GLUT4 vesicle trafficking in rat adipocytes after ethanol feeding: regulation by heterotrimeric G-proteins

Long-term ethanol consumption decreases insulin-stimulated glucose uptake i n isolated rat adipocytes. Here we investigate the mechanisms for this decr ease. Male Wistar rats were fed for 4 weeks with a liquid diet containing 3 5% of the calories from ethanol and compared with pair-fed controls. Stimul ation of 3-O-methylglucose transport in isolated adipocytes by insulin was decreased by 70% after ethanol feeding. However, stimulation by insulin of the tyrosine phosphorylation of the p85 subunit of phosphoinositide 3-kinas e and the phosphorylation of Akt were not affected by ethanol feeding. GLUT 4 was mobilized from intracellular light microsomes in response to insulin in both pair-fed and ethanol-fed rats, resulting in 4.3-fold and 3.3-fold i ncreases in GLUT4 associated with plasma membrane in pair-fed and ethanol-f ed rats respectively. Surface-accessible GLUT4, assessed by a trypsin cleav age assay or cell-surface labelling with bis-mannose photolabel, was increa sed 2.3-fold and 1.6-fold respectively, in pair-fed rats after treatment wi th insulin. In contrast, insulin did not increase surface-accessible GLUT4 in ethanol-fed rats. Treatment of adipocytes with R-phenylisopropyladenosin e, an adenosine A(1) receptor agonist, increased the transport of 3-O-methy lglucose and trypsin-accessible GLUT4, in adipocytes from both pair-fed and ethanol-fed rats. These results demonstrate that whereas the insulin-media ted signalling and translocation of GLUT4 to the plasma membrane is maintai ned after ethanol feeding, the final fusion of GLUT4 vesicles to the plasma membrane is disrupted, preventing the stimulation of glucose uptake by ins ulin. Fusion of GLUT4 with the plasma membrane can be stimulated by the act ivation of adenosine A(1) receptors.