Mechanistic Role of I f Revealed by Induction of Ventricular Automaticity by Somatic Gene Transfer of Gating-Engineered Pacemaker (HCN) Channels

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Abstract
Background— Although If, encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN) channel gene family, is known to be functionally important in pacing, its mechanistic action is largely inferential and indeed somewhat controversial. To dissect in detail the role of If, we investigated the functional consequences of overexpressing in adult guinea pig left ventricular cardiomyocytes (LVCMs) various HCN1 constructs that have been engineered to exhibit different gating properties. Methods and Results— We created the recombinant adenoviruses Ad-CMV-GFP-IRES (CGI), Ad-CGI-HCN1, Ad-CGI-HCN1-ΔΔΔ, and Ad-CGI-HCN1-Ins, which mediate ectopic expression of GFP alone, WT, EVY235-7ΔΔΔ, and Ins HCN1 channels, respectively; EVY235-7ΔΔΔ and Ins encode channels in which the S3–S4 linkers have been shortened and lengthened to favor and inhibit opening, respectively. Ad-CGI-HCN1, Ad-CGI-HCN1-ΔΔΔ, and Ad-CGI-HCN1-Ins, but not control Ad-CGI, transduction of LVCMs led to robust expression of If with comparable densities when fully open (≈−22 pA/pF at −140 mV; P>0.05) but distinctive activation profiles (V1/2=−70.8±0.6, −60.4±0.7, and −87.7±0.7 mV; PPPIf. Correlation analysis revealed the specific biophysical parameters required for If to function as an active membrane potential oscillator. Conclusions— Our results not only contribute to a better understanding of cardiac pacing but also may advance current efforts that focus primarily on automaticity induction to the next level by enabling bioengineering of central and peripheral cells that make up the native sinoatrial node.