Effects of porcine follicular fluid, inhibin-A, and activin-A on goldfish gonadotropin release in vitro.

Inhibin and activin are important reproductive regulators in mammalian species and have been demonstrated to be highly conserved in structure. The present study examines the effects of porcine follicular fluid (pFF; a crude inhibin and activin preparation) and purified porcine inhibin-A and activin-A on goldfish gonadotropin-II (GTH-II) release. In studies using primary cultures of dispersed goldfish pituitary cells in static incubation, treatments with pFF, inhibin-A, and activin-A for 10 h caused dose-dependent increase in GTH-II release. In perifusion studies using goldfish pituitary fragments, basal GTH-II release was significantly elevated after 12-h exposure to 500 micrograms/ml pFF. Furthermore, GnRH-induced GTH-II secretion was potentiated by pretreatment with pFF. When pFF was applied in the form of 5-min pulses, a rapid dose-related stimulation of GTH-II was observed. Similarly, challenges with 2-min pulses of 15, 150, and 1500 pM inhibin-A and activin-A stimulated GTH-II release by goldfish pituitary fragments in a rapid and dose-dependent manner. This acute stimulatory action of inhibin on goldfish GTH-II release was completely abolished after pretreatment with specific inhibin antibodies. The acute actions of inhibin and activin on GTH-II release are probably not due to the release of endogenous GnRH from nerve terminals in the pituitary fragments or binding to the GnRH receptors. First, a specific GnRH antagonist did not block the actions of inhibin and activin. Second, dopamine, a potent inhibitor of GnRH-stimulated GTH-II secretion in goldfish, was only partially effective in decreasing inhibin- and activin-induced GTH-II release. Third, the stimulatory effects of inhibin and GnRH on GTH-II release were additive. These lines of evidence also indicate that the mechanisms mediating inhibin and activin stimulation of goldfish GTH-II release may be somewhat different from those of GnRH. These results demonstrate that in contrast with the usual inhibitory effects of inhibin on GTH release in mammals, both inhibin and activin exert long term and acute stimulatory actions on GTH-II release in the goldfish.