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Epostane, an inhibitor of 3β-hydroxysteroid dehydrogenase, was administered orally to pro-oestrous rats to evaluate further a possible role for preovulatory progesterone secretion in eliciting surges of LH and FSH. Whereas a dose of 10 mg epostane/kg had essentially no effects on preovulatory gonadotrophin surges and ovulation, 200 mg epostane/kg markedly attenuated LH and FSH surges and blocked ovulation. A dose of 50 mg epostane/kg exerted effects on LH and FSH surges and ovulation intermediate between those of doses of 10 and 200 mg/kg. Plasma concentrations of progesterone were significantly lower in all anovulatory epostane-treated rats at 18.00 and 22.00 h on pro-oestrus than those measured in vehicle-treated rats. Concurrent injection of 2 mg progesterone in rats given 200 mg epostane/kg restored gonadotrophin surges to normal, but consistently failed to reverse the inhibitory effects of epostane on ovulation. Peak plasma progesterone levels produced by the progesterone injections were eight- to tenfold higher than the highest levels measured in vehicle-treated rats during the afternoon of pro-oestrus. Insertion of progesterone capsules was less effective than injections of progesterone in restoring gonadotrophin surges to normal, even though peak plasma progesterone concentrations achieved after insertion of two 20 mm long progesterone capsules were double the peak progesterone concentrations measured in control rats. Nevertheless, taken together with recent reports showing attenuation of preovulatory gonadotrophin surges by the progesterone antagonist RU 486 (17β-hydroxy-11β-[4-dimethyl-aminophenyl]-17-[prop-1-ynl]estra-4,9-diene-3-one), the present results provide support for a role of preovulatory progesterone secretion in enhancing oestrogen-dependent LH/FSH surges on pro-oestrus.

J. Endocr. (1988) 118, 59–68

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