Regulation of Ca2+Efflux in Rat Liver Mitochondria

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Abstract
The paper analyzes the relationship between membrane potential (ΔΨ), steady state pCa0(‐log [Ca2+] in the outer aqueous phase) and rate of ruthenium‐red‐induced Ca2+efflux in liver mitochondria. Energized liver mitochondria maintain a pCa0of about 6.0 in the presence of 1.5 mM Mg2+and 0.5 mM Pi. A slight depression ofΔΨresults in net Ca2+uptake leading to an increased steady state pCa0. On the other hand, a more marked depression ofΔΨresults in net Ca2+efflux, leading to a decreased steady‐state pCa0. These results reflect a biphasic relationship betweenΔΨand pCa0, in that pCa0increases ofΔΨup to a value of about 130 mV, whereas a further increase ofΔΨabove 130 mV results in a decrease of pCa0. The phenomenon of Ca2+uptake following a depression ofΔΨis independent of the tool used to affectΔΨwhether by inward K+current via valinomycin, or by inward H+current current through protonophores or through, F1‐ATP synthase, or by restriction of e−flow. The pathway for Ca2+efflux is considerably activated by stretching of the inner membrane in hypotonic media. This activation is accompanied by a decreased pCa0at steady state and by an increased rate of ruthenium‐red‐induced Ca2+efflux. By restricting the rate of e−flow in hypotonically treated mitochondria, a marked dependence of the rate of ruthenium‐red‐induced Ca2+efflux on the value ofΔΨis observed, in that the rate of Ca2+efflux increases with the value ofΔΨ. The pCa0is linearly related to the rate of Ca2+efflux. Activation of oxidative phosphorylation via addition of hexokinase + glucose to ATP‐supplemented mitochondria, is followed by a phase of Ca2+uptake, which is reversed by atractyloside. These findings support the view that Ca2+efflux in steady state mitochondria occurs through an independent,ΔΨ‐controlled pathway and that changes ofΔΨduring oxidative phosphorylation can effectively modulate mitochondrial Ca2+distribution by inhibiting or activating theΔΨ‐controlled Ca2+efflux pathway.