Ryanodine Receptor Ca ²⁺ Leak‐Induced Redistribution of Ca ²⁺ in Dystrophic mdx Mouse Muscle

ABSTRACT Aim The dystrophic mdx mouse is a widely used model of Duchenne muscular dystrophy. Altered Ca 2+ handling is a key feature, including increased Ca 2+ leak through the ryanodine receptor (RyR1's), the primary Ca 2+ release channel in skeletal muscle. Such leak has important downstream consequences for intracellular Ca 2+ homeostasis. Here, we quantified basal compartmentalized Ca 2+ levels in mdx muscle compared with wild‐type (WT). Methods Single extensor digitorum longus muscle fibers from WT and mdx mice were mechanically skinned. Transverse tubule Ca 2+ dynamics were assessed using confocal microscopy with fluorescent Ca 2+ indicators during caffeine‐induced RyR1‐mediated Ca 2+ release. Sarcoplasmic reticulum (SR) and mitochondrial Ca 2+ contents were quantified using established depletion protocols combined with force measurements. Results Consistent with previous reports, mdx fibers exhibited increased RyR1 Ca 2+ leak. Absolute quantification revealed a reduction in SR Ca 2+ content accompanied by a ~4‐fold increase in mitochondrial Ca 2+ content. These shifts indicate a redistribution of intracellular Ca 2+ , triggered by the RyR1 Ca 2+ leak to lower SR Ca 2+ content and increase the Ca 2+ permeability of the t‐system membrane, leading to an elevation in cytoplasmic and mitochondrial Ca 2+ levels in mdx muscle. Conclusion Redistribution of Ca 2+ is a regulated process, proportional to RyR1 Ca 2+ leak. In mdx muscle fibers, there is reduced SR and elevated mitochondrial and cytoplasmic Ca 2+ compared to WT fibers. These alterations contribute to the dystrophic muscle pathology, likely through promotion of oxidative stress through increased reactive oxygen species production.