NDLI: Mechanisms of cardiac cell excitation with premature monophasic and biphasic field stimuli: a model study.

Content Provider	PubMed Central
Author	Fishler, M. G. Sobie, E. A. Tung, L. Thakor, N. V.
Abstract	The mechanisms by which extracellular electric field stimuli induce the (re)excitation of cardiac cells in various stages of refractoriness are still not well understood. We modeled the interactions between an isolated cardiac cell and imposed extracellular electric fields to determine the mechanisms by which relatively low-strength uniform monophasic and biphasic field stimuli induce premature reexcitations. An idealized ventricular cell was simulated with 11 subcellular membrane patches, each of which obeyed Luo-Rudy (phase 1) kinetics. Implementing a standard S1-S2 pulse protocol, strength-interval maps of the cellular excitatory responses were generated for rectangular monophasic and symmetric biphasic field stimuli of 2, 5, 10, and 20 ms total duration. In contrast to previously documented current injection studies, our results demonstrate that a cardiac cell exhibits a significantly nonmonotonic excitatory response to premature monophasic and, to a much lesser degree, biphasic field stimuli. Furthermore, for monophasic stimuli at low field strengths, the cell is exquisitely sensitive to the timing of the shock, demonstrating a classic all-or-none depolarizing response. However, at higher field strengths this all-or-none sensitivity reverts to a more gradual transition of excitatory responses with respect to stimulus prematurity. In contrast, biphasic stimuli produce such graded responses at all suprathreshold stimulus strengths. Similar behaviors are demonstrated at all S2 stimulus durations tested. The generation of depolarizing (sodium) currents is triggered by one or more of the sharp field gradient changes produced at the stimulus edges-i.e., make, break, and transphasic (for biphasic stimuli)-with the magnitude of these edge-induced current contributions dependent on both the prematurity and the strength of the applied field. In all cases, however, depolarizing current arises from the partial removal of sodium inactivation from at least part of the cell, because of either the natural process of repolarization or a localized acceleration of this process by the impressed field.
Related Links	http://dx.doi.org/10.1016/s0006-3495(96)79692-3
Ending Page	1362
Page Count	16
Starting Page	1347
File Format	PDF
ISSN	00063495
e-ISSN	15420086
Journal	Biophysical Journal
Issue Number	3
Volume Number	70
Language	English
Publisher Date	1996-03-01
Access Restriction	Open
Subject Keyword	Biophysics Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Biophysics

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