CYCLIC-AMP ACUTELY STIMULATES TRANSLOCATION OF THE MAJOR INSULIN-REGULATABLE GLUCOSE TRANSPORTER GLUT4

Facilitated glucose transport across plasma membranes is mediated by a family of transporters (GLUT1-GLUT5) that have different tissue distributions and K(m) values for transport. It has been shown that insulin stimulates glucose transport in fat and muscle tissues by causing the redistribution of one of these proteins (GLUT4) from inside the cell to the plasma membrane. Previous studies have shown that agents that change cAMP levels are able to modulate glucose transport in fat cells. The aim of this study was to investigate the mechanisms responsible for modulation of glucose transport by cAMP. 2-Deoxyglucose transport and insulin-regulatable glucose transporter (GLUT4) immunoreactivity in plasma and low density microsomal membranes were measured in adipocytes incubated for 30 min with insulin or dibutyryl-cAMP (Bt2cAMP). Low concentrations of Bt2cAMP (10-mu-M) increased 2-deoxyglucose uptake by translocating GLUT4 from low density microsomal membranes to the plasma membranes. Bt2cAMP at 1000-mu-M inhibited glucose transport below basal but further increased translocation of GLUT4. The effect of Bt2cAMP on translocation was additive to that of 7 nM insulin. We conclude that in rat adipocytes, Bt2cAMP acutely translocates GLUT4 but inhibits its activity to transport glucose.