Changes in cytosolic Ca²⁺ induced by SR Ca²⁺-ATPase inhibition in control and in mdx fibers. (A) Changes in intracellular Ca²⁺ monitored by Fura-2 and induced by 50 µM CPA in a control (C57BL6 mouse) and in an mdx fiber at a holding potential of −80 mV (the fluorescence baselines before addition of CPA were superimposed to make easier the comparison of the fluorescence changes induced by CPA in the two fiber types). (B) Mean rates of CPA-induced Ca²⁺ changes in control (C57BL6 mice) and in mdx fibers in the presence of 2.5 mM Ca²⁺ or in the absence of Ca²⁺ in the external solution. The numbers above each bar indicate the number of cells tested. Mean values in control and in mdx fibers and mean values in the presence of Ca²⁺ and in its absence have been compared using a Student’s unpaired t test. *, P < 0.05; **, P < 0.005; ***, P < 0.0005. Error bars indicate mean ± SEM.