EFFECTS OF NORADRENALINE ON THE CELL-SURFACE GLUCOSE TRANSPORTERS IN CULTURED BROWN ADIPOCYTES - NOVEL MECHANISM FOR SELECTIVE ACTIVATION OF GLUT1 GLUCOSE TRANSPORTERS

Glucose transport into rat brown adipocytes has been shown to be stimu lated directly by the sympathetic neurotransmitter, noradrenaline, wit hout a significant increase in the protein content of either GLUT1 or GLUT4 glucose transporter in the plasma membrane [Shimizu, Kielar, Min okoshi and Shimazu (1996) Biochem. J. 314, 485-490]. In the present st udy, we labelled the exofacial glucose-binding sites of GLUT1 and GLUT 4 with a membrane-impermeant photoaffinity reagent, 2-N-[4-(1 l]-[2-H- 3]1,3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-[H-3]BMPA), to determi ne which isoform is responsible for the noradrenaline-induced increase in glucose transport into intact brown adipocytes in culture. Insulin stimulated the rate of hexose transport by increasing ATB-[H-3]BMPA-l abelled cell-surface GLUT4. In contrast, the noradrenaline-induced inc rease in glucose transport was not accompanied by an increased ATB-[H- 3]BMPA labelling of GLUT4, nor with an increased amount of GLUT4 in th e plasma membrane fraction as assessed by Western blotting, indicating that noradrenaline does not promote the translocation of GLUT4. Howev er, noradrenaline induced an increase in photoaffinity labelling of ce ll-surface GLUT1 without an apparent increase in the immunoreactive GL UT1 protein in the plasma membrane. This is suggestive of an increased affinity of GLUT1 for the ligand. In fact, the K-i value of non-radio active ATB-BMPA for 2-deoxy-D-glucose uptake was significantly decreas ed after treatment of the cells with noradrenaline. The increased phot oaffinity labelling of GLUT1 and increased glucose transport caused by noradrenaline were inhibited by a cAMP antagonist, cAMP-S Rp-isomer. These results demonstrate that noradrenaline stimulates glucose transp ort in brown adipocytes by enhancing the functional activity of GLUT1 through a cAMP-dependent mechanism.