Some effects of ionophore A23187 on energy utilization and the distribution of cations and anions in mitochondria

The effects of ionophore A23187 on the movements of Ca2+, Mg2+, H+ phosphate, and succinate and its effects on energy utilization by mitochondria have been studied as a function of ionophore concentrations. At a low ratio of the compound to mitochondrial protein, below that required for maximal uncoupling, an apparent steady state is established between intra- and extramitochondrial Ca2+ and H+ which is maintained until anaerobiosis or inhibition of the mitochondrial Ca2+ pump. Mg2+ is continuously lost from the mitochondria during the steady state. At higher levels of the compound, both Ca2+ and Mg2+ are released very rapidly. In the medium used, the rate of respiration produced by cyclic Ca2+ uptake and release is a function of the ionophore level between 0 and 0.2 nmol per mg of protein and of the extramitochondrial Ca2+ concentration between 1 and 15 muM. The depletion of mitochondrial Ca2+ by A23187 appears to result in the complete reversal of the total ion movement occurring during energy-dependent Ca2+ accumulation by mitochondria. The ratio of H+ uptake to Ca2+ released is nearly 1. During ionophore-induced Mg2+ depletion, no net H+ uptake is observed, but the mitochondria are depleted of equal molar amounts of phosphate plus succinate. It is proposed that the initial H+ uptake produced by the ionophore is reversed through the combined actions of the mitochondrial phosphate-hydroxide and phosphate-dicarboxylate exchangers. The data are discussed in terms of factors affecting the "turnover number" of A23187, the mechanism of Ca2+ uptake by mitochondria, and some considerations affecting interpretation of data obtained by use of A23187 in cellular systems.