Placental Nitric Oxide Synthase Activity and Abnormal Umbilical Artery Flow Velocity Waveforms WARWICK GILES, MB BS, FRACOG, PhD, STEPHEN O’CALLAGHAN, MB BS, FRACOG, MARK READ, PhD, NEIL GUDE, MSc, ROGER KING, MB BCh, PhD, AND SHAUN BRENNECKE, MB BS, FRACOG, PhD Objective: To assess the nitric oxide synthase activity in placentas from women with either normal or abnormal Doppler ultrasound umbilical artery flow velocity wave- forms who delivered by elective cesarean. Methods: This prospective observational study involved 16 women admitted either for elective cesarean for standard obstetric indications (with normal umbilical artery Doppler waveform studies, n = 8) or with evidence of fetal or maternal complications of pregnancy (with abnormal umbil- ical artery Doppler studies, II = 8). Placental tissue was collected and frozen in liquid nitrogen immediately upon delivery. Following storage at --8OC, nitric oxide synthase activity was analyzed by measuring the conversion of [3HlL- arginine to [3H]L-citrulline. Results: Placentas from women with abnormal umbilical artery flow velocity waveforms showed significantly lower mean nitric oxide synthase activity than did placentas from women with normal umbilical artery flow velocity wave- forms (V,,, = 11.3 pmol/minute/mg protein versus 22.6 pmol/minute/mg protein). Conclusion: There is a statistically significant reduction in nitric oxide synthase activity in placentas from pregnancies with abnormal umbilical artery flow velocity waveforms. (Obstet Gynecol 1997;89:49-52. Copyright 0 1997 by The American College of Obstetricians and Gynecologists.) From the Discipline of Reproductive Medicine, School of Medicine and Health Sciences, University of Newcastle, Callaghan, Australia, the Department of Perinatal Medicine, Royal Women’s Hospital, Carlton, Australia, and the Department of Pharmacology, Monash University, Clayton, Australia. This zoork was supported by an Australian National Health and Medical Research Council Project Grant iNumber 930410), the 3AW Community Service Trust Research Foundation of Victoria, The Sun- shine Foundation of Victoria, and the Maternal Health Research Unit, Uniz)ersify of Newcastle. The authors acknowledge the technical assistance of Ms. Idiana Di lulio. Histopathologic examination of placentas with abnor- mal umbilical artery flow velocity waveforms has shown a reduction in the small arterial or arteriolar vessels.l,’ A long-term, prospective, clinical observa- tional study noted an association between abnormal Doppler studies and poor fetal and neonatal outcomes,3 and the use of Doppler studies in the management of high-risk pregnancies is associatedwith improved peri- natal mortality.” Nitric oxide has been implicated in the maintenance of low placental blood-flow resistance.5-7 Constitutive, calcium-dependent nitric oxide synthase activity has been measured in human placentas,8 and inhibition of its activity by N”-nitro-L-arginine increases placental vascular resistance.5In fetal sheep, inhibition of nitric oxide synthase causesdecreased arterial O2 saturation and pH and increased PCO, as well as changes in umbilical blood flow and resistance.’ In human preg- nancies complicated by pregnancy-induced hyperten- sion and fetal growth restriction (FGR), a decrease in nitric oxide synthase activity has been rep0rted.l’ How- ever, Doppler umbilical artery flow velocity waveforms were not documented in all women included in the normal and hypertensive groups in these studies. This confuses the issue with respect to nitric oxide synthase activity and umbilical artery Doppler waveform stud- ies. We have reported previously by letter (Giles W, O’Callaghan S, Boura A, Walters W. Reduction in human fetal umbilical-placental vascular resistance by glyceryl trinitrate. Lancet 1992;340:856)that abnormal umbilical artery Doppler waveforms improve with the administration of sublingual glyceryl trinitrate (a nitric oxide-donating agent). This finding was confirmed in pregnancies complicated by severe preeclampsia in which women were treated with nitroglycerin.” Based VOL. 89, NO. 1, JANUARY 1997 0029.7844/97/$17.00 49 PII SOO29-7844(96)00371-7