Evaluation of a novel model of skeletal muscle fatigue

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Skeletal muscle fatigue, is a common experience that has fascinated scientists for centuries, yet the cellular processes impacting contractile properties during and following repeated activations are not understood. Most theories of fatigue considered build-up of products of metabolism and inhibition of the contractile response. This is not consistent with the known primary mechanism of fatigue: inhibition of excitation-contraction coupling (E-CC). It is time to study how E-CC is impaired. An innovative explanation for muscle fatigue is proposed: fatigue is a consequence of cellular regulation of E-CC to preserve energy charge in the muscle. Experiments are proposed to test this theory as well as alternative theories. Experiments are proposed using: single fibre, fibre bundles and whole muscle at physiological temperature. It has been demonstrated that fatigue mechanisms at room temperature are not relevant at physiological temperature. A second innovative and practical aspect of the proposed studies is contractions under consideration will be sub maximal, like natural contractions in common movements. ****If the theory is correct, then changing cellular energy charge should result in altered E-CC, on a contraction-to-contraction basis and in steady state. For example, stretching a muscle to increase active force and ATP turnover, without changing cross-membrane ion exchange associated with activation will result in attenuation of subsequent Ca2+ release.  Free [Ca2+] will be measured in single fibre experiments to detect any changes in Ca2+ transients. Measuring average free [Ca2+] during contractions over a range of frequencies reveals the force-pCa2+ relationship. Regulation of the ryanodine receptor is thought to occur by increases in Ca2+-calmodulin binding and/or binding of Mg2+. The potential impact of Ca2+-calmodulin may be limited once RyR inhibition is achieved. Decreased free [Ca2+] will result in less Ca2+-calmodulin. This will be assessed in whole muscle with a bioassay by looking at myosin light chain phosphorylation, permitting the first assessment of Ca2+ transients in whole muscle. The possibility that decreases in light chain phosphorylation contributes to impaired Ca2+ sensitivity will also be considered.  ***The proposed experiments will provide evidence for how the regulation of E-CC can result in fatigue, advancing our knowledge in a field of study that requires some novel thinking. The proposed work represents a new direction for the study of muscle fatigue and should be remarkably revealing in support of a new model of this common yet intriguing property of skeletal muscle.