Effect of SR Ca2+ load on alternans. SR Ca2+ release flux (A), JSR (B) and NSR (C) Ca2+ concentrations, and Ca2+ transport between the NSR and JSR (D) are shown for the following three simulations: C1 (a combination of β-AR-stimulated downstream effects with high release and no alternans), C2 (a combination of β-AR-stimulated downstream effects with relatively high release, manifesting alternans), and C3 (C2 with NSR Ca2+ content changed to that of C1 at the beginning of the plotted period).