Rhythmic exocytosis stimulated by GnRH-induced calcium oscillations in rat gonadotropes

In pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) induces the rhythmic release of Ca²⁺ from an inositol 1,4,5-trisphosphate (IP₃)-sensitive store. Simultaneous measurement of the concentration of cytosolic free Ca²⁺ ([Ca²⁺]_i) and exocytosis in single identified gonadotropes showed that each elevation of [Ca ²⁺]_i induced a burst of exocytosis. These phenomena were largely suppressed by buffering of [Ca²⁺]_i but persisted in the absence of extracellular Ca²⁺. Activation of voltage-gated Ca²⁺ channels by brief depolarizations seldom supplied enough Ca ²⁺ for exocytosis, but [Ca²⁺]_i elevations induced by photolysis of caged IP₃ did trigger exocytosis, confirming that GnRH-stimulated gonadotropic hormone secretion is closely coupled to intracellular Ca²⁺ release. Agonist-induced oscillations of [Ca ²⁺]_i in secretory cells may be a mechanism to optimize the secretory output while avoiding the toxic effects of sustained elevation of [Ca²⁺]_i.