NITRIC-OXIDE SYNTHASE ACTIVITY AND LOCALIZATION DO NOT CHANGE IN UTERUS AND PLACENTA DURING HUMAN PARTURITION

Animal studies have suggested that nitric oxide, a smooth muscle relax ant, is a fundamental mediator in the initiation of parturition. The p urpose of this study was to test the hypothesis that the onset of huma n labour is associated with a reduction in the activity of the enzyme nitric oxide synthase (NOS), within the uterus. Samples of myometrium, placenta, decidua and fetal membranes were collected during Caesarean section from 11 women before and 11 women after the onset of labour a t term. Immunocytochemistry was used to localize each of the three iso forms of NOS (endothelial NOS, brain NOS, and inducible NOS) in each o f these tissues and the intensity of staining was qualitatively assess ed. NOS enzyme activity was determined in homogenates of frozen myomet rium, placenta and fetal membranes (with attached decidua), by measuri ng conversion of radio-labelled L-arginine to L-citrulline. Each of th e three isoforms of NOS was localized in each of the tissues. We found no difference in either the expression or enzyme activity of NOS in m yometrium, placenta or fetal membranes before and during labour at ter m. These results suggest that, in contrast to animal studies, a decrea se in NOS enzyme activity may not be involved in the onset of parturit ion at term in the human.