Nitric oxide inhibits uterine contractility during pregnancy but not during delivery.

Nitric Oxide mediates various biological phenomena, including vascular smooth muscle relaxation. In the present study, we sought to determine if an L-arginine nitric oxide-relaxation system is present in the uterus and if it modulates contractility during pregnancy. The substrate and a donor of nitric oxide and nitric oxide gas caused substantial relaxation of the spontaneous contractility of tissues from the rat uterus in vitro during pregnancy. Inhibitors of nitric oxide synthase and soluble guanylate cyclase reversed the relaxation effects of L-arginine. Nitric oxide was produced by the uterus in organ culture. Relaxation effects of L-arginine on the pregnant rat uterus were diminished at the time of spontaneous labor and postpartum. Nitric oxide production was also substantially reduced during labor. These results show that an L-arginine-nitric oxide-relaxation system is present in the uterus and it inhibits contractility during pregnancy but not during labor.