The Effect of Exercise Training on Skeletal Muscle Glucose Transorter Isoform GLUT4 Concentration in the Obese Zucker Rat

Exercise training has been demonstrated to improve skeletal muscle insulin resistance, however, the mechanisms for this improvement have not been fully chararcterized. The effects of high (HT) and low (LT) intensity exercise training were therefore examined on insulin-stimulated 3-O-methyl-D-glucose (3- OMG) transport and the concentration of glucose transporter isoform GLUT4 in the red (fast-twitch oxidative) and white (fast-twitch glycolytic) guadriceps of the obese Zucker rat. Sedentary obese (SED) and lean (LN) rats served as controls. 3-OMG transport was determined during hindlimb HT and LT rats displayed greater red quadriceps 3-OMG transport rates and GLUT4 concentrations than SED rats. Citrate synthase activity, which is an oxidative enzyme marker for skeletal muscle exercise training, highly correlated with GLUT4 concentration. The 3-OMG transport rate of LN rats was two-fold greater than SED rats despite similar GLUT4 concentration. These results suggest muscle insulin resistance in the obese Zucker rat is not due to a reduced GLUT4 concentration. However, the improved insulin resistance seen with exercise training appears to be due in part to an increased GLUT4 concentration which is only observed in muscle fibers recruited during training.