Caffeine induces a transient inward current in cultured cardiac cells.
Electrical excitation of cardiac muscle may sometimes be due to initiation of inward current by the presence of Ca2+ ions at the inner surface of the cell membrane. During digitalis toxicity and other conditions that abnormally augment cellular Ca2+ stores, premature release of Ca2+ from the sarcoplasmic reticulum leads to a transient inward current, which is large enough to initiate premature beats and is accompanied by a transient contractile response. This inward current may be mediated either by electrogenic sodium-calcium exchange or by specific Ca2+-activated cation channels that have recently been characterized in tissue cultures of cardiac myocytes. An obvious question raised by these observations is whether release of the sequestered Ca2+ stores during each normal beat exerts a similar influence on membrane potential. To explore this, chick embryonic myocardial cell aggregates were voltage-clamped during abrupt exposure to caffeine, which is known to release Ca2+ from the sarcoplasmic reticulum. The speed of the perfusion system and the relative absence of diffusion barriers in the tissue-cultured cells allowed the effects of caffeine-induced Ca2+ release to be studied on a time scale comparable to that of a single normal beat. We report here that abrupt exposure of the cells to caffeine produced a transient inward current having similar features to that of digitalis toxicity, and which was both large enough and rapid enough to potentially contribute to the action potential.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002462:Cell Membrane / Q000187:drug effects; D002478:Cells, Cultured; D002642:Chick Embryo; D004558:Electric Stimulation; D004594:Electrophysiology; D006321:Heart / Q000187:drug effects* / Q000502:physiology*; D007473:Ion Channels / Q000378:metabolism; D008433:Mathematics; D012964:Sodium / Q000378:metabolism
D007473:Ion Channels; D002110:Caffeine; D012964:Sodium; D002118:Calcium