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Department of Physiology, UCLA School of Medicine, Los Angeles, CA 90095, USA
The mdx mouse, a model of the human disease Duchenne muscular dystrophy, has skeletal muscle fibres which display incompletely understood impaired contractile function. We explored the possibility that action potential-evoked Ca2+ release is altered in mdx fibres. Action potential-evoked Ca2+-dependent fluorescence transients were recorded, using both low and high affinity Ca2+ indicators, from enzymatically isolated fibres obtained from extensor digitorum longus (EDL) and flexor digitorum brevis (FDB) muscles of normal and mdx mice. Fibres were immobilized using either intracellular EGTA or N-benzyl-p-toluene sulphonamide, an inhibitor of the myosin II ATPase. We found that the amplitude of the action potential-evoked Ca2+ transients was significantly decreased in mdx mice with no measured difference in that of the surface action potential. In addition, Ca2+ transients recorded from mdx fibres in the absence of EGTA also displayed a marked prolongation of the slow decay phase. Model simulations of the action potential-evoked transients in the presence of high EGTA concentrations suggest that the reduction in the evoked sarcoplasmic reticulum Ca2+ release flux is responsible for the decrease in the peak of the Ca2+ transient in mdx fibres. Since the myoplasmic Ca2+ concentration is a critical regulator of muscle contraction, these results may help to explain the weakness observed in skeletal muscle fibres from mdx mice and, possibly, Duchenne muscular dystrophy patients.
(Received 15 January 2004;
accepted after revision 2 March 2004;
first published online 5 March 2004)
Corresponding author J. L. Vergara: Department of Physiology, UCLA School of Medicine, Los Angeles, CA 90095, USA. Email: jvergara{at}mednet.ucla.edu
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