Mechanisms of subcellular alternans in cardiac myocytes

Cardiac repolarization alternans is a rhythm disturbance of the heart in which rapid stimulation elicits a beat-to-beat alternation in the duration of action potentials and magnitude of intracellular calcium transients in individual cardiac myocytes. Although this phenomenon has been identified as an important precursor to dangerous reentrant arrhythmias and sudden cardiac death, significant uncertainty remains regarding its mechanism and no clinically practical means of halting its occurrence or progression currently exists. In addition to well-characterized tissue and cellular-level manifestations, so-called "subcellular alternans" has been frequently observed, in which rapid pacing causes the calcium transients of adjacent regions of an individual myocyte to alternate with opposite phase. Although its mechanism has yet to be determined, the arrhythmogenic potential of subcellular alternans has already been recognized and has motivated a quickly growing field of its research.;This work uses a combination of mathematical and computational modeling, real-time electrophysiology, and fluorescent imaging to predict and verify a mechanism by which subcellular alternans can be dynamically induced in cardiac myocytes. First, a simplified model is used to analytically predict that a simple feedback control pacing algorithm ("alternans control") will dynamically induce subcellular alternans in isolated cardiac myocytes by a well known, generic mechanism of pattern-formation (a Turing/diffusion-driven instability). Computational modeling using a physiologically realistic model of an isolated cardiac myocyte is then used to support this mechanism as well as to introduce a proposed experimental utility for this pacing protocol in future studies of alternans mechanisms. This protocol is then experimentally implemented using simultaneous real-time electrophysiology and fluorescent calcium-imaging, and shown to robustly induce subcellular alternans in isolated guinea pig ventricular myocytes. Finally, simulations of 1-dimensional fibers of cardiac myocytes are used to demonstrate that the same dynamical mechanism may contribute to subcellular alternans seen in intact tissue.;This work provides the first direct experimental evidence for a mechanism of pacing-induced subcellular alternans formation in cardiac myocytes. In addition to advancing our understanding of fundamental cardiac myocyte dynamics, this identification of a robust means of inducing subcellular alternans in isolated cells introduces a useful tool for future studies of subcellular alternans and related phenomena.