Fig. 4.

Active force plotted against pCa or [Ca²⁺]_i. a Active force–pCa curves simulated based on the equation $F=\frac{1}{1+{10}^{h\left(\text{pCa}-{\text{pCa}}_{50}\right)}}$ (i.e., $y=\frac{1}{1+{10}^{h(x-p{K}_{\text{d}})}}$), where F, pCa₅₀ and h indicate relative force, the mid-point of each curve and the Hill coefficient, respectively (x pCa; y relative force). Three different apparent pK _d values for Ca²⁺-binding to TnC (calculated using hypothetical SL changes) were used for simulation, i.e., 0.1, 1 ,and 10 μM, at h = 1. b Active force–[Ca²⁺] curves simulated based on the equation $F=\frac{{\left[{\text{Ca}}^{2+}\right]}^h}{{\text{EC}}_{50}^h+{\left[{\text{Ca}}^{2+}\right]}^h}$ (i.e., $y=\frac{1}{1+{\left(\frac{K_{\text{d}}}{x}\right)}^h}$), where EC₅₀ indicates the mid-point of each curve (x [Ca²⁺]; y relative force). Three different apparent K _d values for Ca²⁺-binding to TnC (calculated using hypothetical SL changes) were used for simulation, i.e., 0.1, 1, and 10 μM, at h = 1. As shown in the graphs, Ca²⁺ sensitivity is more clearly indicated in (a) using pCa values (see text for details)