Effects of Oxygen and Antioxidants on the Mitochondrial Ca-Retention Capacity

2-Oxoglutarate-supported rat liver mitochondria were loaded with moderate amounts of calcium and submitted to O2 deprivation and reoxygenation. In the presence of acetoacetate, anaerobic energy production maintained Ca2+ retention by mitochondria during the anoxia period unless the Pi concentration of the incubation solution was raised to 4-6 mM. Acetoacetate prompted Ca2+ release from O2-deprived mitochondria at elevated Pi levels, presumably due to occurrence of a permeahility transition of the inner memhrane. Providing 3-hydroxybuty-rate and malate, together with acetoacetate, was found to delay the permeability transition until O2 was reintroduced, i.e., O2 triggered a paradoxical release of Ca2+ from mitochondria under these conditions. Whether initiated by O2 in the presence of Pi or by Pi under aerobic conditions, Ca2+ release was initially activated and subsequently inhibited or reversed in the presence of α-tocopherol (10-90 μmol · g protein−1). Similar effects were exerted by α-tocopherol during Pi-induced Ca2+ release from oligomycin-treated mitochondria supported by succinate (+ rotenone). In addition, the permeability transition was delayed by retinol (3-30 μmol · g protein−1) while β-carotene, ubiquinone, and water-soluble antioxidants, including Trolox C, were ineffective. Other observations suggest that the Ca2+-releasing and/or -retaining effects of α-tocopherol and retinol may be independent from pro- and/or antioxidant activities. Effects resembling those of α-tocopherol were exerted by α-tocopherol succinate, which is devoid of antioxidant activity. Our data indicate that the permeability transition of Ca2+-loaded liver mitochondria may be triggered by O2, in the presence of ketone bodies, and affected by lipid-soluble antioxidants through mechanisms seemingly unrelated to free-radical generation or scavenging.