Overexpression of HCN-encoded pacemaker current silences bioartificial pacemakers

BACKGROUND Current strategies of engineering bioartificial pacemakers from otherwise silent yet excitable adult atrial and ventricular cardiomyocytes primarily rely on either maximizing the hyperpotarization-activated If or on minimizing its presumptive opponent, the inwardly rectifying potassium current I-K1. OBJECTIVE The purpose of this study was to determine quantitativelly 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-Delta Delta Delta 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-Delta Delta Delta (Ad-CGI-HCN1-Delta Delta Delta)-transduced cells demonstrated automaticity (240 +/- 14 bpm) with gradual phase 4 depotarization (143 +/- 28 mV/s), a depolarized maximal diastolic potential (-45.3 +/- 2.2 mV), and substantial I-f at -140 mV (If-140 (mV) = -9.32 +/- 1.84 pA/pF). In the remaining quiescent Ad-CGI-HCN1-Delta Delta Delta-transduced atrial cardiomyocytes, two distinct immediate phenotypes were observed: (1) 13% had a hyperpotarized 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 depotarization that was never seen in controls. Further electrophysiologic analysis indicates that an intricate balance of I-f and I-K1 is necessary for induction of atrial automaticity. CONCLUSION optimized pacing induction and modulation can be better achieved by engineering the I-f/I-K1 ratio rather than the individual currents.