The first column gives the reaction steps and the second and third columns the reaction rate constants for ATP (A), parvalbumin (B), the regulatory sites on troponin (C), the SR Ca²⁺ pump (D, where E denotes Enzyme), and the indicator dye fluo-4 (E). The fourth column gives the dissociation constants of the reactions (k_off/k_on). In A, the indicated rate constants are for [K⁺] = 150 mM, pH = 7, and viscosity = 2 centipoise (Baylor and Hollingworth, 1998). In B, the parvalbumin rate constants are those in model 3 of Harwood et al. (2011). In C, the two regulatory sites on each troponin molecule are assumed to be independent and have identical reaction rate constants with Ca²⁺ (Rome et al., 1996). In D, the reaction steps are adapted from the scheme of Peinelt and Apell (2002), with dissociation constants involving protons expressed as pKs (shown in parentheses); the parentheses on the right-hand side of the last reaction in column 1 denote the two Ca²⁺ ions transported to the lumen of the SR with each reaction cycle. In E, “Pr” denotes protein sites capable of binding fluo-4 (Table 1). The fluo-4 reaction scheme is the same as that for fluo-3 (Harkins et al., 1993); the rate constants are derived from the values given by Hollingworth et al. (2000) for fluo-3 in frog myoplasm at 16°C in combination with information from Woodruff et al. (2002) about the Ca²⁺ dissociation constants (K_d,Ca) of fluo-3 and fluo-4 in a simple salt solution. The latter article reports that the values of K_d,Ca of fluo-3 and fluo-4 are similar, with a similar temperature dependence; the extrapolated value of K_d,Ca at 16°C is 0.8–0.9 µM for both indicators. Harkins et al. (1993) reported a somewhat smaller K_d,Ca for fluo-3 in a simple salt solution at 16°C, 0.51 µM. The value of K_d,Ca for protein-free fluo-4 given above (0.77 µM) was selected to fall within the range reported for fluo-3 and fluo-4 by Harkins et al. (1993) and Woodruff et al. (2002). The reaction rate constants of fluo-4 (second and third columns) are based on those of Hollingworth et al. (2000) for fluo-3 and the assumption that a larger value of K_d,Ca arises from a larger off-rate constant. At the resting values of free [Ca²⁺] (30 nM), free [Mg²⁺] (1 mM), and pH (7), the resting fractional occupancies of the different states of the various model constituents are as follows: (A) ATP (0.909), MgATP (0.091), CaATP (0.000); (B) Parv (0.070), CaParv (0.173), MgParv (0.758); (C) Trop (0.992), CaTrop (0.008); (D) E (0.006), CaE (0.004), Ca₂E (0.002), MgE (0.123), Mg₂E (0.123), HE (0.062), H₂E (0.617), H₃E (0.062), H₄E (0.001); (E) Dye (0.109), CaDye (0.004), PrDye (0.878), CaPrDye (0.009). The model assumes that the rate of Ca²⁺ pumping in the resting state, ∼0.01 µM/ms, is balanced by an equivalent (and constant) leak of Ca²⁺ from the SR into the myoplasm.