Chemical energy usage and myosin light chain phosphorylation in mammalian skeletal muscle

The purpose of this study was to ascertain whether phosphorylation of the regulatory light chain of myosin plays a role in modulating the rate of chemical energy usage in mammalian skeletal muscle. There was no change in the average rate of chemical energy usage with duration of isometric stimulation in the rat extensor digitorum longus (EDL), even though the degree of light chain phosphorylation increased from 5% at rest to above 60% after 7 s of stimulation. When the initial degree of phosphorylation was increased to 73% by prestimulation of the muscle, there was still no change in the chemical energy usage under isometric conditions. In contrast, under the conditions used, the mouse EDL showed changes in the average rate of energy usage that depended upon both tetanus duration and stimulation history. However, there was no consistent relationship between phosphorylation of the light chain and average rate of chemical energy usage. These results suggest that while there are factors which can change crossbridge cycling rate in mammalian skeletal muscle, phosphorylation of the regulatory light chain of myosin is neither necessary nor sufficient to cause such changes.