Involvement of mitochondria in intracellular calcium sequestration by rat gonadotropes

Intracellular Ca2+ ([Ca2+](i)) dynamics were studied in identified rat gonadotropes using the whole-cell patch-clamp technique in conjunction with Indo-1 photometry. The kinetics of depolarization-induced [Ca2+](i) transients vary with Ca2+ load. In addition to a rapid initial decay, large (> 500 nM) [Ca2+](i) transients have a slow plateau phase. Application of the mitochondrial inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP) significantly slows the decay of [Ca2+](i) transients, consistent with stopping uptake of Ca2+ by mitochondria. CCCP causes a small increase of [Ca2+](i) at rest. After a large Ca2+ entry the amount is much larger, consistent with release from a mitochondrial Ca2+ pool that fills during cytoplasmic Ca2+ loading. The rate of Ca2+ uptake by mitochondria is dependent upon [Ca2+](i). Consistent with previous studies, gonadotropin releasing hormone (GnRH) induces [Ca2+](i) oscillations. The mitochondrial inhibitors CCCP and cyanide (CN-) terminate these oscillations. The mitochondrial ATP-synthase inhibitor oligomycin reduces the frequency and increases the amplitude of the oscillations. In the presence of ruthenium red (a non-specific blocker of the mitochondrial Ca2+-uniporter) in the pipette, GnRH does not induce rhythmic [Ca2+](i) oscillations. We suggest that mitochondria play a significant role in the rapid clearance of cytosolic Ca2+ loads in gonadotropes and participate in GnRH-induced periodic [Ca2+](i) oscillations.