The GLUT4 density in slow fibres is not increased in athletes. How does training increase the GLUT4 pool originating from slow fibres?

The influence of training on GLUT4 expression in slow- and fast-twitch skel etal muscle fibres was studied in male endurance-trained athletes and contr ol subjects. The trained state was ensured by elevated maximal oxygen uptak e (29%), as well as citrate synthase (60%) and 3-hydroxy-acyl-CoA dehydroge nase (38%) activities in muscle biopsy samples of the vastus lateralis. GLU T4 densities in slow- and fast-twitch fibres were measured by the use of a newly developed, sensitive method combining immunohistochemistry with morph ometry, and no effect of training was found. GLUT4 density was higher in sl ow-twitch fibres compared to fast-twitch fibres (P <0.05) when biopsy sampl es from untrained subjects were examined. In athletes GLUT4 density was ide ntical 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 14 0% 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 expressio n by means of an elevated slow-twitch fibre mass in human skeletal muscle.