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Biophys J, May 2000, p. 2392-2404, Vol. 78, No. 5
Cardiac Bioelectricity Research and Training Center and Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7207 USA
Sodium overload of cardiac cells can accompany various
pathologies and induce fatal cardiac arrhythmias. We investigate
effects of elevated intracellular sodium on the cardiac action
potential (AP) and on intracellular calcium using the Luo-Rudy model
of a mammalian ventricular myocyte. The results are: 1) During rapid pacing, AP duration (APD) shortens in two phases, a rapid phase without
Na+ accumulation and a slower phase that depends on
[Na+]i. 2) The rapid APD shortening is due to
incomplete deactivation (accumulation) of IKs. 3) The slow
phase is due to increased repolarizing currents
INaK and reverse-mode
INaCa, secondary to elevated
[Na+]i. 4) Na+-overload slows the
rate of AP depolarization, allowing time for greater
ICa(L) activation; it also enhances
reverse-mode INaCa. The resulting increased
Ca2+ influx triggers a greater
[Ca2+]i transient. 5) Reverse-mode
INaCa alone can trigger Ca2+
release in a voltage and [Na+]i-dependent
manner. 6) During INaK block,
Na+ and Ca2+ accumulate and APD shortens due to
enhanced reverse-mode INaCa; contribution of
IK(Na) to APD shortening is negligible. By
slowing AP depolarization (hence velocity) and shortening APD,
Na+-overload acts to enhance inducibility of reentrant
arrhythmias. Shortened APD with elevated
[Ca2+]i (secondary to
Na+-overload) also predisposes the myocardium to
arrhythmogenic delayed afterdepolarizations.
Biophys J, May 2000, p. 2392-2404, Vol. 78, No. 5
© 2000 by the Biophysical Society 0006-3495/00/05/2392/13 $2.00
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