IMMUNOREACTIVE INHIBIN DECREASES FOLLOWING BILATERAL OVARIECTOMY AND DURING THE POSTOVULATORY RISE OF FSH IN SYRIAN-HAMSTERS

Inhibin is a heterodimeric glycoprotein which may regulate FSH synthes is and secretion as well as follicular development and maturation. The source and physiological role of inhibin have not been established fo r the hamster, although several investigators have suggested that this hormone may function in the regulation of FSH in this species. The ma jor objectives of the present studies were to develop a radioimmunoass ay (RIA) for the measurement of inhibin in hamster serum and tissue, t o identify the primary source of inhibin and to examine the relationsh ip between inhibin and FSH during the estrous cycle. A sensitive, accu rate and specific RIA was developed and utilized to measure changes in circulating levels of immunoreactive inhibin (ir-inh-alpha) following bilateral gonadectomy and throughout the estrous cycle. Circulating i r-inh-alpha declined rapidly and significantly following bilateral gon adectomy in female hamsters suggesting a gonadal source. Serum FSH con centrations increased following the decline in serum ir-inh-alpha leve ls. In the adult female hamster circulating ir-inh-alpha increased gra dually throughout diestrus, peaked at the time of the preovulatory gon adotropin surge, then declined to a nadir on the morning of estrus. Ch anges in ovarian inhibin subunit mRNAs were examined throughout the es trous cycle and correlated with changes observed in circulating ir-inh -alpha levels. Observed significant reductions in the relative amount of inhibin mRNAs and serum concentrations of ir-inh-alpha during early estrus may moderate the amount and duration of the secondary FSH rise and thus contribute to the regulation of follicle recruitment in the hamster.