The Role of Phosphate in the Regulation of the Independent Calcium‐Efflux Pathway of Liver Mitochondria

Abstract

The rate of spontaneous efflux of Ca from liver mitochondria incubated in the absence of ATP and Mg increases with time and is associated with a synchronous collapse of membrane potential and with P_i efflux. In the presence of Mg and ATP the ruthenium-red-induced Ca efflux does not change with time. The activity of the Ca efflux pathway in P_i-depleted mitochondria is 15-fold greater than in mitochondria equilibrated with 3.3 mM P_i. 50% inhibition is caused by 0.3 mM P_i. The membrane potential is not affected by changes in P_i concentration, although the steady-state extra-mitochondrial free Ca concentration reflects the alterations in efflux rate. In the presence of P_i, the ruthenium-red-induced efflux rate is independent of the total matrix Ca content; however in P_i-depleted mitochondria, with acetate substituting as permeant anion, the efflux rate increases with total matrix Ca content. The lowered efflux rate in the presence of P_i is not due to a limitation in the rate of dissociation of the matrix Ca-phosphate complex. The efflux pathway is activated by a lowered membrane potential, but the relative effect of P_i is retained. Under the present conditions Na slightly inhibits the efflux rate. The lack of an effect of total matrix Ca content on the efflux rate in the presence of P_i is used as the basis of a highly accurate determination of the activity of the Ca uniporter as a function of external free Ca concentration.