Decrease in Fluo-5N fluorescence induced by CPA in control and in mdx fibers. (A) Effect of removal of external Ca²⁺ on the changes in Fluo-5N fluorescence induced by addition of 50 µM CPA in a control (C57BL6 mouse) fiber. (B) Mean rates of CPA-induced fluorescence decrease in the absence and in the presence of 2.5 mM external Ca²⁺. Mean values have been compared using a Student’s paired t test. (C) Changes in Fluo-5N fluorescence in response to the addition of 50 µM CPA in a control (C57BL6 mouse) and in an mdx fiber. (D) Mean rates of fluorescence decrease in control and in mdx fibers. Mean values have been compared using a Student’s unpaired t test. Images were collected at a frequency of 0.5 Hz in A and C. ***, P < 0.0005. Error bars indicate mean ± SEM.