Atrial myocardial nox2 containing NADPH oxidase activity contribution to oxidative stress in mitral regurgitation: Potential mechanism for atrial remodeling

Background: Oxidative stress is linked with several cardiovascular diseases. However, the NADPH oxidase activity in severe mitral regurgitation patients with and without atrial fibrillation has not yet been explored. Methods: This study involved 16 adult patients (eight patients with persistent atrial fibrillation and eight with sinus rhythm) with severe mitral and moderate-to-severe tricuspid regurgitation and five control patients without mitral and tricuspid disease. Atrial tissues of the right and left atrial appendages were obtained during surgery. Superoxide anion production was measured by lucigenin-enhanced chemiluminescence, and the expression of nox2 containing NADPH oxidase mRNA was measured by quantitative real-time RT-PCR. Additionally, immunohistochemical study was performed. Results: NADPH-stimulated superoxide release was significantly higher than basal superoxide production from right [5671.9±3498.7 vs. 232.7±70.0 relative light units per second per milligram of protein (RLU s^-1 mg protein^-1), P=.008) and left atrial homogenates (6475.1±1890.8 vs. 229.0±79.6 RLU s^-1 mg protein^-1, P=.008) in atrial fibrillation patients. The NADPH-stimulated superoxide release from right atrial homogenates was also significantly higher than basal superoxide production in sinus patients (6809.1±1327.1 vs. 244.2±65.5 RLU s^-1 mg protein ^-1, P=.008). Additionally, there was a borderline significant correlation between NADPH-stimulated superoxide production from left atrial homogenates and left atrial sizes (r=0.683, P=.062) in atrial fibrillation patients. Membrane-bound nox2 containing NADPH oxidase mRNA expression was increased and was similar in both the atrial fibrillation patients and sinus patients. The NADPH-stimulated superoxide production in right atrial homogenates in control atrial samples was 1863.7±137.2 RLU s^-1 mg protein^-1. Immunohistochemical study demonstrated increased expression of nox2 in myocytes with moderate-to-severe myolysis and hypertrophy. Conclusions: Results of this study demonstrate that membrane-bound nox2 containing NADPH oxidase activity and expression in the atrial myocardium is increased in patients with severe mitral regurgitation, possibly contributing to atrial remodeling in this clinical setting.