Simulation of calcium sparks with different calcium handling–related parameters. (A) Typical simulations for cytosolic [Ca²⁺] of 0.05 (top row) and 0.4 µM (bottom row) for (left to right) a threefold increase of D_Ca, j_Ca, and τ_d. (B–G) The calcium dependence of the frequency of triggered sparks (Δϕ_Spark; B), the increase of the average number of open RyRs in triggered sparks $(Δn−O;$ C), spontaneous spark frequency (ϕ_Spark; D), calcium sensitivity of spontaneous spark frequency (∂_Caϕ_Spark; E), average number of open RyRs in spontaneous sparks (⁠$n−O;$ F), and calcium sensitivity of the average number of open RyRs in spontaneous sparks (⁠$∂Can−O;$ G) for control conditions (black symbols) and for decreased (red symbols) and increased (blue symbols) parameters D_Ca, j_Ca, and τ_d. The parameters were changed to 33 and 300% of control. Theoretical predictions (D–G) and predictions of multiple linear regression (B and C) are shown by solid lines. Theoretical values and data from simulations at calcium concentrations used in the experiment (Lukyanenko and Gyorke, 1999) shown in Fig. 1 are shown as open and closed symbols, respectively. Error bars represent mean ± SEM.