Rate-dependent effects of sematilide on ventricular monophasic action potential duration and delayed rectifier K+ current in rabbits

Halothane sensitizes the canine heart to pharmacological IKr blockade

The effects of halothane and pentobarbital on the cardiovascular system were compared using the in vivo canine models. The ventricular repolarization process was longer under the halothane-anesthesia than pentobarbital-anesthesia. Intravenous administration of a selective blocker of rapidly activating delayed rectifier K+ currents (I(Kr)) sematilide prolonged the ventricular repolarization period without affecting the intraventricular conduction under both anesthesia; however, the potency was about 1.5-folds greater under the halothane-anesthesia than pentobarbital-anesthesia. These results suggest that halothane can more effectively sensitize the heart to pharmacological I(Kr) blockade, resulting in the excessive QT interval prolongation. Thus, the halothane-anesthetized canine model can be useful for predicting the in vivo I(Kr) blocking property of new drugs.

The effect of isoproterenol on the class III effect of azimilide in humans

BACKGROUND

Many class III antiarrhythmic agents lose efficacy under beta-adrenergic stimulation and at high heart rates (reverse rate dependence). This effect is thought to be due to selective blockade of the rapidly (I(Kr)), but not the slowly (I(Ks)), activating component of the delayed inward rectifier potassium current. Azimilide is an investigational class III antiarrhythmic agent that blocks both IKr and IKs.

METHODS

We investigated the electrophysiologic effect of azimilide with and without beta-adrenergic stimulation in humans. Right ventricular effective refractory period at cycle lengths of 600 and 400 milliseconds and monophasic right ventricular action potential duration at 90% repolarization at cycle lengths of 250, 300, 400, 500 and 600 milliseconds were measured in 13 patients at baseline. Isoproterenol was then infused to increase the heart rate to 125% of baseline, and the pacing protocol was repeated. Patients then received, in a single-blind randomized manner, azimilide dihydrochloride (4.5 mg/kg intravenous loading dose followed by 0.625 mg/kg/h) plus either isoproterenol at the previous dose, or saline. After measurements were taken, treatment groups were crossed over, the azimilide infusion was continued and the procedure repeated.

RESULTS

Azimilide significantly (P < 0.05) prolonged monophasic action potential duration compared to baseline at all cycle lengths except for the 250 millisecond cycle length. In the presence of isoproterenol, azimilide maintained its class III effect, prolonging the action potential duration at 90% repolarization by a mean of 8.7 +/- 3.9 milliseconds, (3.7 +/- 1.7%), whereas isoproterenol alone shortened the action potential duration at 90% repolarization by -2.6 +/- 3.2 milliseconds (-1.2 +/- 1.4%) (P = 0.0051). Isoproterenol alone shortened the right ventricular effective refractory period by -13.6 +/- 3.4 milliseconds, whereas with isoproterenol in the presence of azimilide, the right ventricular effective refractory period was essentially unaffected (-1.4 +/- 3.4 milliseconds, P = 0.0085).

CONCLUSIONS

Azimilide maintained its class III effect in the presence of isoproterenol and at increased heart rates, suggesting that IKs block may be of particular benefit in these circumstances.

Comparison of the in vivo electrophysiological and proarrhythmic effects of amiodarone with those of a selective class III drug, sematilide, using a canine chronic atrioventricular block model

Amiodarone effectively blocks both the sodium and calcium channels and beta-adrenoceptors, in addition to blocking several potassium currents including IKr, IKs, Ito, IK1, IKACh and IKNa. The incidence of clinical torsade de pointes (TdP) associated with amiodarone has been reported to be low and the present study compared the proarrhythmic potential of amiodarone with that of a selective IKr channel blocker, sematilide, using a canine chronic atrioventrucular block model. Amiodarone or sematilide (3 and 30 mg/kg; n=4 for each group) was administered orally without anesthesia under continuous ECG monitoring. Both drugs prolonged the QT interval, although the onset was faster for sematilide. The high dose of sematilide induced TdP in 3 of 4 animals, which caused their death, but neither the low dose of sematilide nor the 2 dosages of amiodarone induced lethal ventricular arrhythmias. These results suggest that IKr channel inhibition by amiodarone with its additional ion channel blocking action may contribute to the prevention of TdP.

Effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes

The effects of a typical I(Kr) channel blocker sematilide on the relationship between ventricular repolarization, refractoriness and onset of torsades de pointes (TdP) were studied using the canine isolated, blood-perfused ventricular septum preparation with monophasic action potential (MAP) recording. Intracoronary infusion of sematilide (10-100 microg/min) prolonged the repolarization phase and effective refractory period, the extent of which was greater in the former than in the latter, resulting in prolongation of terminal repolarization process. Prolonging the basic pacing cycle length from 400 to 600 ms and/or increasing the drug doses enhanced each of these actions. Reverse use-dependence was obvious in the drug-induced prolongation of MAP duration, but it was less clear in the effective refractory period. More importantly, during sematilide infusion, in preparations paced at longer basic cycle length of 600 - 2000 ms, TdP-like polymorphic ventricular tachycardia was repeatedly induced by an extra-stimulus applied on the terminal repolarization phase, which indicates the appearance of electrically vulnerable period. Prolonging the basic pacing cycle length and/or increasing the drug doses prolonged this electrically vulnerable period in parallel with the terminal repolarization phase. These results suggest that prolongation of the terminal repolarization process by sematilide would enhance the chance of conduction slowing at less complete repolarization levels, which may be associated with a high incidence of TdP induction.

Overexpression of a human potassium channel suppresses cardiac hyperexcitability in rabbit ventricular myocytes

The high incidence of sudden death in heart failure may reflect abnormalities of repolarization and heightened susceptibility to arrhythmogenic early afterdepolarizations (EADs). We hypothesized that overexpression of the human K+ channel HERG (human ether-a-go-go-related gene) could enhance repolarization and suppress EADs. Adult rabbit ventricular myocytes were maintained in primary culture, which suffices to prolong action potentials and predisposes to EADs. To achieve efficient gene transfer, we created AdHERG, a recombinant adenovirus containing the HERG gene driven by a Rous sarcoma virus (RSV) promoter. The virally expressed HERG current exhibited pharmacologic and kinetic properties like those of native IKr. Transient outward currents in AdHERG-infected myocytes were similar in magnitude to those in control cells, while stimulated action potentials (0.2 Hz, 37 degrees C) were abbreviated compared with controls. The occurrence of EADs during a train of action potentials was reduced by more than fourfold, and the relative refractory period was increased in AdHERG-infected myocytes compared with control cells. Gene transfer of delayed rectifier potassium channels represents a novel and effective strategy to suppress arrhythmias caused by unstable repolarization.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

Modulation of quinidine-induced arrhythmias by temperature in perfused rabbit heart

We used low temperature to slow ion channel kinetics and studied the electrophysiological effects of quinidine at different pacing rates in isolated rabbit hearts. Fifteen epicardial electrograms together with an endocardial monophasic action potential were recorded. Epicardial activation and local recovery times were measured. Arrhythmias together with the characteristics of their mode of induction and rate were analyzed by epicardial activation sequence mapping. In the presence of quinidine, arrhythmias consistent with both triggered activity and reentry were observed. At baseline, triggered activity was not inducible, even though at 25 degrees C the recovery time was greater than that in the presence of quinidine at 36 degrees C. Also, with quinidine, the incidence of triggered activity decreased at 30 and 25 degrees C. Therefore prolongation of the recovery time per se does not cause triggered activity. Quinidine's use-dependent effects on conduction and reverse use-dependent effects on recovery time were amplified by low temperatures. These findings can be understood in terms of the known temperature sensitivities of the kinetics of the membrane ion channels responsible for activation and recovery. The results demonstrate that temperature can be used as a tool to elucidate mechanisms of drug action.

Development and evaluation of a fully automated monophasic action potential analysis program

A fully automated program has been developed for analysing digitised cardiac monophasic action potentials (MAPs) of several animal species. Under diverse conditions, the program's performance is evaluated by comparison with high-resolution manual analysis of action potential duration, MAP detection, baseline and plateau determination. For each variable, the differences between both forms of analysis are categorised according to species (rabbit, dog or baboon), intervention (control, sematilide or LY-190147), rhythm source (sinus or paced), and rates. They are then examined using ANOVA (null hypothesis: no difference between methods for any category). Intra-group differences are significant for baseline selection (p < 0.05), and treatment effects are significant (p < 0.05) for one species (baboon). However, the differences found are negligible when compared with expected measurement errors. Thus the program reliably detects and accurately analyses MAPs, independent of species, drug or pacing. The program's strength is its use of simple heuristic algorithms to identify and profile MAP signals while rejecting spurious transients. These algorithms focus on the procedural aspects of automatic MAP detection and validation, with minimum use of complex mathematics. The program's estimated throughput exceeds 8200 MAPs min-1 on a 50 MHz 1486 DX machine. With available data format conversion packages, signals from almost any data-acquisition source can be analysed.