Insulin resistance of obese Zucker rats exercise trained at two different intensities

The effect of exercise intensity on oral glucose tolerance and hindlimb glucose uptake and transport was studied in 26 female obese Zucker rats after a treadmill training program. The rats were randomly assigned to either a low-intensity (LI) or high-intensity (HI) exercise group, with equal work being performed by the two groups. A third group of rats served as sedentary controls (SED). The trained rats demonstrated a significant improvement in oral glucose tolerance while maintaining significantly lower plasma insulin concentrations when compared with the SED rats. However, no significant differences in plasma glucose or insulin concentrations were observed between the LI and HI exercise-trained groups. During hindlimb perfusion (500 μU/ml insulin, 8 mM glucose), the rate of muscle glucose uptake for the HI rats (13.5 ± 0.8 μmol·h^-1·g^-1) was significantly faster than that of the LI rats (11.4 ± 0.8 μmol·h^-1·g^-1), which was significantly faster than that of the SED rats (8.3 ± 0.6 μmol·h^-1·g^-1). The rates of 3-O-methyl-D-glucose (3-MG) transport were substantially greater in the fast-twitch red fibers of the HI (10.11 ± 0.49 μmol·h^-1·g^-1) and LI (9.08 ± 0.46 μmol·h^-1·g^-1) rats when compared with those of the SED rats (6.15 ± 0.41 μmol·h^-1·g^-1). However, only the HI training resulted in a significant increase in the 3-MG transport of the fast-twitch white fibers (HI, 2.37 ± 0.27; LI, 1.48 ± 0.11; SED, 1.31 ± 0.15 μmol·h^-1·g^-1). Only muscles with an increased citrate synthase activity demonstrated an improved insulin-stimulated glucose transport. These results suggest that 1) the improvement in muscle insulin resistance due to exercise training is exercise intensity specific and restricted to muscle recruited during training, and 2) the exercise intensity-related improvement in muscle insulin resistance is not necessarily reflected by an oral glucose tolerance test.