Relationship Between Ca2+ Alternans and T-Wave Alternans: Role of Calsequestrin and Sorcin

Abstract

Several pathologies are associated with defects in Ca2+ handling; One example is T-wave alternans (TW-Alt). TW-Alt is observed as alternating beat-to-beat changes in the T-wave of the electrocardiogram (ECG) and constitutes an important arrhythmogenic mechanism. The likelihood of TW-Alt increases with tachycardia and is thought to be associated with abnormalities in intracellular Ca2+ handling. Thus, the regulation of the Ryanodine Receptor (RyR2) could be essential in understanding the genesis of Ca2+ alternans (Ca-Alt) and its relationship with TW-Alt. To modify Ca2+ signaling we knocked out two Ca2+ regulatory proteins: Sorcin and Calsequestrin (Csq2). Ca2+ transients were measured from the epicardial layer of murine hearts (n=48) using Pulsed Local Field Fluorescence Microscopy. In addition, simultaneous intracellular action potentials (AP) and ECGs were obtained. Ablation of Csq2 or Sorcin alone did not induce any significant changes in the time course of Ca2+ transients or APs. Interestingly, when both proteins were knocked out (Csq/Sorcin KO) a significant change in the time to peak of the transients were observed (WT 16.1±2.4 vs. Csq/Sorcin KO 24.5±1.65). The prolongation of the release can be explained by the modification of the RyR2 gating by Csq2 and Sorcin. The restitution of Ca2+ transients was not modified in Sorcin KO, however Csq KO or Csq/Sorcin KO displayed dramatic changes in the time course of the restitution. Finally, maximum Ca-Alt at 32 °C was significantly shifted to higher heart rates (from 11.5 Hz to 15 Hz and 14 Hz, respectively) in Csq2 KO and Csq/Sorcin KO but not in Sorcin KO. AP repolarization alternans and TW-Alt were also modified in Csq2 KO and Csq/Sorcin KO. We conclude that Ca2+-binding proteins can regulate the RyR2 synergistically, modifying bath the frequency dependency of Ca-Alt and electrical alternans as well.