Ca2+ Efflux in Mitochondria from the Yeast Endomyces magnusii

Abstract

Calcium release pathways in Ca2+-preloaded mitochondria from the yeast Endomyces magnusii were studied. In the presence of phosphate as a permeant anion, Ca2+ was released from respiring mitochondria only after massive cation loading at the onset of anaerobiosis. Ca2+ release was not affected by cyclosporin A, an inhibitor of the mitochondrial permeability transition. Aeration of the mitochondrial suspension inhibited the efflux of Ca2+ and induced its re-uptake. With acetate as the permeant anion, a spontaneous net Ca2+ efflux set in after uptake of ∼150 nmol of Ca2+/mg of protein. The rate of this efflux was proportional to the Ca2+ load and insensitive to aeration, protonophorous uncouplers, and Na+ ions. Ca2+ efflux was inhibited by La3+, Mn2+, Mg2+, tetraphenylphosphonium, inorganic phosphate, and nigericin and stimulated by hypotonicity, spermine, and valinomycin in the presence of 4 mm KCl. Atractyloside and t-butyl hydroperoxide were without effect. Ca2+ efflux was associated with contraction, but not with mitochondrial swelling. We conclude that the permeability transition pore is not involved in Ca 2+ efflux in preloaded E. magnusii mitochondria. The efflux occurs via an Na+-independent pathway, in many ways similar to the one in mammalian mitochondria.