Content area
Full Text
OBJECTIVE-In skeletal muscle, insulin stimulates glucose transport activity three- to fourfold, and a large part of this stimulation is associated with a net translocation of GLUT4 from an intracellular compartment to the cell surface. We examined the extent to which insulin or the AMP-activated protein kinase activator AICAR can lead to a stimulation of the exocytosis limb of the GLUT4 translocation pathway and thereby account for the net increase in glucose transport activity.
RESEARCH DESIGN AND METHODS-Using a biotinylated photoaffinity label, we tagged endogenous GLUT4 and studied the kinetics of exocytosis of the tagged protein in rat and human skeletal muscle in response to insulin or AICAR. Isolated epitrochlearis muscles were obtained from male Wistar rats. Vastus lateralis skeletal muscle strips were prepared from open muscle biopsies obtained from six healthy men (age 39 ± 11 years and BMI 25.8 ± 0.8 kg/m^sup 2^).
RESULTS-In rat epitrochlearis muscle, insulin exposure leads to a sixfold stimulation of the GLUT4 exocytosis rate (with basal and insulin-stimulated rate constants of 0.010 and 0.067 min^sup -1^, respectively). In human vastus lateralis muscle, insulin stimulates GLUT4 translocation by a similar sixfold increase in the exocytosis rate constant, (with basal and insulin-stimulated rate constants of 0.011 and 0.075 min^sup -1^, respectively). In contrast, AICAR treatment does not markedly increase exocytosis in either rat or human muscle.
CONCLUSIONS-Insulin stimulation of the GLUT4 exocytosis rate constant is sufficient to account for most of the observed increase in glucose transport activity in rat and human muscle. Diabetes 58:847-854, 2009
Impairments in skeletal muscle glucose uptake occur in type 2 diabetes (1-4) and are associated with defects in GLUT4 translocation rather than a change in the total amount of GLUT4 protein (2,5). These changes are also associated with defects in cell signaling (6,7). However, in healthy normal glucose-tolerant rela- tives of type 2 diabetic patients, impairments in skeletal muscle insulin-stimulated glucose transport activity occur without alterations in the phosphorylation of Akt or its downstream target protein AS 160 (8). In rodents, marked impairments in insulin signaling to glycogen metabolism occur as a consequence of skeletal muscle GLUT4 knockout (9), suggesting that defects in GLUT4 traffic have the potential to feed back and inhibit early steps in insulin signaling. Changes in GLUT4 translocation may be...