Local Ca²⁺ release from internal stores controls exocytosis in pituitary gonadotrophs

Exocytosis and the cell-averaged cytosolic [Ca²⁺], [Ca²⁺](i), were tracked in single gonadotrophs. Cells released 100 granules/s at 1 μM [Ca²⁺](i) when gonadotropin-releasing hormone (GnRH) activated IP₃- mediated Ca²⁺ release from internal stores, but only 1 granule/s when [Ca²⁺](i) was raised uniformly to 1 μM by other means. Strong exocytosis was then seen only at higher [Ca²⁺], (half-maximal at 16 μM). Parallel second messengers did not contribute to GaRH-induced exocytosis, because IP₃ alone was as effective as GnRH, and because even GnRH failed to trigger rapid exocytosis when the [Ca²⁺](i) rise was blunted by EGTA. When [Ca²⁺](i) was released from stores, exocytosis depended on [Ca²⁺](i) rising rapidly, as if governed by Ca²⁺ flux into the cytosol. We suggest that IP₃ releases Ca²⁺ selectively from subsurface cisternae, raising [Ca²⁺] near exocytic sites 5-fold above the cell average.