Activation of electrogenic Na+/K+ exchange by extracellular K+ in canine cardiac Purkinje fibers.

Abstract

Transient increments in Na pump current were elicited in small voltage-clamped Purkinje fibers suspended in a fast flow system by briefly exposing them to K+-free fluid, to temporarily inhibit the pump and then suddenly returning them to K+-containing fluid. The exponential time course of decay of the current increment provides a measure of the pump rate constant for Na+ extrusion. The dependence of that rate constant, and of the peak amplitude of the increment in pump current, on the extracellular K+ concentration was determined. In cardiac Purkinje cells, as in many other cells, the pump is half-maximally activated by about 1 mM K+. The coupling ratio for Na+/K+ exchange is independent of intracellular Na+ concentration or external K+ concentration. A simple model in which intracellular Na+ concentration is determined by a passive leak, and an active extrusion of Na+, seems sufficient to account for moderate changes in cellular Na+ concentration.