title: Stochastic initiation and termination of calcium-mediated triggered activity in cardiac myocytes.
abstract: Cardiac myocytes normally initiate action potentials in response to a current stimulus that depolarizes the membrane above an excitation threshold. Aberrant excitation can also occur due to spontaneous calcium (Ca2+) release (SCR) from intracellular stores after the end of a preceding action potential. SCR drives the Na+/Ca2+exchange current inducing a "delayed afterdepolarization" that can in turn trigger an action potential if the excitation threshold is reached. This "triggered activity" is known to cause arrhythmias, but how it is initiated and terminated is not understood. Using computer simulations of a ventricular myocyte model, we show that initiation and termination are inherently random events. We determine the probability of those events from statistical measurements of the number of beats before initiation and before termination, respectively, which follow geometric distributions. Moreover, we elucidate the origin of randomness by a statistical analysis of SCR events, which do not follow a Poisson process observed in other eukaryotic cells. Due to synchronization of Ca2+releases during the action potential upstroke, waiting times of SCR events after the upstroke are narrowly distributed, whereas SCR amplitudes follow a broad normal distribution with a width determined by fluctuations in the number of independent Ca2+wave foci. This distribution enables us to compute the probabilities of initiation and termination of bursts of triggered activity that are maintained by a positive feedback between the action potential upstroke and SCR. Our results establish a theoretical framework for interpreting complex and varied manifestations of triggered activity relevant to cardiac arrhythmias.
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095.|Cardiovascular Research Laboratories, David Geffen School of Medicine, University of California, Los Angeles, CA 90095.|Department of Physics, Northeastern University, Boston, MA 02115.|Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA 02115.
authors.forename: Zhen
authors.initials: Z
authors.lastname: Song
authors.affiliation: Department of Medicine, David Geffen School of Medicine, University of California, Los Angeles, CA 90095; a.karma@neu.edu zqu@mednet.ucla.edu.|Cardiovascular Research Laboratories, David Geffen School of Medicine, University of California, Los Angeles, CA 90095.
authors.forename: Zhilin
authors.initials: Z
authors.lastname: Qu
authors.affiliation: Department of Physics, Northeastern University, Boston, MA 02115; a.karma@neu.edu zqu@mednet.ucla.edu.|Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA 02115.
authors.forename: Alain
authors.initials: A
authors.lastname: Karma
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000209:etiology / Q000503:physiopathology; D020013:Calcium Signaling / Q000502:physiology; D003198:Computer Simulation; D055724:Electrophysiological Phenomena; D025461:Feedback, Physiological; D006801:Humans; D007473:Ion Channels / Q000502:physiology; D008955:Models, Cardiovascular; D032383:Myocytes, Cardiac / Q000502:physiology*; D013269:Stochastic Processes
chemical_list: D007473:Ion Channels
keywords: arrhythmias; bistability; calcium wave; delayed afterdepolarization; triggered activity