INHIBIN SUPPRESSES LUTEINIZING-HORMONE (LH)-RELEASING HORMONE SELF-PRIMING - DIRECT ACTION ON FOLLICLE-STIMULATING-HORMONE SECRETION AND OPPOSITION OF ESTRADIOL-ENHANCED LH-SECRETION

Previous studies have suggested that the ovary produces a factor that maintains the pituitary in a state of low LHRH responsiveness that must be overcome by the self-priming action of LHRH. To determine the role of inhibin in maintaining low LHRH responsiveness in pituitaries of diestrous female rats, endogenous inhibin was passively immunoneutralized in vivo, and the pituitaries were removed 18-20 h later and examined for LHRH responsiveness in vitro. Pituitaries from diestrous control rats produced the biphasic pattern of gonadotropin secretion that typifies LHRH self-priming: an initial low secretory response to LHRH (lag phase), followed by a protein synthesis-dependent transition to an enhanced rate of secretion with continued LHRH exposure (primed phase). Immunoneutralization of endogenous inhibin [antiserum (AS) treated] resulted in an increased rate of LH secretion during the lag phase, while no change was observed in the primed phase rate of LH secretion. FSH secretion from pituitaries of AS-treated rats was increased during the lag phase to a rate of secretion similar to that observed during the primed phase of FSH secretion from control pituitaries, and it was increased further during the primed phase of secretion. These results suggest that inhibin is at least partially responsible for the low secretion of LH observed during the lag phase response to LHRH exposure and is totally responsible for the lowered rate of FSH secretion during the lag phase. The observation that the enhanced rate of gonadotropin secretion observed with AS-treated pituitaries during the lag phase was resistant to inhibition of protein synthesis provides further evidence that a partial transition from the lag to the primed phase had already occurred. Pituitaries from ovariectomized rats were also examined in order to place the contribution of inhibin in perspective with the total ovarian influence on pituitary responsiveness to LHRH. Unexpectedly, LH secretion during the lag phase was similar to the low secretion rate of diestrous control pituitaries, and the higher primed rate of secretion failed to fully develop, suggesting that an additional ovarian factor was required to induce and maintain pituitary responsiveness to LHRH in terms of LH secretion. FSH secretion from the ovariectomized rats was similar to that observed from pituitaries of AS-treated rats, thus further supporting the concept that inhibin is fully responsible for the suppression of FSH secretion in response to LHRH. Plasma from the AS-treated rats revealed a 2-fold increase in estradiol levels compared with diestrous control rats. To determine the contribution of estradiol, pituitaries were examined from ovariectomized rats in which estradiol levels were maintained at both the level observed in diestrous controls and that in AS-treated rats. Estradiol replacement completely restored the LH secretion response to LHRH during both the lag and primed phases to the levels observed with pituitaries from AS-treated rats. FSH secretion was again similar to that observed with AS-treated pituitaries. These results demonstrate that estradiol is necessary to maintain pituitary responsiveness to LHRH in terms of LH secretion. The observation that normal diestrous day 2 levels of estradiol in the absence of inhibin (ovariectomized rats with diestrous day 2 level of estradiol maintenance) can induce the same degree of pituitary priming as immunoneutralization of endogenous inhibin in intact rats suggests that inhibin may have a direct action at the level of the pituitary to oppose the stimulatory action of estradiol on pituitary sensitivity to LHRH. In addition, inhibin appears to indirectly oppose the stimulatory action of estradiol on pituitary sensitivity to LHRH by maintaining lowered plasma levels of estradiol. The observation that AS treatment, but not ovariectomy or estradiol maintenance, induced an increased rate of FSH secretion in response to LHRH during the primed phase suggests the presence of an additional ovarian factor that enhances LHRH-induced FSH secretion and is normally opposed by inhibin.