Kinetics of GLUT4 Trafficking in Rat and Human Skeletal Muscle

Abstract

Objective: In skeletal muscle, insulin stimulates glucose transport activity 3-4-fold, 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 AMPK 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 6 healthy men (39 ± 11 years and a BMI of 25.8 ± 0.8).

Results: In rat epitrochlearis muscle, insulin exposure leads to a 6-fold stimulation of the GLUT4 exocytosis rate (with basal and insulin-stimulated rate constants of 0.010 min^-1 and 0.067 min^-1, respectively). In human vastus lateralis muscle, insulin stimulates GLUT4 translocation by a similar 6-fold increase in the exocytosis rate constant (with basal and insulin-stimulated rate constants of 0.011 min^-1 and 0.075 min^-1, respectively). In contrast, AICAR treatment does not markedly increase exocytosis in either rat or human muscle.

Conclusion: 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.