Pro- and anti-arrhythmic effects of TRPM4 inhibition on ventricular fibrillation trigger and substrate mechanisms

Introduction Ventricular Fibrillation (VF) is a major cause of sudden cardiac death. Purkinje fibers (PF) are known to play an important role in triggering and maintaining VF. Previous studies have highlighted the role of TRPM4 channels in repolarization properties of PFs, but their potential impact on arrhythmogenesis and VF dynamics remains unknown. Objective To investigate the role of TRPM4 channels in Purkinje-mediated trigger and substrate mechanisms. Method Endocardial optical mapping experiments were performed in sheep perfused left ventricular wedge preparations using a high-resolution CMOS camera. Purkinje-Muscle Junctions (PMJ) were localized through PF stimulation and identification of the breakthrough sites. Arrhythmias were induced either by isoprenaline perfusion (ISO; 1 μM; n = 7) or by endocardial burst pacing (10 Hz; n = 6) and their spatiotemporal dynamics quantified with or without 9-Phenanthrol (9PHE; 45 μM), a known TRPM4 inhibitor. Results ISO perfusion provoked focal tachy-arrhythmias originating from the PFs confirmed by the co-localization of the exit sites with previously identified PMJs. 9PHE reduced the rate of spontaneous focal activity and activation patterns were no longer correlated to the PMJs. However, following burst pacing, inhibition of TRPM4 led to a higher propensity of VF than ventricular tachycardia (VT). For the induced VT episodes, we observed a significant increase of DF (3.7 ± 0.7 Hz vs. 4.8 ± 0.8 Hz) and a significant decrease of regularity index (0.94 ± 0.05 vs. 0.84 ± 0.07) at the PMJs after 9PHE perfusion, while a significant increase in long lasting rotors was observed (P = 0.044; for rotor number and P = 0.005; for rotor duration). A significant increase in focal activity (15 ± 2 μpixel/ms vs. 30 ± 9 μpixel/ms) was found during VF episodes. Conclusion These results suggest that TRPM4 inhibition has anti-arrhythmic effects by preventing VF triggers from PFs, while at the same time increasing the complexity of induced ventricular arrhythmias.