Depolarization of the mitochondrial membrane potential increases free cytosolic calcium in synaptosomes.

Intracellular calcium transients in synaptosomes, isolated from the guinea pig brain, were measured using entrapped metallochromic indicator arsenazo III. Addition of 1 microM carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) increased rapidly the absorbance of the entrapped arsenazo III, indicating an increase in the cytosolic free calcium. The FCCP-induced increase in cytoplasmic free Ca2+ was not blocked by 200 microM verapamil, while the increment in calcium caused by 40 microM veratridine was verapamil-sensitive. The absorbance changes induced by FCCP were not significantly increased when the extracellular potassium concentration was elevated from 5.4 to 50 mM. These data indicate that in nerve endings of mammalian brain, cytoplasmic free calcium, which is essential for the release of transmitter, is increased on depolarization of major intracellular calcium buffers, mitochondria.