Effect of progesterone on prostaglandin F2α secretion and outcome of pregnancy during cloprostenol-induced abortion in mares

Abstract

Objectives - To determine the role of progesterone in the regulation of endogenous prostaglandin F2α (PGF2α) secretion during cloprostenol-induced abortion and to investigate use of progestins to prevent prostaglandin-associated abortion. Animals - 16 pregnant mares. Procedure - To induce abortion, cloprostenol (250 μg/d) was administered daily until fetal expulsion or for up to 5 days. In experiment 1, 8 mares, 98 to 153 days' pregnant, received progesterone (300 mg/d) at 24-hour intervals for 5 days, starting 18 hours after the first cloprostenol administration. In experiment 2, 8 mares, 93 to 115 days' pregnant, received altrenogest (44 mg/d) at 24-hour intervals, starting 12 hours after the first cloprostenol administration. Historic control mares, 82 to 102 days' pregnant, received cloprostenol (250 μg/d) daily until fetal expulsion. Results - In control mares, fetal expulsion occurred after 2 to 3 cloprostenol administrations and was associated with significant increases in PGF2α secretion. Abortion did not occur in 5 of 8 progesterone-treated mares and 8 of 8 altrenogest-treated mares, and endogenous PGF2α secretion was inhibited, compared with values in aborting mares. Conclusion - Circulating progestogen concentrations may have a role in the outcome of pregnancy during prostaglandin-induced abortion. Altered prostaglandin secretion may be a reflection of a direct effect of progesterone or may be caused by the abortion process. Clinical Relevance - Progestogens might be useful for prevention of abortion in mares in which pregnancy is at risk owing to diseases that are associated with excess prostaglandin secretion.