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The mechanism by which a number of agents such as hydroperoxides, inorganic phosphate, azodicarboxylic acid bis(dimethy1amide) (diamide), 2-methyl-1 ,4-naphthoquinone (menadione) and aging, induce Ca2 + release from rat liver mitochondria has been analyzed by following Ca2 + fluxes in parallel with K + fluxes, matrix swelling and triphenylmethylphosphonium fluxes (as an index of transmembrane potential).1. Addition of hydroperoxides causes a cycle of Ca2+ efflux and reuptake and an almost parallel cycle of A Y depression. The hydroperoxide-induced A Y depression is biphasic. The first phase is rapid and insensitive to ATP and is presumably due to activation of the transhydrogenase reaction during the metabolization of the hydroperoxides. The second phase is slow and markedly inhibited by ATP and presumably linked to the activation of a Ca2+-dependent reaction. The slow phase of d Y depression is paralleled hy matrix K + release and mitochondrial swelling. Nupercaine and ATP reduce or abolish also K + release and swelling. 2. Inorganic phosphate, diamide, menadione or aging also cause a process of Ca2+ efflux which is paralleled by a slow A Y depression, K + release and swelling. All these processes are reduced or abolished by Nupercaine and ATP.3. The slow A Y depression following addition of hydroperoxide and diamide is largely reversible at low Ca2+ concentration but tends to become irreversible at high Ca2+ concentration. The A Y depression increases with the increase of hydroperoxide, diamide and menadione concentration, but is irreversible only in the latter case.4. Addition of ruthenium red before the hydroperoxides reduces the extent of the slow but not of the rapid phase of A Y depression. Addition of ruthenium red after the hydroperoxides results in a slow increase of A y/. Such an effect differs from the rapid increase of A Y due to ruthenium-red-induced inhibition of CaZ+ cycling in A231 87-supplemented mitochondria.5. Metabolization of hydroperoxides and diamide is accompanied by a cycle of reversible pyridine nucleotide oxidation. Above certain hydroperoxide and diamide concentrations the pyridine nucleotide oxidation becomes irreversible. Addition of menadione results always in an irreversible nucleotide oxidation.6. The kinetic correlation between Ca2+ efflux and A Y decline suggests that hydroperoxides, diamide, menadione, inorganic phosphate and aging cause, in the presence of Ca2+, an increase of the permeability for protons of the inner mitochondrial membrane. This is followed by Ca2+ efflux through a pathway which is not the H+/Ca2+ exchange.

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