The dose-response relationship between plasma insulin concentration and total glucose uptake, glucose oxidation, and glucose storage was examined in 22 healthy young volunteers by employing the euglycemic insulin clamp technique in combination with indirect calorimetry. Insulin was infused at five rates to achieve steady-state hyperinsulinemic plateaus of 62 ± 4, 103 ± 5, 170 ± 10, 423 ± 16, and 1132 ± 47 μU/ml. With increasing plasma insulin concentrations within the physiologic range, there was a linear increase in glucose uptake with a half maximally effective insulin concentration of 72 μU/ml. Glucose uptake by all tissues of the body reached 80% of its maximum value (12.6 mg/kg · min) at a plasma insulin concentration of ∼200 μU/ml. In contrast to total glucose uptake, glucose oxidation plateaued more quickly, achieved a maximum rate of only 4.0 mg/kg · min, and displayed a lower half maximally effective insulin concentration of 40 μU/ml. The increase in glucose uptake with progressively increasing plasma insulin levels was primarily the result of an increase in glucose storage, with a half maximally effective insulin concentration of 105 μU/ml and maximum rate of 8.7 mg/kg · min. Glucose storage represented over 60–70% of total glucose uptake at all insulin concentrations. After achieving maximum rates of insulin-mediated glucose uptake (plasma insulin concentration = 1132 μU/ml), hyperglycemia (+125 mg/dl) was superimposed on hyperinsulinemia to further enhance glucose transport. Under these conditions, total glucose uptake (32.5 mg/kg · min, P < 0.001) was markedly augmented but no significant increase in glucose oxidation was observed. These results indicate a true saturation of the glucose oxidation pathway. With pro-gressively increasing doses of insulin, the glucose storage represents the major route of glucose disposal.

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