The Effects of Resistance Exercise on Muscle Fiber Type Composition and GLUT4 Expression in Zucker Diabetic Rat Skeletal Muscle

Abstract

Backgrounds: Skeletal muscle is a major organ that control glucose uptake and utilization, and its metabolic and contractile properties are dependent on its fiber type and mass. In this study, we investigate whether 8 weeks of resistance training could result in an alteration of muscle fiber size, fiber type composition, and glucose transporter protein 4 (GLUT4) which could ameliorate the impaired glucose tolerance in Zucker diabetic fatty (ZDF) rats before and after onset of diabetes. Methods: Five weeks old male Zucker rats were divided into Zucker Lean Control (ZLC-Con, n=7), non-exercised Zucker Diabetic Fatty (ZDF-Con, n=7), and exercised Zucker Diabetic Fatty (ZDF-Ex, n=7) rat groups. ZDF-Ex conducted 8 weeks of ladder climbing resistance training followed by non weight bearing adaptation period for one week. We conducted immunohistochemistry to investigate effect of exercise intervention on muscle fiber size, fiber type composition, and GLUT4 expression in gastrocnemius (GAS) muscle. Western blotting was also performed to detect the changes of GLUT4 expression level in the same muscle. Intraperitoneal glucose tolerance test (IPGTT) was also evaluated before and after diabetes onset. Results: Fasting glucose level and area under the curve (AUC) responses to an IPGTT were significantly increased after onset of diabetes in the ZDF-Con compare to ZLC-Con, while it was decreased in the ZDF-Ex. There was no difference between ZLC-Con and ZDF-Con in GLUT4 expression but ZDF-Ex was increased by 88.5%. The proportion of MHCI was lower, and MHCII was higher in the ZDF-Con than ZLC-Con while there was no alteration of muscle fiber type distribution in the ZDF-Ex. Conclusion: Resistance training improves muscle glucose tolerance in ZDF rats result from up-regulating GLUT4 expression without any changes in muscle fiber type composition. However, muscle hypertrophy and composition were not altered by exercise training. Thus, we concluded that resistance training could be an effective treatment for promoting muscle glucose tolerance and glucose uptake capacity independent of muscle morphology in ZDF rats.