Prostacyclin produced by the pregnant uterus in the dog may act as a circulating vasodepressor substance.

Uterine production of PGI2 (prostacyclin) was quantitated in late pregnant dogs to evaluate if PGI2 could act as circulating vasodepressor substance in pregnancy. In five anesthetized, laparotomized dogs, the uterine venous plasma concentration of 6-keto PGF1 alpha (the in vitro hydrolysis product of PGI2) was 6.7 +/- 1.9 ng/ml and the arterial plasma concentration was 2.6 +/0 0.8 ng/ml. In four nonpregnant female dogs the arterial plasma concentration of 6-keto PGF1 alpha was consistently below 0.2 ng/ml. In eight pregnant dogs we also evaluated the ability of the pregnant uterus to inactivate PGI2 by comparing the hypotensive response to increasing doses of PGI2 infused into the uterine artery to the hypotensive response to increasing doses of PGI2 infused into the inferior vena cava. In addition, the effect of PGI2 infused into the uterine artery on uterine blood flow and intraamniotic fluid pressure was evaluated. The dose-response curves of intrauterine and intravenous PGI2 in causing systemic hypotension were identical suggesting that the pregnant uterus does not inactivate infused PGI2. Intrauterine PGI2 had no consistent effect on uterine hemodynamics although it did increase intraamniotic fluid pressure significantly. These data demonstrate that the pregnant uterus has the capacity to produce large quantities of PGI2 which is not inactivated in the uterus and therefore can appear in the arterial blood to exert a systemic vasodepressor effect.