Effect of carbohydrate supplementation on postexercise GLUT-4 protein expression in skeletal muscle

The effect of carbohydrate supplementation on skeletal muscle glucose trans porter GLUT-4 protein expression was studied in fast-twitch red and white g astrocnemius muscle of Sprague-Dawley rats before and after glycogen deplet ion by swimming. Exercise significantly reduced fast-twitch red muscle glyc ogen by 50%. During a 16-h exercise recovery period, muscle glycogen return ed to control levels (25.0 +/- 1.4 mu mol/g) in exercise-fasted rats (24.2 +/- 0.3 mu mol/g). However,when carbohydrate supplementation was provided d uring and immediately post-exercise by intubation, muscle glycogen increase d 77% above control (44.4 +/- 2.1 mu mol/g). Exercise-fasting resulted in a n 80% increase in fast-twitch red muscle GLUT-4 mRNA but only a 43% increas e in GLUT-4 protein. concentration. Conversely, exercise plus carbohydrate supplementation elevated fast-twitch red muscle GLUT-4 protein concentratio n by 88% above control, whereas GLUT-4 mRNA was increased by only 40%. Neit her a 16-h fast nor carbohydrate supplementation had an effect on fast-twit ch red muscle GLUT-4 protein concentration or on GLUT-4 mRNA in sedentary r ats, although carbohydrate supplementation increased muscle glycogen concen tration by 40% (35.0 +/- 0.9 mu mol/g). GLUT-4 protein in fast-twitch white muscle followed a pattern similar to fast-twitch red muscle. These results indicate that carbohydrate supplementation, provided with exercise, will e nhance GLUT-4 protein expression by increasing translational efficiency. Co nversely, postexercise fasting appears to upregulate GLUT-4 mRNA, possibly to amplify GLUT-4 protein expression on an increase in glucose availability . These regulatory mechanisms may help control muscle glucose uptake in acc ordance with glucose availability and protect against postexercise hypoglyc emia.