Effect of moderate exercise immediately followed by induced hyperglycemia on gene expression and content of the glucose transporter-4 protein in skeletal muscles of horses

To determine the effect of a single bout of exercise and increased substrate availability after exercise on gene expression and content of the glucose transporter-4 (GLUT-4) protein in equine skeletal muscle. 6 healthy adult Thoroughbreds. The study was designed in a balanced, randomized, 3-way crossover fashion. During 2 trials, horses were exercised at 45% of their maximal rate of oxygen consumption for 60 minutes after which 1 group received water (10 mL/kg), and the other group received glucose (2 g/kg, 20% solution) by nasogastric intubation. During 1 trial, horses stood on the treadmill (sham exercise) and then received water (10 mL/kg) by nasogastric intubation. Muscle glycogen concentration and muscle GLUT-4 protein and mRNA content were determined before exercise and at 5 minutes and 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 before and after exercise. Glycogen replenishment was similar in both exercised groups and was not complete at 24 hours after exercise. 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. Under the conditions of this study, neither exercise nor the combination of exercise followed by hyperglycemia induced translation or transcription of the GLUT-4 protein in horses.