Intracellular mechanisms underlying increases in glucose uptake in response to insulin or exercise in skeletal muscle.

This review will provide insight on potential intracellular signalling mechanisms by which insulin and exercise/contraction increases glucose metabolism and gene expression. Glucose transport, the rate limiting step in glucose metabolism, is mediated by glucose transporter 4 (GLUT4) and can be activated in skeletal muscle by two separate and distinct signalling pathways; one stimulated by insulin and the second by muscle contractions. Impaired insulin action on whole body glucose uptake is a hallmark feature of type II (non-insulin-dependent) diabetes mellitus. Defects in insulin signal transduction through the insulin-receptor substrate-1/phosphatidylinositol 3-kinase pathway are associated with reduced insulin-stimulated glucose transporter 4 translocation and glucose transport activity in skeletal muscle from type II diabetic patients. Studies performed using glucose transporter 4-null mice show that this glucose transporter isoform plays a major role in mediating exercise-stimulated glucose uptake in skeletal muscle. Level of physical exercise has been linked to improved glucose homeostasis and enhanced insulin sensitivity. Exercise training leads to alterations in expression and activity of key proteins involved in insulin signal transduction. These changes may be related to increased signal transduction through the mitogen-activated protein kinase (MAPK) signalling cascades. Because MAPK is associated with increased transcriptional activity, these signalling cascades are candidates for these exercise-induced changes in protein expression. Understanding the molecular mechanism for the activation of signal transduction pathways will provide a link for defining new strategies to enhance glucose metabolism and improve health in the general population.