Heterogeneous distribution of type I nitric oxide synthase in pulmonary vasculature of ovine fetus

The nitric oxide/guanosine 3',5'-cyclic monophosphate pathway plays an esse ntial role in mediating pulmonary vasodilation at birth. Small resistance a rteries in the fetal lung are vessels of major significance in the regulati on of pulmonary vascular tone. The present study is to determine that type I nitric oxide synthase (NOS-I) is present in ovine fetal pulmonary vascula ture and that NOS-I is distributed heterogeneously in ovine fetal pulmonary circulation. We used reduced nicotinamide adenine dinucleotide phosphate d iaphorase (NADPH-d) histochemistry and NOS-I immunohistochemistry to locali ze NOS-I in fetal sheep lungs and showed a colocalization for NADPH-d activ ity with NOS-I immunoreactivity, Strong NOS-I immunoreactivity was observed exclusively in the endothelium of the terminal bronchiole and respiratory bronchiole-associated arteries. As a comparison, adult sheep lung did not s how positive immunoreactivity in the pulmonary endothelium. NOS-I was absen t in the umbilical or systemic arteries from the ovine fetus, whereas abund ant NOS-III immunoreactivity was present in these arteries. We conclude tha t NOS-I is present uniquely in the ovine fetal pulmonary circulation as opp osed to the adult pulmonary or the fetal systemic circulation. NOS-I is dis tributed heterogeneously in the ovine pulmonary vasculature. We speculate t hat NOS-I plays an active role in the regulation of perinatal pulmonary cir culation.