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.