Regulation of gonadotropin secretion and number of gonadotropin-releasing hormone receptors by inhibin, activin-A, and estradiol

Primary cultures of ovine pituitary cells were used to characterize the effects of inhibin and activin on the secretion of gonadotropins and on the regulation of number of GnRH receptors in the presence or absence of estradiol. Number of GnRH receptors was determined by the specific binding of a saturating dose of [125I]des-Gly10-D-Trp6-GnRH-ethylamide (GnRH-A). Recombinant human inhibin-A (rh-inhibin-A) or inhibin in porcine and bovine follicular fluid (pFF and bFF, respectively) decreased secretion of FSH in a dose-dependent manner, with maximum inhibition at an inhibin concentration of approximately 0.1 nM. Neither pFF or bFF affected secretion of LH, although rh-inhibin-A caused a modest decrease (p < 0.05) in secretion of LH. Treatment of cells with rh-inhibin-A, bFF, or pFF approximately doubled the number of GnRH receptors. Scatchard analysis indicated that increases in GnRH-A binding were due to an increase in receptor number rather than a change in affinity. Additionally, rh-inhibin-A, at a dose that doubled numbers of GnRH receptors, increased GnRH-induced LH release above that caused by GnRH alone, indicating that the increase in receptor number leads to increased responsiveness to GnRH. Recombinant human activin-A (rh-activin-A) increased secretion of FSH but did not affect secretion of LH. Number of GnRH receptors was not affected by lower concentrations of rh-activin-A but was decreased (p < 0.05) by 3.0 nM activin. The combination of inhibin (5 nM) from bFF and estradiol (0.4 nM) increased the number of GnRH receptors to a level above that achieved by either treatment alone, suggesting an additive effect of inhibin and estradiol. rh-Activin-A (3 nM) had no effect on the estradiol-induced increase in number of GnRH receptors. These results indicate that in ovine pituitary cells inhibin and activin regulate secretion of FSH and suggest that inhibin increases the number of GnRH receptors. Finally, inhibin and estradiol seem to have additive effects on the up-regulation of number of GnRH receptors.