Figure 4.

Models to account for steady-state Ca_T. (A) Ca²⁺ accumulation in the t-system limited by pump thermodynamics. The dashed curves show steady-state Ca_T–Ca_JS relationships for different ΔG_ATP values. The curves show what steady-state Ca_T would be if it was determined only by the ability of the pump to generate a gradient (Eq. 5). The symbols are experimental data (mean ± SEM) determined in the presence of tetracaine; the independent variable for the experimental data was Ca_M, but with RYRs blocked by tetracaine, Ca_M ≈ Ca_JS, which drives the pump, both determined by the Ca²⁺–EGTA equilibrium. (B) Steady-state Ca_T determined by balance between pumping Ca²⁺ into the t-system and a concentration gradient–dependent leak of Ca²⁺ out of t-system. The model used is given by Eqs. 6 and 7. Fitted parameters: c_PMCA, 60 μM; $k_{\mathrm{L}\mathrm{e}\mathrm{a}\mathrm{k}}^{\mathrm{T}},$ 0.023 s⁻¹; and $K_{\mathrm{D}}^{\mathrm{P}\mathrm{M}\mathrm{C}\mathrm{A}},$ 223 μM.