In this study the acute effects of ATI-2001, a recently developed ester derivative of amiodarone, on heart rate, atrioventricular conduction and frequency-dependent prolongation of ventricular conduction, repolarization and refractoriness were investigated in guinea pig isolated heart. Compared with amiodarone, an equimolar concentration of ATI-2001 (1 microM) caused significantly greater slowing of heart rate, depression of atrioventricular and intraventricular conduction and prolongation of ventricular repolarization. Unlike amiodarone, the effects of ATI-2001 were significantly reversed during washout of the drug. ATI-2001 exhibited frequency-independent effects on ventricular repolarization and refractoriness. It prolonged base-line ventricular monophasic action potential duration by 10%, 8%, 9% and 9% and ventricular effective refractory period by 24%, 20%, 22% and 26% at cycle lengths of 350, 300, 250 and 200 msec, respectively. Thus, ATI-2001 (1 microM) increased the ventricular effective refractory period/action potential duration ratio, suggesting both time- and voltage-dependent prolongation of ventricular refractoriness. In addition, ATI-2001 lengthened ventricular conduction times (QRS interval and basic conduction time) significantly more at shorter cycle lengths. Conversely, d-sotalol, a pure class III antiarrhythmic agent, had no effect on ventricular conduction times and exhibited a reverse frequency-dependent effect on ventricular repolarization. In summary, the electrophysiological effects of ATI-2001 were greater and more rapidly reversible than those of amiodarone. The lack of reverse frequency-dependent effects on ventricular repolarization and refractoriness suggests that ATI-2001 may be more efficacious than d-sotalol or other pure class III drugs in treating ventricular tachycardias and less likely to become proarrhythmic at normal or slow heart rates.