Local and Systemic Effects of Embryos on Uterine Tissues During Early Pregnancy in Pigs

In the pig, the periimplantation period is critical for successful establishment of pregnancy. We studied the influence of embryos on oxytocin (OT) and progesterone (P-4) regulated endometrial and myometrial secretion of 1) luteotrophic prostaglandin E-2 (PGE(2)) and 2) luteolytic prostaglandin F-2 alpha and its metabolite (PGFM) on days 12-14 of pregnancy in pigs. We used unilaterally pregnant pigs created by a surgical procedure in which one uterine horn remained intact and the second horn was cut transversely so that part of the horn was detached from the uterine body. The animals were divided into two groups, inseminated gilts (days 12-14 of pregnancy, n=6) and uninseminated cyclic gilts, which were used as controls (days 12-14 of estrous cycle, n=5). Embryos developed only in the patent part of the uterus and not in the occluded horn. The abundance of OTR mRNA was increased in the endometrium and decreased in the myometrium of the gravid uterine horn in the pregnant pigs compared with the non-gravid uterine horn or either uterine horn in the cyclic pigs, indicative of a local effect of the conceptus. The presence of embryos in the uterine horn during the periimplantation period determines endometrial metabolism of PGF(2 alpha) and the local response of the endometrium to OT and P-4. OT stimulates PGF(2 alpha) secretion and PGFM accumulation in endometrial cultures only from the non-gravid uterine horn and controls PGE(2) Secretion from the endometrium and myometrium in the pregnant gilts. The results indicate a more systemic affect of pregnancy on the uterine response to OT and a possibly the local effect of the conceptus in promoting progesterone's inhibition of OT-stimulated PGE(2) secretion and uterine metabolism of PGF(2 alpha).