CA-2+-INDUCED MEMBRANE TRANSITION IN MITOCHONDRIA .3. TRANSITIONAL CA-2+ RELEASE

The efflux of Ca2+ from mitochondria respiring at steady state, and much of uncoupler-induced Ca2+ efflux, is shown to be a consequence of the Ca2+-induced membrane transition (the Ca2+-induced transition is the Ca2+-dependent sudden increase in the nonspecific permeability of the mitochondrial inner membrane which occurs spontaneously when mitochondria are incubated under a variety of conditions (D. R. Hunter, R. A. Haworth, and J. H. Southard, 1976, J. Biol. Chem. 251, 5069-5077)). Ca2+ release from mitochondria respiring at steady state is shown to be transitional by four criteria: (1) Ca2+ release is inhibited by Mg2+, ADP, and bovine serum albumin (BSA), all inhibitors of the transition; (2) release is selective for Ca2+ over Sr2+, a selectivity also found for the transition; (3) the time course of Ca2+ release is identical to the time course of the change in the mitochondrial population from the aggregated to the orthodox configuration; and (4) from kinetics, Ca2+ release from individual mitochondria is shown to occur suddenly, following a lag period during which no release occurs. Ca2+ release induced by uncoupler is shown to be mostly by a transitional mechanism, as judged by four criteria: (1) release of Ca2+ is ruthenium red-insensitive and is an order of magnitude faster than Sr2+ release which is ruthenium red-sensitive; (2) release of Ca2+ is strongly inhibited by keeping the mitochondrial NAD+ reduced; (3) the kinetics of Ca2+ release indicates a transitional release mechanism; and (4) uncoupler addition triggers the aggregated to orthodox configurational transition which, at higher levels of Ca2+ uptake, occurs in the whole mitochondrial population at a rate equal to the rate of Ca2+ release. Na2+-induced Ca2+ release was not accompanied by a configurational change; we therefore conclude that it is not mediated by the Ca2+-induced transition. © 1979.