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 Ca ²⁺]_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.