Comparison of Ca²⁺ movements elicited by APs in slow-twitch and fast-twitch fibers (16°C). (A). Simulated Ca²⁺ movements in slow-twitch and fast-twitch fibers (continuous and dashed traces, respectively) in response to a single AP initiated at zero time. f_CaTrop, the fractional occupancy of the troponin regulatory sites with Ca²⁺, includes the resting occupancy (0.071 in slow-twitch fibers and 0.004 in fast-twitch fibers) plus the change in occupancy during activity. See text for definitions of the other traces. Δ[Ca_T] is the sum of the five lower traces, excluding f_CaTrop, plus Δ[CaATP] and Δ[CaDye] (the change in the concentration of Ca²⁺ bound to the indicator). Δ[CaATP] is approximately equal to 2.2 times Δ[Ca²⁺] in slow-twitch fibers and 3.4 times Δ[Ca²⁺] in fast-twitch fibers. Δ[CaDye] is relatively small (= Δf_CaD × [Dye_T], where [Dye_T], the total indicator concentration in myoplasm, is 100 µM). The units of all traces except f_CaTrop are spatially averaged concentrations referred to the myoplasmic water volume. (B). As in A, but for five APs at 67 Hz.