Effects of HOCl oxidation on excitation–contraction coupling: Implications for the pathophysiology of Duchenne muscular dystrophy: Calcium Signaling and Excitation–Contraction in Cardiac, Skeletal and Smooth Muscle

Duchenne muscular dystrophy (DMD) is a fatal X-linked genetic disease characterized by progressive loss of skeletal muscle. The mechanisms underlying the DMD pathology likely involve the complex interaction between reactive oxygen species (ROS) impaired Ca²⁺ handling and chronic inflammation, characterized by the presence of immune cells such as neutrophils. Hypochlorous acid (HOCl) is a highly reactive form of ROS produced endogenously via the actions of myeloperoxidase, an enzyme secreted by neutrophils. Myeloperoxidase activity is significantly elevated in dystrophic muscle. This study aimed to determine the effect of HOCl exposure on excitation–contraction coupling and its potential contribution to the dystrophic pathology. Isolated extensor digitorum longus (EDL) muscles and single fibers from C57 (wild type) and mdx (dystrophic) mice were used to investigate the effects of HOCl on whole muscle function, intracellular Ca²⁺ handling, and myofilament force production. HOCl exposure significantly decreased maximum specific force in isolated EDL muscles by 26% and 49%, respectively, in C57 and mdx mice (P < 0.0001). In single interosseous fibers, HOCl exposure significantly increased resting intracellular Ca²⁺ concentration by ∼17–19% (P < 0.05) and decreased the amplitude of electrically induced Ca²⁺ transients by ∼45% and 50%, respectively, in C57 and mdx fibers (C57, P < 0.05; mdx, P < 0.01). These effects of HOCl on resting Ca²⁺ could be blocked via application of tetracaine (ryanodine receptor blocker) or Gd³⁺ (stretch-activated channel blocker; C57, P < 0.01; mdx, P < 0.01 for both). The effect of HOCl on Ca²⁺ transient amplitude was significantly reduced by Gd³⁺ (C57, P < 0.05; mdx, P < 0.01). In chemically skinned EDL fibers, HOCl exposure decreased maximum Ca²⁺-activated force by ∼40% in both C57 and mdx fibers (P < 0.001). These results indicate that HOCl potently affects excitation–contraction coupling via impaired Ca²⁺ handling and myofilament force production. Hence, HOCl potentially links the chronic inflammation, oxidative stress, and impaired Ca²⁺ handling that underlies the dystrophic pathology.