Glucose transport and GLUT4 protein distribution in skeletal muscle of GLUT4 transgenic mice.
The aim of the present investigation was to determine whether the subcellular distribution and insulin-stimulated translocation of the GLUT4 isoform of the glucose transporter are affected when GLUT4 is overexpressed in mouse skeletal muscle, and if the overexpression of GLUT4 alters maximal insulin-stimulated glucose transport and metabolism. Rates of glucose transport and metabolism were assessed by hind-limb perfusion in GLUT4 transgenic (TG) mice and non-transgenic (NTG) controls. Glucose-transport activity was determined under basal (no insulin), submaximal (0.2 m-unit/ml) and maximal (10 m-units/ml) insulin conditions using a perfusate containing 8 mM 3-O-methyl-D-glucose. Glucose metabolism was quantified by perfusing the hind limbs for 25 min with a perfusate containing 8 mM glucose and 10 m-units/ml insulin. Under basal conditions, there was no difference in muscle glucose transport between TG (1.10 +/- 0.10 mumol/h per g; mean +/- S.E.M.) and NTG (0.93 +/- 0.16 mumol/h per g) mice. However, TG mice displayed significantly greater glucose-transport activity during submaximal (4.42 +/- 0.49 compared with 2.69 +/- 0.33 mumol/h per g) and maximal (11.68 +/- 1.13 compared with 7.53 +/- 0.80 mumol/h per g) insulin stimulation. Nevertheless, overexpression of the GLUT4 protein did not alter maximal rates of glucose metabolism. Membrane purification revealed that, under basal conditions, plasma-membrane (approximately 12-fold) and intracellular-membrane (approximately 4-fold) GLUT4 protein concentrations were greater in TG than NTG mice. Submaximal insulin stimulation did not increase plasma-membrane GLUT4 protein concentration whereas maximal insulin stimulation increased this protein in both NTG (4.1-fold) and TG (2.6-fold) mice. These results suggest that the increase in insulin-stimulated glucose transport following overexpression of the GLUT4 protein is limited by factors other than the plasma-membrane GLUT4 protein concentration. Furthermore, GLUT4 overexpression is not coupled to glucose-metabolic capacity.
Experimental Diabetes, Metabolism, and Nutrition Section, DB/NIDDK National Institutes of Health, Bethesda, MD 20892-1420, USA.
J T
JT
Brozinick
B B
BB
Yaspelkis
C M
CM
Wilson
K E
KE
Grant
E M
EM
Gibbs
S W
SW
Cushman
J L
JL
Ivy
D000818:Animals; D001692:Biological Transport; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D005947:Glucose / Q000378:metabolism / Q000493:pharmacokinetics*; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D007425:Intracellular Membranes / Q000737:chemistry / Q000378:metabolism; D007536:Isomerism; D008565:Membrane Proteins / Q000302:isolation & purification / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D013268:Stimulation, Chemical; D013347:Subcellular Fractions / Q000378:metabolism
D007328:Insulin; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D005947:Glucose