A re-examination of late outward plateau currents of cardiac Purkinje fibers.

Two outward currents, IX1 and IX2, are thought to be activated by depolarization of the Purkinje fiber. One of these, IX1, is presently believed to play a critical role in repolarization of the action potential. The IX currents were originally analyzed in voltage-clamp experiments in sheep Purkinje fibers. These experiments were designed to minimize interference by other currents, and it was assumed that changes of the net current were produced entirely by the IX currents. We have tried to repeat the original experiments and the analysis that led to acceptance of the existence and roles of the IX currents, without success. Moreover, tests of how membrane current should behave if the IX current hypothesis is correct did not give satisfactory results. Our data suggest the original conclusions about IX1 and IX2 may need substantial revision.