title: Inward calcium current and activation of contraction in mammalian myocardial fibers.
abstract: 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.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D002395:Catecholamines / Q000494:pharmacology; D004553:Electric Conductivity; D006321:Heart / Q000502:physiology; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D008670:Metals / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D012508:Sarcolemma / Q000378:metabolism; D012756:Sheep; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
chemical_list: D002395:Catecholamines; D008670:Metals; D013779:Tetrodotoxin; D002118:Calcium