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T o examine the relationship between plasma insulin concentration and intracellular glucose metabolism in control and diabetic rats, we measured endogenous glucose production, glucose uptake, whole body glycolysis, muscle and liver glycogen synthesis, and rectus muscle glucose-6-phosphate (G-6-P) concentration basally and during the infusion of 2, 3, 4, 12, and 18 mU/kg.min of insulin. The contribution of glycolysis decreased and that of muscle glycogen synthesis increased as the insulin levels rose. Insulin-mediated glucose disposal was decreased by 20-30% throughout the insulin dose-response curve in diabetics compared with controls. While at low insulin infusions (2 and 3 mU/kg.min) reductions in both the glycolytic and glycogenic fluxes contributed to the defective tissue glucose uptake in diabetic rats, at the three higher insulin doses the impairment in muscle glycogen repletion accounted for all of the difference between diabetic and control rats. The muscle G-6-P concentration was decreased (208 +/- 11 vs. 267 +/- 18 nmol/g wet wt; P less than 0.01) compared with saline at the lower insulin infusion, but was gradually increased twofold (530 +/- 16; P less than 0.01 vs. basal) as the insulin concentration rose. The G-6-P concentration in diabetic rats was similar to control despite the reduction in glucose uptake. These data suggest that (a) glucose transport is the major determinant of glucose disposal at low insulin concentration, while the rate-limiting step shifts to an intracellular site at high physiological insulin concentration; and (b) prolonged moderate hyperglycemia and hypoinsulinemia determine two distinct cellular defects in skeletal muscle at the levels of glucose transport/phosphorylation and glycogen synthesis.