title: Overexpression of HCN-encoded pacemaker current silences bioartificial pacemakers.
abstract: BACKGROUND
  Current strategies of engineering bioartificial pacemakers from otherwise silent yet excitable adult atrial and ventricular cardiomyocytes primarily rely on either maximizing the hyperpolarization-activated I(f) or on minimizing its presumptive opponent, the inwardly rectifying potassium current I(K1).
  
  
  OBJECTIVE
  The purpose of this study was to determine quantitatively the relative current densities of I(f) and I(K1) necessary to induce automaticity in adult atrial cardiomyocytes.
  
  
  METHODS
  Automaticity of adult guinea pig atrial cardiomyocytes was induced by adenovirus (Ad)-mediated overexpression of the gating-engineered HCN1 construct HCN1-DeltaDeltaDelta with the S3-S4 linker residues EVY235-7 deleted to favor channel opening.
  
  
  RESULTS
  Whereas control atrial cardiomyocytes remained electrically quiescent and had no I(f), 18% of Ad-CMV-GFP-IRES-HCN1-DeltaDeltaDelta (Ad-CGI-HCN1-DeltaDeltaDelta)-transduced cells demonstrated automaticity (240 +/- 14 bpm) with gradual phase 4 depolarization (143 +/- 28 mV/s), a depolarized maximal diastolic potential (-45.3 +/- 2.2 mV), and substantial I(f) at -140 mV (I(f,-140 mV) = -9.32 +/- 1.84 pA/pF). In the remaining quiescent Ad-CGI-HCN1-DeltaDeltaDelta-transduced atrial cardiomyocytes, two distinct immediate phenotypes were observed: (1) 13% had a hyperpolarized resting membrane potential (-56.7 +/- 1.3 mV) with I(f,-140 mV) of -4.85 +/- 0.97 pA/pF; and (2) the remaining 69% displayed a depolarized resting membrane potential (-27.6 +/- 1.3 mV) with I(f,-140 mV) of -23.0 +/- 3.71 pA/pF. Upon electrical stimulation, both quiescent groups elicited a single action potential with incomplete phase 4 depolarization that was never seen in controls. Further electrophysiologic analysis indicates that an intricate balance of I(K1) and I(f) is necessary for induction of atrial automaticity.
  
  
  CONCLUSIONS
  Optimized pacing induction and modulation can be better achieved by engineering the I(f)/I(K1) ratio rather than the individual currents.
authors.affiliation: Stem Cell Program, University of California, Davis, California, USA.
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Lieu
authors.forename: Yau Chi
authors.initials: YC
authors.lastname: Chan
authors.forename: Chu Pak
authors.initials: CP
authors.lastname: Lau
authors.forename: Hung Fat
authors.initials: HF
authors.lastname: Tse
authors.forename: Chung Wah
authors.initials: CW
authors.lastname: Siu
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Li
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D016275:Atrial Function / Q000502:physiology*; D002304:Cardiac Pacing, Artificial; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000187:drug effects*; D018014:Gene Transfer Techniques; D006168:Guinea Pigs; D006325:Heart Atria / Q000166:cytology / Q000294:innervation*; D006352:Heart Ventricles / Q000166:cytology / Q000294:innervation*; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D008564:Membrane Potentials; D032383:Myocytes, Cardiac / Q000502:physiology*; D010138:Pacemaker, Artificial; D015221:Potassium Channels / Q000187:drug effects*; D023822:Tissue Engineering; D015854:Up-Regulation; D016276:Ventricular Function / Q000502:physiology*
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015221:Potassium Channels