Abstract
Increases in insulin‐mediated glucose uptake following endurance training (ET) and sprint interval training (SIT) have in part been attributed to concomitant increases in glucose transporter 4 (GLUT4) protein content in skeletal muscle. This study used an immunofluorescence microscopy method to investigate changes in subcellular GLUT4 distribution and content following ET and SIT. Percutaneous muscle biopsy samples were taken from the m. vastus lateralis of 16 sedentary males in the overnight fasted state before and after 6 weeks of ET and SIT. An antibody was fully validated and used to show large (> 1 μm) and smaller (<1 μm) GLUT4‐containing clusters. The large clusters likely represent trans‐Golgi network stores and the smaller clusters endosomal stores and GLUT4 storage vesicles (GSVs). Density of GLUT4 clusters was higher at the fibre periphery especially in perinuclear regions. A less dense punctate distribution was seen in the rest of the muscle fibre. Total GLUT4 fluorescence intensity increased in type I and type II fibres following both ET and SIT. Large GLUT4 clusters increased in number and size in both type I and type II fibres, while the smaller clusters increased in size. The greatest increases in GLUT4 fluorescence intensity occurred within the 1 μm layer immediately adjacent to the PM. The increase in peripheral localisation and protein content of GLUT4 following ET and SIT is likely to contribute to the improvements in glucose homeostasis observed after both training modes.
Highlights
Skeletal muscle is the primary site of insulin-mediated glucose uptake (Katz et al 1983; Wasserman et al 2011) and is important in determining whole body glucose disposal rates
Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society
The results show that comparable increases in total skeletal muscle Glucose transporter 4 (GLUT4) content occur following the two training modes through increases in the number and size of large GLUT4 clusters (>1 lm; including trans-Golgi network (TGN) stores) and increases in size of the smaller GLUT4 clusters (
Summary
Skeletal muscle is the primary site of insulin-mediated glucose uptake (Katz et al 1983; Wasserman et al 2011) and is important in determining whole body glucose disposal rates. Glucose transporter 4 (GLUT4) is the major glucose transporter isoform expressed in human skeletal muscle and is the primary insulin-responsive glucose transporter (Mueckler 1994). GLUT4 is distributed between intracellular storage clusters associated with intracellular membranes, the plasma membrane (PM) and T-tubule membranes. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.
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