EFFECTS OF EXERCISE TRAINING ON MUSCLE GLUT-4 PROTEIN-CONTENT AND TRANSLOCATION IN OBESE ZUCKER RATS

The rates of muscle glucose uptake of trained (TR) and untrained (UT) obese Zucker rats were assessed by hindlimb perfusion under basal conditions (no insulin) in the presence of a maximally stimulating concentration of insulin (10 mU/ml) and after muscle contraction elicited by electrical stimulation of the sciatic nerve. Perfusate contained 28 mM glucose and 7.5 muCi/mmol of 2-deoxy-D-[H-3]glucose. Muscle GLUT-4 concentration was determined by Western blot analysis and expressed as a percentage of a heart standard. The rates of insulin-stimulated glucose uptake were significantly higher in the plantaris, red gastrocnemius (RG), and white gastrocnemius (WG), but not the soleus or extensor digatorum longus (EDL) of TR compared with UT rats. After muscle contraction the rates of glucose uptake in the TR rats were significantly higher in the soleus, plantaris, and RG. TR rats had significantly higher GLUT-4 protein concentration and citrate synthase activity than the UT rats in the soleus, plantaris, RG, and WG. Basal plasma membrane GLUT-4 protein concentration of TR rats was 144% above UT rats (P < 0.01). Stimulation by insulin and contraction resulted in a significant increase in plasma membrane GLUT-4 protein concentration in UT rats only. However, plasma membrane GLUT-4 protein concentration in insulin- and contraction-stimulated TR rats remained 53% and 30% greater than that of UT rats, respectively (P < 0.05). Exercise training did not alter basal, insulin-, or contraction-stimulated GLUT-4 functional activity. These results indicate that the increases in insulin- and contraction-stimulated muscle glucose uptake after aerobic exercise training were due in part to an increased plasma membrane GLUT-4 protein concentration and not to an increased GLUT-4 protein translocation or enhanced GLUT-4 functional activity.