title: Synergistic effects of inward rectifier (I) and pacemaker (I) currents on the induction of bioengineered cardiac automaticity.
abstract: BACKGROUND
  Normal heart rhythms originate in the sinoatrial node. HCN-encoded funny current (I(f)) and the Kir2-encoded inward rectifier (I(K1)) counteract each other by respectively oscillating and stabilizing the negative resting membrane potential, and controlling action potential firing. Therefore, I(K1) suppression and I(f) overexpression have been independently exploited to convert cardiomyocytes (CMs) into AP-firing bioartificial pacemakers. Although the 2 strategies have been largely assumed synergistic, their complementarity has not been investigated.
  
  
  RESULTS
  We explored the interrelationships of automaticity, I(f) and I(K1) by transducing single left ventricular (LV) CMs isolated from guinea pig hearts with the recombinant adenoviruses Ad-CMV-GFP-IRES-HCN1-AAA and/or Ad-CGI-Kir2.1 to mediate their current densities via a whole-cell patch clamp technique at 37 degrees C. Results showed that Ad-CGI-HCN1-AAA but not Ad-CGI-Kir2.1 transduction induced automaticity (181.1 +/- 13.1 bpm). Interestingly, Ad-CGI-HCN1-AAA/Ad-CGI-Kir2.1 cotransduction significantly promoted the induced firing frequency (320.0 +/- 15.8 bpm; P < 0.05). Correlation analysis revealed that the firing frequency, phase-4 slope and APD(90) of AP-firing LV CMs were correlated with I(f) (R(2) > 0.7) only when -2 >I(K1) >-4 pA/pF but not with I(K1) over the entire I(f) ranges examined (0.02 < R(2) < 0.4). Unlike I(f), I(K1) displayed correlation with neither the phase-4 slope (R(2)= 0.02) nor phase-4 length (R(2)= 0.04) when -2 > I(f) > -4 pA/pF. As anticipated, however, APD(90) was correlated with I(K1) (R(2)= 0.4).
  
  
  CONCLUSIONS
  We conclude that an optimal level of I(K1) maintains a voltage range for I(f) to operate most effectively during a dynamic cardiac cycle.
authors.affiliation: Cardiology Division, Department of Medicine, University of Hong Kong, Hong Kong.
authors.forename: Yau-Chi
authors.initials: YC
authors.lastname: Chan
authors.forename: Chung-Wah
authors.initials: CW
authors.lastname: Siu
authors.forename: Yee-Man
authors.initials: YM
authors.lastname: Lau
authors.forename: Chu-Pak
authors.initials: CP
authors.lastname: Lau
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Li
authors.forename: Hung-Fat
authors.initials: HF
authors.lastname: Tse
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000235:genetics*; D005260:Female; D005818:Genetic Engineering / Q000379:methods*; D006168:Guinea Pigs; D006329:Heart Conduction System / Q000502:physiology*; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000235:genetics*
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015221:Potassium Channels; D011188:Potassium