Comparative effects of oxodipine and nifedipine in the isolated guinea-pig heart.

The electromechanical effects of 2 dihydropyridines, oxodipine (OXD) and nifedipine, were compared on isolated guinea-pig atrial and ventricular muscle fibres. Intracardiac conduction times were measured in the Langendorff-perfused guinea-pig heart. On isolated atria OXD and nifedipine produced a dose-dependent decrease in rate and amplitude of contractions, the negative inotropic effect being accompanied by a shortening of the time to peak tension and time for total contraction. In both atrial and ventricular muscle fibres OXD and nifedipine shortened the duration of the action potential without altering the resting membrane potential and the Vmax and amplitude of phase 0. OXD also decreased the amplitude and Vmax and shortened the duration of the slow action potentials and decreased the amplitude of the slow contractions elicited by isoprenaline in K-depolarized papillary muscle fibres. In the Langendorff-perfused heart OXD and nifedipine slowed the conduction through the A-V node at concentrations at which it had no effect on intraatrial and intraventricular conduction times. The depression on A-V conduction was more marked with nifedipine than with OXD. All these results indicate that in the guinea-pig heart OXD, a new dihydropyridine, exhibit a potent and selective inhibition of Ca influx via the slow inward current.