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 further uptake of Ca2+ by these mitochondria but induces little or no net Ca2+ efflux; the further addition of Na+, however, induces rapid efflux of Ca2+. The velocity of the Na+-induced efflux of Ca2+ from these mitochondria exhibits a sigmoidal dependence on the [Na+]. Addition of Na+ to mitochondria exhibiting the most active Na+-dependent efflux of Ca2+ (brain and adrenal cortex) also releases Ca2+ in the absence of ruthenium red and, under these conditions, the mitochondria become uncoupled. It is concluded that the efflux of Ca2+ from these mitochondria occurs via a Na+-dependent pathway, possibly a Na+-Ca2+ antiporter, that is distinct from the ruthenium-red-sensitive carrier that catalyses energy-linked Ca2+-influx. The possible role of the Na+-dependent efflux process in the distribution of Ca2+ 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 Ca2+.