in different cardiac cell types and in cells in different regions of the ventricles (see Fig. 3.1 ). These differences, which impact the normal spread of excitation in the myocardium and influence the dispersion of repolarization in the ventricles, primarily reflect regional differences in the densities and/or the biophysical properties of the outward K ⁺ and in the inward (Na _V and Ca _V ) currents ^, expressed in different cells.

Cellular electrophysiologic studies have detailed the properties of the major inward (Na _V and Ca _V ) and outward (K ⁺ ) currents that shape the waveforms of atrial and ventricular action potentials ( Fig. 3.2 ). In contrast to myocardial Na _V and Ca _V currents (see Fig. 3.2 ), however, there are multiple types of voltage-gated (K _V ) and non–voltage-gated K ⁺ channels expressed in mammalian cardiac cells ( Table 3.1 ). Additional inward and outward currents generated by the Na ⁺ /Ca ²⁺ exchanger (NCX; see Fig. 3.2 ) also contribute to shaping myocardial action potential waveforms. Many of the myocardial K ⁺ conductances are differentially expressed (see Fig. 3.2 ), contributing to cell type and regional variations in myocardial action potential waveforms (see Fig. 3.1 ) and refractoriness. In addition, changes in the expression and properties of myocardial K _V channels are evident in inherited and acquired cardiac diseases, and these changes affect action potential repolarization and propagation through the myocardium and decrease rhythmicity, effects that can produce substrates for the generation of life-threatening arrhythmias. There is therefore considerable interest in defining the molecular mechanisms controlling the biophysical properties and the functional cell surface expression of these channels. A large number of K ⁺ channel pore-forming and accessory subunits have been identified, and considerable progress has been made in defining the relationships between these subunits and functional myocardial K _V channels. Importantly, the studies completed to date have revealed that the molecular correlates of the various types of K _V and non–voltage-regulated K ⁺ channels, functionally distinguished based on differences in electrophysiologic and/or pharmacologic properties (see Table 3.1 ), are indeed molecularly distinct. Identifying the molecular compositions of native myocardial K ⁺ channels is critical to efforts aimed at defining the molecular and cellular mechanisms controlling cell type–specific and regional differences in functional K ⁺ channel cell surface expression and properties in the normal (nondiseased) myocardium, detailing the derangements in the expression/functioning of these channels that occur with acquired cardiovascular and systemic diseases; and developing novel therapeutic strategies to treat cardiac arrhythmias by targeting specific K ⁺ channels expressed in different cardiac cell types. This chapter reviews the electrophysiologic and functional diversity of repolarizing myocardial K ⁺ channels and the molecular determinants of native myocardial K ⁺ channels.

Myocardial K _v Channels: Transient Outward K _v Channels

K _V currents, which are activated on membrane depolarization, influence myocardial action potential amplitudes and durations. In most cells, two broad classes of K _V currents have been distinguished: transient outward K ⁺ currents (I _to ) and delayed, outwardly rectifying K ⁺ currents, (I _K ; see Table 3.1 ). The transient currents (I _to ) activate rapidly and underlie early (phase 1) repolarization, whereas the delayed rectifiers (I _K ) determine the latter phase (phase 3) of action potential repolarization (see Fig. 3.2 ) back to the resting membrane potential. These are broad classifications, however, and there are actually multiple types of transient (I _to ) and delayed rectifier (I _K ) K _V currents (see Table 3.1 ) expressed in cardiac cells. ^, Electrophysiologic and pharmacologic studies, for example, have clearly demonstrated two types of transient outward K ⁺ currents, referred to as I _to,fast (I _to,f ) and I _to,slow (I _to,s ), and that these currents are differentially expressed in different cell types in both mouse and human ventricles. Both of these K _V currents activate and inactivate rapidly on membrane depolarizations, and the primary difference between them is in the rates of recovery from inactivation: I _to,f is characterized by rapid recovery from inactivation, whereas I _to,s recovery from inactivation is slow. In addition, I _to,f is readily distinguished from other K _V currents, including I _to,s , in mouse and human ventricular myocytes ^, using pharmacologic agents, such as Heteropoda toxin 2 or 3, Hanatoxin, or Snx-482, which block I _to,f but not I _to,s (see Table 3.1 ).

Although originally identified in Purkinje fibers, I _to is a prominent repolarizing K _V current in atrial and ventricular myocytes, as well as in nodal cells, in most species; notable exceptions are guinea pig and porcine atrial and ventricular myocytes. There are, however, marked regional differences in peak I _to densities, with the highest densities typically observed in atrial myocytes. In addition, there are regional differences in I _to densities within the ventricles. In canine left ventricles, for example, peak I _to density is reportedly approximately sevenfold higher in epicardial and mid-myocardial myocytes than in endocardial myocytes. A transmural gradient of peak I _to density has also been described in (nondiseased) human left ventricles (LVs), although the approximate 1.5-fold epicardial-endocardial difference in I _to density reported is considerably smaller than the approximately sevenfold epicardial to endocardial I _to gradient observed in canine LV. In adult mouse ventricles, peak I _to densities are higher in right than in left ventricular myocytes and, within the LV, peak I _to densities are higher in apex than in base myocytes, although the differences are around twofold ^, (i.e., also considerably smaller than reported in canine ventricles).

In addition, when distinguished electrophysiologically, I _to,f and I _to,s have also been shown to be differentially distributed in mammalian ventricles. In the mouse, for example, I _to,s is only observed in interventricular septum myocytes. ^, All cells from the mouse interventricular septum express I _to,s , and around 80% of the cells also express I _to,f . When present, however, I _to,f density is substantially lower in the interventricular septum than in both right and left ventricular myocytes. Marked regional differences in I _to,f and I _to,s expression have also been identified in (nondiseased) human LVs: in approximately 60% of subepicardial human LV myocytes, for example, only I _to,f is expressed, whereas in approximately 75% of subendocardial human LV myocytes, only I _to,s is present. The remaining LV subepicardial (∼40%) and subendocardial (∼25%) LV myocytes and, in addition, the vast majority of myocytes (epicardial, mid-myocardial, and endocardial) through the thickness of the LV wall in the (nondiseased) human heart express both I _to,f , and I _to,s . In marked contrast with previous studies in canine LVs, however, no measurable transmural differences in I _to,f densities were reported in human LV endocardial, mid-myocardial, and/or epicardial myocytes.

Although the amplitudes and durations of action potentials are quite similar in human LV subepicardial and subendocardial myocytes (see Fig. 3.1 ), there are striking differences in early (phase 1) repolarization. Specifically, there is a prominent phase 1 “notch,” which results in a “spike-and-dome” morphology in subepicardial but not subendocardial, LV myocytes. In addition, the plateau membrane potentials in subepicardial myocytes are more hyperpolarized than in subendocardial myocytes. Similar regional differences in the early phase of repolarization have been described in canine LV myocytes. Although it has also been reported that mid-myocardial (M) cells can be distinguished from surrounding LV myocytes by action potentials that are disproportionately prolonged when pacing rates are slowed, ventricular cells with markedly prolonged action potential durations at slow pacing rates have not been observed in recordings obtained from (nondiseased) human hearts, suggesting that M cells are not present in human ventricles.

Because of their very rapid recovery from inactivation, I _to,f channels contribute to action potential repolarization in mouse ventricular myocytes, despite the very high heart rates (∼600–700 beats/min) in adult mice under baseline physiologic conditions. In contrast, I _to,s channels recover from inactivation very slowly, over a time course of hundreds of milliseconds, resulting in the cumulative inactivation of these channels at normal mouse heart rates. As a result, I _to,s channels contribute only marginally to action potential repolarization in the adult mouse heart. In human LV myocytes, I _to,f and I _to,s are also readily distinguished by markedly (∼100-fold) different rates of recovery from inactivation: at physiologic temperature, the time constants of recovery of I _to,f and I _to,s are approximately 10 ms and 1000 ms, respectively. The contribution of I _to,s to shaping the waveforms of action potentials in human LV myocytes therefore will be highly rate dependent. At a resting heart rate of 60 beats/min, for example, approximately 50% of I _to,s is inactivated; this current therefore contributes little to the total transient outward current and to shaping action potential waveforms. At a faster rate of 120 beats/min, I _to,s density is further diminished (∼75% inactivation), and at heart rates greater than 120 beats/min, I _to,s is almost completely inactivated. The rate-dependent reductions in I _to,s density will be expected to have the greatest effect on action potentials in left subendocardial myocytes at elevated heart rates (>120 beats/min) where essentially all transient outward current is eliminated. In marked contrast, because of the very rapid rate of recovery of I _to,f channels from inactivation, there are no rate-dependent decreases in I _to,f amplitudes at stimulation frequencies corresponding to heart rates between 60 and 120 beats/min; indeed, in the human heart, I _to,f is not expected to exhibit rate-dependent inactivation at physiologic heart rates.

As previously noted, in the human heart, rate-dependent changes in I _to,s density will have the largest effect on peak I _to densities in LV subendocardial myocytes. Because of the differential expression of I _to,f and I _to,s , the peak (total) I _to density gradient across the wall of the LV is therefore rate dependent, becoming increasingly larger at progressively faster heart rates (as I _to,s channels undergo cumulative inactivation). This is in marked contrast to canine LVs, where the I _to density gradient is attributed to regional differences in the expression of a single type of I _to channel. Indeed, the rate-dependent transmural density gradient of I _to has not been previously reported in the LVs of any large animal model and appears unique to human LVs. As discussed further in subsequent sections, I _to,f and I _to,s in adult mouse ventricular myocytes reflect the expression of distinct molecular entities. Interestingly, in spite of heterogeneities in expression levels and functional roles, the biophysical and pharmacologic properties of I _to,f and I _to,s in mouse ^, and human ventricles are remarkably similar, suggesting that the molecular compositions of the underlying (I _to,f and I _to,s ) channels are also very similar.