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The direct influence of the embryo on uterine PGF₂ and PGE₂ production in sheep

In unilateral pregnant ewes, obtained by surgically transecting one uterine horn to isolate it from the rest of the pregnant uterus, the isolated horn develops endometrial oxytocin receptors by day 16 despite being in a hormonal environment equivalent of that of a pregnant ewe. This study examines uterine prostaglandin production in these animals. On day 16 of pregnancy, ewes were anaesthetized and a uterine vein draining each horn and the jugular vein cannulated. Venous blood samples were taken at 10 min intervals for 1 h before and 1 h after an i.v. injection of oxytocin (1 µg). Uterine vein samples were assayed for PGF₂ and PGE₂ and jugular blood samples assayed for 13,14-dihydro,15-keto-PGF₂. All ewes showed high plasma concentrations of 13,14-dihydro,15-keto-PGF₂ after the oxytocin injection. PGF₂ increased in the uterine vein draining the nonpregnant transected horn of a unilaterally pregnant ewe following oxytocin treatment, whereas there was no increase in concentration in plasma draining the pregnant horn. Basal plasma PGF₂ concentrations from the uterine vein draining the pregnant horn were significantly higher than concentrations obtained from the uterine vein of the transected horn while PGE₂ concentrations did not vary between the two horns. After oxytocin injection, there was a significant increase in plasma PGE₂ concentration in venous blood collected from both sides. These results indicate that (1) oxytocin receptors present in the isolated nonpregnant horn can be stimulated by oxytocin to produce PGF₂, despite the steroid environment being that of a pregnant ewe; (2) the presence of the conceptus locally inhibits the endometrial PGF₂ production following the challenge but enhances baseline production; (3) PGE₂ production does not differ between the pregnant and nonpregnant uterine horns of ewes with transected uteri and (4) oxytocin causes a bilateral increase in venous PGE₂ concentrations draining pregnant and nonpregnant uterine horns.

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