The effects of nitric oxide on the contractility of isolated uterine and aortic rings from pregnant rats

OBJECTIVE: The object was to compare the effects of nitric oxide on isolated uterus and aorta of pregnant rats. STUDY DESIGN: Rings of uterus and thoracic aorta without endothelium from Sprague-Dawley rats at mid and late gestation were used for isometric tension recording. The concentration- response curve for diethylamine/nitric oxide was studied in the presence or absence of oxyhemoglobin (10^-5 mol/L), or oxyhemoglobin was added after the response to diethylamine/nitric oxide. RESULTS: Diethylamine/nitric oxide concentration dependently inhibited uterine contractions, and the effect was attenuated by previous treatment with oxyhemoglobin at mid gestation (n = 8). The effects were negligible at late gestation (n = 8). The relaxation of aortic rings by diethylamine/nitric oxide and its attenuation by previous treatment with oxyhemoglobin were similar at mid (n = 6) and late (n = 6) gestation. The sensitivity of aortic rings to diethylamine/nitric oxide is significantly higher than that of uterine rings. Oxyhemoglobin partly restored inhibited diethylamine/nitric oxide phenylephrine tension in aortic rings and had no effect on diethylamine/nitric oxide-inhibited uterine rings. CONCLUSIONS: Uterine smooth muscle is less sensitive to nitric oxide than is aortic smooth muscle. Nitric oxide sensitivity of rat uterus but not aorta decreases toward term.