Antiprogestagen RU486 prevents the LH-dependent decrease in the serum concentrations of inhibin in the rat.

1. In the rat, the LH-dependent ovarian progesterone rise mediates several actions of the primary surge of LH on the ovary. This experiment was aimed at elucidating the effects of the antiprogestagen RU486 on the LH-dependent decrease in both the serum concentrations and the ovarian content of inhibin. 2. All rats in this experiment were treated with an antagonist of LHRH (1 mg/200 microliters saline at 0800 h in proestrus) to suppress the endogenous release of LH. One group of rats received 32 micrograms LH/250 microliters saline at 1200 h in proestrus. Other group was given 4 mg RU486/200 microliters oil at 0800 h in proestrus. The third group was injected with both RU486 and LH. Rats from the control group were injected with 250 microliters saline and 200 microliters oil. Animals were decapitated at 1700 h in proestrus and trunk blood and ovaries collected to determine the serum concentrations of LH, FSH, progesterone, 17 beta-estradiol and inhibin as well as the ovarian content of inhibin. 3. The ovulatory dose of LH in LHRHa-treated rats decreased both the serum concentrations and the ovarian content of inhibin and increased the serum concentrations of FSH. The administration of RU486 blocked the effect of LH on the serum concentrations of inhibin but not that on the ovarian content of inhibin. 4. Since the antiprogestagen RU486 blocked the effect of LH on the serum concentrations of inhibin, we conclude that ovarian progesterone, besides mediating the effects of the primary LH surge on the ovulatory process and luteinization, participates in the LH-dependent drop in the serum concentrations of inhibin in proestrous afternoon.