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Initiation of Sustained Rhythmic Activity by Single Propagated Action Potentials in Canine Cardiac Purkinje Fibers Exposed to Sodium‐Free Solution or to Ouabain

Canine cardiac Purkinje fibers were exposed to sodium-free solutions containing 128 mM tetraethylammonium chloride and 16.2 mM calcium. Some fibers exposed to these solutions were spontaneously active. Other fibers were quiescent, but they could be thrown into sustained rhythmic activity by a single driven action potential; the sustained activity resulted from the depolarizing phase of an oscillatory after-potential. Premature action potentials often showed prolonged plateaus, and their repolarization continued to a more negative level than that seen during the regular sustained activity. This condition caused a temporary slowing of rhythm and, on rare occasions, a cessation of spontaneous activity. Other canine cardiac Purkinje fibers were exposed to normal Tyrode's solution containing 0.125 mg ouabain/liter. In such fibers driven activity caused progressive loss of resting potential. Concomitant with that loss of resting potential, oscillatory after-potentials appeared. These oscillatory after-potentials sometimes grew into sustained rhythmic activity that occurred at a level of resting potential at which the normal sodium-dependent upstroke was presumably inactivated.