title: The GLUT4 density in slow fibres is not increased in athletes. How does training increase the GLUT4 pool originating from slow fibres?
abstract: The influence of training on GLUT4 expression in slow- and fast-twitch skeletal muscle fibres was studied in male endurance-trained athletes and control subjects. The trained state was ensured by elevated maximal oxygen uptake (29%), as well as citrate synthase (60%) and 3-hydroxy-acyl-CoA dehydrogenase (38%) activities in muscle biopsy samples of the vastus lateralis. GLUT4 densities in slow- and fast-twitch fibres were measured by the use of a newly developed, sensitive method combining immunohistochemistry with morphometry, and no effect of training was found. GLUT4 density was higher in slow-twitch fibres compared to fast-twitch fibres (P<0.05) when biopsy samples from untrained subjects were examined. In athletes GLUT4 density was identical in slow- and fast-twitch fibres. Slow-twitch fibre diameters were 10% larger in the athletes (P<0.01), and slow-twitch fibre fractions were 140% of the fraction in the control group. Thus, GLUT4 originating from slow-twitch fibres was increased by 30% (P<0.02) in athletes. We conclude that long-lasting endurance training increases slow-twitch fibre GLUT4 expression by means of an elevated slow-twitch fibre mass in human skeletal muscle.
authors.affiliation: Dept. of Pathology, Odense University Hospital, 5000 Odense, Denmark. Michael.Gaster@ouh.Fyns-amt.dk
authors.forename: M
authors.initials: M
authors.lastname: Gaster
authors.forename: J
authors.initials: J
authors.lastname: Franch
authors.forename: H
authors.initials: H
authors.lastname: Beck-Nielsen
authors.forename: H D
authors.initials: HD
authors.lastname: Schrøder
mesh_terms: D000328:Adult; D051275:Glucose Transporter Type 4; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism / Q000648:ultrastructure; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism* / Q000648:ultrastructure; D009124:Muscle Proteins; D010806:Physical Education and Training; D013177:Sports
chemical_list: D051275:Glucose Transporter Type 4; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; C495692:SLC2A4 protein, human