Insulin-induced translocation of intracellular glucose transporters in the isolated rat adipose cell.
Three techniques have now been used to demonstrate that insulin stimulates glucose transport in isolated rat adipose cells through the translocation of glucose transporters from a large intracellular pool to the plasma membrane. By using a specific D-glucose-inhibitable cytochalasin B-binding assay, most of the basal cell's transporters are found associated with a low-density microsomal membrane fraction. However, although Golgi marker enzyme activities are also enriched in this fraction, their distributions over all fractions do not parallel that of the transporters. In response to insulin, more than half of the intracellular transporters are translocated to the plasma membranes without a corresponding redistribution of marker enzyme activities. Furthermore, although the Kd of the transporters in the plasma membranes remains constant at approximately 100 nM, that of the intracellular transporters decreases from approximately 140 to approximately 100 nM. Nevertheless, transport activity is reconstitutable from, and an affinity-purified rabbit IgG against the purified human erythrocyte transporter cross-reacts with a 45,000-dalton band in, both plasma membranes and the low-density microsomal membrane fraction in proportion to the number of glucose transporters determined by cytochalasin B binding. Thus, intracellular glucose transporters in the rat adipose cell appear to be 1) localized to a unique membrane species, 2) either compartmentalized in two distinguishable pools or processed during their cycling to the plasma membrane in response to insulin, but fully functional and indistinguishable when reconstituted into liposomes, and 3) immunologically similar to the human erythrocyte glucose transporter.