Mitochondria Shape Hormonally Induced Cytoplasmic Calcium Oscillations and Modulate Exocytosis

Pituitary gonadotropes transduce hormonal input into cytoplasmic calcium ([Ca2+]cyt) oscillations that drive rhythmic exocytosis of gonadotropins. Using Calcium Green-1 and rhod-2 as optical measures of cytoplasmic and mitochondrial free Ca2+, we show that mitochondria sequester Ca2+ and tune the frequency of [Ca2+]cyt oscillations in rat gonadotropes. Mitochondria accumulated Ca2+ rapidly and in phase with elevations of [Ca2+]cyt after GnRH stimulation or membrane depolarization. Inhibiting mitochondrial Ca2+ uptake by the protonophore CCCP reduced the frequency of GnRH-induced [Ca2+]cyt oscillations or, occasionally, stopped them. Much of the Ca2+ that entered mitochondria is bound by intramitochondrial Ca2+ buffering systems. The mitochondrial Ca2+ binding ratio may be dynamic because [Ca2+]mit appeared to reach a plateau as mitochondrial Ca2+ accumulation continued. Entry of Ca2+ into mitochondria was associated with a small drop in the mitochondrial membrane potential. Ca2+ was extruded from mitochondria more slowly than it entered, and much of this efflux could be blocked by CGP-37157, a selective inhibitor of mitochondrial Na+-Ca2+ exchange. Plasma membrane capacitance changes in response to depolarizing voltage trains were increased when CCCP was added, showing that mitochondria lower the local [Ca2+]cyt near sites that trigger exocytosis. Thus, we demonstrate a central role for mitochondria in a significant physiological response.