Human placental and fetal membrane nitric oxide synthase activity before, during and after labour at term.

The aim of this study was to determine whether any labour-associated changes in nitric oxide synthase (NOS) activity occur in human placenta and fetal membranes. NOS activity in amnion, choriodecidua, and placenta obtained from women before (at Caesarean section, not in labour), during (at Caesarean section, in labour) and after (spontaneous onset labour, normal vaginal delivery) labour was assessed by measuring conversion of radio-labelled L-arginine to L-citrulline. NOS activity, as judged by its inhibition by the specific NOS inhibitor N omega-nitro-L-arginine, was present in placental and amnionic tissues, but not in choriodecidual tissue specimens. Activity detected in choriodecidua was significantly blocked during incubation with a high concentration of valine, suggesting that L-arginine was being consumed by reactions other than NOS under the experimental conditions in that tissue. There were no significant differences among the labour groups in either amnion or placental NOS activities measured in the presence of 1 microM L-arginine. Amnion NOS activity was significantly less than that in placenta. Placental V(max) and Km values (determined after removal of endogenous L-arginine) did not differ significantly among the different labour groups.