Dual responses of CNS mitochondria to elevated calcium

Isolated brain mitochondria were examined for their responses to calcium ch allenges under varying conditions. Mitochondrial membrane potential was mon itored by following the distribution of tetraphenylphosphonium ions in the mitochondrial suspension, mitochondrial swelling by observing absorbance ch anges, calcium accumulation by an external calcium electrode, and oxygen co nsumption with an oxygen electrode. Both the extent and rate of calcium-ind uced mitochondrial swelling and depolarization varied greatly depending on the energy source provided to the mitochondria. When energized with succina te plus glutamate, after a calcium challenge, CNS mitochondria depolarized transiently, accumulated substantial calcium, and increased in volume, char acteristic of a mitochondrial permeability transition. When energized with 3 mM succinate, CNS mitochondria maintained a sustained calcium-induced dep olarization without appreciable swelling and were slow to accumulate calciu m. Maximal oxygen consumption was also restricted under these conditions, p reventing the electron transport chain from compensating for this increased proton permeability. In 3 mM succinate, cyclosporin A and ADP plus oligomy cin restored potential and calcium uptake. This low conductance permeabilit y was not effected by bongkrekic acid or carboxyatractylate, suggesting tha t the adenine nucleotide translocator was not directly involved. Fura-2FF m easurements of [Ca2+](i) suggest that in cultured hippocampal neurons gluta mate-induced increases reached tens of micromolar levels, approaching those used with mitochondria. We propose that in the restricted substrate enviro nment, Ca2+ activated a low-conductance permeability pathway responsible fo r the sustained mitochondrial depolarization.