Atrial fibrillation increases production of superoxide by the left atrium and left atrial appendage - Role of the NADPH and xanthine oxidases

Background - Atrial fibrillation ( AF) is associated with an increased risk of stroke due almost exclusively to emboli from left atrial appendage ( LAA) thrombi. Recently, we reported that AF was associated with endocardial dysfunction, limited to the left atrium ( LA) and LAA and manifest as reduced nitric oxide ( NO.) production and increased expression of plasminogen activator inhibitor- 1. We hypothesized that reduced LAA NO. levels observed in AF may be associated with increased superoxide ( O-2(.-)) production. Methods and Results - After a week of AF induced by rapid atrial pacing in pigs, O-2(.-) production from acutely isolated heart tissue was measured by 2 independent techniques, electron spin resonance and superoxide dismutase - inhibitable cytochrome C reduction assays. Compared with control animals with equivalent ventricular heart rates, basal O-2(.-) production was increased 2.7- fold ( P < 0.01) and 3.0- fold ( P < 0.02) in the LA and LAA, respectively. A similar 3.0- fold ( P < 0.01) increase in LAA O-2(.-) production was observed using a cytochrome C reduction assay. The increases could not be explained by changes in atrial total superoxide dismutase activity. Addition of either apocyanin or oxypurinol reduced LAA O-2(.-) , implying that NADPH and xanthine oxidases both contributed to increased O-2(.-) production in AF. Enzyme assays of atrial tissue homogenates confirmed increases in LAA NAD( P) H oxidase ( P = 0.04) and xanthine oxidase ( P = 0.01) activities. Although there were no changes in expression of the NADPH oxidase subunits, the increase in superoxide production was accompanied by an increase in GTP-loaded Rac1, an activator of the NADPH oxidase. Conclusions - AF increased O-2(.-) production in both the LA and LAA. Increased NAD( P) H oxidase and xanthine oxidase activities contributed to the observed increase in LAA O-2(.-) production. This increase in O-2(.-) and its reactive metabolites may contribute to the pathological consequences of AF such as thrombosis, inflammation, and tissue remodeling.