Effects of exercise training on glucose transport and cell surface GLUT-4 in isolated rat epitrochlearis muscle

The effects of exercise training on maximal glucose transport activity and cell surface GLUT-4 were examined in rat epitrochlearis muscle. Five days of swim training (2 x 3 h/day) produce a significant increase in citrate synthase activity (24.5 +/- 0.6 vs. 20.1 +/- 0.7 mu mol . min(-1) . g(-1)), GLUT-4 content (22.9 +/- 0.8 vs. 17.4 +/- 0.4% GLUT-4 standard), and glycogen levels (54.3 +/- 9.4 vs. 28.6 +/- 9.4 mu mol/g). Maximally, insulin-stimulated glucose transport activity and cell surface GLUT-4 are increased by 55 (1.50 +/- 0.11 vs. 0.97 +/- 0.10 mu mol . ml(-1) . 20 min(-1)) and 48% [12.0 +/- 0.8 vs. 8.1 +/- 0.9 disintegrations . min(-1) (dpm) . mg(-1)], respectively, in exercise-trained epitrochlearis muscles. In contrast, hypoxia-stimulated glucose transport activity and cell surface GLUT-4 are reduced by 38 (0.78 +/- 0.08 vs. 1.25 +/- 0.14 mu mol . ml(-1) . 20 min(-1)) and 40% (5.7 +/- 0.9 vs. 9.4 +/- 1.2 dpm/mg), respectively, in exercise-trained epitrochlearis muscles. These results demonstrate that changes in insulin- and hypoxia-stimulated glucose transport activity after exercise training are fully accounted for by the appearance of cell surface GLUT-4 and support the concept of two intracellular pools of GLUT-4. Finally, we propose that high levels of muscle glycogen with exercise training may contribute to the decrease in hypoxia-stimulated glucose transport activity.