Rate-limiting steps of 2-deoxyglucose uptake in rat adipocytes

2-Deoxy[1-14C]glucose uptake in rat adipocytes was measured as a function of time in the absence and presence of unlabelled glucose or 2-deoxyglucose. Uptake of tracer alone was linear from 2s to 6 min. At 37 degrees C the rate of uptake in insulin-stimulated cells decreased markedly after a few seconds in the preence of glucose (0.5-10 mM) and after 0.5-2 min in the presence of deoxyglucose (2-10 mM). Similar data were obtained at 22 degrees C. With 10 mM glucose (37 degrees C, 30 s) approx. 80% of the intracellular radioactivity was non-phosphorylated deoxyglucose and with 10 mM deoxyglucose approx. 40% was non-phosphorylated. The results show that deoxy[14C]glucose uptake after a few minutes is mainly limited by hexokinase in the presence of glucose and at least partially in the presence of deoxyglucose. The data suggest caution in using deoxyglucose uptake as a measure of transport, especially in complex kinetic studies. In addition, the initial velocity of tracer * 3-O-methylglucose was found to be approx. 2-fold higher than that of tracer deoxyglucose even though both sugars inhibited the initial velocity of labelled methylglucose half-maximally at a concentration of 5 mM. These data suggest a fundamental difference between deoxyglucose and methylglucose transport.