Biochemical characterization and subcellular distribution of the glucose transporter from rat brain microvessels

This study describes the biochemical characterization and subcellular distribution of glucose transporters from isolated rat brain cortical microvessels. The D-glucose inhibitable 3H]cytochalasin B binding assay was used to quantitate glucose transporter binding sites in plasma membranes, high-density microsomes and low-density microsomes prepared from basal and insulin-stimulated cells. Incubation with insulin for 30 min increased the number of glucose transporters in the high-density microsomes by around 33% but had no effect on the number of glucose transporters in the plasma membrane or low-density microsomes. Prolonged incubation with insulin (2 h), however, resulted in a small but significant redistribution of glucose transporters to the low-density microsomes. Preincubation of cells with cycloheximide blocked this insulin-induced increase in glucose transporter number, suggesting that this effect of insulin was due to the synthesis of new glucose transport proteins. Specific labeling of glucose transporters was achieved by photoincorporation of 3H]cytochalasin B. Labeled membranes from all fractions contained a single D-glucose inhibitable peak, migrating with a molecular size of 55 kDa on SDS-polyacrylamide gel electrophoresis. Isoelectric focusing of the 55 kDa protein revealed one major peak of D-glucose inhibitable radioactivity focusing at pH 6.0 in all fractions.