The Cycling of Calcium, Sodium, and Protons Across the Inner Membrane of Cardiac Mitochondria

Abstract

A method is described that permits simultaneous determination of the net charge transfer associated with Ca²⁺ transport by the ruthenium-red-sensitive carrier and the ionized internal [Ca²⁺] in heart mitochondria. The data indicate that this carrier catalyses a charge-uncompensated flux of Ca²⁺. Full charge compensation for Ca²⁺ influx is provided by the respiration-dependent efflux of H⁺. The net efflux of Ca²⁺ induced by Na⁺ is analysed in terms of two other carriers, a Na⁺-Ca²⁺ antiporter and a Na⁺-H⁺ antiporter. Evidence is presented that these two carriers are separate and that the Na⁺-H⁺ exchange is the more rapid. The fluxes of Ca²⁺, Na⁺ and H⁺ during the Na⁺-induced efflux of Ca²⁺ support a series of events in which the Na⁺-H⁺ exchange enables unidirectional Ca²⁺ fluxes via the uniport and antiport systems to be integrated into a cycle.