Insulin-induced translocation of glucose transporters to the plasma membrane precedes full stimulation of hexose transport

Insulin stimulation of hexose transport in 3T3-L1 adipocytes was studied at 27 degrees C. At this temperature, the transport of 2-deoxyglucose was stimulated 8-fold, with a half-time of 9.5 min. Under the same conditions, the increase in cell surface glucose transporters, as measured by labeling in the intact cell with galactose oxidase and tritiated borohydride, was only 2.6-fold. Moreover, the half-times for the increase in cell surface glucose transporters and for the decrease in transporter number in the intracellular pool were both 4 min. Thus, these processes clearly precede the full stimulation of transport. These data are in agreement with immunolocalization studies of the glucose transporter in this cell line and further support the hypothesis that a second mechanism besides translocation is involved in the stimulation of hexose transport by insulin [Blok, J., Gibbs, E. M., Lienhard, G. E., Slot, J. W., & Gueze, H. J. (1988) J. Cell Biol. 106, 69-76]. The findings presented here indicate that neither the translocation of glucose transporters to, nor their subsequent insertion into, the plasma membrane is the rate-limiting step in the stimulation of hexose transport by insulin. Rather, there is a second mechanism of activation, which is rate limiting and occurs after the transporter is in the plasma membrane.