Acute failure of action potential conduction in mdx muscle reveals new mechanism of contraction-induced force loss

摘要

? A primary feature of skeletal muscle lacking the protein dystrophin, as occurring in Duchenne muscular dystrophy, is a hypersensitivity to eccentric contraction-induced injury. Dystrophin is associated with the plasmalemma which in healthy muscle has the physiological function of maintaining resting membrane potential to facilitate action potential generation and conduction during muscle activation. ? We tested the hypothesis that the physiological function of the plasmalemma is impaired as a result of eccentric contractions in dystrophic skeletal muscles. ? Electromyographic analysis of dystrophic muscle from mdx mice, the murine model of Duchenne muscular dystrophy, during and immediately after eccentric contractions revealed impairment in the muscle's ability to generate and conduct action potentials. ? In agreement with our electromyographic analysis, assessment of resting membrane potentials showed that dystrophic muscle cells are depolarized immediately after an injurious bout of eccentric contractions. ? Our results suggest a major plasmalemma-based mechanism of strength loss underlying eccentric contraction-induced injury in dystrophic muscle. Abstract A primary feature of skeletal muscle lacking the protein dystrophin, as occurring in Duchenne muscular dystrophy, is a hypersensitivity to contraction-induced strength loss. We tested the hypothesis that the extensive strength loss results from an impairment in the electrophysiological function of the plasmalemma specifically impaired action potential development. Anterior crural muscles from mdx and wildtype mice performed a single bout of 100 electrically stimulated eccentric contractions in vivo. Electromyography, specifically the M-wave, was analysed during muscle contraction to assess the ability of the tibialis anterior muscle plasmalemma to generate and conduct action potentials. During eccentric contractions, wildtype mice exhibited a 36% loss in torque about the ankle but mdx mice exhibited a greater torque loss of 73% (P < 0.001). Despite the loss of torque, there was no reduction in M-wave root mean square (RMS) for wildtype mice, which was in stark contrast to mdx mice that had a 55% reduction in M-wave RMS (P < 0.001). This impairment resolved within 24 h and coincided with a significant improvement in strength and membrane integrity. Intracellular measurements of resting membrane potential (RMP) in uninjured and injured extensor digitorum longus muscles were made to determine if a chronic depolarization had occurred, which could lead to impaired fibre excitability and/or altered action potential conduction properties. The distributions of RMP were not different between wildtype uninjured and injured muscle cells (median: -73.2 mV vs. -72.7 mV, P= 0.46) whereas there was a significant difference between mdx uninjured and injured cells (median: -71.5 mV vs. -56.6 mV, P < 0.001). These data show that mdx muscle fibres are depolarized after an injurious bout of eccentric contractions. These findings (i) suggest a major plasmalemma-based mechanism of strength loss underlying contraction-induced injury in Duchenne muscular dystrophy distinctly different from that for healthy muscle, and (ii) demonstrate dystrophin is critical for maintaining action potential generation and conduction after eccentric contractions.