Control of Exercise-stimulated Muscle Glucose Uptake by GLUT4 Is Dependent on Glucose Phosphorylation Capacity in the Conscious Mouse

Abstract

Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during exercise because GLUT4 overexpression does not increase exercise-stimulated MGU. Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity was increased. Thus, C57BL/6J mice with hexokinase II (HKII) overexpression (HKTg), a GLUT4 partial knock-out (G4+/–), or both (HKTg + G4+/–) and wild-type (WT) littermates were implanted with carotid artery and jugular vein catheters for sampling and infusions at 4 months of age. After a 7-day recovery, 5-h fasted mice remained sedentary or ran on a treadmill at 0.6 mph for 30 min (n = 9–12 per group) and received a bolus of 2-deoxy[3H]glucose to provide an index of MGU (Rg). Arterial blood glucose and plasma insulin concentrations were similar in WT, G4+/–, HKTg, and HKTg + G4+/– mice. Sedentary Rg values were the same in all genotypes in all muscles studied, confirming that glucose transport is a significant barrier to basal glucose uptake. Gastrocnemius and soleus Rg were greater in exercising compared with sedentary mice in all genotypes. During exercise, G4+/– mice had a marked increase in blood glucose that was corrected by the addition of HK II overexpression. Exercise Rg (μmol/100g/min) was not different between WT and G4+/– mice in the gastrocnemius (24 ± 5 versus 21 ± 2) or the soleus (54 ± 6 versus 70 ± 7). In contrast, the enhanced exercise Rg observed in HKTg mice compared with that in WT mice was absent in HKTg + G4+/– mice in both the gastrocnemius (39 ± 7 versus 22 ± 6) and the soleus (98 ± 13 versus 65 ± 13). Thus, glucose transport is not a significant barrier to exercise-stimulated MGU despite a 50% reduction in GLUT4 content when glucose phosphorylation capacity is normal. However, when glucose phosphorylation capacity is increased by HK II overexpression, GLUT4 availability becomes a marked limitation to exercise-stimulated MGU. Previous work suggests that normal GLUT4 content is sufficient for increases in muscle glucose uptake (MGU) during exercise because GLUT4 overexpression does not increase exercise-stimulated MGU. Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU when glucose phosphorylation capacity is normal but would do so when glucose phosphorylation capacity was increased. Thus, C57BL/6J mice with hexokinase II (HKII) overexpression (HKTg), a GLUT4 partial knock-out (G4+/–), or both (HKTg + G4+/–) and wild-type (WT) littermates were implanted with carotid artery and jugular vein catheters for sampling and infusions at 4 months of age. After a 7-day recovery, 5-h fasted mice remained sedentary or ran on a treadmill at 0.6 mph for 30 min (n = 9–12 per group) and received a bolus of 2-deoxy[3H]glucose to provide an index of MGU (Rg). Arterial blood glucose and plasma insulin concentrations were similar in WT, G4+/–, HKTg, and HKTg + G4+/– mice. Sedentary Rg values were the same in all genotypes in all muscles studied, confirming that glucose transport is a significant barrier to basal glucose uptake. Gastrocnemius and soleus Rg were greater in exercising compared with sedentary mice in all genotypes. During exercise, G4+/– mice had a marked increase in blood glucose that was corrected by the addition of HK II overexpression. Exercise Rg (μmol/100g/min) was not different between WT and G4+/– mice in the gastrocnemius (24 ± 5 versus 21 ± 2) or the soleus (54 ± 6 versus 70 ± 7). In contrast, the enhanced exercise Rg observed in HKTg mice compared with that in WT mice was absent in HKTg + G4+/– mice in both the gastrocnemius (39 ± 7 versus 22 ± 6) and the soleus (98 ± 13 versus 65 ± 13). Thus, glucose transport is not a significant barrier to exercise-stimulated MGU despite a 50% reduction in GLUT4 content when glucose phosphorylation capacity is normal. However, when glucose phosphorylation capacity is increased by HK II overexpression, GLUT4 availability becomes a marked limitation to exercise-stimulated MGU. Muscle glucose uptake (MGU) 1The abbreviations used are: MGU, muscle glucose uptake; GLUT, facilitative glucose transporter; GLUT4+/–, heterozygous GLUT4 knock-out (mice); HK, hexokinase; HKTg, HK transgene (mice overexpressing HK II); SVL, superficial vastus lateralis; Kg, concentration-independent index of MGU; Rg, concentration-dependent index of MGU; NEFA, non-esterified fatty acid; [2-3H]DG, 2-deoxy[3H]glucose; [2-3H]DGP, [2-3H]DG-6-phosphate; WT, wild type.1The abbreviations used are: MGU, muscle glucose uptake; GLUT, facilitative glucose transporter; GLUT4+/–, heterozygous GLUT4 knock-out (mice); HK, hexokinase; HKTg, HK transgene (mice overexpressing HK II); SVL, superficial vastus lateralis; Kg, concentration-independent index of MGU; Rg, concentration-dependent index of MGU; NEFA, non-esterified fatty acid; [2-3H]DG, 2-deoxy[3H]glucose; [2-3H]DGP, [2-3H]DG-6-phosphate; WT, wild type. can be separated into three sequential steps, i.e. delivery of glucose from the blood to the muscle, transport across the sarcolemma by a GLUT, and irreversible phosphorylation to glucose-6-phosphate by an HK isozyme. Each of these steps can serve as a barrier to MGU and, thus, are important in regulating glucose influx. During resting conditions, the transport step exerts the most control in regulating MGU, as GLUT1 (1Hansen P.A. Marshall B.A. Chen M. Holloszy J.O. Mueckler M. J. Biol. Chem. 2000; 275: 22381-22386Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, 2Marshall B.A. Ren J.M. Johnson D.W. Gibbs E.M. Lillquist J.S. Soeller W.C. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 18442-18445Abstract Full Text PDF PubMed Google Scholar, 3Gulve E.A. Ren J.M. Marshall B.A. Gao J. Hansen P.A. Holloszy J.O. Mueckler M. J. Biol. Chem. 1994; 269: 18366-18370Abstract Full Text PDF PubMed Google Scholar, 4Ren J.M. Marshall B.A. Gulve E.A. Gao J. Johnson D.W. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 16113-16115Abstract Full Text PDF PubMed Google Scholar) or GLUT4 (5Hansen P.A. Gulve E.A. Marshall B.A. Gao J. Pessin J.E. Holloszy J.O. Mueckler M. J. Biol. Chem. 1995; 270: 1679-1684Abstract Full Text Full Text PDF PubMed Google Scholar, 6Treadway J.L. Hargrove D.M. Nardone N.A. McPherson R.K. Russo J.F. Milici A.J. Stukenbrok H.A. Gibbs E.M. Stevenson R.W. Pessin J.E. J. Biol. Chem. 1994; 269: 29956-29961Abstract Full Text PDF PubMed Google Scholar) overexpression augments basal MGU. Previous work suggests that normal GLUT4 content is sufficient for increases in MGU during exercise, because GLUT4 overexpression alone does not further increase exercise-stimulated MGU (7Fueger P.T. Bracy D.P. Malabanan C.M. Pencek R.R. Wasserman D.H. Am. J. Physiol. 2004; 286: E77-E84Crossref PubMed Scopus (47) Google Scholar). Instead of glucose transport, glucose phosphorylation is a primary limitation of exercise-stimulated MGU (7Fueger P.T. Bracy D.P. Malabanan C.M. Pencek R.R. Wasserman D.H. Am. J. Physiol. 2004; 286: E77-E84Crossref PubMed Scopus (47) Google Scholar, 8Halseth A.E. Bracy D.P. Wasserman D.H. J. Appl. Physiol. 1998; 85: 2305-2313Crossref PubMed Scopus (43) Google Scholar, 9Halseth A.E. Bracy D.P. Wasserman D.H. Am. J. Physiol. 1999; 276: E70-E77PubMed Google Scholar). Heterozygous GLUT4 knock-out mice serve as a useful tool for examining the impact of reductions in glucose transport capacity on MGU (10Stenbit A.E. Tsao T.S. Li J. Burcelin R. Geenen D.L. Factor S.M. Houseknecht K. Katz E.B. Charron M.J. Nat. Med. 1997; 3: 1096-1101Crossref PubMed Scopus (273) Google Scholar). The aim of this study was to further define the role that GLUT4 plays in controlling MGU during exercise, when muscle glucose phosphorylation capacity is altered. It was hypothesized that a partial ablation of GLUT4 would not impair exercise-stimulated MGU because glucose phosphorylation capacity is such a marked barrier to MGU but that it do so would when glucose phosphorylation capacity is increased by HK II overexpression. Mouse Maintenance and Genotyping—All procedures performed were approved by the Vanderbilt University Animal Care and Use Committee. Male FVB/NJ mice that selectively overexpress HK II (HKTg) in skeletal muscle using a transgene containing the human HK II cDNA driven by the rat muscle creatine kinase promoter that yields a 3-, 5-, and 7-fold increase in HK activity in the soleus, gastrocnemius, and SVL muscles, respectively, were obtained from Dr. David E. Moller (11Chang P.Y. Jensen J. Printz R.L. Granner D.K. Ivy J.L. Moller D.E. J. Biol. Chem. 1996; 271: 14834-14839Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar) and backcrossed onto the C57BL/6J background for at least seven generations. Male C57BL/6J mice with a complete ablation of the GLUT4 gene (12Katz E.B. Stenbit A.E. Hatton K. DePinho R. Charron M.J. Nature. 1995; 377: 151-155Crossref PubMed Scopus (392) Google Scholar) were bred with HKTg females to obtain heterozygous GLUT4 knock-out mice with (HKTg + GLUT4+/–) or without (GLUT4+/–) the HK II transgene. Mice from this mating were bred to create offspring with one of four genotypes, i.e. wild-type (WT), GLUT4+/–,HKTg, and HKTg + GLUT4+/–. Litter mates were separated by gender following a 3-week weaning period and maintained in microisolator cages. Genotyping was performed with the polymerase chain reaction on genomic DNA obtained from a tail biopsy and isolated by a DNeasy Tissue Kit (Qiagen, Valencia, CA). All mice were fed standard chow ad libitum, handled at least twice per week, and studied at ∼4 months of age. Immunoblotting—Total GLUT4 and HKII protein content was determined on representative gastrocnemius and SVL muscles. Muscles were homogenized in a solution containing 10% glycerol, 20 mm sodiumpyrophosphate, 150 mm NaCl, 50 mm Hepes (pH 7.5), 1% Nonidet P-40, 20 mm β-glycerophosphate, 10 mm NaF, 2 mmol/liter EDTA (pH 8.0), 2 mm phenylmethylsulfonyl fluoride, 1 mm CaCl2, 1 mm MgCl2, 10 μg/ml aprotinin, 10 μg/ml leupeptin, 2 mm Na2VO3, and 3 mm benzamide. After centrifugation (1 h at 4500 × g) pellets were discarded, and supernatants were retained for protein determination using a Pierce BCA protein assay kit. Proteins (30 μg) were separated by electrophoresis on an SDS polyacrylamide gel and then transferred to a polyvinylidene difluoride membrane. Membranes were blocked, blotted with rabbit α-GLUT4 (1:1000; Alpha Diagnostic International; San Antonio, TX) and α-HKII (1:1000; Chemicon International; Temecula, CA), and then incubated with anti-rabbit horseradish peroxidase (1:20000; Pierce). Densitometry was performed using ImageJ software (NIH). Surgical Procedures—The surgical procedures utilized are similar to those described previously (9Halseth A.E. Bracy D.P. Wasserman D.H. Am. J. Physiol. 1999; 276: E70-E77PubMed Google Scholar, 13Rottman J.N. Bracy D. Malabanan C. Yue Z. J. Wasserman D.H. Am. J. Physiol. Scholar, M. C. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). Mice were with The carotid artery was for sampling of blood with a of into the and The jugular vein was for infusions with a The of catheters were the to the of the were to of were and with were with of containing and 5 were and was an period in was 10% of mice were to treadmill with a of exercise were performed 2 following the treadmill following on the of the mice were in a with and fasted for 5 1 h to an was to the and mice were on an = a blood was for the of blood glucose CA), and plasma insulin and The blood were with containing 10 and The mice remained sedentary (n = WT, + GLUT4+/–) or ran (n = WT, + GLUT4+/–) on the treadmill for 30 The work is to increase to of J. Physiol. 1993; PubMed Scopus Google Scholar). Exercise was so that the same of work was performed work in to create similar = 5 a bolus of was to provide an index of glucose uptake. = and 20 of blood was to blood glucose and plasma = 30 a blood was and mice were with an of The soleus and gastrocnemius and and SVL and muscles D.L. C. W.C. Am. J. Physiol. PubMed Google Scholar) were in and at of and Muscle insulin was with a PubMed Scopus Google Scholar). were by an assay with and of the plasma was determined by with as the Muscle were and homogenized in were and with was to and was with and to and into the and then to is the between the The of was to The of [2-3H]DG, or Kg, and the or Rg, were as described previously D.E. Am. J. Physiol. Google Scholar) and in 1 and 2) is the in the muscle in is the the plasma using the in and is the blood glucose in mm during the and Rg are used as concentration-independent and of MGU, Muscle was determined by the of and PubMed Scopus Google Scholar) on the gastrocnemius and SVL muscles. not be because both of the muscles were used to assay for and are as ± between were determined by of The was at of partial GLUT4 knock-out GLUT4 content to in gastrocnemius and SVL muscles as compared with WT when with HK II overexpression, a of a of the GLUT4 gene not GLUT4 protein content in the SVL to HK II overexpressing mice. However, GLUT4 content in the SVL was not different between the of with a partial GLUT4 knock-out and HKTg + HKII overexpression, alone or with a partial GLUT4 increased HKII content and in gastrocnemius and SVL muscles, HK II content from skeletal muscles. was performed to HK II protein content in the gastrocnemius and SVL muscles of WT, GLUT4+/–, HKTg, and HKTg + representative is Densitometry are ± for four mice per versus WT and of 5-h fasted mice are in one GLUT4 overexpressing HK or the of those not or glucose and insulin GLUT4 knock-out with and without HK II overexpression in the fasted in 5-h fasted C57BL/6J + ± ± ± ± ± ± ± ± ± ± ± ± ± ± versus WT and HKTg ± ± versus WT and HKTg versus WT and HKTg mice. in a of GLUT4 during and mice had similar glucose concentrations compared with WT mice the sedentary period mice had similar sedentary and Rg values compared with WT mice in all muscles However, muscle was by partial GLUT4 knock-out in sedentary mice glucose + ± ± ± ± ± ± ± ± ± ± ± ± ± versus sedentary ± versus sedentary ± versus sedentary versus WT, and HKTg + ± versus sedentary ± versus sedentary ± versus sedentary ± versus sedentary versus WT, and HKTg + ± versus sedentary ± versus sedentary ± versus sedentary versus WT ± versus sedentary versus WT ± versus sedentary versus WT versus sedentary versus WT, and HKTg + versus WT mice. in a and exercise-stimulated glucose uptake in skeletal muscles from mice. WT, GLUT4+/–, HKTg, and HKTg + were and to from for a 5-h and a bolus of of during a sedentary or exercise Rg was as described are ± for 9–12 mice per versus sedentary a versus versus + ± ± versus WT and HKTg ± ± versus WT and HKTg versus ± ± versus WT ± ± versus WT and HKTg versus ± versus sedentary ± versus sedentary ± versus sedentary ± versus WT and HKTg ± versus sedentary ± versus WT and HKTg versus sedentary ± ± versus WT and HKTg versus WT and HKTg versus versus WT versus sedentary mice. in a Arterial glucose to the of the treadmill exercise in both WT and mice Exercise increased and Rg and content in all of the muscles studied of WT and mice. partial GLUT4 ablation not impair and Rg during exercise II and In SVL and Rg were increased in compared with WT mice during exercise to mice had concentrations following exercise compared with WT littermates However, between sedentary and exercise was similar between WT and mice. of GLUT4 on the of HK II during and + mice had similar glucose concentrations as compared with those of WT and HKTg mice the sedentary period HKTg + mice had similar sedentary and Rg values as compared with those of WT, GLUT4+/–, and HKTg mice in all of the muscles Muscle was in HKTg + mice as compared with that in WT, GLUT4+/–, HKTg mice in the sedentary In to mice in glucose HKTg + mice a glucose during exercise 10 min of exercise, glucose was that of mice and remained so the of were compared with WT and HKTg mice. Exercise increased and Rg compared with the sedentary in all of the muscles studied of HKTg and HKTg + mice. HK II overexpression exercise-stimulated and Rg compared with WT mice in the gastrocnemius and soleus and compared with WT mice in the of exercise-stimulated and Rg was by partial GLUT4 knock-out in the gastrocnemius II and and soleus II and but not in the SVL II and Muscle was in sedentary HKTg + mice compared with genotypes. not to a significant of the of exercise, HKTg + mice had muscle compared with WT and HKTg mice Each step of the glucose uptake glucose transport, and can be as a barrier to muscle glucose in Mice overexpressing GLUT1 (1Hansen P.A. Marshall B.A. Chen M. Holloszy J.O. Mueckler M. J. Biol. Chem. 2000; 275: 22381-22386Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, 2Marshall B.A. Ren J.M. Johnson D.W. Gibbs E.M. Lillquist J.S. Soeller W.C. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 18442-18445Abstract Full Text PDF PubMed Google Scholar, 3Gulve E.A. Ren J.M. Marshall B.A. Gao J. Hansen P.A. Holloszy J.O. Mueckler M. J. Biol. Chem. 1994; 269: 18366-18370Abstract Full Text PDF PubMed Google Scholar, 4Ren J.M. Marshall B.A. Gulve E.A. Gao J. Johnson D.W. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 16113-16115Abstract Full Text PDF PubMed Google GLUT4 (5Hansen P.A. Gulve E.A. Marshall B.A. Gao J. Pessin J.E. Holloszy J.O. Mueckler M. J. Biol. Chem. 1995; 270: 1679-1684Abstract Full Text Full Text PDF PubMed Google Scholar, 6Treadway J.L. Hargrove D.M. Nardone N.A. McPherson R.K. Russo J.F. Milici A.J. Stukenbrok H.A. Gibbs E.M. Stevenson R.W. Pessin J.E. J. Biol. Chem. 1994; 269: 29956-29961Abstract Full Text PDF PubMed Google Scholar, 1995; PubMed Scopus Google Scholar, M. 1995; PubMed Scopus Google Scholar, Gibbs E.M. Milici A.J. Stukenbrok H.A. McPherson R.K. J.L. Pessin J.E. 1993; PubMed Scopus Google Scholar, B.A. Mueckler Am. J. Physiol. 1994; PubMed Google or HK II (1Hansen P.A. Marshall B.A. Chen M. Holloszy J.O. Mueckler M. J. Biol. Chem. 2000; 275: 22381-22386Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, P.T. Bracy D.P. Malabanan C.M. Pencek R.R. Wasserman D.H. Am. J. Physiol. 2004; 286: E77-E84Crossref PubMed Scopus (47) Google Scholar, 9Halseth A.E. Bracy D.P. Wasserman D.H. Am. J. Physiol. 1999; 276: E70-E77PubMed Google Scholar, P.Y. Jensen J. Printz R.L. Granner D.K. Ivy J.L. Moller D.E. J. Biol. Chem. 1996; 271: 14834-14839Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, Moller D. M. J. J. 1997; PubMed Scopus Google Scholar) used to the of to MGU in a of The used in these was that a step is a primary of to MGU, then overexpression of the that that step increase MGU. is to increase the to a step by the or activity of those same and such a MGU. was for glucose phosphorylation by HK II M. M. E. J. M. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, P.T. Bracy D.P. Malabanan C.M. Pencek R.R. M. Wasserman D.H. Am. J. Physiol. PubMed Scopus Google and it was that glucose phosphorylation is a primary of to MGU during exercise, because partial HK II knock-out mice had glucose in muscles P.T. Bracy D.P. Malabanan C.M. Pencek R.R. M. Wasserman D.H. Am. J. Physiol. PubMed Scopus Google Scholar). that a partial GLUT4 ablation in C57BL/6J mice sedentary exercise-stimulated MGU in of the muscles studied at 4 months of age. when the barrier to glucose phosphorylation was by HK II overexpression glucose transport to MGU during The in 5 the impact of such on to MGU and MGU. basal sedentary conditions, muscle glucose transport as the primary barrier to glucose to the of glucose in the a with to glucose from in D.E. Am. J. Physiol. 1999; PubMed Google Scholar, D.E. J. R. J. 1996; PubMed Scopus Google Scholar) and A.E. Bracy D.P. Wasserman D.H. Am. J. Physiol. 2000; Google Scholar, A.E. Bracy D.P. Wasserman D.H. Am. J. Physiol. Scholar) using glucose and overexpression in mice (1Hansen P.A. Marshall B.A. Chen M. Holloszy J.O. Mueckler M. J. Biol. Chem. 2000; 275: 22381-22386Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, 2Marshall B.A. Ren J.M. Johnson D.W. Gibbs E.M. Lillquist J.S. Soeller W.C. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 18442-18445Abstract Full Text PDF PubMed Google Scholar, 3Gulve E.A. Ren J.M. Marshall B.A. Gao J. Hansen P.A. Holloszy J.O. Mueckler M. J. Biol. Chem. 1994; 269: 18366-18370Abstract Full Text PDF PubMed Google Scholar, 4Ren J.M. Marshall B.A. Gulve E.A. Gao J. Johnson D.W. Holloszy J.O. Mueckler M. J. Biol. Chem. 1993; 268: 16113-16115Abstract Full Text PDF PubMed Google Scholar, P.A. Gulve E.A. Marshall B.A. Gao J. Pessin J.E. Holloszy J.O. Mueckler M. J. Biol. Chem. 1995; 270: 1679-1684Abstract Full Text Full Text PDF PubMed Google Scholar, 6Treadway J.L. Hargrove D.M. Nardone N.A. McPherson R.K. Russo J.F. Milici A.J. Stukenbrok H.A. Gibbs E.M. Stevenson R.W. Pessin J.E. J. Biol. Chem. 1994; 269: 29956-29961Abstract Full Text PDF PubMed Google Scholar, 1995; PubMed Scopus Google Scholar, M. 1995; PubMed Scopus Google Scholar, Gibbs E.M. Milici A.J. Stukenbrok H.A. McPherson R.K. J.L. Pessin J.E. 1993; PubMed Scopus Google Scholar, B.A. Mueckler Am. J. Physiol. 1994; PubMed Google Scholar). The partial ablation of the overexpression of HK and the of the in the study had on sedentary of MGU and is with a (10Stenbit A.E. Tsao T.S. Li J. Burcelin R. Geenen D.L. Factor S.M. Houseknecht K. Katz E.B. Charron M.J. Nat. Med. 1997; 3: 1096-1101Crossref PubMed Scopus (273) Google Scholar). a of GLUT4 in the plasma basal and basal glucose transport be GLUT1 or as glucose a 50% reduction in GLUT4 content was not to a impact on sedentary MGU. are with the that the glucose during sedentary can be with the of GLUT4 and GLUT1 or glucose a GLUT4 ablation (12Katz E.B. Stenbit A.E. Hatton K. DePinho R. Charron M.J. Nature. 1995; 377: 151-155Crossref PubMed Scopus (392) Google Scholar) or a of GLUT4 by D. D. R. C. R.R. PubMed Scopus Google Scholar) basal MGU; thus, of GLUT4 and of WT is for normal of basal glucose influx. the in the study are with the that glucose transport and not glucose phosphorylation is a of to basal MGU. During exercise, the increases glucose delivery J.L. Am. J. Physiol. Google Scholar, N.A. Physiol. 3: PubMed Scopus Google and GLUT4 to the sarcolemma P.A. C.M. J. Appl. Physiol. PubMed Scopus Google Scholar, PubMed Scopus Google Scholar, Holloszy J.O. PubMed Scopus Google Scholar) increases glucose The of these is that MGU increases and the primary barrier to glucose in the muscle to glucose when HK II is the to glucose phosphorylation is and, MGU is enhanced during exercise when GLUT4 content is by MGU is not during exercise MGU is increased in However, the enhanced exercise-stimulated or Rg from HK II overexpression is not when with partial GLUT4 Thus, glucose transport becomes a of to MGU when glucose phosphorylation capacity is increased. partial ablation of GLUT4 not MGU during sedentary or exercising conditions, it MGU. in isolated muscle (10Stenbit A.E. Tsao T.S. Li J. Burcelin R. Geenen D.L. Factor S.M. Houseknecht K. Katz E.B. Charron M.J. Nat. Med. 1997; 3: 1096-1101Crossref PubMed Scopus (273) Google Scholar, T.S. Stenbit A.E. Factor S.M. Chen Charron M.J. 1999; PubMed Scopus (47) Google Scholar) and for the muscle observed in and HKTg + mice. The in blood glucose during exercise observed in mice for reductions in glucose increase in glucose increase the from the plasma to the of the and increase glucose delivery and glucose transport and glucose influx. The for this is but a to be in this a from the muscle that to the to increase glucose for such in mice with a reduction of GLUT4 in E. Nature. PubMed Scopus Google Scholar). reductions in mice had a during exercise similar to that of WT mice. HKTg + not or not during exercise, most to the observed in the basal In a similar the be the of glucose uptake by to for fatty J. Houseknecht Stenbit A.E. Katz E.B. Charron M.J. J. 2000; PubMed Scopus (47) Google Scholar). The on fatty as a and in a reduction in the glucose-6-phosphate with of all increases in MGU. of in and plasma glucose in glucose transport by partial GLUT4 knock-out and, MGU to normal these the of the HK II transgene is when with a reduction in GLUT4 The in this study are across muscles with one The exercise-stimulated Rg from SVL muscles is increased in as compared with WT are in soleus and gastrocnemius muscles. be a of the during exercise in mice. on the and reduction in GLUT4 protein content in this muscle, these are because GLUT4 content in a muscle would be MGU during further the of this i.e. that glucose phosphorylation is the primary barrier to MGU during exercise and that the role of glucose phosphorylation in MGU is on GLUT4 for these is by the that HK II overexpression increases exercise-stimulated MGU in all muscles studied and that in a muscle GLUT4 content is not the transgene In a partial ablation of GLUT4 in C57BL/6J mice not impair sedentary or exercise-stimulated of MGU and It the enhanced exercise-stimulated MGU by HK II overexpression. despite a 50% reduction of normal GLUT4 glucose transport is not a significant barrier to exercise-stimulated MGU. However, a 50% reduction in GLUT4 availability glucose transport a limitation to exercise-stimulated MGU when the glucose phosphorylation barrier is by HK II overexpression. The of these is that in MGU with insulin not or in glucose transport are with the that HK II is a for the of that the insulin is not with a glucose transport and of the Vanderbilt for the insulin and Malabanan of the Vanderbilt Mouse for