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‐C MV‐G FP‐I RES‐HCN1‐ÄÄÄ and/or Ad‐CGI‐Kir2.1 to mediate their current densities via a whole‐cell patch clamp technique at 37°C. Results showed that Ad‐CGI‐HCN1‐ÄÄÄ but not Ad‐CGI‐Kir2.1 transduction induced automaticity (181.1 ± 13.1 bpm). Interestingly, Ad‐CGI‐HCN1‐ÄÄÄ/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 APD₉₀ of AP‐firing LV CMs were correlated with I_f (R² > 0.7) only when ?2 >I_K1 >?4 pA/pF but not with I_K1 over the entire I_f ranges examined (0.02 < R² < 0.4). Unlike I_f, I_K1 displayed correlation with neither the phase‐4 slope (R²= 0.02) nor phase‐4 length (R²= 0.04) when ?2 > I_f > ?4 pA/pF. As anticipated, however, APD₉₀ was correlated with I_K1 (R²= 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.