Reverse Rate Dependence Research Articles

Electrophysiological effects of risperidone in mammalian cardiac cells.

In this study, the effects of risperidone, the widely used antipsychotic drug, on isolated canine ventricular myocytes and guinea-pig papillary muscles were analyzed using conventional microelectrode and whole cell voltage-clamp techniques. Risperidone concentration-dependently lengthened action potential duration in guinea-pig papillary muscles (EC(50)=0.29+/-0.02 micro M) and single canine ventricular myocytes (EC(50)=0.48+/-0.14 micro M). This effect was reversible, showed reverse rate dependence, and it was most prominent on the terminal portion of repolarization. No significant effect of risperidone on the resting membrane potential, action potential amplitude or maximum rate of depolarization was observed. In voltage-clamped canine ventricular myocytes risperidone caused concentration-dependent block of the rapid component of the delayed rectifier K(+) current ( I(Kr)), measured as outward current tails at -40 mV, with an IC(50) of 0.92+/-0.26 micro M. Suppression of I(Kr) was not associated with changes in activation or deactivation kinetics. High concentration of risperidone (10 micro M) suppressed also the slow component of the delayed rectifier K(+) current ( I(Ks)) by 9.6+/-1.5% at +50 mV. These effects of risperidone developed rapidly and were readily reversible. Risperidone had no significant effect on the amplitude of other K(+) currents ( I(K1) and I(to)). The inhibition of cardiac I(Kr) current by risperidone may explain the cardiac side-effects observed occasionally with the drug. Our results suggest that risperidone displays class III antiarrhythmic properties, and as such, may produce QTc prolongation, especially in patients with long QT syndrome. Therefore, in psychotic patients having also cardiac disorders, ECG control may be suggested during risperidone therapy.

Role of β-adrenoceptors in reverse rate-dependency of dofetilide-induced ERP lengthening in guinea-pig atrium, in vitro

Role of β-adrenoceptors in reverse rate-dependency of dofetilide-induced ERP lengthening in guinea-pig atrium, in vitro

Synthesis and Preliminary Characterization of a Novel Antiarrhythmic Compound (KB130015) with an Improved Toxicity Profile Compared with Amiodarone

Recent developments in antiarrhythmic therapy have indicated that the best approach to pharmacologically controlling supraventricular arrhythmias and life-threatening ventricular tachyarrhythmias is by prolonging cardiac repolarization rather than by blocking conduction. In this context, amiodarone has emerged as the most potent compound, but its universal use has been limited by its toxicity profile. There are data to suggest that an important component of amiodarones antiarrhythmic action might be mediated via inhibition of thyroid hormone action in the heart. Therefore, a new series of carboxymethoxybenzoyl and benzyl derivatives of benzofuran has been prepared and evaluated as thyroid hormone receptor antagonists. Within this series, 2-methyl-3-(3,5-diiodo-4-carboxymethoxybenzyl)benzofuran KB130015 (7) was found to reveal the most promising in vitro data. It inhibits the binding of (125)I-T(3) to the human thyroid hormone receptors (hThR) alpha(1) and beta(1). T(3)-Antagonism was confirmed in reporter cell assays employing CHOK1 cells (Chinese hamster ovary cells) stably transfected with hThR alpha(1) or hThR beta(1) and an alkaline phosphatase reporter gene downstream a thyroid response element. The derived IC(50) values were 2.2 microM for hThR alpha(1) and 4.1 microM for hThR beta(1). Compound 7 was selected for further characterization of chronic effects on ventricular papillary muscle by transmembrane electrophysiology after daily intraperitoneal injection of the ligand (40 mg/kg body weight) in guinea pigs. Compound 7 was found to prolong the action potential duration at 90% (APD(90)) repolarization time (219 +/- 22 ms, control: 186 +/- 9 ms, p < 0.01) without exhibiting any reverse rate dependency of action in a manner similar to that of amiodarone. In general, preliminary tolerance experiments with 7 demonstrated an improved safety profile compared to that of amiodarone. In summary, 7 appears to be less toxic than amiodarone while maintaining its electrophysiologic properties consistent with antiarrhythmic activity. Its potential antiarrhythmic actions warrant further investigations.

Rate dependence of the effect of antiarrhythmic drugs delaying cardiac repolarization: an overview.

Prolongation of the cardiac action potential and refractoriness (class III effect) is a potentially beneficial electrophysiological mechanism of action. However, this effect may be diminished or eliminated at rapid heart rates, so-called 'reverse rate dependence' of prolongation of repolarization. Action potential duration normally shortens as heart rate increases, due to increases in outward repolarizing currents, and/or decreases in inward depolarizing currents. The assessment of the effect of drugs on action potential duration is complicated by inter-species differences in ionic currents mediating repolarization, heterogeneity within the heart in repolarizing currents, and differential effect of drugs in different species, during differing experimental conditions, and at different rates of stimulation. In general, most drugs which predominantly block the IKr repolarizing current exhibit reverse rate-dependent effects on cardiac repolarization. Drugs or combinations of drugs which produce multiple ionic channel blocks and possibly those which block the IKs current, may be less prone to this potentially undesirable effect.

Chronic amiodarone effects on epicardial conduction and repolarization in the isolated porcine heart.

Amiodarone is a potent antiarrhythmic agent with complex chronic effects, notably on repolarization and conduction, that are not fully understood. Its low arrhythmogenic potential has been related to a lack of increase in repolarization dispersion. Since its effects are not documented in pigs we conducted a mapping study of activation and repolarization in isolated perfused porcine hearts. Amio20 female pigs (n = 7) received amiodarone 20 mg/kg per day over 4 weeks while Amio50 female pigs (n = 7) received 50 mg/kg per day over 4 weeks. Concentrations of the drug encompassed values found in clinical studies. Then, activation patterns and activation-to-recovery intervals (ARI) were mapped epicardially from 128 unipolar electrograms in isolated perfused hearts in corroboration of epicardial action potential recordings. Mean ARI was longer in Amio20 experiments compared to the seven control hearts (325 +/- 11 ms vs 288 +/- 5 ms at 1,000 ms), whereas ARI dispersion was not different, being comprised between 7 and 11 ms and generating smooth gradients. In Amio50 experiments, mean ARI was further prolonged (390 +/- 10 ms at 1,500 ms) with an exaggerated reverse rate dependence concomitant with a depressant effect on the plateau of the action potential. Again, ARI dispersion did not differ from controls. Finally, the drug depressed the maximal rate of depolarization (Vmax) and slowed conduction in a rate dependent and concentration dependent fashion. In conclusion, chronic amiodarone induces Class I and Class III antiarrhythmic effects in ventricular porcine epicardium that are concentration dependent but does not affect dispersion of repolarization. This may partly explain its low arrhythmogenic potential.

Effects of mitoxantrone on action potential and membrane currents in isolated cardiac myocytes.

1. The effects of mitoxantrone (MTO), an anticancer drug, on the membrane electrical properties of cardiac myocytes were investigated using the whole-cell clamp technique. 2. In isolated guinea-pig ventricular myocytes, 30 microM MTO induced a time-dependent prolongation of action potential duration (APD) which was occasionally accompanied by early afterdepolarizations. APD prolongation was preserved in the presence of 10 microM tetrodotoxin and showed reverse rate-dependence. 3. Both the inward rectifier K+ current (I(KI)) and the delayed rectifier K+ current (I(K)) of guinea-pig ventricular myocytes were significantly depressed by 30 microM MTO. The rapidly activating component of I(k) (I(Kr)) seemed to be preferentially blocked by MTO. The transient outward current was not affected by MTO in rat ventricular myocytes. 4. Thirty microM MTO had no direct effect on the L-type Ca2+ current (I(Ca(L))), but reversed the inhibitory effect of 1 microM carbamylcholine but not the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (1 microM) on I(Ca(L)) enhanced by 50 nM isoprenaline in guinea-pig ventricular myocytes. In guinea-pig atrial mycotyes, 30 microM MTO inhibited by 93% the muscarinic receptor gated K+ current (I(K,ACh)) evoked by 1 microM carbamylcholine, whereas I(K,ACh) elicited by 100 microM GTPgammaS, a nonhydrolysable GTP analogue, was only decreased by 12%. 5. The specific binding of [3H]QNB, a muscarinic receptor ligand, to human atrial membranes was concentration-dependently displaced by MTO (1-1000 microM). 6. In conclusion, MTO blocks cardiac muscarinic receptors and prolongs APD by inhibition of I(KI) and I(Kr). The occasionally observed early afterdepolarizations may signify a potential cardiac hazard of the drug.

Rate-dependence of antiarrhythmic and proarrhythmic properties of class I and class III antiarrhythmic drugs.

Rate or frequency-dependence is a characteristic property of antiarrhythmic drugs belonging to the Vaughan William classes I and III. The rate-dependence of class I drugs (i.e., increasing blockade of fast Na(+)-channels with faster rates) results from periodical drug binding to Na(+)-channel sites which are preferably available in the activated and/or inactivated channel states (use-dependence). With respect to their binding and unbinding kinetics, class I drugs can be subdivided into three groups (group 1-group 3) which differ in their block-frequency relations as well as in their onset kinetics of channel blockade. These properties can serve as predictors of the anti- and proarrhythmic potential of class I drugs. Class III drugs (blockers of potassium channels) are mostly characterized by reverse rate-dependence (loss of class III action at faster rates). However, this property cannot be attributed to reverse use-dependence, i.e., binding to channels in the rested state. It is more likely due to different rate-dependent contributions of the two components of the delayed rectifier potassium current to repolarization, when the rapidly activating, the rectifying component IKr is specifically blocked by class III drugs, while the slowly activating component IKs remains unchanged. In spite of their reverse rate-dependence, class III drugs exert an antifibrillatory effect when fibrillation is induced by frequent stimulation. This can be attributed to the slow time course of the decline (offset kinetics) of the class III effect accompanying a sudden increase in frequency. Proarrhythmic effects of class III drugs result from the delay in repolarization that may favor the development of early afterdepolarizations. The proarrhythmic potential of class III drugs is species dependent and is favored if the contribution of IKr to the repolarization phase of the action potential is comparatively large.

Dynamic analysis of dofetilide-induced changes in ventricular repolarization.

To use dynamic electrocardiographic (ECG) techniques to study the influence of heart rate on dofetilide-induced QT prolongation among healthy volunteers. The extent to which heart rate modulates QT prolongation induced by the new class III antiarrhythmic drug dofetilide is a matter of debate. Ten healthy volunteers underwent two 24-hour ECG recordings, one in the absence of dofetilide and the other after a single oral dose of 0.5 mg dofetilide. Two 4-hour periods were defined during the second recording: Dh, which corresponded to stable high concentration of the drug, and D1, which corresponded to low concentration of the drug. Corresponding baseline recording periods, Ch and C1, matched by time with Dh and D1 were selected from the control ECG recording in the absence of dofetilide. QT versus R-R relations were compared in the presence and absence of dofetilide. The QT versus R-R relation slope was used as an index of the rate dependence QT prolongation. Rate-independent changes in QT duration were also analyzed. During Dh, dofetilide induced a mean 12% lengthening of ventricular repolarization. Dynamic ECG analysis showed that this prolongation increased as R-R cycles became longer, a phenomenon known as reverse rate dependence. However, QT prolongation persisted at the shortest (600 ms) R-R cycle length that could be analyzed. During D1, dynamic ECG analysis showed a persistent, although small, effect of dofetilide on both QT prolongation (3%) and reverse rate dependence of this effect. Dofetilide prolongs QT duration, and this class III effect is influenced by heart rate. Although dofetilide-induced QT prolongation decreases when the R-R cycle shortens, this reverse rate dependence is only partial because marked QT prolongation persists at an R-R cycle of 600 ms. The results of our study indicated that dynamic ECG techniques can be useful in detection of subtle, drug-induced changes in the duration of ventricular repolarization.

Differential effects of d-sotalol on subendocardial Purkinje myocytes isolated from normal or 10 to 14 days postinfarction canine hearts: role of extracellular potassium concentration.

Electrophysiologic properties of surviving Purkinje cardiomyocytes in the late postmyocardial-infarction phase are not well established. By using standard microelectrode techniques, we evaluated the effects of the class III agent d-sotalol on action potential parameters of single Purkinje cardiomyocytes isolated from normal canine hearts or those 10-14 days after infarction. Measurements were obtained at 2.5, 3.5, and 6 mM extracellular potassium concentrations. Action-potential parameters recorded at baseline did not differ significantly between normal and infarct-surviving Purkinje cardiomyocytes. At 3.5 and 6 mM extracellular potassium concentrations, surviving Purkinje cells appeared to be more sensitive to the effects of d-sotalol than normal Purkinje cells. In contrast, at 2.5 mM extracellular potassium concentration, the differential responses of normal and infarct-surviving Purkinje cells to d-sotalol was abolished. Reverse rate dependence was more prominent in normal than in postinfarction Purkinje cells, independent of the extracellular potassium concentration studied. The previously described enhanced sensitivity of subacutely infarcted tissue to class III agents seems to persist on a cellular level 10-14 days after myocardial infarction, even after full normalization of baseline action-potential parameters. Differential membrane-regulation mechanisms, dependent on the extracellular potassium concentrations, may account for the increased susceptibility to antiarrhythmia agents in the late postinfarction phase.

Divergent effect of acute ventricular dilatation on the electrophysiologic characteristics of d,l-sotalol and flecainide in the isolated rabbit heart.

The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l-sotalol and flecainide. The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid-filled balloon placed in the left ventricle. Pacing was performed alternately using epi- and endocardial monophasic action potential (MAP)-pacing catheters at cycle lengths from 1,000 to 300 msec. d,l-Sotalol (10 microM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5% +/- 3.1%), MAP duration (+14.9% +/- 3.2%), and QT interval (+15.5% +/- 4.1%) (mean +/- SEM at 1,000 msec). These effects had a reverse rate-dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of AVD, d,l-sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2 microM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate-dependent increase of pacing thresholds (+47.6% +/- 8.2%), prolongation of QRS (+48.8% +/- 5.6%), and conduction time (+78.6% +/- 8.6%) (mean +/- SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate-dependent prolongation of QRS (+16.7% +/- 5.5%) and conduction time (+17.1% +/- 4.3%). Our data demonstrate that, during AVD, the Class III effect of d,l-sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide-related proarrhythmia in certain clinical situations.

Class III Antiarrhythmic Effects of Dofetilide in Rabbit Atrial Myocardium

BACKGROUND: Dofetilide is a new class III antiarrhythmic agent with demonstrated efficacy in ventricular and atrial tachyarrhythmias. We investigated its class III actions and their modulation by stimulation rate in rabbit atrial myocardium. METHODS AND RESULTS: Standard microelectrode techniques were used to record action potentials from rabbit atrial tissue at varying stimulation rates. Dofetilide produced a dose-dependent prolongation of action potential duration at concentrations from 1 nM to 1 µM at an interstimulus interval of 1000 ms. Action potential duration at 90% repolarization (action potential duration) was prolonged from 116 +/- 11.7 ms in control solutions to 13.9 ms at 1 nM dofetilide and 186 +/- 49.3 ms at 1 µM dofetilide (P <.05 for 1 nM vs control; P <.01 for 1 µM vs control). Reduction of interstimulus interval to 500 ms has no significant effect on action potential duration prolongation by dofetilide (P <.05 for 1 nM vs control; P <.01 for 1 µM vs control). Reduction of interstimulus interval to 500 ms had no significant effect on action potential duration prolongation by dofetilide. At faster rates than this, and particularly at an interstimulus interval less than 330 ms, a marked "reverse rate dependence" of the class III effect was observed. Specifically, the high therapeutic concentration of 10 nM showed no effect on action potential duration at interstimulus interval of 250 ms or 200 ms, and even at a concentration of 30 nM, the small class III effect was no longer statistically significant at these rates. CONCLUSIONS: Dofetilide prolongs action potential duration in rabbit atrial myocardium, but this effect is significantly attenuated at stimulation rates above 2 Hz.

Electrophysiological, rate dependent, and autonomic effects of the class III antiarrhythmic almokalant after myocardial infarction in the pig.

Ventricular arrhythmias remain a major problem, in particular in patients with left ventricular dysfunction or heart failure. In this group of patients, Class I drugs were shown to be ineffective, and they even increased mortality during chronic treatment. New antiarrhythmic agents should preferably not only have pure antiarrhythmic effects, but should also be free from adverse autonomic properties. In the present study, the electrophysiological, rate dependent and autonomic effects of intravenously administered almokalant, a new Class III antiarrhythmic drug, were investigated in nine pigs surviving a myocardial infarction. The ventricular effective refractory period (VERP) increased after almokalant (loading dose: 0.05 mumol.kg-1.min-1, continuous infusion: 0.0025 mumol.kg-1.min-1) from 292 +/- 25 to 308 +/- 13 ms (pacing cycle length [PCL] 500 ms + 1 extrasystole [ES]), from 249 +/- 19 to 261 +/- 16 ms (PCL 400 ms +1ES), and from 209 +/- 18 to 219 +/- 18 ms (PCL 300 ms +1ES). The VERPs increased most after three ES at PCL 400 ms: from 167 +/- 27 to 186 +/- 29 ms (P < 0.05) and at PCL 300 ms: from 150 +/- 29 to 174 +/- 27 ms (P < 0.05). The ventricular monophasic action potential durations (MAPD) were similarly prolonged and the ratio VERP/MAPD did not change. Prolongation of MAPD after almokalant remained present at short pacing cycle lengths. Before almokalant infusion, sustained monomorphic ventricular tachycardia (VT) was inducible in two pigs, and nonsustained VT in a third animal. After almokalant, only one pig remained inducible. Two weeks after myocardial infarction, heart rate variability and baroreflex sensitivity were reduced. Furthermore, subsequent electrophysiological testing transiently reduced these parameters of autonomic activity. During almokalant however, no changes in autonomic functions were observed after programmed stimulation. Heart rate variability decreased after myocardial infarction from 6.3 +/- 2.5 ms to 5.4 +/- 4.2 ms (P = NS). After programmed stimulation, it further decreased to 2.8 +/- 2.0 ms (P = 0.028). Almokalant infusion prevented autonomic deterioration: 3.3 +/- 2.2 ms before stimulation and 3.3 +/- 1.3 after stimulation (P = NS). In postinfarct pigs, almokalant prolongs VERP and MAPD at shorter pacing cycle lengths. The results indicate absence of reverse rate dependence and of adverse autonomic changes.

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.

Effects of Sematilide, a Novel Class III Antiarrhythmic Agent, on Action Potential in Guinea Pig Atrium

Electrophysiological effects of sematilide, a novel class III antiarrhythmic agent, were examined and compared with those of (±)sotalol in guinea pig left atrium by a conventional microelectrode technique. Application of 0.1-1000 μM sematilide or 1-1000 μM (±)sotalol concentration-dependently prolonged the duration of action potentials (APD) that were elicited by electrical stimulation at 1 Hz. Other parameters of action potentials such as the maximum upstroke velocity of phase 0 depolarization, action potential amplitude and resting membrane potential were not affected significantly by these drugs in the concentration ranges employed. The prolongation of APD by sematilide or (±)sotalol was accompanied by a corresponding increase in the effective refractory period (ERP). Approximately a 30% increase in ERP was obtained by the treatment with 5 μM sematilide or 100 μM (±)sotalol, suggesting that sematilide as a class III antiarrhythmic agent is approximately 20 times more potent than (±)sotalol on a molecular basis. When the stimulation rate was increased stepwise from 0.2 to 2 Hz, the relative increase in APD at 90% repolarization by the treatment with sematilide and (±)sotalol was slightly larger at 2 Hz than at 0.2 Hz, indicating that “reverse rate-dependence” was not observed under these conditions. These results may suggest a possibility that sematilide effectively blocks atrial arrhythmia.

Block of the delayed rectifier current (IK) by the 5‐HT3 antagonists ondansetron and granisetron in feline ventricular myocytes

1. We investigated the effects of two 5-HT3 antagonists, ondansetron and granisetron, on the action potential duration (APD) and the delayed rectifier current (IK) of feline isolated ventricular myocytes. Whole-cell current and action potential recordings were performed at 37 degrees C with the patch clamp technique. 2. Ondansetron and granisetron blocked IK with a KD of 1.7 +/- 1.0 and 4.3 +/- 1.7 microM, respectively. At a higher concentration (30 microM), both drugs blocked the inward rectifier (IKl). 3. The block of IK was dependent on channel activation. Both drugs slowed the decay of IK tail currents and produced a crossover with the pre-drug current trace. These results are consistent with block and unblock from the open state of the channel. 4. Granisetron showed an intrinsic voltage-dependence as the block increased with depolarization. The equivalent voltage-dependency of block (delta) was 0.10 +/- 0.04, suggesting that granisetron blocks from the intracellular side at a binding site located 10% across the transmembrane electrical field. 5. Ondansetron (1 microM) and granisetron (3 microM) prolonged APD by about 30% at 0.5 Hz. The prolongation of APD by ondansetron was abolished at faster frequencies (3 Hz) showing reverse rate dependence. 6. In conclusion, the 5-HT3 antagonists, ondansetron and granisetron, are open state blockers of the ventricular delayed rectifier and show a clear class III action.