Inhibin is a heterodimeric glycoprotein which may regulate FSH synthesis and secretion as well as follicular development and maturation. The source and physiological role of inhibin have not been established for the hamster, although several investigators have suggested that this hormone may function in the regulation of FSH in this species. The major objectives of these studies were to develop a radioimmunoassay (RIA) for the measurement of inhibin-a in hamster serum and tissue, to identify the primary source of inhibin and to examine the relationship between inhibin and FSH during postnatal development. A sensitive, accurate and specific RIA was developed and utilized to measure changes in circulating levels of inhibin-a following bilateral gonadectomy, in developing hamsters and in studies on inhibin throughout the estrous cycle. Circulating inhibin-a declined rapidly and significantly following bilateral gonadectomy in male and female hamsters suggesting a gonadal source. Serum FSH concentrations increased following the decline in serum inhibin-a levels. An inverse relationship between inhibin and FSH was observed in early postnatal development. Male hamsters exhibited high serum concentrations of inhibin-α and low concentrations of FSH prior to the third postnatal week. Young female hamsters exhibited low concentrations of circulating inhibin-α and early, high levels of FSH. With follicular stimulation prior to the first ovulation, ovarian and serum inhibin concentrations increased and serum FSH was suppressed suggesting a possible endocrine regulatory role for inhibin. Administration of PMSG to peripubertal female hamsters increased ovarian and serum levels of inhibin-α, and increased steady-state levels of inhibin subunit mRNAs. Elevated levels of immunoreactive inhibin-α in PMSG-treated hamsters were inversely correlated with serum FSH concentrations, providing evidence for the reciprocal relationship between inhibin and FSH. Immunoreactive inhibin-a increased gradually throughout diestrus in the hamster. Inhibin-α concentrations peaked in the circulation at the time of the preovulatory gonadotropin surge, then declined to a nadir on the morning of estrus. Significant reductions in serum concentrations of inhibin-α during early estrus may moderate the amount and duration of the secondary FSH rise and thus contribute to the regulation of follicle recruitment
