Abstract 1083: Targeted SERCA2a Gene Overexpression Identifies Mechanism and Potential Therapy for Arrhythmogenic Cardiac Alternans
Background: Beat to beat alternation of the cardiac action potential (Vm-Alt) attributable to alternation in cellular calcium cycling (Ca-Alt) is a recognized mechanism of ventricular fibrillation (VF). However, the molecular mechanisms underlying arrhythmogenic cardiac alternans remain unclear. We hypothesized that in-vivo gene therapy targeting sarcoplasmic reticulum Ca ATPase (SERCA2a), by improving SR Ca uptake, can suppress cellular alternans and decrease susceptibility to VF.
Methods: In-vivo gene transfer using adenoviral vectors, with the transgene for SERCA2a (Ad.SERCA2a), was performed in young guinea pigs. Four days after gene transfer, isolated myocyte and whole heart electrophysiological studies were performed. Ampho-tericin perforated patch technique was used to measure membrane voltage under current clamp conditions and Indo-1AM was used to measure Ca transients from isolated myocytes. Vm-ALT and Ca-Alt were measured simultaneously from isolated myocytes using a ramp pacing protocol. High resolution optical mapping was used to measure Vm-ALT in the intact Langendorff-perfused heart.
Results: Isolated myocytes transduced with Ad.SERCA2a (p<0.01) demonstrated faster SR calcium uptake and enhanced Ca release compared to controls (p<0.01). Importantly, SERCA2a overex-pression inhibited cellular alternans as evidenced by significantly increased heart rate thresholds for both Vm-ALT and Ca-ALT compared to controls (265±17 bpm vs 349±22 bpm, p<0.01). Additionally, action potential duration did not differ between Ad.SERCA2a and control myocytes, suggesting a direct effect on SR Ca cycling rather than sarcolemmal currents. Similarly, the heart rate threshold for Vm-ALT increased significantly (503±41 vs 411±8 bpm) following SERCa2a overexpression in the whole heart, reaffirming that SERCA2a plays an important role in the mechanism of alternans. Moreover, although all (n=5) control animals developed pacing-induced VF, SERCA2a overexpression suppressed pacing-induced VF in 3 of 3 hearts (p < 0.01).
Conclusions: These data provide new evidence that SERCA2a gene transfer can directly modulate susceptibility to Ca-ALT, suggesting a possible mechanism and therapeutic target for arrhythmogenic Vm-ALT.