Placental nitric oxide metabolism.

There is increasing evidence that nitric oxide (NO) has a role in pregnancy. NO is synthesized from L-arginine by NO synthase (NOS), which can exist either as a calcium-dependent or a calcium-independent isoform of the enzyme. Both isoforms are present in placental villi and the authors have measured NOS activities in tissues from early and term normal, pre-eclamptic and growth-retarded pregnancies. Higher activities were seen in first trimester placental villi than at term. An impairment of NO metabolism occurred in placental villi from pre-eclamptic and growth-retarded pregnancies. Smoking also results in decreased NOS activities in the placental villi, suggesting that problems attributed to smoking during pregnancy could be linked to NO metabolism. Polyamines arginine and citrulline (all of which are important metabolites in the NO pathway) were also measured in placental villous tissues. The data presented in this review article are from work carried out in the authors' laboratories and suggest that alterations in the placental arginine-NO pathway may not only play a role in the physiological changes of advancing gestation but may also contribute to the pathophysiology of pre-eclampsia and fetal growth retardation.