PERSISTENT ENOS IN LUNG HYPOPLASIA CAUSED BY LEFT PULMONARY-ARTERY LIGATION IN THE OVINE FETUS

Because increased flow and shear stress upregulate endothelial (e) nit ric oxide synthase (NOS) in adult endothelial cells in vivo and in vit ro, we hypothesized that decreased pulmonary blood flow would decrease eNOS content in the late-gestation ovine fetus. To investigate the ef fects of decreased blood flow and the potential role of altered eNOS c ontent in lung hypoplasia, we studied an animal model of lung hypoplas ia after left pulmonary artery (LPA) ligation in nine fetal lambs (114 -124 days gestation; term=147 days). After at least 14 days, animals w ere killed, and lungs were harvested for histology, immunostaining, We stern blot analysis for eNOS protein content, and biochemical assays o f NOS activity. LPA ligation markedly reduced left lung size. Histolog y demonstrated loose connective tissue and airway immaturity in the le ft lungs. eNOS immunostaining demonstrated equal staining in the left pulmonary vessels compared with the right. Solitary endothelial cells staining for eNOS and factor VIII-related antigen were observed throug hout the mesenchyme of left, but not right, lungs. eNOS protein conten t and activity were similar in left and right lungs. We conclude that, despite the absence of pulmonary blood flow and marked lung hypoplasi a, eNOS content and NOS activity were not reduced after LPA ligation i n the late fetal lung. We speculate that low pulmonary blood flow does not downregulate fetal pulmonary vascular eNOS expression and that ot her factors, such as paracrine or autocrine stimuli, may account for t he persistence of eNOS in the developing lung circulation.