Contribution of delayed rectifier and inward rectifier to repolarization of the action potential: pharmacologic separation

Local Control Model of a Human Ventricular Myocyte: An Exploration of Frequency-Dependent Changes and Calcium Sparks

Calcium (Ca2+) sparks are the elementary events of excitation-contraction coupling, yet they are not explicitly represented in human ventricular myocyte models. A stochastic ventricular cardiomyocyte human model that adapts to intracellular Ca2+ ([Ca2+]i) dynamics, spark regulation, and frequency-dependent changes in the form of locally controlled Ca2+ release was developed. The 20,000 CRUs in this model are composed of 9 individual LCCs and 49 RyRs that function as couplons. The simulated action potential duration at 1 Hz steady-state pacing is ~0.280 s similar to human ventricular cell recordings. Rate-dependence experiments reveal that APD shortening mechanisms are largely contributed by the L-type calcium channel inactivation, RyR open fraction, and [Ca2+]myo concentrations. The dynamic slow-rapid-slow pacing protocol shows that RyR open probability during high pacing frequency (2.5 Hz) switches to an adapted "nonconducting" form of Ca2+-dependent transition state. The predicted force was also observed to be increased in high pacing, but the SR Ca2+ fractional release was lower due to the smaller difference between diastolic and systolic [Ca2+]SR. Restitution analysis through the S1S2 protocol and increased LCC Ca2+-dependent activation rate show that the duration of LCC opening helps modulate its effects on the APD restitution at different diastolic intervals. Ultimately, a longer duration of calcium sparks was observed in relation to the SR Ca2+ loading at high pacing rates. Overall, this study demonstrates the spontaneous Ca2+ release events and ion channel responses throughout various stimuli.

Comparison of the effect of class IA antiarrhythmic drugs on transmembrane potassium currents in rabbit ventricular myocytes

BACKGROUND

The blockade of cardiac transmembrane potassium channels, which is commonly seen with various antiarrhythmic drugs, plays an important role in their mechanism of action. We studied and compared the less-explored effects of three Class IA antiarrhythmics on the transient outward current (I(to)) and on the inward rectifier (I(kl)), ATP sensitive (I(KATP)), and delayed rectifier (I(K)) potassium currents in rabbit ventricular myocytes.

METHODS AND RESULTS

Transmembrane currents were measured by applying the whole-cell configuration of the patch-clamp technique at 37 degrees C in myocytes enzymatically isolated from rabbit ventricular preparations. Quinidine (10 microM), disopyramide (10 microM), and procainamide (50 microM) were studied at concentrations close to or exceeding the therapeutic plasma level. All studied drugs significantly decreased the amplitude of I(KATP) (activated by 50 microM pinacidil) and I(K) currents. None of them influenced significantly I(kl). The amplitude of I(to) was decreased by quinidine and disopyramide but was not considerably altered by procainamide. The fast inactivation of I(to) was not changed by procainamide and was significantly accelerated by quinidine and disopyramide.

CONCLUSION

Although quinidine, disopyramide, and procainamide are all classified as Class IA antiarrhythmics, these drugs had different effects on various potassium currents, which may partially explain their distinct effect on repolarization in various cardiac tissues and on cardiac arrhythmias in clinical settings.

Terikalant and barium decrease the area of vulnerability to ventricular fibrillation induction by T-wave shocks

The area of vulnerability (AOV) to ventricular fibrillation (VF) induction by high-voltage shocks has been proposed as a measure of vulnerability to VF. Biphasic shocks spanning the T wave and ranging between 50 V and the upper limit of vulnerability (ULV) to VF were delivered before and after terikalant (1 mg/kg) and barium (1.1 mg/kg load followed by 0.05-0.10 mg/kg/min maintenance) or vehicle in dogs. The AOV decreased by 34% and 28% (p < 0.01) after terikalant and barium (n = 8 dogs each), respectively. Mean ULV, defibrillation threshold (DFT), and ventricular vulnerability period (VVP) decreased by 16%, 23%, and 31% (p < 0.01), respectively, after terikalant, and by 25%, 17% (p < 0.01), and 13% (p = 0.08), respectively, after barium. Vehicle (n = 14) did not significantly alter any of these variables. The ULV was correlated with the DFT before and after terikalant (r = 0.78, p < 0.01) and barium (r = 0.83, p < 0.01). Potassium channel blockers of the current reduce the ability to induce VF; this effect may be related to the anti-fibrillatory action of class III anti-arrhythmic drugs and their ability to decrease DFT.

The Novel Class III Antiarrhythmic Agent MS-551 Blocks the Cardiac Inward Rectifier With Greater Potency Than Sotalol or E-4031: Possible Relevance to Reverse Use Dependence

BACKGROUND: The tendency for the electrophysiologic effect of class III antiarrhythmic agents (action potential prolongation) to be diminished at faster heart rates represents a major drawback of this class of drug and is usually referred to as "reverse use dependence." A novel class III agent, MS-551, has recently been reported to exhibit less reverse use dependence than E-4031. We set out to investigate whether this observation may be due to differential blockade of the inward rectifier current (i(K1)) by these drugs. METHODS AND RESULTS: We recorded i(K1) using single channel methods and cell attached patch configurations, with standard patch clamp technology. Neither E-4031 nor racemic sotalol in concentrations up to 100 µM had any significant effect on the open probability or kinetics of i(K1) without altering the single-channel conductance. Openings to subconductance levels were abolished in three of six patches in which they had been frequently present in the absence of drug. MS-551 had no effect on mean channel open time but increased the slower component of the closed time. CONCLUSIONS: MS-551, unlike E-4031 and sotalol, appears to produce significant blockade of the inwardly rectifying potassium channel at clinically relevant concentrations. We propose that this might provide a partial explanation for the observed differences in their response to rate changes.

Mechanism of action potential prolongation by RP 58866 and its active enantiomer, terikalant. Block of the rapidly activating delayed rectifier K+ current, IKr

BACKGROUND

The class III antiarrhythmic agent RP 58866 and its active enantiomer, terikalant, are reported to selectively block the inward rectifier K+ current, IK1. These drugs have demonstrated efficacy in animal models of cardiac arrhythmias, suggesting that block of IK1 may be a useful antiarrhythmic mechanism. The symmetrical action potential (AP)-prolonging and bradycardic effects of these drugs, however, are inconsistent with a sole effect on IK1.

METHODS AND RESULTS

We studied the effects of RP 58866 and terikalant on AP and outward K+ currents in guinea pig ventricular myocytes. RP 58866 and terikalant potently blocked the rapidly activating delayed rectifier K+ current, IKr, with IC50S of 22 and 31 nmol/L, respectively. Block of IK1 was approximately 250-fold less potent; IC50S were 8 and 6 mumol/L, respectively. No significant block of the slowly activating delayed rectifier, IK1, was observed at < or = 10 mumol/L. The phenotypical IKr currents in mouse AT-1 cells and Xenopus oocytes expressing HERG were also blocked 50% by 200 to 250 nmol/L RP 58866 or terikalant, providing further conclusive evidence for potent block of IKr. RP 58866 < or = 1 mumol/L and dofetilide increased AP duration symmetrically, consistent with selective block of IKr. Only higher concentrations (> or = 10 mumol/L) of RP 58866 slowed the rate of AP repolarization and decreased resting membrane potential, consistent with an additional but substantially less potent block of IK1.

CONCLUSIONS

These data demonstrate that RP 58866 and terikalant are potent blockers of IKr and prompt a reinterpretation of previous studies that assumed specific block of IK1 by these drugs.

HERG, a primary human ventricular target of the nonsedating antihistamine terfenadine

BACKGROUND

Administration of the antihistamine terfenadine (Seldane) to patients may result in acquired long QT syndrome and ventricular arrhythmias. One human cardiac target is Kv1.5, which expresses the ultrarapid outward K+ current (Ikur) in atrium but may play only a minor role in ventricle. Another possible target is HERG, the human ether-a-go-go-related gene that expresses a delayed rectifier current (IKr) in human ventricle and produces hereditary long QT syndrome when defective.

METHODS AND RESULTS

We therefore heterologously expressed Kv1.5 and HERG in Xenopus oocytes to compare the sensitivity of each to terfenadine. We found that HERG was 10 times more sensitive than Kv1.5 to terfenadine block. The apparent Kd values for HERG and Kv1.5 currents were 350 nmol/L and 2.7 mumol/L, respectively. These Kd values compare well with values reported for terfenadine block of IKr and IKur currents in human atrial myocytes. The Kd value for HERG block is relevant to the toxicity of the antihistamine, since the clinical terfenadine concentrations in human plasma may reach the 100 nmol/L range.

CONCLUSIONS

Terfenadine carboxylate, the major metabolite of terfenadine, does not block either HERG or Kv1.5, which agrees with the hypothesis that the buildup of parent terfenadine is the likely explanation for its cardiotoxicity. We propose that the blocking of HERG by terfenadine explains the acquired long QT syndrome. HERG is likely to be the primary target for the known cardiotoxic effects of other, related antihistamines.

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

Two components of delayed rectifier current in canine atrium and ventricle. Does IKs play a role in the reverse rate dependence of class III agents?

Because the number and characteristics of delayed rectifier K+ current (IK) components vary between species, the role of each component in the action potential and modulation by class III agents is uncertain. To address these issues, IK was assessed in adult isolated canine ventricular and and atrial myocytes by using whole-cell and perforated-patch techniques. IK components were characterized by using two complementary approaches: a kinetic approach (based on biexponential fits to deactivating tail currents) and a pharmacological approach approach (using the methanesulfonanilide compound E-4031). In ventricular myocytes, two exponential tail current components were distinguished; these components differed in the voltage and time dependence of activation and the effect of lower (K+). Both kinetic components contributed equally to peak tail current amplitude (measured at -35 mV) after a single 300-ms pulse to 5 mV, simulating an action potential. By use of E-4031, rapidly and slowly activating components described kinetically were identified. The activation kinetics and rectification properties of canine IKr and IKs are qualitatively similar to those described previously for guinea pigs. In contrast, canine IKr and IKs deactivation kinetics differed markedly from those found in guinea pigs, with canine IKr deactivating slowly (time constant tau, 2 to 3 s near -35 mV) and IKs deactivating rapidly (tau, 150 ms near -35 mV and decreasing to 30 ms near -85 mV). E-4031 elicited reverse rate-dependent effects (greater drug-induced prolongation of the action potential at slower stimulation rates); this effect is inconsistent with the hypothesis attributing reverse rate dependence to incomplete IKs deactivation during rapid stimulation (due to rapid deactivation of canine IKs). Two IK components with characteristics comparable to those found in ventricular myocytes were also observed in atrial myocytes. In conclusion, (1) IKr- and IKs-like components of IK are present in canine atrial and ventricular myocytes, with deactivation kinetics strikingly different from those found in guinea pigs, and (2) the rapid deactivation kinetics of canine IKs do not support its role in reverse rate dependence with class III agents in this species.

Two components of the delayed rectifier K+ current in ventricular myocytes of the guinea pig type. Theoretical formulation and their role in repolarization

Two distinct delayed rectifier K+ currents, IKr and IKs, were found recently in ventricular cells. We formulated these currents theoretically and investigated their roles in action potential repolarization and the restitution of action potential duration (APD). The Luo-Rudy (L-R) model of the ventricular action potential was used in the simulations. The single delayed rectifier K+ current in the model was replaced by IKr and IKs. Our results show that IKs is the major outward current during the plateau repolarization. A specific block of either IKr or IKs can effectively prolong APD to the same degree. Therefore, either channel provides a target for class III antiarrhythmic drugs. In the simulated guinea pig ventricular cell, complete block of IKr does not result in early afterdepolarizations (EADs). In contrast, > 80% block of IKs results in abnormal repolarization and EADs. This behavior reflects the high IKs-to-IKr density ratio (approximately 8:1) in this cell and can be reversed (ie, IKr block can cause EADs) by reducing the ratio of IKs to IKr. The computed APD restitution curve is consistent with the experimental behavior, displaying fast APD variation at short diastolic intervals (DIs) and downward shift at longer DIs with the decrease of basic drive cycle length (BCL). Examining the ionic currents and their underlying kinetic processes, we found that activation of both IKr and IKs is the primary determinant of the APD restitution at shorter DIs, with Ca2+ current through L-type channels (ICa) playing a minor role. The rate of APD change depends on the relative densities of IKr and IKs; it increases when the IKr-to-IKs density ratio is large. The BCL-dependent shift of restitution at longer DIs is primarily attributed to long-lasting changes in [Ca2+]i. This in turn causes different degrees of Ca(2+)-dependent inactivation of ICa and different degrees of Ca(2+)-dependent conductance of IKs at very long DIs (> 5 s) for different BCLs. This BCL dependence of ICa and IKs that is secondary to long-lasting changes in [Ca2+]i is responsible for APD changes at long DIs and can be viewed as a "memory property" of cardiac cells.

Regulation of potassium channels by nonsedating antihistamines

BACKGROUND

Terfenadine and astemizole are widely prescribed nonsedating antihistamines that have been associated with QT-interval prolongation and ventricular arrhythmias. Since potassium channels are intrinsically involved in repolarization, this study was designed to evaluate the effect of the nonsedating antihistamines on potassium channel modulation.

METHODS AND RESULTS

The whole-cell patch-clamp technique was used to study K+ currents in enzymatically isolated rat and guinea pig ventricular myocytes. Three distinct K+ channels were examined: the inward rectifier (IK1), the delayed rectifier (IK), and the transient outward (I(to)) currents. The dialyzing pipette solution was buffered with EGTA, and ionic channels other than potassium were pharmacologically inhibited or electrically inactivated. Both astemizole and terfenadine suppressed the IK1 channel by 17% to 50% in a voltage-dependent manner in rat and guinea pig myocytes. Ito was evaluated in rat ventricular myocytes. Both drugs also inhibited the maintained component of I(to) to a lesser extent, by 23%, in a dose-dependent, reversible manner. IK was examined mainly in guinea pig myocytes. Terfenadine but not astemizole slightly inhibited IK, by 9%, and only at higher drug concentrations. The medications had dose-dependent inhibitory actions, with specific K+ channel suppression evident only beginning at concentrations > 0.1 mumol/L.

CONCLUSIONS

These findings suggest that the mechanism of action of the rare proarrhythmic effects of the nonsedating antihistamines appears to be secondary to potassium channel blockade. A significant voltage-dependent blockade of the IK1 channel was demonstrated, as well as additional inhibitory effects on I(to) and IK channels. These actions lead to delayed repolarization, QT interval prolongation, and enhanced susceptibility to the development of premature ventricular depolarizations. Caution is advised in the prescription of nonsedating antihistamines, particularly in patients at risk of elevated serum levels of the antihistamine or patients with existing repolarization abnormalities.

Cloning and functional expression of an inwardly rectifying K+ channel from human atrium

The cardiac inward rectifier current (IK1) contributes to the shape and duration of the cardiac action potential and helps to set the resting membrane potential. Although several inwardly rectifying K+ channels (IRKs) from different tissues have been cloned recently, the nature and number of K+ channels contributing to the cardiac IK1 are presently unknown. To address this issue in human heart, we have used the reverse-transcriptase-polymerase chain reaction (PCR) technique with human atrial total RNA as a template to identify two sequences expressed in heart that are homologous to previously cloned IRKs. One of the PCR products we obtained was virtually identical to IRK1 (cloned from a mouse macrophage cell line); the other, which we named hIRK, exhibited < 70% identity to IRK1. A full-length clone encoding hIRK was isolated from a human atrial cDNA library and functionally expressed in Xenopus oocytes. This channel, like IRK1, exhibited strong inward rectification and was blocked by divalent cations. However, hIRK differed from IRK1 at the single-channel level: hIRK had a single-channel conductance of 36 pS compared with 21 pS for IRK1. We have identified single channels of 41, 35, 21, and 9 pS in recordings from dispersed human atrial myocytes. However, none of these atrial inward rectifiers exhibited single-channel properties exactly like those of cloned hIRK expressed in oocytes. Our findings suggest that the cardiac IK1 in human atrial myocytes is composed of multiple inwardly rectifying channels distinguishable on the basis of single-channel conductance, each of which may be the product of a different gene.

Frequency-dependent effects of E-4031, almokalant, dofetilide and tedisamil on action potential duration: no evidence for "reverse use dependent" block

Antiarrhythmic drugs with class III action are incriminated by "reverse use dependency" which implies preferential block of resting channels (Hondeghem and Snyders 1990). The purpose of the present study was to investigate the frequency dependence of the effects of four new antiarrhythmic compounds on action potential duration (APD) in guinea-pig papillary muscle and on delayed rectifier in guinea-pig ventricular myocytes in order to scrutinize the concept of reverse use dependency and to obtain evidence for drug-channel interaction. In guinea-pig papillary muscles, E-4031 (1-[2-(6-methyl-2-pyridyl)ethyl]-4- (4-methylsulfonyl-aminobenzoyl)piperidine), almokalant, dofetilide and tedisamil prolonged APD in a concentration-dependent manner. Drug-induced APD prolongation was not affected significantly by low rates of stimulation (0.2 to 0.5 Hz. In order to investigate whether drug-channel interaction takes places during rest, regular stimulation (1 Hz) was interrupted by three 30-min periods of quiescence. Drug was added at the beginning of the second period of rest, the third period was interposed at steady state of drug action. With E-4031 and dofetilide no change in shape of the first AP after the initial 30 min of drug exposure was observed as compared with pre-drug control, but regular stimulation was required for the full effect to develop. APD did not recover to pre-drug values after the third period of quiescence. With almokalant and tedisamil, however, the first APD after wash-in was already prolonged and the effects increased further with regular pacing. Only with almokalant but not with tedisamil did APD recover during rest.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of disopyramide and flecainide on the kinetics of inward rectifier potassium channels in rabbit heart muscle

1. Standard patch-clamp techniques were used to study the interaction of therapeutic concentrations of flecainide and disopyramide with single inwardly-rectifying potassium channels in cell-attached membrane patches from rabbit ventricular myocytes. 2. Under drug-free conditions, the potassium channels had a conductance of 31 +/- 2 pS (n = 13), a mean open time of 230 +/- 6 ms (n = 11) recorded at the resting cell potential, and an open probability of 0.66 +/- 0.20 (n = 39). The resting potential of the cells studied was -68.5 +/- 3.6 mV (n = 32). 3. Disopyramide did not reduce the open probability of the channel when the cell was voltage-clamped at the resting cell potential. However, disopyramide increased the mean open time of the channel, recorded at the resting cell potential, by 15% at 5 microM and by 29% at 20 microM. The action potential prolonging actions of disopyramide in therapeutic concentrations appear not to be due to blocking the inward rectifier K+ channel. 4. Flecainide (3.0 microM, but not at 0.5 microM) decreased the open probability without changing the conductance of the channel, at 3 microM (51.0 +/- 7.2%, n = 6, P = 0.03) at the resting cell potential. Flecainide increased the mean open time of the channel, recorded at the resting cell potential, by 12% at 3.0 microM. 5. We propose that flecainide stabilized the inward rectifier K+ channel in an inactivated state, without plugging the conducting pore. In addition, it appeared to bind to an open conformation of the channel,since some of the reduction in open probability could be accounted for by the lengthening of the mean open time. The changes in open-state kinetics suggest that this binding may be in the region of the activation gate.

Is the sigma 2 receptor in rat brain related to the K+ channel of class III antiarrhythmic drugs?

The sigma 2 receptor subtype was studied in rat cerebral cortex and in C6 glioma cells homogenates using various compounds including class III antiarrhythmic drugs. The characteristics of (+)-[3H]-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ((+)-[3H]-3-PPP) binding were assessed in competition experiments with different displacers which revealed the presence of sigma 2 receptors. Various class III antiarrhythmic drugs inhibited (+)-[3H]-3-PPP binding with high affinity and their binding affinity was found to correlate with the potency of these compounds to increase the duration of action potentials measured in Purkinje fibers in electrophysiological studies. Since class III antiarrhythmic drugs are known to interact with voltage-dependent K+ channels, the present results provide evidence that the (+)-[3H]-3-PPP binding sites in rat brain possess the characteristics of K+ channels of class III antiarrhythmic drugs.

Specific IK1 blockade: a new antiarrhythmic mechanism? Effect of RP58866 on ventricular arrhythmias in rat, rabbit, and primate

BACKGROUND

The effectiveness of blockade of the inwardly rectifying K+ current (IK1) in prevention of arrhythmias is unknown. We have examined the antiarrhythmic potential of a new selective IK1 blocker, RP58866, in rat, rabbit, and primate (marmoset) isolated hearts in the settings of acute ischemia and reperfusion.

METHODS AND RESULTS

In concentration-response studies (n = 12 per group), the drug reduced ischemia-induced ventricular fibrillation (VF) in rat from control incidence of 100 to 50%, 17% (p < 0.05), and 0% (p < 0.05) at 1, 3, and 10 mumol/L, respectively. RP58866 produced significant bradycardia at the 3- and 10-mumol/L concentrations and significant QT interval widening at all three concentrations (p < 0.05). When rat hearts (n = 12 per group) were paced (5 Hz) via the left atrium to prevent bradycardia, the antiarrhythmic effects of 10-mumol/L RP58866 were unmodified (ischemia-induced VF incidence was reduced by drug from 83% in control hearts to 8%; p < 0.05). Similarly, pacing did not prevent the drug's QT-widening activity at 90% repolarization (QT90 was 64 +/- 3 msec in control hearts versus 128 +/- 17 msec in the presence of 10 mumol/L of drug after 10 minutes of ischemia; p < 0.05). These values are similar to equivalent values in unpaced hearts (65 +/- 3 msec in control hearts versus 159 +/- 15 msec with 10 mumol/L of drug; p < 0.05). In separate groups of rat hearts (n = 10 per group) subjected to 10 minutes of ischemia, reperfusion-induced VF incidence was reduced from 90% in control hearts to 10% (p < 0.05), 0% (p < 0.05), and 0% (p < 0.05) by 1-, 3-, and 10-mumol/L RP58866. To examine whether drug actions were species-specific, we performed further studies in rabbit and primate using the middle concentration of RP58866 (3 mumol/L). Ischemia-induced VF incidence was too low in these species to assess the effects of the drug. However, RP58866 widened QT interval (p < 0.05), slowed heart rate (p < 0.05), and reduced the incidence of reperfusion-induced VF from 67% to 8% (p < 0.05) in rabbit. Furthermore, in the more clinically relevant primate species (marmoset; n = 9-12 per group), RP58866 (3 mumol/L) abolished ischemia-induced VT (36% incidence in control hearts; p < 0.05) and significantly reduced the incidence of ischemia-induced ventricular premature beats from 91% to 33% (p < 0.05). The drug was also effective against reperfusion VF in primates (incidence reduced from 64% in control hearts to 11%; p < 0.05). As in rat and rabbit, RP58866 significantly widened QT interval in primate and caused bradycardia before and during ischemia. RP58866 had no significant influence on coronary flow in any species. Finally, in further studies on rat, QT widening by RP58866 was found to persist relatively unmodified in nonischemic hearts perfused with solution containing K+ elevated to 8 mmol/L to mimic the early ischemic milieu.

CONCLUSIONS

RP58866, a selective IK1 blocker, is a potent and efficacious new antiarrhythmic drug in ischemia and reperfusion in rat, rabbit, and primate. When tested in rat, pharmacological activity was undiminished by cardiac pacing or elevation of extracellular K+.