Secretion of ovarian inhibin and its physiologic roles in the regulation of follicle-stimulating hormone secretion during the estrous cycle of the female guinea pig

To characterize inhibin secretion during the estrous cycle in guinea pigs, the concentrations of plasma inhibin, estradiol, progesterone, and FSH were determined. A significant positive correlation was observed between inhibi n and estradiol throughout the estrous cycle. Plasma inhibin and estradiol started to increase a few days before ovulation (Day 0 day of estimated ovu lation), and decreased after ovulation. These two hormones remained low dur ing the luteal phase. The immunoreactivity of inhibin alpha, beta(A), and b eta(B) subunits was colocalized in the granulosa cells of one or two health y large follicles in the ovary before ovulation. There was no positive reac tion of inhibin alpha and beta subunits in the corpora lutea or other folli cles. Ovariectomy resulted in an abrupt decrease in plasma inhibin and a si gnificant increase in plasma FSH. Injection of anti-inhibin serum into adul t female guinea pigs induced an elevation in plasma FSH in a dose-dependent manner. This report presents the first description of sequential changes i n plasma inhibin and estradiol during the estrous cycle of guinea pigs. Res ults suggest that inhibin is secreted mainly by granulosa cells of a few he althy large follicles in the ovary and that it plays an important role in t he regulation of FSH secretion during the estrous cycle in guinea pigs.