Calcium Homeostasis and Contraction of the Uterine Artery: Effect of Pregnancy and Chronic Hypoxia

The present study tested the hypothesis that chronic hypoxia alters pregnancy-mediated adaptation of Ca^2+ homeostasis and contractility in the uterine artery. Uterine arteries were isolated from nonpregnant and nearterm pregnant ewes of normoxic control or high-altitude (3820 m) hypoxic (oxygen pressure in the blood [PaO2], 60 mm Hg) treatment for 110 days. Contractions and intracellular-free Ca^2+ concentration ([Ca^2+]i) were measured simultaneously in the same tissue. In normoxic animals, pregnancy increased norepinephrine (NE), but not 5-hydroxy-thymide (5-HT) or KCl, contractile sensitivity in the uterine artery. Chronic hypoxia significantly attenuated NE-induced contractions in the pregnant, but not nonpregnant, uterine arteries. Similarly, 5-HT-mediated contractions of nonpregnant arteries were not changed. In the pregnant uterine artery, chronic hypoxia significantly increased NE-mediated Ca^2+ mobilization, but decreased the Ca^2+ sensitivity. In addition, hypoxia increased the calcium ionophore A23187-induced relaxation in pregnant, but not nonpregnant, uterine arteries. However, the A23187-mediated reduction of [Ca^2+]i was significantly impaired in hypoxic arteries. In contrast, hypoxia significantly increased the slope of the [Ca^2+]i-tension relationship of A23187-induced reductions in [Ca^2+]i and tension in the pregnant uterine artery. The results suggest that the contractility of nonpregnant uterine artery is insensitive to moderate chronic hypoxia, but the adaptation of sympathetic tone that normally occurs in the uterine artery during pregnancy is inhibited by chronic hypoxia. In addition, changes in Ca^2+ sensitivity of myofilaments play a predominant role in the adaptation of uterine artery contractility to pregnancy and chronic hypoxia.