Overexpression of HCN-encoded pacemaker current silences bioartificial pacemakers

BackgroundCurrent strategies of engineering bioartificial pacemakers from otherwise silent yet excitable adult atrial and ventricular cardiomyocytes primarily rely on either maximizing the hyperpolarization-activated If or on minimizing its presumptive opponent, the inwardly rectifying potassium current IK1.ObjectiveThe purpose of this study was to determine quantitatively the relative current densities of If and IK1 necessary to induce automaticity in adult atrial cardiomyocytes.MethodsAutomaticity of adult guinea pig atrial cardiomyocytes was induced by adenovirus (Ad)-mediated overexpression of the gating-engineered HCN1 construct HCN1-ΔΔΔ with the S3-S4 linker residues EVY235-7 deleted to favor channel opening.ResultsWhereas control atrial cardiomyocytes remained electrically quiescent and had no If, 18% of Ad-CMV-GFP-IRES-HCN1-ΔΔΔ (Ad-CGI-HCN1-ΔΔΔ)–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 If at −140 mV (If,−140 mV = −9.32 ± 1.84 pA/pF). In the remaining quiescent Ad-CGI-HCN1-ΔΔΔ–transduced atrial cardiomyocytes, two distinct immediate phenotypes were observed: (1) 13% had a hyperpolarized resting membrane potential (−56.7 ± 1.3 mV) with If,−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 If,−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 IK1 and If is necessary for induction of atrial automaticity.ConclusionOptimized pacing induction and modulation can be better achieved by engineering the If/IK1 ratio rather than the individual currents.