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Am J Physiol Cell Physiol 290: C116-C122, 2006. First published August 17, 2005; doi:10.1152/ajpcell.00291.2005
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CELLULAR METABOLISM

µ-Calpain and calpain-3 are not autolyzed with exhaustive exercise in humans

Robyn M. Murphy,1 Rodney J. Snow,2 and Graham D. Lamb1

1Department of Zoology, La Trobe University, Victoria; 2School of Exercise and Nutrition Sciences, Deakin University, Burwood, Victoria, Australia

Submitted 15 June 2005 ; accepted in final form 16 August 2005

µ-calpain and calpain-3 are Ca2+-dependent proteases found in skeletal muscle. Autolysis of calpains is observed using Western blot analysis as the cleaving of the full-length proteins to shorter products. Biochemical assays suggest that µ-calpain becomes proteolytically active in the presence of 2–200 µM Ca2+. Although calpain-3 is poorly understood, autolysis is thought to result in its activation, which is widely thought to occur at lower intracellular Ca2+ concentration levels ([Ca2+]i; ~1 µM) than the levels at which µ-calpain activation occurs. We have demonstrated the Ca2+-dependent autolysis of the calpains in human muscle samples and rat extensor digitorum longus (EDL) muscles homogenized in solutions mimicking the intracellular environment at various [Ca2+] levels (0, 2.5, 10, and 25 µM). Autolysis of calpain-3 was found to occur across a [Ca2+] range similar to that for µ-calpain, and both calpains displayed a seemingly higher Ca2+ sensitivity in human than in rat muscle homogenates, with ~15% autolysis observed after 1-min exposure to 2.5 µM Ca2+ in human muscle and almost none after 1- to 2-min exposure to the same [Ca2+]i level in rat muscle. During muscle activity, [Ca2+]i may transiently peak in the range found to autolyze µ-calpain and calpain-3, so we examined the effect of two types of exhaustive cycling exercise (30-s "all-out" cycling, n = 8; and 70% VO2 peak until fatigue, n = 3) on the amount of autolyzed µ-calpain or calpain-3 in human muscle. No significant autolysis of µ-calpain or calpain-3 occurred as a result of the exercise. These findings have shown that the time- and concentration-dependent changes in [Ca2+]i that occurred during concentric exercise fall near but below the level necessary to cause autolysis of calpains in vivo.

Ca2+-dependent proteases; proteolysis


Address for reprint requests and other correspondence: R. M. Murphy, Dept. of Zoology, La Trobe Univ., Victoria 3086, Australia (e-mail: r.murphy{at}latrobe.edu.au)




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