Mechanisms of weakness in Mdx muscle following in vivo eccentric contractions

Skeletal muscle of the dystrophin-deficient mdx mouse is hypersensitive to eccentric (ECC) contraction-induced strength loss due to plasmalemmal electrical dysfunction. Despite plasmalemmal inexcitability being a logical mechanism responsible for weakness, it remains unclear if processes up- and/or down-stream remain functionally intact in injured mdx muscle. The purpose of this study was to analyze additional processes necessary for excitation-contraction coupling that are potentially disrupted by ECC contractions. Anterior crural muscles (tibialis anterior, extensor digitorum longus [EDL], and extensor hallucis muscles) of wildtype (WT) and mdx mice were injured in vivo with 50 ECC contractions and torque was measured immediately before and after the contraction bout. Following the in vivo assessment, EDL ex vivo isometric and caffeine forces were analyzed. In vivo isometric torque and ex vivo force in WT muscle were reduced 38 and 30% (p < 0.001), while caffeine force was also reduced (p = 0.021), albeit to a lesser degree (9%). In contrast, in vivo isometric torque, ex vivo isometric force and ex vivo caffeine-induced force were all reduced 56-67% (p < 0.001) in mdx muscle and did not differ from one another (p = 0.114). Disproportional reductions in isometric strength and caffeine-induced force confirm that ECC contractions uncoupled the plasmalemma from the ryanodine receptors (RyRs) in WT muscle. In mdx muscle, the proportional reductions in isometric strength and caffeine-induced force following ECC contractions reveal that dysfunction occurs at and/or distal to the RyRs immediately post-injury. Thus, weakness in injured mdx muscle cannot be isolated to one mechanism, rather several steps of muscle contraction are disrupted.