Modulation of GLUT4 and GLUT1 recycling by insulin in rat adipocytes: kinetic analysis based on the involvement of multiple intracellular compartments.

The trafficking kinetics of GLUT4 and GLUT1 in rat epididymal adipocytes were analyzed by a four-compartment model based upon steady-state pool sizes of three intracellular fractions and one plasma membrane fraction separated and assessed under both basal and insulin-stimulated states. The steady-state compartment sizes provided relative values of the kinetic coefficients characterizing the rate of each process in the loop. Absolute values of these coefficients were obtained by matching the simulated half-times to those observed experimentally and reported in the literature for both basal and insulin-stimulated states. Our analysis revealed that insulin modulates the GLUT4 trafficking at multiple steps in the rat adipocyte, not only reducing the endocytotic rate constant 3-4-fold and increasing the exocytotic rate 8-24-fold but also increasing the two rate coefficients coupling the three intracellular compartments 2-6-fold each. Furthermore, GLUT1 was completely segregated from GLUT4 in two of the three intracellular compartments, and its steady-state distribution is consistent with a four-compartment model of GLUT1 recycling involving an insulin sensitive endocytosis step in common with the GLUT4 system, but with all other processes being insensitive to insulin.