GLUT-4 AND INSULIN-RECEPTOR BINDING AND KINASE-ACTIVITY IN TRAINED HUMAN MUSCLE

1. Physical training enhances sensitivity and responsiveness of insulin-mediated glucose uptake in human muscle. This study examines it this effect of physical training is due to increased insulin receptor function or increased total concentration of insulin-recruitable glucose transporter protein (GLUT 4). 2. Seven healthy young subjects carried out single leg bicycle training for 10 weeks at 70% of one leg maximal oxygen uptake (V(O2,max)). Subsequently biopsies were taken from the vastus lateralis muscle of both legs. 3. Single leg V(O2,max) increased for the trained leg (46+/-3 to 52+/-2 ml min-1 kg-1 (means+/-S.E. M., P < 0.05), and cytochrome c oxidase activity was higher in this compared to the untrained leg (2.0+/-0.1 vs. 1.4+/-0.1 nmol s-1 (mg muscle)-1, P < 0.05). Insulin binding as well as basal- and insulin-stimulated receptor kinase activity did not differ between trained and untrained muscle. The concentration of GLUT 4 protein was higher in the former (14.9+/-1.9 vs. 11.6+/-1.0 arbitrary units (mug protein)-1 in crude membranes, P < 0.05). The training-induced increase in GLUT 4 (26+/-11%) matched a previously reported increase in maximum insulin-stimulated leg glucose uptake (25+/-7%) in the same subjects, and individual values of the two variables correlated (correlation coefficient (r) = 0.84, P < 0.05). 4. In conclusion, in human muscle training induces a local contraction- dependent increase in GLUT 4 protein, which enhances the effect of insulin on glucose uptake. On the other hand, insulin receptor function in muscle is unlikely to be affected by training.