Properties of the calcium-extruding mechanism of liver cells.

1. The movements of Ca2+ between liver slices and the suspending medium have been followed with a Ca2+-specific electrode and with measurements of the efflux of 45Ca. 2. A net entry of Ca2+ into the tissue occurred during anaerobiosis. Reoxygenation of the medium resulted in a net extrusion of Ca2+ which was reversed by the addition of a respiratory inhibitor (Amytal), an uncoupler of oxidative phosphorylation (pentachlorophenol) or the divalent-cation-specific ionophore, A23187. 3. The net extrusion of Ca2+ was dependent on the presence of Mg2+ in the medium, but was not prevented by ouabain or by incubation in a Na+-free medium. Extrusion was not prevented by cytochalasin B or colchicine. 4. The undirectional efflux of 45Ca required Mg2+ to attain its maximal rate, and the magnitude was not affected by the presence of ouabain or the absence of Na+ from the medium. 5. It is concluded that extrusion of Ca2+ from liver cells is a metabolically dependent transport process that occurs independently of the exchange of Na+ between tissue and medium.