title: Single-channel properties support a potential contribution of hyperpolarization-activated cyclic nucleotide-gated channels and If to cardiac arrhythmias.
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.
  
  
  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.
authors.affiliation: Department of Internal Medicine III, University of Cologne, Cologne, Germany.
authors.forename: Guido
authors.initials: G
authors.lastname: Michels
authors.forename: Fikret
authors.initials: F
authors.lastname: Er
authors.forename: Ismail
authors.initials: I
authors.lastname: Khan
authors.forename: Michael
authors.initials: M
authors.lastname: Südkamp
authors.forename: Stefan
authors.initials: S
authors.lastname: Herzig
authors.forename: Uta C
authors.initials: UC
authors.lastname: Hoppe
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology; D000368:Aged; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000378:metabolism / Q000503:physiopathology*; D020517:Atrial Appendage / Q000166:cytology; D016466:CHO Cells / Q000502:physiology; D002478:Cells, Cultured / Q000502:physiology; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D003412:Cricetulus; D054815:Cyclic Nucleotide-Gated Cation Channels; D005260:Female; D006325:Heart Atria / Q000378:metabolism*; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000737:chemistry / Q000502:physiology*; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D009124:Muscle Proteins / Q000737:chemistry / Q000502:physiology*; D009419:Nerve Tissue Proteins; D018408:Patch-Clamp Techniques; D015221:Potassium Channels; D020033:Protein Isoforms / Q000737:chemistry / Q000502:physiology; D055503:Protein Multimerization; D011993:Recombinant Fusion Proteins / Q000502:physiology; D014162:Transfection
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; C578710:HCN2 protein, human; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D009419:Nerve Tissue Proteins; D015221:Potassium Channels; D020033:Protein Isoforms; D011993:Recombinant Fusion Proteins; D005576:Colforsin; D015761:4-Aminopyridine