Overdrive excitation and cellular calcium load in canine cardiac Purkinje fibers.

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.