title: Phenytoin reduces calcium current in the cardiac Purkinje fiber.
abstract: We have investigated the effects of phenytoin on the electrical and mechanical activity of isolated calf and dog cardiac Purkinje fibers. Phenytoin (5-100 microM) lowers and shortens the plateau phase of the Purkinje fiber action potential and reduces twitch tension. We studied the ionic basis of these effects by combining a conventional two-microelectrode voltage clamp procedure with pharmacological techniques that allow separation of time-dependent plateau membrane currents. We find that phenytoin reduces voltage-dependent calcium current in the Purkinje fiber. In addition, this drug slightly reduces two time-dependent outward currents. These effects of phenytoin on membrane current account, at least in part, for its influence on the action potential and twitch tension.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002417:Cattle; D004285:Dogs; D006329:Heart Conduction System / Q000187:drug effects*; D066298:In Vitro Techniques; D009200:Myocardial Contraction / Q000187:drug effects; D010672:Phenytoin / Q000494:pharmacology*; D011690:Purkinje Fibers / Q000187:drug effects*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology
chemical_list: D000644:Quaternary Ammonium Compounds; D010672:Phenytoin; C009405:tetrabutylammonium; D002118:Calcium