ACTIVATION OF NEUROHYPOPHYSEAL VASOPRESSIN RELEASE BY CA-2+ INFLUX AND INTRACELLULAR CA-2+ ACCUMULATION IN THE RAT

1. Isolated rat neurohypophyses were stimulated electrically in an in vitro perifusion system. A Ca2(+)‐sensitive microelectrode was placed in the centre of each neurohypophysis and [Ca2+]o decrease evoked by the stimulation were determined. 2. In neurohypophyses injected with Fura‐2 AM (acetoxymethyl ester), increases and decreases in fluorescence excited at 340 and 380 nm, respectively, were evoked by stimulation. The time course of the fluorescence changes was similar to that of [Ca2+]o decreases, suggesting that the [Ca2+]o changes mirrored [Ca2+]i increases. 3. Calcium influx into neurosecretory axons and terminals was estimated as the difference in [Ca2+]o decrease rates immediately before and after train pulse stimulation. 4. Vasopressin release from the neurohypophysis, measured by specific radioimmunoassay, was facilitated by stimulation in parallel with a parameter of [Ca2+]o decrease multiplied by Ca2+ influx. 5. The O2 consumption rate, estimated as rate of PO2 decrease in the tissue, was facilitated by stimulation in parallel with [Ca2+]o decreases. 6. Possible calcium‐dependent mechanisms of vasopressin release, and the energy‐dependent step of the release by Ca2+, are discussed. © 1990 The Physiological Society