Estimation and kinetic analysis of insulin-independent glucose uptake in human subjects.

Using the glucose clamp technique, glucose uptake was determined isotopically in normal human volunteers at plasma glucose concentrations of congruent to 60, 95, and 160 mg/dl during insulin infusions that increased plasma insulin to congruent to 20, 80, and 160 microU/ml. Because glucose uptake was found to be a linear function of plasma insulin at each plasma glucose concentration (r greater than 0.92, P less than 0.01), glucose uptake at 0 plasma insulin was estimated by linear regression analysis. The values thus derived (1.30, 1.62, and 2.59 mg . kg-1 . min-1 for plasma glucose concentrations of 60, 95, and 160 mg/dl, respectively) produced a linear Eadie-Hofstee plot, suggesting that insulin-independent glucose uptake followed Michaelis-Menten kinetics. The Km for glucose uptake at 0 plasma insulin (congruent to 10 mM) was similar to those observed for glucose uptake at the other plasma insulin concentrations studied (congruent to 9-12 mM), but its Vmax was less (5.2 vs. 6.4, 18.5, and 26.8 mg . kg-1 . min-1 for congruent to 20, 80, and 160 U/ml, respectively). These results indicate that in postabsorptive human subjects 75-85% of glucose uptake is noninsulin-mediated and provide additional support for the concept that insulin may increase glucose uptake merely by providing additional transport sites. The method described herein provides an assessment of insulin-independent glucose uptake in vivo that may prove useful in distinguishing between intrinsic defects of the glucose transport system and those due to defects in insulin action.