Circulatory Changes in the Reproductive Tissues of Ewes during Pregnancy

Abstract

The blood flows to reproductive organs were measured by means of radionuclide-labeled microspheres in 24 pregnant ewes with gestational ages ranging from 38 to 141 days. The microspheres were injected in the left ventricle of the non-anesthetized animal 4–7 days after surgery and while uterine blood flow was recorded continously by means of electromagnetic probes on both uterine arteries. The blood flow to ovaries with corpus luteum was maximum in early pregnancy and greater than in ovaries without corpus luteum. Placental blood flow increased from ∼30 to ∼300 ml/min during the stage of placental growth (40–90 days). In the last 50 days of pregnancy placental weight declined, whereas placental flow continued to rise and attained ∼1,500 ml/min near term. The growth curves of fetal weight and placental blood flow followed a similar pattern, but the early increase in placental flow was more rapid than the increase in fetal weight. Hence placental flow per gram of fetus was larger in early pregnancy (∼9 ml/min·g) than near term (∼0.25 ml/min·g). In twin pregnancies the placenta of each fetus received a smaller blood flow than the placenta of a singleton of comparable age. Mammary blood flow demonstrated no change until the last 60 days, when it increased from ∼10 to ∼200 ml/min. This phenomenon coincided with an increase in mammary weight and the final progressive increase of placental blood flow.

Fetoplacental growth and vascular development in overnourished adolescent sheep at day 50, 90 and 130 of gestation

“…Furthermore, both sets of data are similar to the observations of Stegeman (1974) both in pattern and magnitude of the increase in placental vascularization that she determined using a microscopic point counting method. Moreover, the increases in placentome capillary area density and in cotyledonary capillary number and surface density from early to late gestation are in broad agreement with previously documented cross-sectional measurements of changes in uterine (maternal) and umbilical (fetal) blood flows in vivo, measured at mid and late gestation (Rosenfeld et al 1974, Molina et al 1990.…”

Nitric oxide contributes to estrogen-induced vasodilation of the ovine uterine circulation.

“…Since placental estrogen synthesis is substantial throughout pregnancy, local tissue concentrations probably exceed plasma levels. Exogenous estrogen increases uteroplacental blood flow 40-50% in a pattern similar to that seen in nonpregnant animals (4,9).This also occurs after substrate-induced increases in endogenous placental estrogen production (4,(55)(56)(57). Although we have confirmed that E 2 ␤ increases UBF in pregnant ewes and have now demonstrated that, as in nonpregnant animals, this is associated with a rise in uterine venous cGMP, these data do not provide conclusive evidence that local estrogens are necessary for maintaining blood flow.…”

“…However, it was not until the studies of Killam et al (5), using chronically instrumented nonpregnant ewes studied remote from surgery and anesthesia, that the magnitude of these effects on the uterine vascular bed were clearly demonstrated, uterine blood flow (UBF) 1 increasing Ͼ 10-fold after 1 g/kg of intravenous (iv) estradiol-17 ␤ (E 2 ␤ ) rather than the 50-100% previously reported (4, 6, 7). Rosenfeld et al (4,8,9), using the microsphere technique for measuring regional blood flows, not only confirmed these observations, but also demonstrated that E 2 ␤ had significant vasodilatory effects on several nonreproductive as well as reproductive tissues in nonpregnant, pregnant, and postpartum sheep and increased cardiac output 15-20%, thereby demonstrating substantial systemic responses. Importantly, the uterine responses to E 2 ␤ are locally mediated and independent of its effects on systemic vascular resistance (5,10,11).…”

“…Thus vasodilation is a general response after estrogen administration. Nonetheless, not all vascular beds respond similarly (4,8,9), which may reflect differences in estrogen receptor expression. Estrogen receptors are expressed in uterine artery endothelium and media (29)(30)(31); however, the E 2 ␤-induced rise in UBF has not been clearly shown to be mediated through the estrogen receptor.…”

Vasopressin and Catecholamine Secretion during Metabolic Acidemia in the Ovine Fetus

“…4.3 days of gestation (term -145 days), respectively. The surgical procedure for the preparation of the chronically instrumented sheep model has been previously described in detail (18,19). In brief, after sedation with intravenous ketamine hydrochloride (2 mg/kg), spinal anesthesia was induced with hyperbaric pontocaine hydrochloride (12 mg) and supplemented with intravenous sodium pentobarbital as needed (1 5 mg/kg).…”