Calcium efflux mechanism in sperm mitochondria

This paper reports an investigation on calcium efflux mechanism in ram sperm mitochondria. Energized sperm mitochondria take up Ca²⁺ via the ruthenium-red sensitive uniporter, and possess a ruthenium-red insensitive efflux mechanism. Extramitochondrial Na⁺ did not affect the rate of Ca²⁺ efflux indicating that Na⁺/Ca²⁺ exchange mechanism is not involved. Depolarization of inner mitochondrial membrane induced by the uncoupler carbonylcyanide-p-trifluoromethoxyphenyl hydrazone (FCCP) or by the organomercurial SH-reagent mersalyl, causes high stimulation in Ca²⁺ efflux. This stimulated Ca²⁺ efflux determined in the presence of ruthenium-red and phosphate, is not inhibited by cyclosporin A (CSA), indicating that mitochondrial permeability transition pore (MTP) is not involved in this Ca²⁺ efflux mechanism. The stimulated Ca²⁺ efflux is inhibited by ADP or atractyloside suggesting that the Ca²⁺ transport mechanism might be intrinsic to the ADP/ATP carrier (AAC). Thus, the data indicate that sperm mitochondria contain a Ca²⁺ efflux mechanism operated via AAC and regulated by mitochondrial membrane potential and by ADP concentration.