Antioxidant and Vasodilatory Effects of Heme Oxygenase on Mesenteric Vasoreactivity Following Chronic Hypoxia

Objective: Chronic hypoxia (CH) results in impaired vasoconstriction associated with increased expression of heme oxygenase (HO). We hypothesized that enhanced HO activity minimizes reactive oxygen species (ROS) in arteries from CH rats, thereby normalizing endothelium-dependent vasodilation and concurrently produces carbon monoxide (CO), resulting in tonic vasodilation. Methods: ROS were quantified in mesenteric arteries from control and CH Sprague-Dawley rats. Reactivity to the endothelium-dependent vasodilator, acetylcholine (ACh), and the vasoconstrictor, phenylephrine (PE), were also assessed. Results: Basal ROS levels did not differ between groups and were similarly increased by HO inhibition. In contrast, catalase inhibition increased ROS in CH rats only. Vasodilatory responses to ACh were not different between groups. Combined inhibition of catalase and HO impaired PE-induced vasoconstriction in both groups. CH-induced impairment of vasoconstriction was reversed by either catalase or HO inhibition supporting the protective roles of the HO and catalase pathways following CH. Increased vascular smooth muscle calcium was observed with inhibition in the CH group, suggesting that catalase and HO-derived CO elicit reduced calcium influx, leading to the impaired vasoconstriction. Conclusions: Our data suggest that although the HO pathway is an important antioxidant influence, impaired vasoconstriction following CH appears to be due to effects of ROS and HO-derived CO.