Endothelium-derived reactive oxygen species and endothelin-1 attenuate NO-dependent pulmonary vasodilation following chronic hypoxia.

Vasodilatory responses to exogenous nitric oxide (NO) are diminished following exposure to chronic hypoxia (CH) in isolated, perfused rat lungs. We hypothesized that both endothelium-derived reactive oxygen species (ROS) and endothelin-1 (ET-1) mediate this attenuated NO-dependent pulmonary vasodilation following CH. To test this hypothesis, we examined vasodilatory and vascular smooth muscle (VSM) Ca2+ responses to the NO donor spermine NONOate in UTP-constricted, isolated pressurized small pulmonary arteries from control and CH rats. Consistent with our previous findings in perfused lungs, we observed attenuated NO-dependent vasodilation following CH in endothelium-intact vessels. However, in endothelium-denuded vessels, responses to spermine NONOate were augmented in CH rats compared with controls, thus demonstrating an inhibitory influence of the endothelium on NO-dependent reactivity following CH. Whereas both the ROS scavenger tiron and the ETA receptor antagonist BQ-123 augmented NO-dependent reactivity in endothelium-intact vessels from CH rats, neither fully restored vasodilatory responses to those observed following endothelium denudation in vessels from CH rats. In contrast, the combination of tiron and BQ-123 or the nonselective ET receptor antagonist PD-145065 enhanced NO responsiveness in endothelium-intact vessels from CH rats similar to that observed following endothelium denudation. We conclude that both endothelium-derived ROS and ET-1 attenuate NO-dependent pulmonary vasodilation following CH. Furthermore, CH augments pulmonary VSM reactivity to NO.