Gonadotropic Stimulation of Inhibin Secretion by the Human Ovary During the Follicular and Early Luteal Phase of the Cycle*

Abstract

We studied the pattern of secretion of inhibin bioactivity from the ovary into peripheral blood during the follicular and early luteal phase of the menstrual cycle in women receiving gonadotropin therapy. Multiple follicular development was stimulated in 5 women undergoing in vitro fertilization and embryo transfer for tubal infertility using three different treatments designed to vary the concentration of FSH and LH (14 cycles). The women received clomiphene citrate (150 mg/day) from days 2-6 alone or supplemented with either exogenous human menopausal gonadotropin (28 IU/3 h) or pure FSH (28 IU/3 h) from day 6 until the day of follicle aspiration. Inhibin concentrations increased 10-fold in parallel with those of estradiol, from 0.2-0.3 U/mL on day 2 (before the onset of treatment) to 4-5 U/mL on day 14 of the cycle (time of the peak LH level). Coincidental to the LH surge, the inhibin concentration declined 2- to 3-fold before increasing again early in the luteal phase. The concentration of inhibin was higher in the gonadotropin-treated group (clomiphene plus human menopausal gonadotropin/FSH) than in the group treated with only clomiphene during the follicular phase. The number of follicles stimulated was significantly higher (P less than 0.001) in the group given exogenous gonadotropins [4.8 +/- 0.4 (SE)] than in the clomiphene alone group (2.2 +/- 0.4). These data strongly suggest that both the Graafian follicles and the corpus luteum secrete inhibin, which together with estradiol and progesterone may play a role in the regulation of FSH secretion during the luteal phase.