A gonadotropin-releasing hormone insensitive, thapsigargin-sensitive Ca2+ store reduces basal gonadotropin exocytosis and gene expression: comparison with agonist-sensitive Ca2+ stores

We examined whether distinct Ca2+ stores differentially control basal and gonadotropin (GTH-II)-releasing hormone (GnRH)-evoked GTH-II release, long-term GTH-II secretion and contents, and GTH-II-beta mRNA expression in goldfish. Thapsigargin (Tg)-sensitive Ca2+ stores mediated neither caffeine-evoked GTH-II release, nor salmon (s)GnRH- and chicken (c)GnRH-II-stimulated secretion; the latter responses were previously shown to involve ryanodine (Ry)-sensitive Ca2+ stores. Surprisingly, Tg decreased basal GTH-II release. This response was attenuated by prior exposure to sGnRH and caffeine, but was insensitive to the phosphatase inhibitor okadaic acid, the inhibitor of constitutive release brefeldin A and cGnRH-II. GTH-II-beta mRNA expression was decreased at 24 h by 2 microm Tg, and by inhibiting (10 microm Ry) and stimulating (1 nm Ry) Ry receptors. Transient increases in GTH-II-beta mRNA were observed at 2 h and 12 h following 10 microm and 1 nm Ry treatment, respectively. Effects of Tg, Ry and GnRH on long-term GTH-II secretion, contents and apparent production differed from one another, and these changes were not well correlated with changes in GTH-II-beta mRNA expression. Our data show that GTH-II secretion, storage and transcription can be independently controlled by distinct Ca2+ stores.