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Chronotropic effect of nizofenone fumarate in rabbit sino‐atrial node in vitro

1 The effects of nizofenone fumarate were studied on the membrane potentials and currents of rabbit sino‐atrial node preparations by means of the double‐microelectrode voltage clamp method. 2 In spontaneously firing pacemaker cells, nizofenone (above 1 μm) decreased the heart rate. Above 3 μm, nizofenone reduced the maximum upstroke velocity, the amplitude of the action potential and the slope of the phase 4 depolarization, and prolonged the action potential duration at 50% repolarization. 3 Under voltage clamp conditions, nizofenone decreased the slow inward current and the time‐dependent potassium outward current in a dose‐dependent manner. 4 These findings suggest that nizofenone exerts an inhibitory action on the automaticity of sinoatrial node preparations via effects on both inward and outward currents.