Gestational Changes in Nitric Oxide Synthase Activity in the Rat Placenta

OBJECTIVE: To assess the importance of nitric oxide (NO) generated in the placenta on pregnancy, nitric oxide synthase (NOS) activities were measured in the rat placentas of different gestational ages. MATERIALS AND METHODS: NOS activity was determined by [3H] L-arginine to [3H]-citrulline conversion assay on rat placenta of day 5, 10, 15 and 21 of gestation. RESULTS: NOS activity distributed both in the soluble and particulate fractions. Inhibition of NOS activity by L-arginine analogs confirmed the substrate specificity. The requirement of calcium/calmodulin for the maximal activity indicated that the rat placenta NOS was of a constitutive calcium/calmodulin dependent isoform. The activities in both fractions were higher in the earlier gestational age placentas, decreasing with progression of gestation, and the lowest in the term placentas. CONCLUSION: Although NOS activity was detected in the placenta throughout gestation, it was highest in the early gestational age placenta, suggesting a possible significant role of NO in early gestation.