All concentrations are spatially averaged and referenced to the myoplasmic water volume in the myofibrillar region, except for free [Ca²⁺] and free [Mg²⁺], for which total concentrations are given. The concentrations are taken from Nelson et al. (2014); those for troponin and Parv have been adjusted for measured anatomical, biochemical, and physiological differences between midshipman and toadfish swimbladder fibers (see Materials and methods). The concentration of Mag-fluo-4 was estimated from a comparison of the resting fluorescence of Mag-fluo-4 in midshipman fibers with that in frog twitch fibers, with an appropriate scaling for the measured differences in fiber diameters; the 100-µM value applies to measurements made early in an experiment (as in Figs. 1 and 3 and Table 2), whereas the 50-µM value applies to measurements made later in an experiment (cf. Figs. 4, 5, and 6).