Electrogenic Suppression of Automaticity in Sheep and Dog Purkinje Fibers

Abstract

Driving Purkinje fibers at a fast rate is followed by a temporary suppression of spontaneous activity, "overdrive suppression." The mechanism of this suppression was studied in Purkinje fibers perfused in vitro and stimulated for variable periods of time at selected rates. The overdrive procedures caused an initial decrease in maximum diastolic potential below control followed by a late increase above control. After the overdrive, the slope of diastolic depolarization was decreased and the threshold voltage more positive₂, these changes were responsible for the temporary suppression of spontaneous activity. After the cessation of 2-minute overdrive, the increase in maximum diastolic potential subsided gradually over a period of 2 to 5 minutes. At higher [K]_o, the effect of overdrive on membrane potential was reduced. Substitution of Na⁺ with Li⁺ or exposure to 2,4-dinitrophenol abolished the late hyperpolarization during overdrive. The membrane resistance was not altered after an overdrive period. The results suggest that driving ventricular Purkinje fibers at a rate higher than their intrinsic rate causes an initial loss of K⁺ and the activation of an electrogenic Na⁺ pump. The activation of the electrogenic pump is the major mechanism responsible for overdrive suppression.