The Interrelations between the Transport of Sodium and Calcium in Mitochondria of Various Mammalian Tissues

Addition of ruthenium red to mitochondria isolated from brain, adrenal cortex, parotid gland and skeletal muscle inhibits the further uptake of Ca 2+ by these mitochondria but induces little or no net Ca 2+ efflux; the further addition of Na + , however, induces rapid efflux of Ca 2+ . The velocity of the Na + ‐induced efflux of Ca 2+ from these mitochondria exhibits a sigmoidal dependence on the [Na + ]. Addition of Na + to mitochondria exhibiting the most active Na + ‐dependent efflux of Ca 2+ (brain and adrenal cortex) also releases Ca 2+ in the absence of ruthenium red and, under these conditions, the mitochondria become uncoupled. It is concluded that the efflux of Ca 2+ from these mitochondria occurs via a Na + ‐dependent pathway, possibly a Na + ‐Ca 2+ antiporter, that is distinct from the ruthenium‐red‐sensitive carrier that catalyses energy‐linked Ca 2+ influx. The possible role of the Na + ‐dependent efflux process in the distribution of Ca 2+ between the mitochondria and the cytosol is discussed. In contrast, mitochondria from liver, kidney, lung, uterus muscle and ileum muscle exhibit no Na + ‐dependent efflux of Ca 2+ .