Inducibility, but not stability, of atrial fibrillation is increased by NOX2 overexpression in mice

Aims: Gp91-containing NADPH oxidases (NOX2) are a significant source of myocardial superoxide production. An in- crease in NOX2 activity accompanies atrial fibrillation (AF) induction and electrical remodelling in animal models and predicts incident AF in humans; however, a direct causal role for NOX2 in AF has not been demonstrated. Accordingly, we investigated whether myocardial NOX2 overexpression in mice (NOX2-Tg) is sufficient to gener- ate a favourable substrate for AF and further assessed the effects of atorvastatin, an inhibitor of NOX2, on atrial superoxide production and AF susceptibility. Methods NOX2-Tg mice showed a 2- to 2.5-fold higher atrial protein content of NOX2 compared with wild-type (WT) and results controls, which was associated with a significant (twofold) increase in NADPH-stimulated superoxide production (2-hydroxyethidium by HPLC) in left and right atrial tissue homogenates (P = 0.004 and P= 0.019, respectively). AF susceptibility assessed in vivo by transoesophageal atrial burst stimulation was modestly increased in NOX2-Tg com- pared with WT (probability of AF induction: 88% vs. 69%, respectively; P= 0.037), in the absence of significant alter- ations in AF duration, surface ECG parameters, and LV mass or function. Mechanistic studies did not support a role for NOX2 in promoting electrical or structural remodelling, as high-resolution optical mapping of atrial tissues showed no differences in action potential duration and conduction velocity between genotypes. In addition, we did not observe any genotype difference in markers of fibrosis and inflammation, including atrial collagen content and Col1a1, Il-1b, Il-6, and Mcp-1 mRNA. Similarly, NOX2 overexpression did not have consistent effects on RyR2 Ca2þ leak nor did it affect PKA or CaMKII-mediated RyR2 phosphorylation. Finally, treatment with atorvastatin signifi- cantly inhibited atrial superoxide production in NOX2-Tg but had no effect on AF induction in either genotype. Conclusion Together, these data indicate that while atrial NOX2 overexpression may contribute to atrial arrhythmogenesis, NOX2-derived superoxide production does not affect the electrical and structural properties of the atrial myocardium.