Effects of a K ionophore, lonomycin A, on the action potential in canine cardiac Purkinje fibers.

Effects of a K ionophore, lonomycin A (K+:Na+ = 6:1), on the action potential and contractile force in canine cardiac Purkinje fibers were examined. Lonomycin A increased the developed and resting tensions. The agent shortened the action potential, decreased the plateau height and suppressed the automaticity. In addition, lonomycin A also shortened the duration of the slow action potential response, and it shortened the action potential duration to about the same extent in the presence and absence of tetrodotoxin. In quiescent Purkinje fibers, lonomycin A decreased K content in both 1.8 mM Ca Tyrode's solution (contracture observed) and 0.18 mM Ca Tyrode's solution (contracture not observed). These results suggest that lonomycin A-induced changes in the action potential configuration of Purkinje fibers is mainly due to the increase in K conductance, and the increase is induced irrespective of the degree of the increased [Ca2+]i.