Length of Acute Exposure to Insulin Regulates the Rate of Deactivation of Stimulated Glucose Transport in Isolated Rat Adipocytes*

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Abstract
The effects of the duration of exposure of rat adipocytes to insulin on the temporal relationships between insulin dissociation from receptors and deactivation of insulin-stimulated glucose transport were studied. Cells were incubated with [125I]insulin plus unlabeled insulin at a total insulin concentration of either 1 or 10 ng/ml at 37.degree. C. After various times of association, the cells were washed, and the rate of dissociation of bound hormone and subsequent decrease in insulin-stimulated glucose transport activity (deactivation) were measured. One nanogram of insulin/ml is a submaxillary effective concentration and stimulates glucose transport to .apprx. 80% of maximal values. Dissociation of insulin from receptors and deactivation of glucose transport activity were measured after 10, 30 and 60 min of exposure of cells to 1 ng/ml insulin. Dissociations rates were unchanged after the various times of exposure to insulin, while prolonging the preincubation time from 10 to 60 min slowed the rate of deactivation 5-fold. Dissociation and deactivation were also measured after 5-, 10-, 30- and 60-min periods of incubation with a maximally stimulating hormone concentration (10 ng/ml). The dissociation rates were similar after the various periods of association, while the deactivation rate was 2-fold slower after 30 and 60 min of preincubation compared to that after 5 min. There is a time-dependent slowing of the rate of deactivation of insulin-stimulated glucose transport that is not accounted for by differences in the amount of insulin bound or the dissociation rate of insulin from its receptors. Therefore, the length of exposure to insulin can influence the rate of decline in glucose transport activity, and this may be important in determining biological effects of pulsatile insulin secretion vs. prolonged insulin exposure on insulin target tissues. Apparently with time, insulin causes an increase in some intracellular factor or other cellular perturbation, the level of which modulates the rate of deactivation of the glucose transport system.