The biochemical mechanism of insulin action on glucose transport in the rat adipose cell has been studied using a photochemical crosslinking agent to covalently bind (3H)cytochalasin B to the glucose transporter. The data demonstrate that: 1) there is a heterogeneity of glucose transporter species in the intracellular pool while the plasma membrane transporters are more uniform in structure. 2) The pH 5.6 glucose transporter isoform is translocated by insulin from the low-density microsomes to the plasma membrane but the pH 6.4 isoform is not sensitive to insulin. 3) Differential sensitivity of the glucose transporter isoforms to neuraminidase suggests that the heterogeneity is at least partially due to differences in the glycosylation state. 3T3-L1 fibroblasts differentiate in culture to resemble adipose cells both morphologically and biochemically. The number of glucose transporters has been measured in subcellular membrane fractions from these cells during differentiation. The data suggest that the glucose transporter undergoes differential processing and that functional, insulin-responsive glucose transporters may be different from the insulin-insensitive (basal) glucose transporter. The effect of glucose (Glc) deprivation (starvation) on hexose transporter (GT) polypeptide(s) (pp) was studied in 3T3-C2 murine fibroblasts. The results suggest that the accumulation of total GT pp induced by Glc deprivation is the result of specialized and sensitive adaptation. The GT pp synthesized during chronic Glc deprivation has an Mr of 42000; fed cells synthesize a Mr 55000 GT pp. Neither the level of in vitro translatable GT mRNA nor the rate of GT pp synthesis are increased by Glc deprivation. It is likely, therefore, that the accumulation of GT pp during Glc deprivation is the result of decreased degradation of GT pp. In a preliminary series of experiments, insulin appears to stimulate glucose transport in isolated human adipose cells by a translocation mechanism similar to that observed in rat adipose cells and diaphragm.
