ANTIPROGESTAGEN RU4X6 PREVENTS THE LH-DEPENDENT DECREASE IN THE SERUMCONCENTRATIONS OF INHIBIN IN THE RAT

1. In the rat, the LH-dependent ovarian progesterone rise mediates sev eral actions of the primary surge of LH on the ovary. This experiment was aimed at elucidating the effects of the antiprogestagen RU486 on t he LH-dependent decrease in both the serum concentrations and the ovar ian content of inhibin. 2. All rats in this experiment were treated wi th an antagonist of LHRH (1 mg/200 mu l saline at 0800 h in proestrus) to supress the endogenous release of LH. One group of rats received 3 2 mu g LH/250 mu l saline at 1200 h in proestrus. Other group was give n 4 mg RU486/200 mu l oil at 0800 h in proestrus. The third group was injected with both RU486 and LH. Rats from the control group were inje cted with 250 mu l saline and 200 mu l oil. Animals were decapitated a t 1700 h in proestrus and trunk blood and ovaries collected to determi ne the serum concentrations of LH, FSH, progesterone, 17 beta-estradio l and inhibin as well as the ovarian content of inhibin. 3. The ovulat ory dose of LH in LHRHa-treated rats decreased both the serum concentr ations and the ovarian content of inhibin and increased the serum conc entrations of FSH. The administration of RU486 blocked the effect of L H on the serum concentrations of inhibin but not that on the ovarian c ontent of inhibin. 4. Since the antiprogestagen RU486 blocked the effe ct of LH on the serum concentrations of inhibin, we conclude that ovar ian progesterone, besides mediating the effects of the primary LH surg e on the ovulatory process and luteinization, participates in the LH-d ependent drop in the serum concentrations of inhibin in proestrous aft ernoon.