Heme Oxygenase Decreases Reactive Oxygen Species in Mesenteric Resistance Arteries

Bilirubin is a potent antioxidant that acts in a cytoprotective manner to diminish levels of reactive oxygen species (ROS). Heme oxygenase (HO) produces bilirubin through the breakdown of heme into biliverdin, which is subsequently converted to bilirubin by biliverdin reductase. These enzymatic pathways are present in many tissues including the vascular endothelium. We hypothesized that inhibition of HO would result in increased ROS levels within the vasculature. To quantify ROS, isolated mesenteric resistance arterioles from Sprague‐Dawley rats were superfused with the fluorescent indicator DCF. Specificity of DCF fluorescence as a measure of ROS was verified using Tiron and catalase, scavengers of superoxide and hydrogen peroxide, respectively. Addition of these compounds to the superfusate significantly decreased ROS levels in mesenteric arterioles from 109.2 ± 7.36 arbitrary units (AU), to 24.37 ± 7.22 AU. Catalase alone decreased ROS levels to a similar extent as combined Tiron and catalase suggesting that the major ROS formed in these vessels is hydrogen peroxide. Supportive of our hypothesis, superfusion with the HO inhibitor zinc protoporphyrin IX (ZnPPIX; 0.5μM) resulted in augmented DCF fluorescence (250.2 ± 25.19 AU). These data suggest that the HO system is an important regulator of ROS levels within the resistance vasculature.