[Effects of extracellular sodium concentration on the sodium-pump activity in Purkinje fibres of sheep].

Intracellular sodium activity (aiNa), extracellular potassium activity (aok) and resting membrane potential (Vm) were measured in sheep cardiac Purkinje fibres exposured intermittently to calcium-free solution containing different concentrations of low sodium. In Ca-free low sodium solution, aiNa decreased, the rates and amplitudes of aiNa changes were related to [Na]o linearly. The linear correlation between the stable values of aiNa (aiNa(S)) at 6th minute of aiNa decrease and [Na]o meant that the amplified active Na+ extrusion and pump current resulting from decreased [Na]o, kept finally Na+ gradient across the membrane relatively stable. Hence, it was proposed that it was the Na+ gradient across the membrane but not the internal Na+ that regulated the sodium pump activity. The membrane hyperpolarization which was incident with the increase of active Na+ extrusion was caused mainly by the amplified pump current and embellished by the intercellular K+ depletion and accumulation simultaneously, which was suggested by the mutual relation of time courses of changes in aiNa, Vm, aok during the exposure of the fibre to 1.3Na Ca-free solution.