Chronic atrial fibrillation alters the functional properties of I _f in the human atrium

Human Chronic Atrial Fibrillation and I_f Introduction Despite the evidence that the hyperpolarization-activated current (I_f) is highly modulated in human cardiomyopathies, no definite data exist in chronic atrial fibrillation (cAF). We investigated the expression, function, and modulation of I_f in human cAF. Methods and Results Right atrial samples were obtained from sinus rhythm (SR, n = 49) or cAF (duration >1 year, n = 31) patients undergoing corrective cardiac surgery. Among f-channel isoforms expressed in the human atrium (HCN1, 2 and 4), HCN4 mRNA levels measured by RT-PCR were significantly reduced. However, protein expression was preserved in cAF compared to SR (+85% for HCN4); concurrently, miR-1 expression was significantly reduced. In patch-clamped atrial myocytes, current-specific conductance (g_f) was significantly increased in cAF at voltages around the threshold for I_f activation (-60 to -80 mV); accordingly, a 10-mV rightward shift of the activation curve occurred (P < 0.01). β-Adrenergic and 5-HT₄ receptor stimulation exerted similar effects on I_f in cAF and SR cells, while the ANP-mediated effect was significantly reduced (P < 0.02), suggesting downregulation of natriuretic peptide signaling. Conclusions In human cAF modifications in transcriptional and posttranscriptional mechanisms of HCN channels occur, associated with a slight yet significant gain-of-function of I_f, which may contribute to enhanced atrial ectopy.