Effect of placental tissue on inhibition of uterine contraction by nitric oxide donors

OBJECTIVE: Our purpose was to test the hypothesis that placental tissue mod ulates the effect of nitric oxide on spontaneous uterine contractility in p regnant rats. STUDY DESIGN: Rings (approximately 4 mm) of uterus taken from rats on day 1 4 (midpregnancy, n = 6), day 18 (late pregnancy, n = 4), and day 22 (term, n = 4) of gestation were placed in organ chambers filled with Krebs-bicarbo nate buffer bubbled with 5% carbon dioxide in air (37 degrees C, pH approxi mately 7.4) for isometric tension recording. In some rings a piece of place nta was left attached to the uterine wall. In the other rings the fetuses, placentas, and membranes were removed completely. Change of spontaneous con tractions of the rings (percentage change of basal integral activity for 10 minutes) in response to cumulative concentrations of the nitric oxide dono rs diethylamine-nitric oxide and nitroglycerin (10(-6) mol/L to 10(-4) mol/ L) were compared between rings with and without placenta. RESULTS: Diethylamine-nitric oxide and nitroglycerin inhibited spontaneous uterine contractions in rings from midpregnancy, in both the absence and th e presence of placenta. In rings from midpregnancy, the maximal inhibition of contractions by diethylamine-nitric oxide but not by nitroglycerin was s ignificantly (P<.05) higher in the presence (26.7% +/- 3.5% of basal activi ty) than in the absence (39.6% +/- 3.3%) of placenta. Inhibition of contrac tion by nitric oxide donors in rings from late and term pregnancy was less than in midpregnancy, and the presence of placental tissue did not influenc e the responses. CONCLUSIONS: The presence of placental tissue enhances inhibition of uterin e contractility by agents that spontaneously release nitric oxide, such as diethylamine-nitric oxide, but not by nitroglycerin, which requires metabol ic transformation for nitric oxide to be released. Refractoriness to nitric oxide near or at term does not depend on the presence or absence of placen tal tissue.