Endogenous production of nitric oxide (NO) by the endothelial Ca2+-dependent NO synthase (eNOS) seems to be developmentally regulated in the pulmonary vasculature. Whether decreased production or activity of endogenous NO contributes to sustained elevations of pulmonary vascular resistance (PVR) and the failure of postnatal adaptation in persistent pulmonary hypertension of the newborn (PPHN) is unknown. In the present work we measured eNOS protein content and NOS activity in lungs from normal fetal, newborn and adult lambs. The same measurements were made in animals subjected to a model of perinatal pulmonary hypertension caused by intrauterine compression of the ductus arteriosus (DA), performed between 125-128 days of gestation (147 days, term), and maintained for 9-13 days. The eNOS protein expression, as determined by western blot analysis, increased 2.23 fold between 0.53 and 0.95 term, and maintained in the postnatal animals. Ca2+-dependent NOS activity, as determined by L-14C-arginine to14 C-citrulline conversion assay, increased 6.9 fold between 0.53 and 0.95 term (P < 0-.01) and 3.48 fold between 0.95 term and newborn animals(P < 0.01). No differences were observed in NOS activity between newborn and adult lambs. After chronic intrauterine pulmonary hypertension, eNOS immunostaining demonstrated persistent presence of eNOS protein. However, the protein expression was reduced by 48 ± 4%, and Ca2+-dependent NOS activity was decreased by 23.15 ± 12.3% (P < 0.01 vs. age-matched controls). We conclude that chronic pulmonary hypertension impairs the normal development of eNOS protein expression and activity in late-gestation fetal lambs. We speculate that decreased NO production contributes to altered pulmonary vasoreactivity in utero, sustained elevation of PVR at delivery, and perhaps hypertensive structural changes in small pulmonary arteries in experimental PPHN.
