Comparisons of Nitric Oxide Synthases in Normal Human Placenta from 37 to 41 Weeks Gestation: Qualitative and...

> Objective: The purpose of this study was to determine the type of nitric oxide synthase isoform and to measure the quantities of nitric oxide synthase mRNA in the human placenta. Methods: The isoforms of nitric oxide synthase were determined in ten placentas of normal pregnant women using information regarding calcium dependence and inhibition by arginine analogs and findings from Western blot analyses and reverse transcriptase-polymerase chain reaction. Results: The activity of nitric oxide synthase was largely calcium dependent, although a small element of calcium-independent activity was observed. A stronger inhibition was shown with Nomega-nitro-l-arginine than with Nomega-monomethyl-l-arginine. On Western blots endothelial nitric oxide synthase was detected as a band of 140 kilodaltons, without evidence of inducible nitric oxide synthase. Messenger RNA of endothelial nitric oxide synthase was readily detected by reverse transcriptase-polymerase chain reaction, but that of inducible nitric oxide synthase was barely detected. The quantity of mRNA of inducible nitric oxide synthase was about 100-fold less than that of endothelial nitric oxide synthase. Conclusions: These results point to the constitutive endothelial type as the predominant isoform of nitric oxide synthase in human placenta from 37 to 41 weeks gestation, although protein and mRNA of inducible nitric oxide synthase may exist.