Prostaglandin E(2) and F(2 alpha) production by equine conceptuses and concentrations in conceptus fluids and uterine flushings recovered from early pregnant and dioestrous mares.

A growing equine conceptus must suppress the cyclical release of PGF(2 alpha) from the endometrium to effect maternal recognition of its presence in the uterus. Paradoxically, the conceptus itself secretes PGF(2 alpha), together with other prostaglandins. In this study, the PGF(2 alpha) and PGE(2) content of, and production in vitro by, day 10-32 equine conceptuses were measured and the influence of pregnancy on the concentrations of these prostaglandins in the uterine lumen was examined. In vitro, the release of both prostaglandins per mg conceptus tissue was very high on day 10 after ovulation and lower thereafter. However, while PGF(2 alpha) production decreased further after day 18 of gestation, PGE(2) production remained high until day 32. Prostaglandin concentrations in yolk sac fluid were unaffected by gestational age and PGE(2) concentrations in this compartment were two to five times higher than PGF(2a) concentrations. PGF(2 alpha) concentrations reached high values in uterine flushings recovered from cyclic mares during days 14-16 after ovulation, the expected time of luteolysis, but were negligible in flushings recovered from pregnant mares at this time. Beyond day 18 of gestation, PGF(2 alpha) concentrations in uterine flushings were high and strikingly similar to those recorded during cyclical luteolysis. It is concluded that the equine conceptus effects maternal recognition of pregnancy primarily by inhibiting the ability of the endometrium to release PGF(2 alpha) during days 12-16 after ovulation. However, the conceptus appears to delay, rather than prevent, the development of the uterine PGF(2 alpha) release pathway and an alternative mechanism must prevent luteolysis from being triggered during days 18-32 of gestation.