Bicarbonate-dependent superoxide release and pulmonary artery tone
Pulmonary vasoconstriction is influenced by inactivation of nitric oxide (NO) with extracellular superoxide (O 2- ·). Because the short-lived O 2- · anion cannot diffuse across plasma membranes, its release from vascular cells requires specialized mechanisms that have not been well delineated in the pulmonary circulation. We have shown that the bicarbonate (HCO 3- )-chloride anion exchange protein (AE2) expressed in the lung also exchanges O 2- · for HCO 3- . Thus we determined whether O 2- · release involved in pulmonary vascular tone depends on extracellular HCO 3- . We assessed endothelium-dependent vascular reactivity and O 2- · release in the presence or absence of HCO 3- in pulmonary artery (PA) rings isolated from normal rats and those exposed to hypoxia for 3 days. Lack of extracellular HCO 3- in normal PA rings significantly attenuated endothelial O 2- · release, opposed hypoxic vasoconstriction, and enhanced acetylcholine-mediated vasodilation. Release of O 2- · was also inhibited by an AE2 inhibitor (SITS) and abolished in normoxia by an NO synthase inhibitor (N G -nitro-L-arginine methyl ester). In contrast, hypoxia increased PA AE2 protein expression and O 2- · release; the latter was not affected by N G -nitro-L-arginine methyl ester or other inhibitors of enzymatic O 2- · generation. Enhanced O 2- · release by uncoupling NO synthase with geldanamycin was attenuated by hypoxia or by HCO 3- elimination. These results indicate that O 2- · produced by endothelial NOS in normoxia and unidentified sources in hypoxia regulate pulmonary vascular tone via AE2.