Effect of nitric oxide and carbon monoxide on uterine contractility during human and rat pregnancy

OBJECTIVE: We sought to study the effects of authentic nitric oxide and carbon monoxide on the contractile activity of pregnant human and rat myometrium. STUDY DESIGN: Strips were prepared from uterine biopsy specimens of 10 pregnant, nonlaboring women at term gestation undergoing cesarean delivery. In addition, rings were prepared from the uteri of pregnant rats at midterm (day 14) and at term (day 22) gestation (n = 10-12). The tissues were mounted in organ chambers filled with Krebs-Henseleit solution continuously aerated with 5% carbon dioxide in air (37°C, pH -7.4) for isometric tension recording. The effects of nitric oxide and carbon monoxide gases on spontaneous contractile activity were studied. Responses to hemin (hemoxygenase substrate), which produces endogenous carbon monoxide, were also examined. Responses to nitric oxide and carbon monoxide were also studied in aortic and tail artery rings from pregnant rats after contraction with phenylephrine. RESULTS: Nitric oxide significantly inhibited contractility of human myometrium at term (area under the concentration- response curve, 145.36 ± 30.02 vs 40.56 ± 22.81 in controls; P < .05) and rat myometrium at midterm gestation (264.23 ± 47.86 vs 121.82 ± 23.50; P < .05) but not at term. No statistically significant inhibition was induced in human or rat myometrium by carbon monoxide, whereas hemin significantly attenuated contractility in human myometrium at term and in rat myometrium at midterm gestation (P < .05). Nitric oxide, carbon monoxide, and hemin relaxed aortic and tail artery rings. CONCLUSIONS: Authentic nitric oxide inhibits rat uterine contractile activity at midterm gestation but not at term. However, nitric oxide inhibits human myometrium activity at term. Authentic carbon monoxide does not appear to modulate uterine contractility, whereas hemin may have some inhibitory properties.