Mitochondria shape hormonally induced cytoplasmic calcium oscillations andmodulate exocytosis

Pituitary gonadotropes transduce hormonal input into cytoplasmic calcium ([ Ca2+](cyt)) oscillations that drive rhythmic exocytosis of gonadotropins, U sing Calcium Green-1 and rhod-2 as optical measures of cytoplasmic and mito chondrial free Ca2+, we show that mitochondria sequester Ca2+ and tune the frequency of [Ca2+](cyt) oscillations in rat gonadotropes. Mitochondria acc umulated Ca2+ rapidly and in phase with elevations of [Ca2+](cyt) after GnR H stimulation or membrane depolarization. Inhibiting mitochondrial Ca2+ upt ake by the protonophore CCCP reduced the frequency of GnRH-induced [Ca2+](c yt) oscillations or, occasionally, stopped them. Much of the Ca2+ that ente red mitochondria is bound by intramitochondrial Ca2+ buffering systems. The mitochondrial Ca2+ binding ratio may be dynamic because [Ca2+](mit), appea red to reach a plateau as mitochondrial Ca2+ accumulation continued. Entry of Ca2+ into mitochondria was associated with a small drop in the mitochond rial membrane potential. Ca2+ was extruded from mitochondria more slowly th an it entered, and much of this efflux could be blocked by CGP-37157, a sel ective inhibitor of mitochondrial Na+-Ca2+ exchange. Plasma membrane capaci tance changes in response to depolarizing voltage trains were increased whe n CCCP was added, showing that mitochondria lower the local [Ca2+](cyt) nea r sites that trigger exocytosis. Thus, we demonstrate a central role for mi tochondria in a significant physiological response.