Single-Channel Properties Support a Potential Contribution of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels and I f to Cardiac Arrhythmias

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Abstract
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.