Evidence that downregulation of hexose transport limits intracellular glucose in 3T3-L1 fibroblasts.
Measurements of initial glucose entry rate and intracellular glucose concentration in cultured cells are difficult because of rapid transport relative to intracellular volume and a substantial extracellular space from which glucose cannot be completely removed by quick exchanges of medium. In 3T3-L1 cells, we obtained good estimates of initial entry of [14C]methylglucose and D-[14C]glucose with 1) L-[3H]glucose as an extracellular marker together with the [14C]glucose or [14C]methylglucose in the substrate mixture, 2) sampling times as short as 2 s, 3) ice-cold phloretin-containing medium to stop uptake and rinse away the extracellular label, and 4) nonlinear regression of time courses. Methylglucose equilibrated in two phases--the first with a half-time of 1.7 s and the second with a half-time of 23 s; it eventually equilibrated in an intracellular space of 8 microliters/mg protein. Entry of glucose remained almost linear for 10 s, making its transport kinetics easier to study (Km = 5.7 mM, Vmax = 590 nmol.s-1.ml-1 cell water). Steady-state intracellular glucose concentration was 75-90% of extracellular glucose concentration. Cells grown in a high-glucose medium (24 mM) exhibited a 67% reduction of glucose-transport activity and a 50% reduction of steady-state ratio of intracellular glucose to extracellular glucose.
Whitesell
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RR
Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
Regen
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DM
Pelletier
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D
Abumrad
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NA
D000818:Animals; D001693:Biological Transport, Active; D002250:Carbon Radioisotopes; D002460:Cell Line; D005347:Fibroblasts / Q000378:metabolism; D005947:Glucose / Q000378:metabolism*; D007700:Kinetics; D008757:Methylglucosides / Q000378:metabolism; D051379:Mice; D011865:Radioisotope Dilution Technique; D014316:Tritium
D002250:Carbon Radioisotopes; D008757:Methylglucosides; D014316:Tritium; D005947:Glucose