Impaired Developmental Expression of Lung Endothelial Nitric Oxide Synthase and Abnormal Alveolarization in the Fawn-Hooded Rat : Implications for the Risk of Developing Pulmonary Hypertension 1689

The fawn-hooded rat (FHR) strain is characterized by platelet abnormalities and systemic hypertension, and has been used to study genetic risk factors for pulmonary hypertension (PH). Unlike other strains, FHR develop progressive PH when exposed to slight decreases in alveolar PaO₂. We hypothesized that developmental abnormalities in lung vasoreactivity or growth may predispose the FHR for PH. Since endogenous nitric oxide (NO) modulates pulmonary vasoreactivity and growth, we further hypothesized that early decreases in lung endothelial NO synthase (eNOS) content may contribute to the development of PH in FHR. We measured lung eNOS content and growth in FHR and control rat strains (Fischer and Sprague-Dawley (SDR)). Lungs were harvested from late fetal and postnatal ages (days 1 & 7) and eNOS protein content measured by western blot analysis. Immunostaining for eNOS was performed with paraffin sections and the eNOS monoclonal antibody. Histology was assessed after tracheal installation of formalin at constant pressure (10cm water) and radial alveolar counts performed. Lung to body weight ratios were reduced in fetal and newborn FHR by 20-46% compared to controls (p<0.05). In comparison with SDR, FHR lung eNOS protein was decreased to 69±8%, 56±8%, and 68±7% at fetal, and postnatal days 1 and 7, respectively (p<0.05 at each age). Immunostaining studies demonstrated selective staining for eNOS only in vascular endothelium and no difference in the pattern of eNOS expression between strains. Histology revealed striking changes in lung maturation and growth in FHR at all ages, including reduced alveolarization(FHR 58±2% of SDR at day 7, p<0.05). We conclude that in comparison with other rat strains, lung eNOS content is decreased in the FHR fetus and newborn. In addition, the FHR lung has decreased alveolarization and histologic signs of delayed lung maturation. We speculate that reduced vascular surface area and eNOS increase the risk for developing PH, and that decreased NO production may be associated with abnormal lung growth and development.