[Role of intracellular Ca-depots in the mechanism of action of caffeine on excitation-contraction coupling in the frog myocardium].

Caffeine inotropic effects on frog atrial and ventricular muscle were studied under different patterns of stimulation. It was found that in the course of repetitive stimulation (0.5/s) caffeine produced transient potentiation and following inhibition of contractions which were accompanied by an increase in the rate of tension development, a decrease of the time to peak tension, an increase in relaxation duration and a decrease in the relaxation rate. Caffeine reduced the increment of contraction amplitude in Bowdich staircase under stimulation of the heart muscle with a series of impulses. It was concluded that caffeine caused the depletion of calcium in the intracellular stores. On the basis of literature and our data a scheme was proposed to account for two-phase caffeine action on the frog myocardium. According to this scheme caffeine acts simultaneously on both stages of excitation-contraction coupling: it alters calcium current and prevents Ca accumulation in the intracellular stores.