THE EFFECTS OF BARIUM ON THE ACTION POTENTIAL AND THE MEMBRANE CURRENT OF SHEEP HEART PURKINJE FIBERS

¹ Department of Physiology, Yamaguchi University, School of Medicine, Ube, Japan

The effects of Ba⁺⁺ on quiescent Purkinje fibers from sheep heart were examined using membrane potential recording and membrane current measurement by means of a voltage clamp. Depolarization was observed in either Na⁺ or Na⁺ -free solution containing 0.02 mM Ba⁺⁺. The rate of depolarization was accelerated in the presence of Na⁺ and by repeated stimulation. Threshold potential was lowered and action potential duration was prolonged by Ba⁺⁺. Changes caused by Ba⁺⁺ were suppressed to some extent by Ca⁺⁺. Inward currents found only in Na⁺-Tyrode's solution were increased and outward currents observed in Na⁺ -free Tyrode's solution were decreased by Ba⁺⁺. The increased inward currents were more evident and lasted longer than the decreased outward current. The results suggest that the changes caused by Ba⁺⁺ are mainly brought about by the increased Na⁺ current; a more detailed explanation of these changes is given by considering the competitive action between Ca⁺⁺ and Ba⁺⁺.