Periodic Calcium Waves in Cardiac Myocytes Enhance Susceptibility to Arrhythmia Triggers

Abstract

Intracellular calcium (Ca) waves in cardiac myocytes can cause delayed afterdepolarizations (DADs), which are known triggers of cardiac arrhythmias. How these Ca waves are modulated by diffusive Ca-mediated coupling among Ca release units (CRUs) and promote DADs is not fully understood. Here, we hypothesized that myocytes are most susceptible to DADs due to periodic Ca wave activity at intermediate levels of Ca overload. To test this hypothesis, we strengthened CRU coupling by progressively raising intracellular free [Ca] and studied the transition from Ca sparks to waves using both confocal Ca imaging experiments in permeabilized mouse ventricular myocytes and computer simulations of a homogeneous 3D array (100x20x10) of diffusively-coupled CRUs. As free Ca was increased in experiments from 100 nM to 1,500 nM, intracellular Ca release activity evolved through four stages: Stage 1- random sparks and macrosparks arising from multiple sites; Stage 2- irregular aborted Ca waves arising from multiple sites; Stage 3- periodic full Ca waves arising from a small number of sites mostly near cell borders; Stage 4- high frequency “fibrillatory” Ca waves exhibiting mixed focal and reentrant features. Raising virtual intracellular Ca in computer simulations reproduced Stages 1-3 but not Stage 4, which may require spatial heterogeneities to occur. In both experiments and simulations, Stage 3 produced the largest whole-cell Ca transients and most synchronous Ca release, making this intermediate stage of Ca overload the most likely to generate DADs of sufficient amplitude to trigger arrhythmias. High frequency “fibrillatory” waves under severe Ca overload in Stage 4 diminished Ca transient amplitudes and reduced Ca release synchrony. In conclusion, our findings suggest that ventricular myocytes are most susceptible to DADs when intracellular Ca overload is intermediate rather than mild or severe.