Loss of Myocardial nNOS Mediated by Upregulation of miR-31 in Human Atria Contributes to Begetting of Atrial Fibrillation

Rationale: Neuronal nitric oxide synthase (nNOS) expression and activity in atria are markedly reduced from patients and goats with atrial fibrillation (AF). Whether loss of nNOS contributes to AF-induced atrial electrical remodelling and its upstream mechanism remain unclear.Methodology: Whole-cell patch clamp was used to record action potentials (APs) and ion currents. N/n: number of patients or mice/ number of myocytes.Results: Inhibition of nNOS by S-methylthiocitrulline (SMTC, N/n: 12/45) induces a significant reduction in AP duration (APD) and APD rate-dependent adaptation in human right atrial myocytes with sinus rhythm (SR, N/n: 14/52). In mice, nNOS inhibition (N/n: 4/9) or nNOS-/- (N/n: 9/28) reduce APD50 and 90 by 53% and 35%, respectively.Ionic investigations show SMTC increases atrial IKur by 60%, Ito by 34% and IK1 by 27% with no change in ICa or IKr. Computer modelling shows mimicking SMTC-induced increases in IKur and IK1 successfully retrieves experimental phenotype. Furthermore blocking IKur abolishes the effect of nNOS inhibition on APD and APD rate-dependent adaptation (SMTC: N/n: 6/11 vs vehicle: N/n: 8/16). Additionally, In vivo atrial burst stimulation shows nNOS-/- mice exhibit 2 folds higher AF inducibility.In AF, upregulation of atrial specific miR-31 results in accelerating nNOS mRNA and protein decay. Inhibition of miR-31 recovers the SMTC suppressed APD90 (N/n: 3/12 in each group) and APD rate-dependent adaptation. These effects are NO-mediated as they are reversed by SMTC. Whereas in SR patients, increasing miR-31(N/n: 4/30) reduces nNOS, shortens APD and suppresses APD rate-dependent adaptation (N/n: 3/14).Conclusions: In human and mammalian atrial myocytes, a reduction in nNOS, mediating by upregulation of miR31, is an important factor retrieving key hallmarks of atrial electrical remodelling and contributing to the atrial phenotype begetting AF.