Inward calcium current and activation of contraction in mammalian myocardial fibers.

Electrophysiological and tracer studies show an increase in Ca permeability of cardiac cell membranes during excitation. The Ca influx (ICa) during the plateau of the cardiac action potential is a voltage- and time-dependent electrogenic process. The following results were obtained by simultaneous recordings of twitch tension (T) and membrane ionic currents under voltage clamp conditions. 1) The threshold potentials for T and ICa are the same. 2) T decreases when the reversal potential by ICa is approached during depolarization. 3) The voltage and time dependences of recovery of ICa and of T from inactivation are very similar. 4) Catecholamines and methylxanthines which exert a positive inotropic effect increase ICa. 5) Substances which reduce ICa (La3+, Co2+, Mn2+, verapamil, and D 600) cause a negative inotropic effect. Despite these parallelisms the relation between ICa and T is indirect since during repetitive identical depolarizations ICa decreases while T increases (staircase). This can be explained by a store (sarcoplasmic reticulum?) into which Ca ions flow and from which they are released. Other factors like Na-Ca exchange influence the filling of these stores.