ELECTROGENIC SODIUM EXTRUSION IN CARDIAC PURKINJE-FIBERS

Thin canine cardiac Purkinje fibers in a fast flow chamber were exposed to K-free fluid for 15 s to 6 min to initiate Na loading, then returned to K-containing fluid to stimulate the Na pump. The electrophysiological effects of enhanced pump activity may result from extracellular K depletion caused by enhanced cellular uptake of K or from an increase in the current generated as a result of unequal pumped movements of Na and K. The effects of pump stimulation were studied 3 conditions in which lowering the external K concentration ([K]o) causes changes opposite to those expected from an increase in pump content. The resting potential of Purkinje fibers may have either a high value or a low (less negative) value: at the low level of potential, experimental reduction of [K]o causes depolarization, but an increase in pump current should cause hyperpolarization. In regularly stimulated Purkinje fibers, lowering [K]o prolongs the action potential, but an increase in outward pump current should shorten it. Lowering [K]o enhances spontaneous pacemaker activity in Purkinje fibers, but an increase in outward pump current should reduce or abolish spontaneous activity. Under all 3 conditions, the effects of temporary stimulation of the Na pump were those expected from a transient increase in outward pump current, not those expected from K depletion.