Activation of Electrogenic Na+/K+ Exchange by Extracellular K+ in Canine Cardiac Purkinje Fibers

Transient increments in sodium 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. The results indicate: that in cardiac Purkinje cells, as in many other cells, the pump is half-maximally activated by about 1 mM K⁺; that the coupling ratio for Na⁺/K⁺ exchange is independent of either intracellular Na⁺ concentration or external K⁺ concentration; and that 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.