Roles of inhibin and estradiol in the regulation of follicle-stimulating hormone and luteinizing hormone secretion during the estrous cycle of the rat

The relative contributions of inhibin and estradiol in the regulation of FSH and LH secretion were examined at various stages of the estrous cycle in the rat. At 1100 h on metestrus, diestrus, or estrus or at 0500 h on proestrus, rats were ovariectomized or given an injection of normal goat serum, antiserum to inhibin (inhibin-AS), antiserum to estradiol (estradiol-AS), or both antisera to examine the role of gonadal hormones in the regulation of tonic gonadotropin secretion. Plasma samples were collected before and at 6, 12, and 24 h after the treatments. Further, to examine the effects of the treatments on preovulatory gonadotropin surges, the five treatments described were carried out at 0500 h on proestrus and blood samples were collected from 1100 h to 2000 h on the same day at 1.5-h intervals. There was a significant rise in the concentration of plasma FSH after injection of inhibin-AS as well as after ovariectomy on each day of the estrous cycle. These treatments, however, had less effect on estrous FSH secretion. The rise in FSH was greater with immunoneutralization against both inhibin and estradiol than with immunoneutralization against inhibin alone on diestrus and proestrus. Basal levels of LH were increased at all stages of the cycle through immunoneutralization against inhibin and were also increased through immunoneutralization against estradiol except at estrus. Especially on diestrus, a remarkable increase in LH secretion was induced at 6 h after immunoneutralization against both inhibin and estradiol (1449.3 +/- 100.3% vs. control). The magnitude of the LH surge increased in inhibin-immunized rats, decreased in estradiol-immunized or ovariectomized rats, and remained at normal levels after injections of both antisera. The magnitude of the primary FSH surge increased very markedly in the inhibin-immunized group and decreased in the estradiol-immunized group. These results suggest that both estradiol and inhibin play a role in the regulation of LH secretion and that inhibin is a major regulator of FSH secretion during the estrous cycle of the rat. Furthermore, it is suggested that one or more extragonadal factors suppress estrous FSH secretion.