Abstract 5328: Intracellular Calcium Mediated Late-Phase 3 Early Afterdepolarization Induces Ventricular Fibrillation in Isolated Rabbit Hearts with Activated K ATP Channel

Introduction: The mechanism and importance of late-phase 3 early after depolarization (EAD) in ventricular arrhythmogenesis and the corresponding intracellular calcium dynamics are unknown. Methods: We studied 22 Langendorff-perfused isolated normal rabbit hearts with and without K ATP channels opener, pinacidil (80 μM). In group I (n=10), we performed simultaneous dual optical mapping of transmembrane potential (V m ) and intracellular calcium (Ca i ) transient on left ventricular endocardium. In group II (n=6), glass microelectrode single cell recording of transmembrane potential (TMP) was performed. In group III (n=6), we examined the effects of BAPTA-AM, a calcium chelator, on late-phase 3 EAD and VF induction. Results: Ventricular endocardium was paced at PCL between 60 ms and 350 ms for 29 beats. Pinacidil shortened both APD 90 (192±11 ms at baseline to 98±9 ms) and duration of Ca i at 50% of amplitude (DCaT 50 , 152±10 ms at baseline to 84±9 ms) at PCL of 250 ms. At pacing rate < 200 ms, the first post-pacing spontaneous beat showed persistent APD shortening and late-phase 3 EAD accompanied by large Ca i elevation. There was a positive correlation between the EAD amplitudes and the amplitude of Ca i elevation (r=0.899). At a critical Ca i elevation magnitude, the late-phase 3 EAD then induced triggered activity and VF in 10 episodes. The single cell TMP recording confirmed the optical mapping data of late-phase 3 EAD-induced VF. Infusion of BAPTA-AM significantly reduced the maximal Ca i amplitude in the mapped region (26.4±3.5% of the control; p<0.001). Due to the lowered Ca i level, neither EAD in the first post-pacing beat nor VF induction was observed after pacing train at any PCLs. Conclusions: K ATP channel activation leads to shortened APD with correspondingly large Ca i elevation, resulting in late-phase 3 EAD in first spontaneous beat at the cessation of rapid pacing. Reducing intracellular calcium handling by BAPTA-AM prevented EAD and VF induction/reinitiation. Because acute myocardial ischemia activates the K ATP channel, the calcium mediated late-phase 3 EAD induced triggered activity may be a novel mechanism for VF and sudden death during acute myocardial ischemia. This research has received full or partial funding support from the American Heart Association, AHA National Center.