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 If extrapolated from maximum likelihood fits indicated a multistate gating scheme comprising 5 closed- and 4 open-channel states. Single-channel characteristics of If in human atrial myocytes closely resembled those of HCN4 or HCN2 + HCN4, supporting the hypothesis that native If 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.