The Ruthenium Red-insensitive efflux of Ca2+ from previously loaded rat liver mitochondria was studied as a function of the added Na+ concentration and ADP present. Stimulation of Ca2+ efflux is sigmoidally dependent on the Na+ concentration; maximal stimulation of efflux was observed with 12–15 mM-NaCl. Na+-stimulated Ca2+ efflux from liver mitochondria is about one-tenth that from cardiac mitochondria. No synergistic effect of K+ on the Na+-stimulated efflux was found. The alkali-metal cations other than Na+ did not stimulate efflux and did not prevent stimulation by Na+. In the absence of Na+, Ca2+ efflux was diminished by added ADP, but the Na+-stimulated efflux was made correspondingly greater as ADP concentration was increased to 16 microM. The Na+-stimulated Ca2+ efflux was inhibited by 70% by oligomycin and was not observed in the presence of antimycin. It is suggested that failure to observe Na+-stimulation of Ca2+ efflux from liver mitochondria by some investigators is attributable to a high basal efflux existing before addition of the Na+ salt.
