Ca2+ acting at the external side of the inner mitochondrial membrane can stimulate mitochondrial permeability transition induced by phenylarsine oxide

Mitochondrial permeability transition (MPT) induced by the thiol cross-linker phenylarsine oxide (PhAsO) in Ca2+-depleted mitochondria incubated in the presence of ruthenium red, an inhibitor of the Ca2+ uniporter, is stimulated by the addition of extramitochondrial Ca2+. The presence of extramitochondrial Ca2+ stimulates the reaction of mitochondrial membrane protein thiol groups with PhAsO. Both Ca2+-induced increase in mitochondrial membrane permeabilization and protein thiol group reaction with PhAsO are dependent on time (5-10min to be complete) and the concentration of Ca2+ (1-25 mu M). Mitochondrial permeabilization induced by PhAsO (15 mu M) and extramitochondrial Ca2+ is inhibited by ADP, cyclosporin A, dibucaine and Mg2+, while mitochondrial permeabilization induced by high concentrations of PhAsO (60 mu M) in the absence of Ca2+ is inhibited only by ADP and cyclosporin A. These results suggest that dibucaine and Mg2+ can inhibit mitochondrial permeabilization by antagonizing the effect of Ca2+ on the mitochondrial membrane. Once mitochondrial permeabilization induced by 15 mu M PhAsO and extramitochondrial Ca2+ has already occurred, the addition of the Ca2+ chelator EGTA restores mitochondrial membrane potential (MPT pore closure), suggesting that the presence of Ca2+ is essential for the maintenance of the permeability of the mitochondrial membrane to protons (MPT pore opening). In conclusion, the results presented indicate that low Ca2+ concentrations acting at the external side of the inner mitochondrial membrane can stimulate mitochondrial permeability transition induced by PhAsO, due to increased accessibility of protein thiol groups to the reaction with PhAsO and increased probability of MPT pore opening. (C) 1997 Elsevier Science B.V.