Nitric oxide synthases in pregnant rat uterus

The conversion of [C-14]arginine into [C-14]citrulline as an indicator of n itric oxide synthesis was studied in uteri isolated from rats on different days of gestation, after labour and during dioestrus. Nitric oxide synthesi s was present in uterine tissues isolated at each stage of gestation and al so in tissues collected during dioestrus and after labour. Expression of ne uronal nitric oxide synthase was not detectable at any of the stages studie d. Endothelial nitric oxide synthase was present at all the stages studied, but there was a significant increase on day 13 of gestation and a decrease thereafter, with the lowest expression recorded on the day after labour. I nducible nitric oxide synthase expression in rat uteri increased substantia lly during pregnancy, with the highest expression on day 13 of gestation; e xpression decreased at term and after labour. The changes in expression of inducible nitric oxide synthase were coincident with the changes in nitric oxide synthase activity in uteri treated with aminoguanidine. Thus, these f indings indicate that an increase in expression of inducible nitric oxide s ynthase in the uterus may be important for maintenance of uterine quiescenc e during pregnancy and its decrease near the time of could have an effect o n the start of uterine during labour court contractility.