MECHANISMS AND SPATIOTEMPORAL EVOLUTION OF INTRACELLULAR Ca2+ ALTERNANS IN CARDIAC TISSUE
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Instabilities in cardiac repolarization, presented as T-wave alternans (TWA) in the ECG, are a known precursor of re-entrant cardiac arrhythmias, e.g. ventricular tachycardia (VT) and ventricular fibrillation (VF). Alternations in intracellular Ca2+ cycling have been suggested recently as direct contributors to instabilities in action potential duration (APD) and TWA. Furthermore, both computer simulations and experiments have shown that subtle fine-scale intracellular Ca2+ alternans emerge much earlier than detectable VT and VF episodes, and can promote the initiation of VT/VF. Recent experimental results from our lab and others revealed that the alternans regions may be persistent during VT. Overall, these findings corroborate the notion that the genesis and evolution of Ca2+ alternans are tightly linked to VT/VF development. We hypothesize that alternans in intracellular Ca2+ are not all-or-nothing response, but evolve gradually over time and space, and are influenced by several important factors, e.g. cellular coupling and Ca2+-Vm kinetics. The goal of this research is to quantify the influences of these factors with the long-term objective to predict arrhythmia development based on spatiotemporal profiles of early-stage alternans. Experimental testing of this hypothesis, particularly the capture and characterization of the early-stage fine-scale alternans, which are dynamically changing, requires specialized technical tools that were specifically developed for this project - e.g. automated computer detection/characterization algorithms for fine alternans in conjunction with ultra-high spatiotemporal resolution macroscopic imaging system. Furthermore, mathematical models were employed as complementary tools to validate and expand the experimental findings. Finally, a new technological development by our lab - an optogenetic approach (use of exogenous light-sensitive ion channels) was used to provide optical instead of electrical stimulation of cardiac tissue and to probe for the role of virtual electrodes in the emergence of fine-scale alternans. This research is the first to tackle the challenge of automatically identifying and tracking the spatiotemporal characteristics of cell-level calcium alternans over a large field of view, at the early stage of onset. By developing a theoretical and experimental framework for the effects of cellular coupling and Ca2+-Vm coupling on the spatiotemporal evolution of alternans, this work aims to provide new predictive tools to systematically correlate early stage alternans to later arrhythmia development.