Inward calcium current and activation of contraction in mammalian myocardial fibers
Reuter, H.
Recent Advances in Studies on Cardiac Structure and Metabolism 5: 13-18
1975
ISSN/ISBN: 0363-5872 PMID: 1103242 Document Number: 88169
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.