Advancement of uterine secretion of prostaglandin E2 by treatment with progesterone and transfer of asynchronous embryos.

Effects of treatment with progesterone early after estrus and the age of the blastocyst in the uterus on day-6 postestrus on uteroovarian venous concentration of prostaglandin E2 (PGE2) during days 7-12 postestrus were examined. At estrus, 24 ewes were assigned randomly to mating with fertile rams and daily injections of progesterone from days 1-3 postestrus, mating with fertile rams and daily injections of vehicle from days 1-3 postestrus, no mating, injection of progesterone from days 1-3 postestrus and transfer of a day-10 blastocyst into the uterus on day-6 postestrus, and no mating, injection of vehicle from days 1-3 postestrus and transfer of a day-10 blastocyst into the uterus on day-6 postestrus. PGE2 increased beginning on day 8 in progesterone-treated ewes that received a day-10 blastocyst on day-6 postestrus, but did not increase until day 12 in mated ewes with confirmed pregnancy. PGE2 remained at basal concentrations in mated progesterone-treated ewes (with a day-6 blastocyst on day-6) and showed only a transient rise on day-11 in vehicle-treated ewes which received a day-10 blastocyst on day-6 postestrus. It is concluded that progesterone regulates the ability of the uterus to release PGE2 in response to a blastocyst of appropriate age.