Abstract 8300: SkM1 Gene Transfer Into Canine Ventricular Myocardium Decreases the Threshold Potential for Action Potential Initiation

Introduction: We have previously shown that combined HCN2/SkM1 gene expression in canine left bundle branch (LBB) results in a biological pacemaker having a faster rate than that based on HCN2 alone. We now tested the hypothesis that expressing Nav1.4 channels (encoded by the SkM1 gene) decreases the threshold potential (TrP) for action potential (AP) initiation. Thus, in the LBB expressing Nav1.4, the same slope of phase 4 depolarization should bring membrane potential to TrP faster, resulting in a more rapid automatic rate. We now tested this hypothesis in left ventricular subepicardial (LV) preparations suitable for current injection and TrP measurement. Methods: Adenoviral constructs of HCN2, SkM1 or HCN2/SkM1 were implanted into canine anterior LV. At 7d, myocardial fascicles (~0.5 mm diameter, 6-10 mm long) were isolated from injected sites and transferred to a tissue bath having 2 compartments separated by a plastic membrane with a hole permitting passage of the preparation. Transmembrane potential was recorded by microelectrodes. Preparations were driven at 1 Hz and every 10th stimulus was substituted by a 30 ms pulse of extracellular depolarizing current. The magnitude of depolarizing current was gradually increased until AP was initiated and TrP recorded. Results: For regularly paced AP, there were no differences in maximum diastolic potential (MDP) among groups whereas maximum upstroke velocity (V max ) was higher in SkM1 and HCN2/SkM1 than in HCN2 (Table, * P <0.05 vs HCN2). TrP was significantly more negative and V max at TrP was higher in SkM1 and HCN2/SkM1 than in HCN2 (Table, * P <0.05 vs HCN2). Conclusions: Expression of Nav1.4 Na+ channels can accelerate the rate of HCN2 based biological pacemakers by decreasing TrP for AP initiation.