EXPRESSION OF NITRIC-OXIDE SYNTHASE BY SYNCYTIOTROPHOBLAST IN HUMAN PLACENTAL VILLI

The endogenous biosynthesis of nitric oxide (NO) is increased during g estation. To begin our investigation of a possible tissue source (or s ources), we examined the placenta. We postulated that analogous to the endothelium of blood vessels, the syncytiotrophoblast (STr) cell laye r that lines the intervillous blood space of the human placenta would express NO synthase. Our results show that human placental villi expre ss a calcium- and calmodulin-sensitive form of NO synthase, located ma inly in the microsomal cell fraction. By in situ hybridization using a riboprobe generated from human endothelial NO synthase cDNA, we obser ve NO synthase mRNA expression in STr. The STr also shows NADPH-diapho rase staining, indicating the presence of NO synthase, and most likely other flavin-containing enzymes involved in sex steroid metabolism. N O synthase activity was also detected in the villi of a complete mole placenta (which lacks fetal vessels), further supporting a trophoblast ic origin. Our findings suggest a previously unrecognized role for STr -derived NO in placental function.