Studies of glucose transport system of fat cells: effects of insulin and insulin mimickers.

The rate of uptake of [U-14C]glucose or 3-O-[14C]methylglucose by isolated fat cells was rapidly measured (within 30 s) by an oil-centrifugation method. Insulin and the chemical agents, H2O2 (0.3 mM), vitamin K5 (48 micron), and spermine (0.3 mM), enhanced the uptake of both sugars. Each agent required a characteristic time period of incubation with the cells to produce maximum activation of transport. Insulin and the chemical agents increased the equilibrium exchange rate of methylglucose by the cells. The increase in rate elicited by the hormone and the reagents resulted primarily from increasing the Vmax of the exchange process. The Kt ranged from 7.5 to 10 mM after treatment of cells with insulin or the other agents. Basal Kt values were more difficult to measure, but in a sugar concentration range of 5-15 mM the values were similar to those found under conditions of stimulated exchange. Overall, these findings suggest that the chemical agents may be useful as probes to study certain aspects of insulin-mediated activation of glucose transport. However, because of the differences in time courses needed for activation, the actual steps that lead to stimulation may differ for the chemical agents and insulin.