Normal glucose-induced suppression of glucose production but impaired stimulation of glucose disposal in type 2 diabetes: Evidence for a concentration-dependent defect in uptake

The present studies were undertaken to determine whether people with type 2 diabetes are resistant to the effects of glucose as well as insulin. Diabetic and nondiabetic subjects were studied on three occasions. Hormone secretion was inhibited with somatostatin. Insulin concentrations were kept at 'basal' levels (referred to as low insulin infusion) from 0 to 180 min then increased to ~200 pmol/l from 181 to 360 min (referred to as high insulin infusion). Glucose concentrations were clamped at either ~95, ~130, or ~165 mg/dl on each occasion. In the presence of basal insulin concentrations, a progressive increase in glucose from 95 to 130 to 165 mg/dl was accompanied by a comparable and progressive decrease (P = 0.001 to 0.003 by analysis of variance [ANOVA]) in endogenous glucose production (measured with [6-³H]glucose) and total glucose output (measured with [2-³H]glucose) and incorporation of ¹⁴CO₂ into glucose (an index of gluconeogenesis) in both diabetic and nondiabetic subjects, indicating normal hepatic (and perhaps renal) response to glucose. In the nondiabetic subjects, an increase in glucose concentration from 95 to 130 to 165 mg/dl resulted in a progressive increase in glucose disappearance during both the low (19.9 ± 1.8 to 23.6 ± 1.8 to 25.4 ± 1.6 μmol kg^-1 min^-1; P = 0.003 by ANOVA) and high (36.4 ± 3.1 to 47.6 ± 4.5 to 61.1 ± 7.0 μmol kg^-1 min^-1; P = 0.001 by ANOVA) insulin infusions. In contrast, in the diabetic subjects, whereas an increase in glucose from 95 to 130 mg/dl resulted in an increase in glucose disappearance during both the low (P = 0.001) and high (P = 0.01) dose insulin infusions, a further increase in glucose concentration to 165 mg/dl had no further effect (P = 0.41 and 0.38) on disappearance at either insulin dose (low: 14.2 ± 0.8 to 18.2 ± 1.1 to 18.7 ± 2.4 μmol kg^-1 min^-1; high: 21.0 ± 3.2 to 33.9 ± 6.4 to 32.5 ± 8.0 μmol kg^-1 min^{- 1} for 95, 130, and 165 mg/dl, respectively). We conclude that whereas glucose-induced stimulation of its own uptake is abnormal in type 2 diabetes, glucose-induced suppression of endogenous glucose production and output is not. The abnormality in uptake occurs in the presence of both basal and high insulin concentrations and is evident at glucose concentrations above but not below 130 mg/dl, implying a defect in a glucose-responsive step.