Figure 9.

Summary of steady-state Ca²⁺ flows. Flows of Ca²⁺ between compartments of the model fiber are indicated by arrows and quantified by flow rates in μM s⁻¹. Arrow colors: blue, passive leaks; red, active pumps; green, diffusion; black, division of flows within compartments. Pumps: P, PMCA; S, SERCA. Leaks are identified by adjacent leak rate constant symbols. Rates in each compartment are expressed with concentrations relative to that compartment’s volume; −, efflux from a compartment; +, influx into a compartment. Differences in rates between the ends of arrows that span compartment boundaries reflect the difference in volumes of the two compartments. The following values were used to construct the diagram: Ca_SR, 400 μM; Ca_T, 1,200 μM; $k_{\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{k}}^{\mathrm{T}},$ 0.013 s⁻¹; $k_{\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{k}}^{\mathrm{R}\mathrm{y}\mathrm{R}},$ 0.035 s⁻¹; and $k_{\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{k}}^{\mathrm{S}\mathrm{R}},$ 0.145 s⁻¹.