Role of nitric oxide in regulating the ductus arteriosus caliber in fetal rats

The role of nitric oxide (NO) on the ductus arteriosus (DA) patency was exa mined in fetal rats at various stages of gestation. NO-nitro-L-arginine met hyl ester (L-NAME, 50 mg/kg, ip), an NO synthase (NOS) inhibitor, or indome thacin (3 mg/kg, po), a cyclooxygenase inhibitor, was administered at 3 hr before cesarean section to pregnant rats ranging from day 17 to day 21 of g estation. Dams were decapitated and the fetuses were obtained by cesarean s ection. The fetuses were rapidly frozen in an acetone-dry ice mixture. Usin g rapid-freezing and shaving methods, the calibers of the DA and pulmonary artery were measured. The constrictive effect of L-NAME on the fetal DA cal iber was stronger than that of indomethacin in 19-day-old and immature fetu ses. In near-term fetuses, the constrictive effects of L-NAME were reduced, while indomethacin caused marked DA constriction. We conclude that endogen ous NO may play a major role in regulating the patency of the DA in earlier fetal stages, while dilator prostaglandins may play a greater role in regu lating the ductal patency in the near-term fetus.