Delayed rectifier potassium channels in ventricle and sinoatrial node of the guinea pig: molecular and regulatory properties

Modeling the chronotropic effect of isoprenaline on rabbit sinoatrial node

Introduction: β-adrenergic stimulation increases the heart rate by accelerating the electrical activity of the pacemaker of the heart, the sinoatrial node (SAN). Ionic mechanisms underlying the actions of β-adrenergic stimulation are not yet fully understood. Isoprenaline (ISO), a β-adrenoceptor agonist, shifts voltage-dependent I_f activation to more positive potentials resulting in an increase of I_f, which has been suggested to be the main mechanism underlying the effect of β-adrenergic stimulation. However, ISO has been found to increase the firing rate of rabbit SAN cells when I_f is blocked. ISO also increases I_CaL, I_st, I_Kr, and I_Ks; and shifts the activation of I_Kr to more negative potentials and increases the rate of its deactivation. ISO has also been reported to increase the intracellular Ca²⁺ transient, which can contribute to chronotropy by modulating the “Ca²⁺ clock.” The aim of this study was to analyze the ionic mechanisms underlying the positive chronotropy of β-adrenergic stimulation using two distinct and well established computational models of the electrical activity of rabbit SAN cells. Methods and results: We modified the Boyett et al. (2001) and Kurata et al. (2008) models of electrical activity for the central and peripheral rabbit SAN cells by incorporating equations for the known dose-dependent actions of ISO on various ionic channel currents (I_CaL, I_st, I_Kr, and I_Ks), kinetics of I_Kr and I_f, and the intracellular Ca²⁺ transient. These equations were constructed from experimental data. To investigate the ionic basis of the effects of ISO, we simulated the chronotropic effect of a range of ISO concentrations when ISO exerted all its actions or just a subset of them. Conclusion: In both the Boyett et al. and Kurata et al. SAN models, the chronotropic effect of ISO was found to result from an integrated action of ISO on I_CaL, I_f, I_st, I_Kr, and I_Ks, among which an increase in the rate of deactivation of I_Kr plays a prominent role, though the effect of ISO on I_f and [Ca²⁺]_i also plays a role.

Essential role of diastolic oscillatory potentials in adrenergic control of guinea pig sino-atrial node discharge

Background

The diastolic oscillatory after-potential V_os and pre-potential ThV_os play an essential role in the pacemaker mechanism of sino-atrial node (SAN). The aim of this study was to investigate whether these oscillatory potentials are also involved in adrenergic control of SAN discharge.

Methods

V_os and ThV_os were visualized by superfusing guinea pig SAN in high [K⁺]_o. The actions of adrenergic agonists on oscillatory potentials were studied by means of a microelectrode technique. Statistical significance was determined by means of Student's paired t-test.

Results

In non-spontaneous SAN, norepinephrine (NE) decreased the resting potential into a voltage range ("oscillatory zone") where increasingly larger ThV_os appeared and initiated spontaneous discharge. In slowly discharging SAN, NE gradually increased the rate by increasing the amplitude and slope of earlier-occurring ThV_os and of V_os until these oscillations fused with initial diastolic depolarization (DD₁). In the presence of NE, sudden fast rhythms were initiated by large V_os that entered a more negative oscillatory zone and initiated a large ThV_os. Recovery from NE exposure involved the converse changes. The β-adrenergic agonist isoproterenol had similar actions. Increasing calcium load by decreasing high [K⁺]_o, by fast drive or by recovery in Tyrode solution led to growth of V_os and ThV_os which abruptly fused when a fast sudden rhythm was induced. Low [Ca²⁺]_o antagonized the adrenergic actions. Cesium (a blocker of I_f) induced spontaneous discharge in quiescent SAN through ThV_os. In spontaneous SAN, Cs⁺increased V_os and ThV_os, thereby increasing the rate. Cs⁺ did not hinder the positive chronotropic action of NE. Barium increased the rate, as Cs⁺ did.

Conclusion

Adrenergic agonists: (i) initiate SAN discharge by decreasing the resting potential and inducing ThV_os; (ii) gradually accelerate SAN rate by predominantly increasing size and slope of earlier and more negative ThV_os; (iii) can induce sudden fast rhythms through the abrupt fusion of large V_os with large ThV_os; (iv) increase V_os and ThV_osby increasing cellular calcium; and (v) do not modify the oscillatory potentials by means of the hyperpolarization-activated current I_f. The results provide evidence for novel mechanisms by which the SAN dominant pacemaker activity is initiated and enhanced by adrenergic agonists.

Regulation of basal and reserve cardiac pacemaker function by interactions of cAMP-mediated PKA-dependent Ca2+ cycling with surface membrane channels

Decades of intensive research of primary cardiac pacemaker, the sinoatrial node, have established potential roles of specific membrane channels in the generation of the diastolic depolarization, the major mechanism allowing sinoatrial node cells to generate spontaneous beating. During the last three decades, multiple studies made either in the isolated sinoatrial node or sinoatrial node cells have demonstrated a pivotal role of Ca(2+) and, specifically Ca(2+) release from sarcoplasmic reticulum, for spontaneous beating of cardiac pacemaker. Recently, spontaneous, rhythmic local subsarcolemmal Ca(2+) releases from ryanodine receptors during late half of the diastolic depolarization have been implicated as a vital factor in the generation of sinoatrial node cell spontaneous firing. Local Ca(2+) releases are driven by a unique combination of high basal cAMP production by adenylyl cyclases, high basal cAMP degradation by phosphodiesterases and a high level of cAMP-mediated PKA-dependent phosphorylation. These local Ca(2+) releases activate an inward Na(+)-Ca(2+) exchange current which accelerates the terminal diastolic depolarization rate and, thus, controls the spontaneous pacemaker firing. Both the basal primary pacemaker beating rate and its modulation via beta-adrenergic receptor stimulation appear to be critically dependent upon intact RyR function and local subsarcolemmal sarcoplasmic reticulum generated Ca(2+) releases. This review aspires to integrate the traditional viewpoint that has emphasized the supremacy of the ensemble of surface membrane ion channels in spontaneous firing of the primary cardiac pacemaker, and these novel perspectives of cAMP-mediated PKA-dependent Ca(2+) cycling in regulation of the heart pacemaker clock, both in the basal state and during beta-adrenergic receptor stimulation.

Mechanisms of adrenergic control of sino-atrial node discharge

Among the mechanisms proposed for the increase in discharge of sino-atrial node (SAN) by norepinephrine (NE) are an increase in the hyperpolarization-activated current I(f) and in the slow inward current I(Ca,L). If I(f) is the primary mechanism, cesium (a blocker of I(f)) should eliminate the positive chronotropic effect of NE. If I(Ca,L), is involved, [Ca(2+)](o) should condition NE effects. We studied the electrophysiological changes induced by NE in isolated guinea pig SAN superfused in vitro with Tyrode solution (both SAN dominant and subsidiary pacemaker mechanisms are present) as well as with high [K(+)](o), higher Cs(+) or Ba(2+) (only the dominant pacemaker mechanism is present). In Tyrode solution, NE (0.5-1microM) increased the SAN rate and adding Cs(+) (approximately 12 mM) caused a decaying voltage tail during diastole in subsidiary pacemakers. NE enhanced the Cs(+)-induced tail, and increased the rate but less than in Tyrode solution. In higher [Cs(+)](o) (15- 18 mM), Ba(2+) (1 mM) or Ba(2+) plus Cs(+) (10 mM) dominant action potentials (not followed by a tail) were present and NE accelerated them as in Tyrode solution. In high [K(+)](o), NE increased the rate in the absence and presence of Cs(+), Ba(2+) or Ba(2+) plus Cs(+). In these solutions, NE increased the overshoot and maximum diastolic potential of dominant action potentials (APs) and increased the rate by steepening diastolic depolarization and shifting the threshold for upstroke to more negative values. High [Ca(2+)](o) alone increased the rate and NE enhanced this action, whereas low [Ca(2+)](o) reduced or abolished the increase in rate by NE. In SAN quiescent in high [K(+)](o) plus indapamide, NE induced spontaneous discharge by decreasing the resting potential and initiating progressively larger voltage oscillations. Thus, NE increases the SAN rate by acting primarily on dominant APs in a manner consistent with an increase of I(Ca,L) and I(K) and under conditions where I(f) is either blocked or not activated. NE INITIATES spontaneous discharge by inducing voltage oscillations unrelated to I(f).

Potentiation of slow component of delayed rectifier K(+) current by cGMP via two distinct mechanisms: inhibition of phosphodiesterase 3 and activation of protein kinase G

1. Regulation of the slowly activating component of delayed rectifier K(+) current (I(Ks)) by intracellular guanosine 3'5' cyclic monophosphate (cGMP) was investigated in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp method. 2. When a cell was dialyzed with pipette solution containing 100 micro M cGMP, I(Ks) started to gradually increase and reached a maximum increase of a factor of 2.37 +/- 0.39 (n = 4) about 10-15 min after rupture of patch membrane. Atrial natriuretic peptide (ANP, 100 nM) also potentiated I(Ks), consistent with intracellular cGMP-induced enhancement of I(Ks). 3. Bath application of a selective blocker of the cGMP-inhibited phosphodiesterase (PDE3) milrinone (100 microM) enhanced I(Ks) by a factor of 1.50 +/- 0.09 (n = 4) but failed to further enhance I(Ks) after a maximum stimulation by intracellular cGMP (100 microM), suggesting that blockade of PDE3 activity is involved in the enhancement of I(Ks). A potent but nonspecific PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 microM) further increased I(Ks) stimulated by 100 microM milrinone, indicating that PDE subtypes other than PDE3 are also involved in the regulation of basal I(Ks) in guinea-pig SA node cells. 4. Bath application of 100 microM 8-bromoguanosine 3'5' cyclic monophosphate (8-Br-cGMP) increased I(Ks) by a factor of 1.48 +/- 0.11 (n = 5) and this stimulatory effect was totally abolished by cGMP-dependent protein kinase (PKG) inhibitor KT-5823 (500 nM), suggesting that the activation of PKG also mediates cGMP-induced potentiation of I(Ks). 5. These results strongly suggest that intracellular cGMP potentiates I(Ks) not only by blocking PDE3 but also by activating PKG in guinea-pig SA node cells.

Rapidly and slowly activating components of delayed rectifier K(+) current in guinea-pig sino-atrial node pacemaker cells

The components and properties of the delayed rectifier K(+) current (I(K)) in isolated guinea-pig sino-atrial (SA) node pacemaker cells were investigated using the whole-cell configuration of the patch-clamp technique. An envelope of tails test was conducted by applying depolarizing pulses from a holding potential of -50 mV to +30 mV for various durations ranging from 40 to 2000 ms. The ratio of the tail current amplitude elicited upon return to the holding potential to the magnitude of the time-dependent outward current activated during depolarizing steps was dependent on the pulse duration, while after exposure to the selective I(Kr) inhibitor E-4031 (5 microM) this current ratio became practically constant irrespective of the pulse duration. These observations are consistent with the presence of the E-4031-sensitive, rapidly activating and E-4031-resistant, slowly activating components of I(K) (I(Kr) and I(Ks), respectively) in guinea-pig SA node cells. The activation range for I(Kr), defined as the E-4031-sensitive current (half-maximal activation voltage (V(1/2)) of -26.2 mV) was much more negative than that for I(Ks), defined as the E-4031-resistant current (V(1/2) of +17.2 mV). I(Kr) exhibited a marked inward rectification at potentials positive to -50 mV, whereas I(Ks) showed only a slight rectification. In the current-clamp experiments, bath application of E-4031 (0.5 and 5 microM) initially slowed the repolarization at potentials negative to approximately -30 mV and produced a significant depolarization of the maximum diastolic potential, followed by the arrest of electrical activity, thus indicating that the late phase of the repolarization leading to the maximum diastolic potential at around -60 mV in spontaneous action potentials is primarily produced by I(Kr) in guinea-pig SA node cells. External application of the selective I(Ks) inhibitor 293B (30 microM) also delayed the repolarization process at potentials negative to about -20 mV and induced moderate depolarization of the maximum diastolic potential leading to the arrest of the spontaneous activity. These results provide evidence to suggest that both I(Kr) and I(Ks) are present and play crucial roles in the spontaneous electrical activity of guinea-pig SA node pacemaker cells.

Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium

In the mammalian heart, Ca2+-independent, depolarization-activated potassium (K+) currents contribute importantly to shaping the waveforms of action potentials, and several distinct types of voltage-gated K+ currents that subserve this role have been characterized. In most cardiac cells, transient outward currents, Ito,f and/or Ito,s, and several components of delayed reactivation, including IKr, IKs, IKur and IK,slow, are expressed. Nevertheless, there are species, as well as cell-type and regional, differences in the expression patterns of these currents, and these differences are manifested as variations in action potential waveforms. A large number of voltage-gated K+ channel pore-forming (alpha) and accessory (beta, minK, MiRP) subunits have been cloned from or shown to be expressed in heart, and a variety of experimental approaches are being exploited in vitro and in vivo to define the relationship(s) between these subunits and functional voltage-gated cardiac K+ channels. Considerable progress has been made in defining these relationships recently, and it is now clear that distinct molecular entities underlie the various electrophysiologically distinct repolarizing K+ currents (i.e. Ito,f, Ito,s, IKr, IKs, IKur, IK,slow, etc.) in myocyardial cells.

Differentiation of cardiomyocytes in floating embryoid bodies is comparable to fetal cardiomyocytes

Embryonic stem cells will cluster and differentiate into embryoid bodies, which can develop spontaneous rhythmic contractions. From these embryoid bodies, cardiomyocytes can be isolated based on density by a discontinuous Percoll gradient. These cardiomyocytes differentiate into ventricular myocytes, which is demonstrated by the expression of the ventricular specific isoform of the myosin light chain 2 gene. In this study the functional expression of ion channels was compared between fetal cardiomyocytes (in vivo) and stem cell derived cardiomyocytes (in vitro). Sodium and calcium currents together with transient potassium currents could be detected in early developmental stages (<day 14) both in vivo and in vitro. In the early stages, we found a limited number of cells expressing I(Kr)and virtual absence of I(Ks). The characteristics and distribution of currents are similar in both cell types. The current characteristics were identical for ventricular compared to atrial or undifferentiated stem cell derived cardiomyocytes, despite differences in expression of regulatory myosin light chain proteins. The myocyte differentiation was verified in a limited number of cardiomyocytes following the patch clamp procedure by immunocytochemistry.

Properties of the delayed rectifier potassium current in porcine sino-atrial node cells

Whole-cell currents were recorded in single, spontaneously active cells dissociated from porcine sino-atrial node, and the conductance and gating properties of the delayed rectifier K+ current (IK) were investigated. The isolated cells exhibited spontaneous action potentials at a rate of 80.5 +/- 5.4 min-1 (mean +/- s.e.m., n = 11). Under Ca2+ current block, depolarization from -40 mV to various potentials activated a time-dependent outward current (IK). The activation curve of IK showed a half-activation potential (V½) of 20.5 +/- 2.1 mV and a slope factor (S) of 16.4 +/- 1.2 mV (n = 8). As the duration of the depolarizing pulse to either +10 or +60 mV was prolonged, the amplitude of the tail current increased in proportion to that of the activated outward current during depolarization. E4031 (2-5 µM), a selective blocker for the rapidly activating component of IK (IK,r), hardly affected IK, but chromanol 293B, a selective blocker for the slowly activating component (IK,s), inhibited IK with an IC50 of 8.79 µM. The reversal potential of IK was -75.2 +/- 2.3 mV with 5.4 mM external and 150 mM internal K+. The time courses of activation and deactivation of IK were fitted by the sum of two exponential functions at various potentials. The relationship between the time constants and membrane potential showed a bell-shaped curve with a peak at around -10 mV for both fast and slow components. The results indicate that in porcine sino-atrial node cells IK is largely derived from IK,s and that IK,s plays a functional role in the slow diastolic depolarization. IK,s may, in part, account for the relatively slower heart rate of pigs than that of rabbit in which IK,r is a functionally dominant component of IK.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Independent and exclusive modulation of cardiac delayed rectifying K+ current by protein kinase C and protein kinase A

Expression of minK in Xenopus oocytes results in a current similar to the cardiac slow delayed rectifying K+ (IKs) current. Modulation of the IKs current in cardiac myocytes has been studied extensively because of its role in shaping the cardiac action potential. The human and cat minK cDNA have been cloned, but their regulation by protein kinases has not been characterized. We report here on the complex modulation of human and cat IKs currents by protein kinase C (PKC) and protein kinase A (PKA). Activation of PKC by phorbol ester (100 nmol/L phorbol 12,13-didecanoate [PDD]) produces an increase in IKs current that peaks after 20 minutes and then subsequently decreases to approximately 50% of the control level after 1 hour. PKA activation only produces a sustained increase in IKs current. Interestingly, premodulation by PKC prevents IKs current modulation by PKA, and PKC has no effect on IKs current after potentiation by PKA. This shows that the IKs current is modulated by PKC and PKA in a mutually exclusive manner and suggests that multiple interacting phosphorylation sites are involved. Activation of PKC by diacylglycerol analogues only produces a slow decrease in IKs current. The biphasic effects of PKC on IKs current activated by PDD can also be separated by dose and duration. Low doses of PDD (5 nmol/L) or brief applications (5 minutes) of 100 nmol/L PDD only produces IKs current activation. These data suggest that there are at least 2 independent PKC phosphorylation sites in the minK-KvLQT1 channel. Additionally, long-term activation of PKC strongly attenuates the IKs current expression even when the corresponding changes in capacitance are taken into account.

Regulation of voltage-gated K+ channel expression in the developing mammalian myocardium

As in neurons, depolarization-activated, Ca2+-independent outward K+ currents play prominent roles in shaping the waveforms of action potentials in myocardial cells. Several different types of voltage-gated K+ currents that contribute to the distinct phases of action potential repolarization have been characterized in myocardial cells isolated from different species, as well as in cells isolated from different regions of the heart in the same species. Important among these are the transient outward current, I(to), similar to the neuronal K+ current IA, and several components of delayed rectification, including I(Kr)[IK(rapid)], I(Ks)(IK(slow)], and I(Kur)[IK(ultrarapid)]. The properties of these currents in different species and cell types are remarkably similar, suggesting that the molecular correlates of functional voltage-gated K+ channel types are also the same. A number of voltage-gated K+ channel (Kv) pore-forming (alpha) and accessory (beta) subunits have now been cloned from heart cDNA libraries, and a variety of experimental approaches are being exploited to determine the molecular relationships between these subunits and functional voltage-gated myocardial K+ channels. Considerable progress has been made recently in defining these relationships, and the results obtained to date indeed suggest that distinct molecular entities underlie the different types of voltage-gated K+ channels characterized electrophysiologically in myocardial cells. Marked changes in the densities and/or the properties of voltage-gated K+ currents occur during normal cardiac development, as well as in conjunction with myocardial damage or disease, and there is considerable interest in understanding the molecular mechanisms underlying these changes. Although there is evidence for transcriptional, translational, and posttranslational regulation of functional voltage-gated K+ channel expression, we are only beginning to understand the underlying mechanisms; further studies focussed on delineating the molecular mechanisms controlling functional K+ channel expression are clearly warranted.

Abnormal heart rate regulation in GIRK4 knockout mice

Acetylcholine (ACh) released from the stimulated vagus nerve decreases heart rate via modulation of several types of ion channels expressed in cardiac pacemaker cells. Although the muscarinic-gated potassium channel I(KACh) has been implicated in vagally mediated heart rate regulation, questions concerning the extent of its contribution have remained unanswered. To assess the role of I(KACh) in heart rate regulation in vivo, we generated a mouse line deficient in I(KACh) by targeted disruption of the gene coding for GIRK4, one of the channel subunits. We analyzed heart rate and heart rate variability at rest and after pharmacological manipulation in unrestrained conscious mice using electrocardiogram (ECG) telemetry. We found that I(KACh) mediated approximately half of the negative chronotropic effects of vagal stimulation and adenosine on heart rate. In addition, this study indicates that I(KACh) is necessary for the fast fluctuations in heart rate responsible for beat-to-beat control of heart activity, both at rest and after vagal stimulation. Interestingly, noncholinergic systems also appear to modulate heart activity through I(KACh). Thus, I(KACh) is critical for effective heart rate regulation in mice.

Inner ear defects induced by null mutation of the isk gene

The isk gene is expressed in many tissues. Pharmacological evidence from the inner ear suggests that isk mediates potassium secretion into the endolymph. To examine the consequences of IsK null mutation on inner ear function, and to produce a system useful for examining the role(s) IsK plays elsewhere, we have produced a mouse strain that carries a disrupted isk locus. Knockout mice exhibit classic shaker/waltzer behavior. Hair cells degenerate, but those of different inner ear organs degenerate at different times. Functionally, we show that in mice lacking isk, the strial marginal cells and the vestibular dark cells of the inner ear are unable to generate an equivalent short circuit current in vitro, indicating a lack of transepithelial potassium secretion.

Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart

The delayed rectifier potassium current (IK) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of IK in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at -40 mV and 5 microM external nifedipine was used to block L-type calcium current (ICa,L). Significant IK was observed with pulses to potentials more positive than -30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of IK occurred at -4.9 and -4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant IK tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100-120 ms in both cell types. Pulses of longer duration did not activate further IK and this suggested that only the rapid component of IK, called IKr, was present in either cell type. Moreover, IK tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of IKr. The reversal potential of IK varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of -81, -56 and -32 mV, respectively, which are close to values predicted if the IK channel were highly selective for K. The time constants for deactivation of IK in ventricle and AVN on return to -40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of IK in AVN cells was a distinguishing feature and suggests that there may be differences in the IKr channel protein between ventricular and AVN cells.

Developmental changes in ionic channel activity in the embryonic murine heart

We have isolated murine embryonic atrial and ventricular cells derived from timed-pregnant females at different periods and used patch-clamp procedures to investigate age- and chamber-specific expression of ionic channels in the developing fetal mouse. Our data indicate that L-type Ca2+ channels play a dominant role in excitation during early murine cardiac embryogenesis and that Na+ channel expression increases dramatically just before birth. K+ channel expression is particularly sensitive to changes during development. Neither atrial nor ventricular cells express a slowly activating component of delayed rectification (IKs) until just before birth, and inwardly rectifying channel activity, associated with determination of cellular resting potential, is not markedly apparent until late stages of embryogenesis. Instead, we find robust expression of the ATP-regulated K+ channel at early and late states of embryonic development, which may indicate a novel functional role for this channel during morphogenesis of the heart. These results have important implications for the physiology and development of the murine cardiac conduction system and will also serve as a baseline for future studies designed to investigate developmental changes of ion channel expression in the myocardium of both wild-type and genetically modified mice.