Abstract 193: Role of Sarcoplasmic Reticulum Calcium Cycling in Arrhythmias in a 2-D Experimental Model of Calcium Overload

Abstract

Ca 2+ overload and/or altered sarcoplasmic reticulum (SR) Ca cycling has been argued to underlie ventricular tachyarrhythmias seen in patients with mutations of RyR2 and calsequestrin2 as well as heart failure. Aim : We investigated the role of SR Ca cycling on triggered activity and spiral wave dynamics in a 2-D model of Ca overload. Methods: We studied control neonatal rat ventricular myocytes (NRVM-only) monolayers and co-cultures of skeletal myoblasts and NRVMs. Initially, Ca overload was induced by superfusion with BAYK8644 (BayK, 2.5μM) and Isoproterenol (ISO, 1μM) for 5 mins. Subsequently, SR Ca cycling was modulated by 10mM caffeine for 5 min followed by Caffeine + 1μM thapsigargin (CAF/THAP) for 5 mins. Triggered activity and reentry were induced by rapid pacing. Optical mapping (voltage or Ca imaging) was performed. Degree of meandering was assessed using mutual information analysis of tip trajectories. Results : Ca imaging during 2Hz pacing revealed that caffeine produced a sharp rise in Ca transients followed by a return to steady state pre-caffeine levels; addition of CAF/THAPS produced a sharp and permanent decrease in Ca transients. Triggered activity (DADs) was observed in co-cultures (n=10) and controls (n=10) with 3–5 Hz pacing in the presence of BAYK/ISO. In co-cultures, DADs occurred from 2 or more sites and tended to drift; pseudo EKGs resembled polymorphic VT, while in controls it was stationary and pseudo EKGs resembled monomorphic VT. Surprisingly, DADs continued to occur even after abolition of SR Ca cycling by CAF/THAPS. Caffeine (n=4) significantly increased reentry pathlength (PL) by 59% but did not significantly change reentry cycle length (CL), conduction velocity (CV), wavelength (WL), or wavefront curvature. Addition of CAF/THAP (n=3) increased reentry WL by 17%, PL by 73%; decreased reentry CV by 28% and increased tip meandering without affecting reentry CL. Additional rotors and wavebreaks were observed during perfusion of BayK/ISO as well as after addition of CAF/THAP; power spectrum showed a single dominant frequency. Conclusion : DADs can occur from cytoplasmic Ca overload even in the absence of SR Ca cycling. Abolition of SR Calcium cycling in the presence of BayK/ISO produced wavebreak but did not induce spiral wave breakup.