Effects of glucose, insulin, and insulin-like growth factor-I on glucose transport activity in cultured rat vascular smooth muscle cells

Abstract Glucose transport activity in cultured rat vascular smooth muscle cells (VSMCs) was examined under various concentrations of d -glucose, insulin, and insulin-like growth factor-I (IGF-I). Confluent cultures of VSMCs were incubated with serum-free medium containing 0–25 mmol/1 of d -glucose for 24–49 h. The basal rate of 2-deoxyglucose uptake was reduced in association with increasing concentrations of d -glucose. Uptake of 2-deoxyglucose into the cells was linear between 0 and 15 min of incubation regardless of the glucose concentrations. The uptake was inhibited by the addition of 10 μmol/1 cytochalasin B or 100 mmol/1 d -glucose indicating that the effects were mediated by specific integral glucose carriers. The effect of d -glucose was time-dependent and reversible. Insulin increased the uptake of 2-deoxyglucose in a dose-dependent manner, and its effect was dependent on the preincubation dose of d -glucose. Insulin-stimulated uptake was lower in the cells pre-exposed to 25 mmol/1 d -glucose than in the cells pre-exposed to concentrations of d -glucose below 5.5 mmol/1. After a long-term incubation with insulin, the insulin-stimulated glucose transport was inhibited. Recovery 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 preincubated with 25 mmol/1 glucose than in the cells preincubated with 5.5 mmol/1 glucose. The effect of the glucose conditioning was reversible and dependent on the preincubation dose of d -glucose. IGF-I was a more potent stimulator of glucose transport than insulin. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), inhibited the uptake of glucose stimulated by insulin or IGF-I in a dose-dependent manner. Our results suggest that d -glucose regulates its own uptake independently 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 of the glucose transport effector system. Activation of wortmannin-sensitive PI3-kinase may be involved in the signaling pathways of the insulin- and IGF-I-stimulated glucose uptake in VSMCs. This mechanism of insulin resistance may be relevant to the formation of cellular defects in the vascular wall in patients with diabetes mellitus.