Nitric oxide synthase isoforms in the rat uterus: Differential regulation during pregnancy and labour

Production of nitric oxide in the uterus is increased during pregnancy and decreased during delivery. In this study the isoform of nitric oxide synthase which may be responsible for the changes in nitric oxide production was investigated in relation to pregnancy and delivery. Monoclonal antibodies were used to measure changes in neuronal nitric oxide synthase (NOS I), macrophage nitric oxide synthase (NOS II) and endothelial cell nitric oxide synthase (NOS III) protein in the rat uterus by densitometric scanning of specific bands. Results show that: (I) NOS II protein concentrations in the uterus were substantially increased during pregnancy and were decreased during delivery, both at term and preterm (induced by RU486); (2) NOS III protein was present at all stages examined but the concentrations were unchanged; (3) NOS I was present in the rat uterus during the nonpregnant stage but not during pregnancy and delivery. The changes in uterine NOS II protein concentrations during pregnancy and delivery were further confirmed by the changes in the Ca2+-independent, but not Ca2+-dependent, nitric oxide synthase activity. Therefore, an increase in NOS II, thus in nitric oxide production during pregnancy, may play a role in maintaining uterine quiescence.