Reduction in transmembrane Ca influx induced by the negative inotropic action of nicardipine in frog ventricular muscle.

The effects of nicardipine, a new 1,4-dihydropyridine derivative, on electrical and mechanical properties of frog ventricular muscle were examined. Nicardipine (3 X 10(-7) M) reduced the twitch tension, and this reduction was frequency dependent, and considerable, in case of high frequencies. The resting potential was not affected by nicardipine (3 X 10(-7) M), but the plateau height of the action potential was decreased and the duration of the action potential was shortened. The suppression of this plateau height was frequency dependent. The nicardipine-induced suppression of tension and action potential could be almost completely antagonized by raising the concentration of [Ca]o or by applying isoprenaline (8 X 10(-7) M). These results suggest that the negative inotropic action of nicardipine is induced mainly by a reduction in the transmembrane Ca influx.