Rapid and extensive release of Ca2+ from energized mitochondria induced by EGTA

A novel mitochondrial Ca2+ release phenomenon is reported. When rat liver mitochondria (oxidizing succinate) are allowed to accumulate Ca2+ in excess of 40 nmol/mg protein and are then treated with excess EGTA, a fraction of the accumulated cation is rapidly (approximately 1 nmol/s/mg protein) released. The size of the released fraction is an apparent function of the extramitochondrial Ca2+ concentration at the time of EGTA addition and can attain a maximal value of approximately 30 nmol/mg protein. Release is inhibited by ruthenium red (I50 approximately 50 pmol/mg protein) and is not dependent on the presence of Na+ or K+ in the medium. During the period of rapid release, O2 consumption is inhibited, membrane potential increases, and apparent H+ accumulation occurs at a ratio of approximately 2H+ per Ca2+ released. It is proposed that this chelator-induced Ca2+ release occurs by reverse uniport with H+ back diffusion to the matrix space providing charge movement compensation.