Effects of insulin incubation on insulin binding, glucose transport, and insulin degradation by isolated rat adipocytes. Evidence for hormone-induced desensitization at the receptor and postreceptor level.

We have examined the effect of in vitro hyperinsulinemia on insulin binding, glucose transport, and insulin degradation in isolated rat adipocytes. When cells were incubated with insulin for 2 or 4 h at 37 degrees C, followed by washing in insulin-free buffer to remove extracellular and receptor-bound insulin, a time and dose-dependent decrease in insulin receptors was observed, which was accompanied by a reduced ability of cells to degrade insulin. Furthermore, the quantitatively predicted rightward shift in the insulin-glucose transport dose-response curve could be demonstrated. In addition to this reduction in insulin sensitivity, a striking decrease in maximal insulin-stimulated glucose transport was observed in the 4-h insulin-treated cells, indicating an abnormality distal to the insulin receptor. Thus, in vitro insulin-induced insulin resistance in adipocytes is caused by both receptor and postreceptor abnormalities. The post-receptor defect is most likely at the level of the glucose transport system per se because the insulinlike agents, spermine and antiinsulin receptor antibodies, also had a markedly reduced ability to stimulate glucose transport in 4-h insulin-treated cells. On the other hand, when cells were incubated with 100 ng/ml insulin for up to 4 h, after which time 2-deoxy glucose uptake was measured without removing buffer insulin or allowing receptor-bound insulin to dissociate, no decrease in maximal insulin-stimulated glucose transport was found. In conclusion, (a) insulin leads to a dose-dependent loss of insulin receptors in freshly isolated adipocytes accompanied by the predicted functional consequence of decreased receptors, i.e., a rightward shift in the insulin-glucose transport dose-response curve, (b) prolonged incubation with insulin causes a marked postreceptor defect in the glucose transport system, (c) maintenance of the activated state of the glucose transport system prevents the expression of the post-receptor defect, (d) the location of the postreceptor abnormality is most likely in the glucose transport system per se, and (e) insulin-induced receptor loss is accompanied by a decrease in insulin degradation.