Mechanism of Abnormal Sarcoplasmic Reticulum Calcium Release in Canine Left-Ventricular Myocytes Results in Cellular Alternans

Electrocardiographic alternans are known to predispose to increased susceptibility to life threatening arrhythmias and sudden cardiac death. While deficiencies in Ca(2+) transport processes have been implicated in the genesis of cellular alternans, the underlying mechanisms have been elusive, and are the goal of this study. A novel reverse engineering approach that applies a simultaneous action potential (AP) and [Ca(2+)](i) clamp of experimentally obtained data, to a previously described left-ventricular canine myocyte model, is employed to isolate the molecular and cellular mechanisms underlying cardiac alternans. The model-derived sarcoplasmic reticulum (SR) Ca(2+) in control beats (102.1 +/- 12.9 nM, n = 639), although larger, is not statistically significantly different as compared to beats corresponding to small [Ca(2+)](i) (99.3 +/- 35.4 nM, n = 310, p = NS), but is significantly smaller as compared to beats corresponding to large [Ca(2+)](i) (122.6 +/- 31.0 nM, n = 311, p < 0.000001) during alternans. The model indicates that the increased SR Ca(2+) in these beats triggers multiple ryanodine receptor (RyR) channel openings and delayed Ca(2+) release that subsequently triggers an inward depolarizing current, a subthreshold early after depolarization, and AP prolongation. In conclusion, the results presented in this study support the idea that aberrant RyR openings on alternate beats are responsible for the [Ca(2+)](i) alternans-type oscillations, which, in turn, give rise to AP alternans.