title: Muscle glycogen and cell function--Location, location, location.
abstract: The importance of glycogen, as a fuel during exercise, is a fundamental concept in exercise physiology. The use of electron microscopy has revealed that glycogen is not evenly distributed in skeletal muscle fibers, but rather localized in distinct pools. In this review, we present the available evidence regarding the subcellular localization of glycogen in skeletal muscle and discuss this from the perspective of skeletal muscle fiber function. The distribution of glycogen in the defined pools within the skeletal muscle varies depending on exercise intensity, fiber phenotype, training status, and immobilization. Furthermore, these defined pools may serve specific functions in the cell. Specifically, reduced levels of these pools of glycogen are associated with reduced SR Ca(2+) release, muscle relaxation rate, and membrane excitability. Collectively, the available literature strongly demonstrates that the subcellular localization of glycogen has to be considered to fully understand the role of glycogen metabolism and signaling in skeletal muscle function. Here, we propose that the effect of low muscle glycogen on excitation-contraction coupling may serve as a built-in mechanism, which links the energetic state of the muscle fiber to energy utilization.
authors.affiliation: Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster, University of Southern Denmark, Odense, Denmark.|Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Sweden.
authors.forename: N
authors.initials: N
authors.lastname: Ørtenblad
authors.affiliation: Department of Sports Science and Clinical Biomechanics, SDU Muscle Research Cluster, University of Southern Denmark, Odense, Denmark.
authors.forename: J
authors.initials: J
authors.lastname: Nielsen
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D000066670:Cell Plasticity; D056966:Excitation Contraction Coupling; D015444:Exercise / Q000502:physiology*; D006003:Glycogen / Q000378:metabolism*; D006801:Humans; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000648:ultrastructure; D009210:Myofibrils / Q000378:metabolism / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000378:metabolism
chemical_list: D006003:Glycogen; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D002118:Calcium
keywords: E-C coupling; Glycogen granule; energy compartmentalization; exercise