The upregulating effect of insulin and vanadate on cell surface insulin receptors in rat adipocytes is modulated by glucose and energy availability

The aim of this study was to further characterize the rapid effects of insu lin and the tyrosine phosphatase inhibitor vanadate to amplify cell surface insulin binding capacity in isolated rat adipocytes. The effect of 20 min insulin treatment (1000 mu U/ml) was 2- to 3-fold (p < 0.01) when cells wer e treated in medium containing 5.6 mM D-glucose, but it was totally absent in glucose-free medium. Other carbon energy sources, such as fructose and p yruvate, could only partly substitute For D-glucose, with an approximately 1.5-fold insulin effect. Moreover, inhibiting transmembrane glucose transpo rt with cytochalasin B completely blocked the effect of insulin to enhance cell surface binding. The effect of vanadate was only partly glucose-depend ent, since a submaximal effect (1.5- to 2-fold, p < 0.05) was seen also in the absence of glucose. The tyrosine kinase inhibitor genistein markedly bl unted the effect of vanadate (from 3- to 4-fold to approximately 2-fold, p < 0.05) also indicating the importance of tyrosine phosphorylation-related mechanisms in the upregulation of cell surface insulin binding. Glycosylati on of insulin receptors as a mechanism for this effect appears unlikely sin ce neither the effect of insulin nor that of vanadate was altered by the gl ycosylation inhibitor tunicamycin. The time course for the insulin effect d isplayed a long duration (at feast 6 h), suggesting a maintenance role of i nsulin keeping its receptors accessible for ligand binding at the cell surf ace. In conclusion, the effect of insulin and vanadate to upregulate cell-s urface insulin receptors is energy-dependent and to some extent specificall y glucose-dependent.