Effects of a new antiarrhythmic agent, OPC‐88117, on the conduction properties and excitability of Langendorff‐perfused rabbit hearts were compared with those of lignocaine. OPC‐88117 above 10−5 m caused a significant prolongation of atrio‐His bundle conduction time (A‐H interval) as well as His bundle‐ventricular conduction time (H‐V interval). Lignocaine above 10−5 m caused a similar prolongation of H‐V interval with a minimum change of A‐H interval. The conduction time from His bundle stimulus to the left ventricle (St‐LV) at a low frequency (1 Hz) was also prolonged by these drugs around 10−4 M. OPC‐88117 above 3 × 10−6 m prolonged the effective and functional refractory periods of His bundle (ERPHB, FRPHB) as well as the effective refractory period of left ventricular muscle (ERPVM) in a dose‐dependent manner. ERPHB, FRPHB and ERPVM were, shortened by low concentrations of lignocaine (3 × 10−6‐10−5 M), but were prolonged by high concentrations (3 × 10−5‐10−4 M). Lignocaine (3 × 10−6‐10−4 M) induced an upward displacement of the His bundle refractory curve, indicating a greater intraventricular conduction delay for premature excitation. In experiments with OPC‐88117, such an upward displacement was observed only at 10−4 M. These results suggest that the primary electrophysiological effect of OPC‐88117 is a lengthening of ventricular refractoriness, and that at high concentrations it may also exert lignocaine‐like inhibition of ventricular conduction.

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