title: Overdrive excitation and cellular calcium load in canine cardiac Purkinje fibers.
abstract: The induction of spontaneous activity by drive ("overdrive excitation") was studied by means of a microelectrode technique in canine cardiac Purkinje fibers exposed to an enhanced calcium load. The following results were obtained: 1) in quiescent fibers, a single action potential is followed by a prolonged transitory depolarization ("slow afterdepolarization") that may initiate a slow spontaneous rhythm; 2) during short drives, the maximum diastolic potential (Emax) gradually decreases and diastolic depolarization becomes steeper due to a superimposed oscillatory potential of progressively greater amplitude; 3) after the drive, the oscillatory potential either initiates a fast repetitive activity or is followed by a slow repolarization to the original resting level; 4) the cessation of induced activity is associated with an increase in Emax; 5) during longer and faster drives, Emax increases and the oscillatory potential becomes smaller, peaks sooner and may fail to excite; 6) repetitive activity may also be induced at a depolarized level. We conclude that overdrive excitation involves an increased cellular calcium, can occur at normal or depolarized levels, and is induced by an oscillatory potential superimposed on a slow afterdepolarization. It is most easily initiated by a short and fast drive because at that time the oscillatory potential and slow afterdepolarization are optimally combined to induce activity.
authors.forename: F
authors.initials: F
authors.lastname: Valenzuela
authors.forename: M
authors.initials: M
authors.lastname: Vassalle
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002304:Cardiac Pacing, Artificial / Q000379:methods*; D004285:Dogs; D004562:Electrocardiography / Q000379:methods*; D005260:Female; D006329:Heart Conduction System / Q000502:physiology*; D007473:Ion Channels / Q000502:physiology; D008297:Male; D011690:Purkinje Fibers / Q000502:physiology*
chemical_list: D007473:Ion Channels; D002118:Calcium