Effects of estrogens on systemic and regional circulations in normal and renal hypertensive sheep

Effects of estrogen administration on systemic and regional circulation were studied in normotensive and renal hypertensive, chronically instrumented nonpregnant and pregnant ewes. Arterial pressure, cardiac output, and uterine, renal, superior mesenteric, and iliac blood flows, as well as uterine oxygen transfer, were monitored before and after intravenous administration of Premarin or estradiol-17 beta. The results show that: (1) estrogen administration produces a marked decrease in uterine vascular resistance and increase in uterine blood flow and oxygen transfer, lasting for about 2 hours; (2) arterial pressure, cardiac output, and other regional blood flows were not affected by estrogens; (3) the magnitude of uterine vasodilatation produced by estrogens was greater in the hypertensive than in the normotensive animals; it was also greater in the nonpregnant than in the pregnant state; these findings indicate that the magnitude of uterine vasodilatation depends on the status of the uterine vascular resistance in the resting state; (4) blockade of the autonomic nervous system at various levels, as well as administration of a mild antihistaminic agent, failed to alter the magnitude of the estrogen-induced uterine vasodilatation. These results indicate that estrogens act directly on the uterine vascular bed and produce a redistribution of flows and resistances in the body; the precise sites of this redistribution are not as yet determined.