EFFECTS OF GLUCOSE, INSULIN, AND INSULIN-LIKE GROWTH-FACTOR-I ON GLUCOSE-TRANSPORT ACTIVITY IN CULTURED RAT VASCULAR SMOOTH-MUSCLE CELLS

Glucose transport activity in cultured rat vascular smooth muscle cell s (VSMCs) was examined under various concentrations of D-glucose, insu lin, and insulin-like growth factor-I (IGF-I). Confluent cultures of V SMCs were incubated with serum-free medium containing 0-25 mmol/l of D -glucose for 24-49 h. The basal rate of 2-deoxyglucose uptake was redu ced in association with increasing concentrations of D-glucose. Uptake of 2-deoxyglucose into the cells was linear between 0 and 15 min of i ncubation regardless of the glucose concentrations. The uptake was inh ibited by the addition of 10 mu mol/l cytochalasin B or 100 mmol/l D-g lucose indicating that the effects were mediated by specific integral glucose carriers. The effect of D-glucose was time-dependent and rever sible. Insulin increased the uptake of 2-deoxyglucose in a dose-depend ent manner, and its effect was dependent on the preincubation dose of D-glucose. Insulin-stimulated uptake was lower in the cells pre-expose d to 25 mmol/l D-glucose than in the cells pre-exposed to concentratio ns of D-glucose below 5.5 mmol/l. After a long-term incubation with in sulin, the insulin-stimulated glucose transport was inhibited. Recover y of glucose transport activity was assessed by incubating cells with D-glucose for 24-48 h to induce desensitization. After a 24 h glucose conditioning, the uptake of 2-deoxyglucose was lower in the cells prei ncubated with 25 mmol/l glucose than in the cells preincubated with 5. 5 mmol/l glucose. The effect of the glucose conditioning was reversibl e and dependent on the preincubation dose of D-glucose. IGF-I was a mo re potent stimulator of glucose transport than insulin. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), inhibited th e uptake of glucose stimulated by insulin or IGF-I in a dose-dependent manner. Our results suggest that D-glucose regulates its own uptake i ndependently of insulin and modulates the ability of insulin to induce insulin resistance in the cultured rat VSMCs. Glucose attenuated the effect of insulin, and led to a progressive decrease in the activity o f the glucose transport effector system. Activation of wortmannin-sens itive PI3-kinase may be involved in the signaling pathways of the insu lin- and IGF-I-stimulated glucose uptake in VSMCs. This mechanism of i nsulin resistance may be relevant to the formation of cellular defects in the vascular wall in patients with diabetes mellitus.