Effect of chronic hypoxia on myometrial responsiveness in the pregnant rat

Mechanisms involving the timing of normal parturition are not well understood in most animal species. To gain a greater understanding of the mechanisms, we employed hypoxia to perturb the normal system of parturition. The present study was designed to investigate the effects of chronic hypoxia on myometrial contractility in the near-term pregnant rat. Rats were exposed to room air (control) or to continuous hypoxia (10.5% O₂) either from experimental days 19 through 21 (2-day exposure) or from experimental days 15 through 21 (6-day exposure). On day 21, blood was collected for hormone assays, and the uterine horns were collected from each dam. One horn was snap-frozen in liquid nitrogen for oxytocin (OT) receptor analysis, and the other was used for in vitro assessment of myometrial contractile responses to cumulative doses of OT or arginine vasopressin (AVP). Hypoxic exposure resulted in ~60% reduction of the maximal myometrial contractile response to OT and a significant reduction in OT binding sites from 256.9 ± 34.9 to 84.9 ± 21.3 fmol/mg protein (P < 0.01). In contrast, the contractile response to AVP was unaffected after exposure to chronic hypoxia (P > 0.05). Additionally, we observed no difference in the plasma concentrations of estrogen, progesterone, and corticosterone. We conclude that chronic hypoxia decreased the effectiveness of OT-specific contractile mechanisms, at least partially through a decrease in OT binding sites.