Cyclic Changes in Serum Follicle-Stimulating Hormone, Luteinizing Hormone and Inhibin During the Guinea Pig Estrous Cycle.

Abstract

We proposed to describe the cyclic changes in serum follicle-stimulating hormone (FSH), luteinizing hormone (LH) and inhibin during the guinea pig estrous cycle. Serum gonadotropin concentrations had not been determined routinely because antibodies to guinea pig gonadotropins were rarely available. Serum gonadotropin concentrations provide important information about the stimuli received by the ovary. Inhibin plays a major role in modulating FSH synthesis and secretion at the anterior pituitary; and inhibin concentrations can be used as a marker for follicular development. We used only sexually mature guinea pigs that showed at least two consecutive normal 15-17 day cycles. The day of maximal vaginal cornification, the day of ovulation, was designated as day 0. Blood samples were taken every other day at 0900 hours throughout one estrous cycle. Serum was analyzed for FSH, LH, and inhibin by radioimmunoassay. Cyclic changes in peripheral FSH, LH, and inhibin concentrations were observed. Serum inhibin rose to a peak on day 7 of the guinea pig estrous cycle. Inhibin increased again, although not significantly, on day 13. Thus, this study confirms the biphasic phenomenon of ovarian follicle development in guinea pigs. Since serum FSH concentrations declined as serum inhibin concentrations reached a peak, it is also suggested that inhibin regulates FSH secretion in this species. This is the first study in which all three hormones were measured during the estrous cycle of the guinea pig.