Effect of inhibitors of slow calcium current on resting calcium efflux from cat papillary muscles

Examination of influence of divalent cations (Co2+, Ni2+, Mn2+) and organic blockers (verapamil and D600) on calcium efflux from resting mammalian myocardium shows that they either inhibit or increase transiently 45Ca2+ efflux, depending on the site of action. It seems that those agents whose sites of action are limited to the sarcolemma inhibit Ca2+ efflux. Co2+, Ni2+, verapamil and D600 belong to this group. Mn2+ ions which act also apparently on intracellular structures increase transiently Ca2+ efflux. Such a finding illustrates a diversity in mechanisms of action in a group of agents generally classified as calcium channel blockers.