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Skeletal Muscle and Exercise |
1 Institute for Biomedical Sciences, School of Medical Sciences, University of Sydney F13, NSW 2006, Australia
The aim of this study was to further characterize the reduction of myofibrillar Ca2+ sensitivity in mouse muscle which has been observed after fatigue at 37°C. Muscle bundles and single fibres were isolated from mouse flexor digitorum brevis muscle and studied at 37°C. The single fibres were injected with the Ca2+ indicator indo-1. Muscle fatigue was produced by 0.4 s tetani repeated at 4 s intervals until force had fallen to less than 50% of initial. Excitation–contraction coupling was assessed by measuring the cytosolic calcium concentration ([Ca2+]i) during tetani, and the maximum Ca2+-activated force and the myofibrillar Ca2+ sensitivity were estimated from a series of tetani at different stimulation frequencies. Two main results were found. (i) The reduction of Ca2+ sensitivity only occurred when the muscle was intensely stimulated leading to fatigue. When the muscle was rested for 10 min at 37°C there was no significant change in Ca2+ sensitivity. (ii) If the membrane-permeant thiol-specific reducing agent dithiothreitol (0.5 mM) was applied to the muscle for 2 min following the fatigue protocol, the reduction in Ca2+ sensitivity was reversed. Dithiothreitol had no effect on Ca2+ sensitivity in unfatigued preparations. There was no effect of fatigue or dithiothreitol on tetanic [Ca2+]i or on the maximum Ca2+-activated force. These results suggest that intense activity of skeletal muscle at 37°C causes the production of reactive oxygen species which oxidize a target protein. We propose that critical sulphydryl groups on the target protein(s) are converted to disulphide bonds and this reaction reduces Ca2+ sensitivity.
(Received 1 November 2005;
accepted after revision 1 December 2005;
first published online 8 December 2005)
Corresponding author D. G. Allen: Institute for Biomedical Sciences, School of Medical Sciences, University of Sydney F13, NSW 2006, Australia. Email: davida{at}physiol.usyd.edu.au
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