Relationship between muscle fibre composition, glucose transporter protein4 and exercise training: possible consequences in non-insulin-dependent diabetes mellitus

Skeletal muscle is composed of different fibre types, which differ in contr actile as well as in metabolic properties. The myosin molecule, which exist s in several different isoforms, is of major importance in determining the contractile properties of the muscle cell. The plasticity of skeletal muscl e is reflected in this tissue's adaptability to changes in the functional d emand. In both rats and humans, a decrease in activity level will in most c ases change the muscle fibre composition towards faster myosin isoforms and an increase in activity level (such as seen with exercise training) will i nduce an increase in slower myosin isoforms. The glucose transporter protei n 4 (GLUT4), which is the major insulin regulatable glucose transporter in mammalian skeletal muscle, is found in larger amounts in slow muscle fibres compared with fast muscle fibres. An increase in activity level will incre ase the GLUT4 protein expression and a decrease in activity level will in m ost cases decrease GLUT4. Thus, there seems to be some kind of relationship between the muscle fibre type and GLUT4. However, the main factor regulati ng both the GLUT4 protein expression and the muscle fibre composition seems to be the activity level of the muscle fibre. Patients suffering from non- insulin-dependent diabetes mellitus (NIDDM) are insulin resistant in their skeletal muscles but are generally normal when it comes to skeletal muscle fibre composition and the GLUT4 protein expression. There is good evidence that exercise training beneficially impacts on insulin sensitivity in healt hy individuals and in patients with type II diabetes. An increase in the GL UT4 protein expression in skeletal muscle may at least partly explain this effect of training.