Mitochondria from mammalian tissues possess an elaborate system for 2+ 2+ transporting Ca across their inner membrane which consists of Ca import, 2+ via the Ca uniporter, in response to the mitochondrial membrane 2+ + potential and of Ca release by an antiport system in exchange for H + 9,23 or Na (see Fig. l) . Because the uniporter is dependent upon the external 2+ 2+ 2+ 2+ Ca concentration ( Ca ]), mitochondria accumulate Ca until the Ca ] o o 2+ decreases to the level at which the uniporter activity balances the Ca efflux. 2+ The Ca ] at which the uniporter and efflux activities are...