Synergistic Effects of Inward Rectifier (IK1) and Pacemaker (If) Currents on the Induction of Bioengineered Cardiac Automaticity

Introduction: 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. Methods and 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). Conclusion: 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. (J Cardiovasc Electrophysiol, Vol. 20, pp. 1048-1054).