RAPID REVERSAL OF ADAPTIVE INCREASES IN MUSCLE GLUT-4 AND GLUCOSE-TRANSPORT CAPACITY AFTER TRAINING CESSATION

Previous studies have shown that when exercise is stopped there is a r apid reversal of the training-induced adaptive increase in muscle gluc ose transport capacity. Endurance exercise training brings about an in crease in GLUT-4 in skeletal muscle. The primary purpose of this study was to determine whether the rapid reversal of the increase in maxima lly insulin-stimulated glucose transport after cessation of training c an be explained by a similarly rapid decrease in GLUT-4. A second purp ose was to evaluate the possibility, suggested by previous studies, th at the magnitude of the adaptive increase in muscle GLUT-4 decreases w hen exercise training is extended beyond a few days. We found that bot h GLUT-4 and maximally insulin-stimulated glucose transport were incre ased approximately twofold in epitrochlearis muscles of rats trained b y swimming for 6 h/day for 5 days or 5 wk. GLUT-4 was 90% higher, citr ate synthase activity was 23% higher, and hexokinase activity was 28% higher in triceps muscle of the 5-day trained animals compared with th e controls. The increases in GLUT-4 protein and in insulin-stimulated glucose transport were completely reversed within 40 h after the last exercise bout, after both 6 days and 5 wk of training. In contrast, th e increases in citrate synthase and hexokinase activities were unchang ed 40 h after 5 days of exercise. These results support the conclusion that the rapid reversal of the increase in the insulin responsiveness of muscle glucose transport after cessation of training is explained by the short half-life of the GLUT-4 protein.