Effects of inhibiting 3 beta-hydroxysteroid dehydrogenase on plasma progesterone and other steroids in the pregnant mare near term.

Epostane, a competitive inhibitor of 3 beta-HSD was administered intravenously to a pregnant mare between 292 and 330 days of gestation at doses of 1-3 mg/kg/min. Plasma progesterone concentrations fell rapidly during epostane infusion in both the artery and uterine vein and remained significantly depressed for 4-5 h after the start of infusion. The venous arterial (V-A) plasma concentration difference in progesterone across the uterus also decreased significantly in response to epostane infusion. There were no significant changes in plasma progesterone or in the V-A concentration difference in control animals infused with vehicle alone. The plasma concentration of total unconjugated oestrogens in the uterine vein was reduced after administration of epostane but remained virtually unchanged in the control experiments. Uterine venous plasma concentrations of PGFM did not change significantly in the control or epostane-treated animals. Arterial plasma cortisol levels fell initially after epostane treatment but then rose to values significantly greater than before infusion. A similar increase in arterial plasma cortisol was observed in the control animals. None of the mares delivered after epostane treatment even at the highest dose. These observations demonstrate that inhibition of 3 beta-HSD alters steroidogenesis but has little effect on the length of gestation in the pregnant mare.