No abstract

The effect of chronic hypoxia-induced pulmonary hypertension on nitric oxide synthase (NOS) in the lung is controversial. To clarify the regulation of endothelial and inducible NOS (eNOS and iNOS) expression in the chronically hypoxic lung, Northern and Western blot analyses were performed on mRNA and total protein from lungs of rats exposed to 3 wk of hypoxia (10% O2, normobaric) or normoxia. Expression of the mRNA and protein for eNOS was significantly increased (1.6-fold and 2.1-fold, respectively) by hypoxia. Immunohistochemistry with an isoform-specific antibody demonstrated de novo expression of eNOS in the endothelium of resistance vessels in the pulmonary vasculature of the hypoxic rats. eNOS was detected in the endothelium of large vessels in both normoxic and hypoxic rat lungs. The level of mRNA and protein for iNOS was also found to be significantly increased (1.9-fold and 1.4-fold, respectively). In addition to the 4.4-kilobase (kb) iNOS mRNA species, a novel 4.0-kb species was also induced by hypoxia. We conclude that expression of eNOS and iNOS was increased in the lungs of rats subjected to chronic hypoxia, and that there was de novo expression of eNOS protein in the microvascular endothelium.

Expression and localization of nitric oxide synthase (NOS) in the lungs of chronically hypoxic and normoxic rats were studied using both immunohistochemistry and NADPH diaphorase (NADPH-d) staining techniques. In the normoxic and in the hypoxic rat, NOS was detected by both methods in the endothelium of large pulmonary vessels and in the epithelium of bronchi and bronchioli. NOS expression was not detected in the endothelium of normoxic pulmonary resistance vessels but was prominently expressed in the endothelium of these vessels after 2-4 wk of chronic hypoxia. In contrast to small pulmonary vessels, the endothelium of small bronchial vessels exhibited NOS immunostaining in both normoxic and hypoxic lungs. Hypoxia was also found to induce de novo NOS expression in the smooth muscle of large and small pulmonary vessels and in bronchial smooth muscle. NOS enzyme activity in lung homogenates was assessed by [3H]arginine to [3H]citrulline conversion. The activity of soluble NOS, but not particulate NOS, was increased in the hypoxic lungs. These results demonstrate chronic hypoxia-induced upregulation of NOS protein expression and activity in the rat lung, suggesting a potentially important role of nitric oxide in adaptation of the pulmonary circulation to chronic hypoxia. The lack of immunostaining in small pulmonary resistance vessels is also consistent with physiological studies suggesting that NO may not be involved in the mechanism for maintaining the normally low pulmonary vascular resistance.

Alterations in nitric oxide signaling have been hypothesized to have an etiologic role in the development of hypoxic pulmonary hypertension. However, changes in the expression of nitric oxide synthase (NOS) in hypoxic lungs remains controversial. In this study, we used (1) Northern and Western analyses to measure NOS mRNA and protein expressions, (2) lung histology together with measurements of lung and heart weights to monitor pulmonary vascular remodeling, and (3) immunohistochemistry to localize NOS proteins. The data demonstrated that endothelial NOS mRNA and protein were upregulated over 1 to 7 days of hypoxia that temporally correlated with and preceded the vascular remodeling that occurred in the course of the development of hypoxic pulmonary hypertension. Hypoxia also induced brain NOS in bronchial epithelium and inducible NOS in vascular smooth muscle but did not affect inducible NOS expression in macrophages or basal guanylyl cyclase activity in the lung. These findings showed that upregulation of endothelial NOS was tightly correlated with the vascular remodeling induced by hypoxia, suggesting a role for nitric oxide in the development of pulmonary hypertension.