The effects of fasting and refeeding on the glucose transport response to insulin in isolated rat adipose cells have been examined using 3-O-methylglucose transport in intact cells and cytochalasin B binding and Western blotting in subcellular membrane fractions. After a 72-h fast, basal glucose transport activity decreases slightly and insulin-stimulated activity decreases >85%. Following 48 h of fasting, insulin-stimulated glucose transport activity is diminished from 3.9 ± 0.5 to 1.3 ± 0.3 fmol/cell per min (mean ± SEM). Similarly, the concentrations of glucose transporters are reduced with fasting in both the plasma membranes from insulin-stimulated cells from 38 ± 5 to 18 ± 3 pmol/mg of membrane protein and the low density microsomes from basal cells from 68 ± 8 to 34 ± 9 pmol/mg of membrane protein. Ad lib. refeeding for 6 d after a 48-h fast results in up to twofold greater maximally insulin-stimulated glucose transport activity compared with the control level (7.1 ± 0.4 vs. 4.5 ± 0.2 fmol/cell per min), before returning to baseline at 10 d. However, the corresponding concentration of glucose transporters in the plasma membranes is restored only to the control level (45 ± 5 vs. 50 ± 5 pmol/mg of membrane protein). Although the concentration of glucose transporters in the low density microsomes of basal cells remains decreased, the total number is restored to the control level due to an increase in low density microsomal protein. Thus, the insulin-resistant glucose transport in adipose cells from fasted rats can be explained by a decreased translocation of glucose transporters to the plasma membrane due to a depleted intracellular pool. In contrast, the insulin hyperresponsive glucose transport observed with refeeding appears to result from (a) a restored translocation of glucose transporters to the plasma membrane from a repleted intracellular pool and (b) enhanced plasma membrane glucose transporter intrinsic activity.
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