Suppression of myometrial contractile responses to oxytocin after different durations of chronic hypoxia in the near-term pregnant rat

OBJECTIVE: In light of our previous finding that chronic hypoxia decreases the myometrial contractile response to oxytocin in the near-term pregnant rat, we designed the current study (1) to investigate the effect of duration of hypoxic exposure on the contractile response to oxytocin and oxytocin binding sites and (2) to examine the effect of prolonged hypoxia on the contractile response to aluminum fluoride. STUDY DESIGN: Rats were exposed to room air (control) or to continuous hypoxia (10.5% oxygen) from day 19 through day 21 (48-hour exposure), from day 20 through day 21 (24- hour exposure), or midday 20 through day 21 of gestation (12-hour exposure). On day 21 the uterine horns were used for oxytocin receptor analysis and for in vitro study of myometrial contractile responses to cumulative doses of oxytocin (10^-10 to 10^-6 mol/L) or aluminum fluoride (0.5 to 4.0 mmol/L sodium fluoride in 10 μmol/L aluminum chloride). RESULTS: The maximal contractile tensions for the control and 12-hour exposure showed no difference. In contrast, 24-hour hypoxic exposure resulted in a reduction of the maximal contractile tension from 143 ± 11 (control) to 116 ± 7 gm x sec/cm². By 48 hours the maximal contractile tension was reduced, even further to 44 ± 13 gm x sec/cm². Oxytocin binding sites followed a similar trend with values changing from 256.9 ± 34.9 for control to 122.9 ± 26.1 and 84.9 ± 21.3 fmol/mg protein for the 24- and 48-hour exposure groups, respectively (p < 0.01, analysis of variance), with no change in the 12-hour group. The contractile responses to aluminum fluoride were not altered. CONCLUSIONS: The suppression in the myometrial contractile response to oxytocin and oxytocin binding sites depends on the duration of hypoxic exposure. Chronic hypoxic exposure did not affect the myometrial response to aluminum fluoride.