Comparison of type I and type III nitric oxide synthase (NOS) immunoreactivity in pulmonary vascular endothelium of ovine fetus

Nitric Oxide (NO) is an important factor responsible for decreased pulmonary vascular resistance at birth. We examined the distribution of type I and type III nitric oxide synthase in the lung of the late gestation ovine fetus(136 days) to study potential sites for NO production within the fetal lung. Using NADPH diaphorase, we screened for NOS within the lung. A polyclonal rabbit anti-rat type I NOS antibody (Affinity Bioreagent) and a mouse anti-bovine aortic endothelial type III NOS antibody (H32, from Dr. J.S.Pollock) were used to detect immunoreactivity for type I and type III NOS, respectively, in the fetal sheep lung. NADPH diaphorase staining colocalized with immunostaining for type I NOS and both showed more intense staining in endothelium of small vessels (intraacinar and bronchiole-associated) than larger vessels. Immunostaining of type I NOS differed from that of type III NOS in endothelium of different generations of pulmonary vessels. For type I NOS, intraacinar vessels stained more intensely than bronchiole-associated vessels but negatively for cartilaginous airway-associated vessels and capillaries, whereas all size vessels stained positively with a similar intensity for type III NOS except for capillaries(no staining). Surprisingly, our results suggest the presence, and a potential role for type I NOS, in addition to type III NOS, in the regulation of pulmonary vascular tone at birth.