Hexose transport in human adipocytes: Factors influencing the response to insulin and kinetics of methylglucose and glucose transport

Abstract

Optimal experimental conditions were defined for measuring the initial uptake rate of the non-metabolizable sugar analogue 3-O-methylglucose in non-stimulated and insulin-stimulated human adipocytes. The permeability of the adipocyte plasma membrane for tracer methylglucose (100 μmol/l) was 2.9×10^-7 cm x s^-1 at 37 °C and slightly lower at 20 °C. At 37 °C and pH 7.4 insulin (5 nmol/l) increased the permeability about twofold (range 1.5 to fivefold) with half maximal effect at about 100 pmol/l. At pH 7.0 the dose response curve for the insulin effect on the uptake rate of methylglucose was shifted about 2.5-fold to the right. The permeability to L-glucose due to simple diffusion was estimated as 3.0×10^-10 cm x s^-1 suggesting that uptake of methylglucose occurs almost exclusively by facilitated diffusion. The K_m for methylglucose equilibrium exchange in insulin stimulated cells was about 4.8 mmol/l. The initial uptake of tracer methylglucose in insulin-stimulated cells was inhibited by unlabelled methylglucose and by D-glucose with inhibition constants of about 3.8 and 7.7 mmol/l, respectively. Uptake of tracer 2-deoxyglucose (50 umol/l) in insulin-stimulated adipocytes was linear from 10 s to 5 min whereas the rate of uptake in the presence of 3 mmol/l of D-glucose was markedly decreased suggesting that deoxyglucose uptake after a few minutes is mainly limited by hexokinase in the presence of glucose.