Enhancement of glucose transport by insulin at 37 °C in rat adipocytes is accounted for by increased Vmax

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Abstract
The kinetics of 3-O-methyl-D-glucose and D-glucose transport was reinvestigated at 37 °C in rat adipocytes exhibiting very slow transport rates in the absence of insulin and approximately 30-fold insulin stimulation. The apparent maximal velocity for 3-O-methylglucose was increased at least 15-fold from 0.04±0.01 to 0.73±0.17 mmol ·1−1·s−1 (SD, n=7). The apparent half saturation constant was decreased by insulin from 8.1±1.6 mmol/l to 3.3±0.8 mmol/l. However, approximately 10% of the 3-O-methylglucose at a very low concentration was transferred by routes other than the glucose transporter, e. g. via nonmediated diffusion or via the fructose transporter, and the estimated value for the half saturation constant was therefore too high in the nonstimulated cells. After appropriate correction this value was estimated as 5.0±1.8 mmol/l. The inhibition constant of 3-O-methylglucose on the conversion of 12 μmol/l 14C-labelled glucose to cell-associated products was about 4 mmol/l and was not changed significantly by insulin. Transport experiments with labelled glucose gave results similar to those obtained for transport of 3-O-methylglucose. It is concluded that insulin can cause a 15- to 30-fold increase in the maximal velocity for transport of 3-O-methylglucose and glucose, and that a concomitant decrease in the half saturation constant, if present, is insignificant.

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