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 (O2*-) production. METHODS AND RESULTS: After a week of AF induced by rapid atrial pacing in pigs, O2*- 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 O2*- 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 O2*- 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 O2*-, implying that NADPH and xanthine oxidases both contributed to increased O2*- 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 O2*- production in both the LA and LAA. Increased NAD(P)H oxidase and xanthine oxidase activities contributed to the observed increase in LAA O2*- production. This increase in O2*- and its reactive metabolites may contribute to the pathological consequences of AF such as thrombosis, inflammation, and tissue remodeling.

Downregulation of endocardial nitric oxide synthase expression and nitric oxide production in atrial fibrillation: potential mechanisms for atrial thrombosis and stroke.

BACKGROUND: In the arterial endothelium, laminar flow and cyclical stretch induce expression of NO synthase (NOS). We hypothesized that atrial fibrillation (AF) causes a downregulation of atrial endocardial NOS expression and NO* production. Because NO* has antithrombotic properties, this may contribute to thromboembolism in AF. METHODS AND RESULTS: In pigs, AF was produced with rapid atrial pacing at 600 bpm for 1 week, whereas controls had atrial pacing at 100 bpm. All animals production from freshly isolated tissue was measured by a NO*-specific microelectrode. Left atrial basal and stimulated NO* production was decreased in AF by 73% and 71% (P<0.01). Endocardial NOS expression, determined by Western analysis, was also significantly decreased by 46%. Expression of the prothombotic protein plasminogen activator inhibitor 1 (PAI-1) is known to be regulated by NO* and was increased in the left atrium by 1.8-fold in AF (P<0.05). NO* concentration was decreased in the left atrial appendage, although NOS expression was not affected. Neither NOS concentration, NO* levels, nor PAI-1 expression were altered in the aortas or right atria of animals with AF. CONCLUSIONS: AF is associated with a marked decrease in endocardial NOS expression and NO* bioavailability and an increase in PAI-1 expression in the left atrium. These data suggest that organized atrial contraction is needed to maintain normal endocardial expression of NOS. It is likely that loss of this antithrombotic enzyme contributes to the thromboembolic phenomena commonly observed in AF.