Nitric oxide synthase activity in the gravid rat uterus decreases a day before the onset of parturition, , ,

OBJECTIVE: Our purpose was to determine the timing, tissue location, and isoform of the uterine nitric oxide synthase activity decrease at term in gravid rat uteri. STUDY DESIGN: Nitric oxide synthase specific activity was assayed in rat uteri 11 through 22 daysʹ gestation by the difference in radiolabeled arginine to citrulline conversion with and without the cofactor reduced nicotinamide adenine dinucleotide phosphate. Nitric oxide synthase isoform was assessed by calcium sensitivity and subcellular location. RESULTS: Rat uterine nitric oxide synthase activity decreased between days 15 and 21 of gestation but did not decrease further at term (day 22), before and after the onset of labor. Decidual nitric oxide synthase activity exceeded the myometrial activity at 15 daysʹ gestation, but then the two were equal at 18 through 22 daysʹ gestation. The nitric oxide synthase activity was calcium insensitive except for half the decidual cytosolic activity on day 15. CONCLUSION: The decrease in pregnant rat uterine nitric oxide synthase activity coincides with the preparation of the uterus for parturition rather than the final activation of labor. (Am J Obstet Gynecol 1996;175:1661-7.)