Mechanisms of excitation-contraction uncoupling relevant to activity-induced muscle fatigue.

If the free [Ca2+] in the cytoplasm of a skeletal muscle fiber is raised substantially for a period of seconds to minutes or to high levels just briefly, it leads to disruption of the normal excitation-contraction (E-C) coupling process and a consequent long-lasting decrease in force production. It appears that the disruption to the coupling occurs at the triad junction, where the voltage-sensor molecules (dihydropyridine receptors) normally interact with and open the Ca2+ release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum (SR). This disruption results in inadequate release of SR Ca2+ upon stimulation. Such E-C uncoupling may underlie the long-duration low-frequency fatigue that can occur after various types of exercise, as well as possibly being a contributing factor to the muscle weakness in certain muscle diseases. The process or processes causing the disruption of the coupling between the voltage sensors and the release channels is not known with certainty, but might be associated with structural changes at the triad junction, possibly caused by activation of the Ca2+-dependent protease, micro-calpain.