Effect of aging and exercise on GLUT-4 glucose transporters in muscle.

This study was conducted to investigate whether changes in muscle glucose transporter GLUT-4 protein might be associated with a previously reported deterioration in glucose tolerance with aging, and, furthermore, to determine whether exercise training could increase GLUT-4 protein levels in older animals. GLUT-4 protein concentration was measured in soleus, gastrocnemius, and extensor digitorum longus muscles of trained (10 or 15 wk treadmill running) and untrained young (6-8 mo), middle-aged (15-17 mo), and old (27-29 mo) Fischer 344 rats. All GLUT-4 protein values were expressed as a percent of the mean for the young untrained group. Two-way analysis of variance demonstrated significant main effects of both training and aging in the gastrocnemius and soleus muscles. Exercise training produced significant increases in GLUT-4 protein in the soleus muscle of young (273 +/- 32.9 vs. 100 +/- 38.5%) and middle-aged rats (215 +/- 19.9 vs. 108 +/- 33.2%) compared with sedentary controls. Similar significant increases were also found in the gastrocnemius muscle of young (169 +/- 20.1 vs. 100 +/- 5.8%) and middle-aged rats (167 +/- 46.7 vs. 60 +/- 7.9%) with training. In the oldest rats, GLUT-4 was not significantly increased with training, but the trend toward an increase was apparent in all three muscle types. The main effect of aging was primarily due to a statistically significant difference between the old trained and young trained rats. A trend toward decreased GLUT-4 with aging was apparent in the untrained animals, but this was not statistically significant.