Glucose starvation is required for insulin stimulation of glucose uptake and metabolism in cultured microvascular endothelial cells

Abstract

In the present study we determined the uptake and disposition of glucose in serum-deprived rabbit coronary microvessel endothelial (RCME) cells. RCME cells exhibited stereospecific hexose uptake inhibited by cytochalasin B. Pretreatment of the RCME cells with potassium cyanide or 2,4-dinitrophenol inhibited 2-deoxyglucose uptake but not 3- O-methylglucose transport. A major proportion (30–60%) of the 2-deoxyglucose present in the RCME cells was not phosphorylated. These two observations suggested that the rate-limiting step in the uptake of 2-deoxyglucose was not transport but rather the phosphorylation of 2-deoxyglucose to 2-deoxyglucose 6-phosphate. When glucose-deprived cells were incubated 2 hr with [U- 14C]glucose the disposition of the label was as follows: glycogen 60%, acid-soluble fraction 30%, and lipid <5%. In contrast glucose-fed cells exhibited lower overall glucose incorporation, and a slightly different disposition: glycogen 45%, acid-soluble fraction 50%, and lipid 5%. Glucose-deprived RCME cells also exhibited greater basal levels of 2-deoxyglucose uptake compared to glucose-fed cells. RCME cells incubated in the absence of glucose and serum for 16 hr exhibited dose-dependent insulin stimulation of hexose uptake and subsequent metabolism to macromolecules (i.e., glycogen and the acid-soluble fraction). Significant effects of insulin were observed with concentrations as low as 2 × 10 −10 M, well within the physiological range. In contrast, cells preincubated in serum-free culture medium containing 5.5 m M glucose did not exhibit insulin-enhanced hexose uptake or glucose metabolism (even at doses as high as 10 −7 M). These studies indicate that the effects of insulin on rabbit coronary microvascular endothelial cell glucose uptake and metabolism require both serum and glucose deprivation.