CONTRIBUTIONS OF ENDOGENOUS INHIBIN AND ESTRADIOL TO THE REGULATION OF FOLLICLE-STIMULATING-HORMONE AND LUTEINIZING-HORMONE SECRETION IN THE PREGNANT RAT

To examine the contributions of endogenous inhibin and estradiol to th e regulation of FSH and LH secretion in the pregnant rat, some rats we re passively immunized against inhibin and/or estradiol, and others we re ovariectomized, on Days 5, 10, 15, and 20 of pregnancy. Ovarian and uterine venous blood was collected separately to confirm the sources of inhibin and steroid hormones during pregnancy. Immunoreactivity of inhibin in the placenta was also examined by RIA. Levels of inhibin in ovarian venous plasma were significantly higher than those in periphe ral plasma during pregnancy. No difference was observed between the le vels of inhibin in uterine venous plasma and peripheral plasma. No imm unoreactivity of inhibin was detected in placental homogenate from rat s at Days 10, 15, and 20. FSH secretion significantly increased after immunoneutralization of inhibin during pregnancy. A marked increase in FSH secretion was noted on Days 5 and 20, and the smallest increase w as observed on Day 15. Administration of estradiol antiserum (AS) alon e did not induce a significant increase in FSH secretion on any day of pregnancy. However, a synergistic effect of estradiol AS and inhibin AS was observed on Day 20. On Days 5, 10, and 20, administration of in hibin AS or estradiol AS induced a significant increase in LH secretio n. A synergistic effect of inhibin AS and estradiol AS on LH secretion was observed on Day 5. On Days 5 and 10, significantly high LH secret ion was noted in ovariectomized rats as compared with that in rats tre ated with both inhibin AS and estradiol AS, indicating that other ovar ian hormones such as progesterone may be involved in the suppression o f LH secretion in these stages of pregnancy. These data indicate that both inhibin and estradiol, predominantly secreted from the ovary, are involved in the regulation of gonadotropin secretion during pregnancy as during the estrous cycle in the rat.