Control of uterine and ovarian blood flow throughout the estrous cycle and pregnancy of ewes, sows and cows.

Current investigations strongly indicate a relationship between the function of adrenergic (vasoconstrictor) nerves innervating uterine and ovarian arteries and local concentrations of estrogen and progesterone. During the estrous cycle of ewes, cows and sows, the higher the estrogen:progesterone ratio in systemic blood, the greater is the quantity of blood flowing through the uterine vascular bed. When converted to a catechol form, estrogen may reduce uterine vasoconstriction by binding to and suppressing perivascular alpha-adrenergic receptor activity. Progesterone, in contrast to estrogen, augments the responsiveness of vascular smooth muscle to norepinephrine. Lymphatic vessels, which are in close linear apposition to the adventitia of the uterine vasculature may serve to transport steroids to the periarterial nerves. Vasodilation associated with steroid-induced changes in autonomic function appears to be modulated and amplified by estrogen-dependent changes in prostaglandin biosynthesis. During early pregnancy in ewes, cows and sows, vasodilation, mediated by the conceptus, occurs in the luteal and (or) uterine vascular beds on days critical for continuation of pregnancy in each species. In cows and sows, this vasodilatory effect of the conceptus is associated with increased concentrations of estrogens in the uterine lumen, as well as the lymph and blood draining the uterus. After the initial increase in blood flow to the uterus during early pregnancy, changes in the estrogen:progesterone ratio in fetal and maternal fluids appear to be unrelated to changes in uterine blood flow (UBF). This lack of association between the estrogen:progesterone ratio and UBF may result from a progressive reduction in number of adrenergic nerves observed in the uterus throughout pregnancy. If one accepts that the estrogen:progesterone ratio controls UBF through alterations in the function of periarterial adrenergic nerves, the impaired function of the nerves during mid and late gestation may preclude the effects of these steroids.