The activity-induced reduction of myofibrillar Ca sensitivity in mouse skeletal muscle is reversed by dithiothreitol

The aim of this study was to further characterize the reduction of myofibrillar Ca sensitivity in mousemuscle 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 injectedwith the Ca indicator indo-1. Muscle fatigue was producedby 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 ([Ca]) during tetani, and the maximum Ca-activated force and the myofibrillar Ca sensitivity were estimated from a series of tetani at different stimulation frequencies. Two main results were found. (i) The reduction of Ca 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 Ca 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 Ca sensitivity was reversed. Dithiothreitol had no effect on Ca sensitivity in unfatigued preparations. There was no effect of fatigue or dithiothreitol on tetanic [Ca] or on the maximum Ca-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 Ca sensitivity. © 2006 The Authors. Journal compilation © 2006 The Physiological Society.