Comparison of the modeled Ca²⁺–troponin reactions in slow-twitch and fast-twitch fibers. (A and B) Schemes for the reaction of Ca²⁺ with the regulatory sites on slow-twitch troponin (A) and fast-twitch troponin (B). The text gives the values of the reaction rate constants. (C and D) Theoretical single-compartment responses of the slow-twitch (C) and fast-twitch (D) troponin regulatory sites at 16°C to a pulse of [Ca²⁺] that starts at 50 nM, rises to 5 µM for 50 ms, and returns to 50 nM; the response in D considers only the doubly occupied troponin state in B. The total site concentrations are 120 µM (C) and 240 µM (D).