Glucose utilization in muscle is a function of glucose availability to the muscle, glucose transport capacity of the surface membrane, and glucose metabolism. Transport of glucose through the sarcolemma appears to be the primary rate-limiting step for glucose metabolism in striated muscle. This study examined the effect of a single bout of exercise and of increased substrate availability after exercise on equine skeletal muscle GLUT-4 gene expression and protein content.
Six horses were used in a balanced, randomized, three-way cross over design. During two trials horses were exercised at 45% Vo2max for 60 minutes after which one group received water (10 ml/kg) and the other group received glucose (2 g/kg, as 20% solution) by nasogastric intubation. During a third trial horses stood on the treadmill (“sham” exercise) and then received water by nasogastric intubation. Muscle glycogen concentration and muscle GLUT-4 protein and mRNA content were determined before and 5 min, 4, 8 and 24 hours after exercise. Although exercise resulted in a 30% reduction in muscle glycogen concentration, no significant difference was detected in muscle GLUT-4 protein or mRNA content when pre-exercise and post-exercise samples were compared. Glycogen replenishment was similar in both exercised groups, and was not complete at 24 hours after exercise. The horses that received glucose had significantly higher plasma glucose and insulin concentrations for 3 hours after exercise, but no effect of hyperglycemia was detected on muscle GLUT-4 protein or mRNA content, or on muscle glycogen replenishment.
We conclude that, under the conditions of this study, neither exercise nor the combination of exercise and hyperglycemia induced translation or transcription of GLUT-4 in horses.