EQUINE EXERCISE PHYSIOLOGY 6 Equine vet. J., Suppl. 34 (2002) 199-204 199 Summary This study examined changes in skeletal muscle GLUT4 content glucose transport in isolated muscle membranes (GT) from horses before and 2 min after standardised submaximal exercise tests (SET) prior to and after completion of 6 weeks of training. Seven horses, age 3–9 years, body mass mean ± s.e. 530 ± 19 kg, and sedentary for at least 4 months, completed 6 weeks of training on a treadmill. An initial SET (UT) was performed on a 4° incline at a speed equivalent to 55% of pretraining VO 2max and was repeated post-training at the same absolute workload (ABS). A third SET (REL) was performed at 55% of post-training VO 2max . There was no significant pre- to postexercise change in GLUT4 content before or after training. Following training, total GLUT4 content was increased 2- or 3-fold in pre-exercise biopsies (pre UT: 0.30 ± 0.05; pre ABS: 1.05 ± 0.32; pre REL: 1.34 ± 0.28 arbitrary units) (P<0.05) with similar increases in postexercise GLUT4 content (P<0.05) (post UT: 0.33 ± 0.06; post ABS: 1.19 ± 0.44; post REL: 1.43 ± 0.31). GT increased 2.5- to 6-fold in postexercise muscle membrane vesicles in UT over a range of glucose concentrations. After training (ABS and REL), there was a 25–50% attenuation (P<0.05) in membrane GT in response to exercise in ABS and REL. These finding indicate that moderate intensity exercise training increased middle gluteal muscle GLUT4 content, but this change was not reflected in an increase in muscle membrane glucose transport activity in postexercise muscle samples. Introduction During exercise, skeletal muscle glucose utilisation reflects the combined effect of glucose supply, glucose transport capacity and glucose metabolism. Regardless of training state, maintenance of glucose supply to working skeletal muscle (from muscle glycogen and plasma glucose) is critical to sustain work output during moderately intense exercise (>60–65% VO 2max , in human subjects) and, when hepatic glucose output fails to match the demands of working muscle, the decline in plasma glucose supply contributes to the onset of fatigue. Exercise training influences the pattern of substrate utilisation during exercise with one of the main adaptations being a decrease in glucose utilisation. In human athletes, this decrease has been attributed to a reduction in glucose transport activity (Kristiansen et al. 1998). A crucial and often rate-limiting step in glucose metabolism in skeletal muscle is the ability to transport glucose across the plasma membrane into the cell (DeFronzo et al. 1981; Richter et al. 2001). Such transport is facilitated by a family of glucose transporter proteins, of which GLUT4 and GLUT1 have been identified as the isoforms of importance in skeletal muscle. GLUT1 is responsible for basal glucose transport independent of insulin and is primarily associated with the plasma membrane. By comparison, the GLUT4 isoform is insulin-responsive and is translocated from intracellular endosomes either by a complex signalling pathway triggered by insulin or by an undefined pathway triggered by exercise (Marette et al. 1992). In rats and man, GLUT4 content is increased with physical conditioning and there is a concomitant increase in insulin-stimulated glucose transport activity (Hirshman et al. 1993) suggesting a mechanism for improved glucose clearance in the trained state. However, evidence of a post-training reduction in glucose utilisation during submaximal exercise indicates that more information is needed to understand the link between the content of GLUT4 in the muscle and its intrinsic activity. In horses, there are few data pertaining to the effects of exercise training on regulation of glucose transport into muscle during submaximal exercise. It is not known whether training results in a reduction in the utilisation of, or increased reliance on, plasma glucose for maintenance of carbohydrate supply to working muscle. An increase in glucose transporter protein content and/or glucose uptake could improve the availability of glucose to the muscle cell. Therefore, the objective of this study was to determine the effects of moderate intensity exercise training on mechanisms that affect glucose transport in skeletal muscle during exercise. Specifically, our objective was to determine the effects of a 6 week period of exercise training in horses on skeletal muscle GLUT4 content and muscle membrane glucose transport capacity in samples of middle gluteal muscle obtained before and after exercise in the untrained and trained state. Our hypothesis was that a 6 week period of exercise training would result in increases in skeletal muscle GLUT4 protein content and muscle membrane glucose transport capacity. As training results in an increase in maximum aerobic capacity, the pattern of fuel utilisation and, as a result, the expectations for glucose uptake could be altered in the trained state. Therefore, in this study, post-training testing was performed at the same relative and same absolute workload. Changes in skeletal muscle GLUT4 content and muscle membrane glucose transport following 6 weeks of exercise training L. J. MCCUTCHEON*, R. J. GEOR † and K. W. HINCHCLIFF † Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario N1G 2W1, Canada and † Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, USA. Keywords: horse; glucoregulation; insulin; glycogen *Author to whom correspondence should be addressed.