Fig. 4

Active force plotted against pCa or [Ca²⁺]_i. a Active force–pCa curves simulated based on the equation \( F = \frac{1}{{1 + 10^{{h{\kern 1pt} \left( {{\text{pCa}} - {\text{pCa}}_{50} } \right)}} }} \) (i.e., \( y = \frac{1}{{1 + 10^{{h{\kern 1pt} (x - pK_{\text{d}} )}} }} \)), 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)