Regulation of GLUT4 protein expression and glycogen storage after prolonged exercise

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Abstract
The purpose of this study was to determine the time course of GLUT4 protein accumulation following an exercise-carbohydrate supplementation regimen, and to evaluate the effect of this regimen on GLUT4 mRNA regulation. Rats were exercised by swimming and intubated with 1 mL of a 50% glucose solution immediately post-exercise. Exercise significantly reduced muscle glycogen by 50%. By 1.5 h of recovery, muscle glycogen was normalized, but continued to increase above the control level during the next 16 h. A faster and larger repletion of glycogen occurred in the fast-twitch red compared with the fast-twitch white muscle during the 16 h of recovery. GLUT4 protein concentration in fast-twitch red muscle was significantly increased above control by 1.5 h of recovery, and progressively increased throughout the recovery period. Fast-twitch white muscle demonstrated a similar trend, but the increase in GLUT4 protein did not reach significance until 5 h of recovery. Fast-twitch red muscle GLUT4 mRNA was increased by 53% above control immediately post-exercise, but returned to the control level by 1.5 h of recovery. GLUT4 mRNA associated with polysomes, however, increased significantly during this time and remained elevated for a minimum of 5 h. The results suggest that the increased GLUT4 protein expression following a regimen of exercise-carbohydrate supplementation occurs sufficiently fast to contribute to the resynthesis of muscle glycogen, and is controlled by both pre-translational and translational mechanisms.