Letter regarding article by Michels et al, "Single-channel properties support a potential contribution of hyperpolarization-activated cyclic nucleotide-gated channels and If to cardiac arrhythmias".

BACKGROUND The pacemaker current I(f) is present in atrial and ventricular myocytes. However, it remains controversial whether I(f) overexpression in diseased states might play a role for arrhythmogenesis, because first I(f) activation in whole-cell recordings hardly overlapped the diastolic voltage of working myocardium. METHODS AND RESULTS To obtain further insight into I(HCN) and I(f) properties, we provide for the first time detailed single-channel analysis of heterologously expressed hyperpolarization-activated cyclic nucleotide-gated (HCN) isoforms and native human I(f). HCN subtypes differed significantly in single-channel amplitude, conductance, and activation kinetics. Interestingly, threshold potentials of HCN isoforms were more positive than would have been expected from whole-cell measurements. Single-channel properties of cells cotransfected with HCN2 and HCN4 were distinct from cells expressing HCN2 or HCN4 alone, demonstrating that different HCN isoforms can influence current properties of a single HCN channel complex, thus providing direct functional evidence for HCN heteromerization. Pooled data of homomeric and heteromeric HCN channels and of native I(f) extrapolated from maximum likelihood fits indicated a multistate gating scheme comprising 5 closed- and 4 open-channel states. Single-channel characteristics of I(f) in human atrial myocytes closely resembled those of HCN4 or HCN2+HCN4, supporting the hypothesis that native I(f) channels in atrial myocardium are heteromeric complexes composed of HCN4 and/or HCN2. Most interestingly, half-maximal activation of single-channel atrial I(f) (-68.3+/-4.9 mV; k=-9.9+/-1.5; n=8) was well within the diastolic voltage range of human atrial myocardium. CONCLUSIONS These observations support a potential contribution of HCN/I(f) to the arrhythmogenesis of working myocardium under pathological conditions.