Journal of Applied Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

J Appl Physiol 101: 1803-1805, 2006; doi:10.1152/japplphysiol.00817a.2006
8750-7587/06 $8.00
This Article
Right arrow Full Text (PDF) Free
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wasserman, D. H.
Right arrow Articles by Fueger, P. T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wasserman, D. H.
Right arrow Articles by Fueger, P. T.

POINT-COUNTERPOINT

Point-Counterpoint: Glucose phosphorylation is/is not a significant barrier to muscle glucose uptake by the working muscle

David H. Wasserman

Department of Molecular Physiology and Biophysics and Mouse Metabolic Phenotyping Center
Vanderbilt University School of Medicine
Nashville, Tennessee
e-mail: david.wasserman{at}vanderbilt.edu

Patrick T. Fueger

Sarah W. Stedman Nutrition and Metabolism Center
Department of Pharmacology and Cancer Biology
Duke University Medical Center
Durham, North Carolina

POINT: GLUCOSE PHOSPHORYLATION IS A SIGNIFICANT BARRIER TO MUSCLE GLUCOSE UPTAKE BY THE WORKING MUSCLE

The assertion that membrane transport of glucose is rate limiting for muscle glucose uptake (MGU) appears throughout the scientific literature (2, 18, 20, 21, 25, 28–30). There is no debate that facilitated glucose transport is essential to MGU. In particular, the quantity of glucose transport protein, GLUT4, in the sarcolemma is closely related to MGU (11). Glucose phosphorylation, as the first committed step, is also essential to MGU. This step is catalyzed by hexokinase (HK) I and HK II. The issue is whether the ability to phosphorylate glucose is always adequate to handle flux across the sarcolemma or whether there are times of high glucose flux, like exercise, when it is not. This has not been a simple issue to resolve because of the close coupling of glucose transport and phosphorylation and the existence of glucose compartmentalization and spatial gradients in the muscle. Our contention is that exercise is a condition where the sarcolemma is sufficiently permeable to permit glucose entry at rates high enough to challenge phosphorylation capacity.

Muscle membranes of sedentary subjects have a low permeability to glucose. In response to exercise, the sarcolemma becomes considerably more permeable to glucose as GLUT4 translocation to it is accelerated (1, 3, 26). There is evidence from our laboratory that the working muscle becomes freely permeable to glucose and therefore offers no barrier to MGU under these conditions (5, 14). In contrast, it is difficult to know whether the capacity of muscle to phosphorylate glucose is increased with exercise. An increase in soluble HK II activity has been reported (19). However, the only identified allosteric regulator of HK II, glucose 6-phosphate, is inhibitory. This inhibitor can be increased under conditions such as exercise where muscle glycogenolysis is high. Thus exercise creates a situation where glucose should easily pass through the sarcolemma, whereas the ability to phosphorylate it may change very little or be inhibited. These cellular events predict that the site of resistance is shifted from membrane transport to glucose phosphorylation.

The paradigm above could be validated if intracellular glucose, which is the product of membrane glucose transport and the substrate for glucose phosphorylation, were known. However, intracellular glucose can neither be directly measured nor realistically calculated. This is because it requires the difference between two comparatively large numbers (total muscle glucose and interstitial glucose), each requiring inherent measurement errors and assumptions. These create an insurmountable signal-to-noise ratio. The problem gets more complicated when one considers that there is likely to be compartmentalization or spatial gradients of glucose in interstitial and intracellular space. A change could occur in one part of the cell (e.g., inner membrane surface), which may be missed or underestimated because measurements reflect the entire tissue water.

We used two independent rodent models to test the functional control of MGU. The first approach uses isotopic glucose analogs to obtain a surrogate for intracellular glucose in catheterized conscious rats. This is accomplished by applying the isotopic glucose analogs 3-0-[3H]methyl-glucose (3-0-[3H]MG), U-[14C]mannitol (U-[14C]MN), and 2-deoxy[3H]glucose (2-[3H]DG) to principles of glucose countertransport (12–14, 23, 24). Countertransport is defined as the difference in the steady-state distribution of one sugar between intracellular and extracellular water induced by a transmembrane gradient of a second sugar (22). With this technique, the distribution of trace 3-0-[3H]MG between intracellular and extracellular water is determined at steady state to assess the trans-sarcolemmal glucose gradient (TSGG). Because 3-0-[3H]MG is not metabolized, its plasma and interstitial concentrations are equal. Thus plasma 3-0-[3H]MG is combined with tissue measurements to calculate intracellular 3-0-[3H]MG. U-[14C]MN is a membrane-impermeable marker used to calculate the ratio of extracellular to intracellular water. The glucose concentration of the outer ([G]om) and inner ([G]im) surfaces of the sarcolemma can be calculated from this approach and used to determine the TSGG and intracellular glucose available for phosphorylation [G]im. Muscle glucose influx (Rg) or a rate constant for the process (Kg) is assessed from the accumulation rate of phosphorylated 2-[3H]DG. Measurements of TSGG and [G]im can be combined with Rg to assess resistances to glucose flux through the sarcolemma and intracellular metabolism using a variation of Ohm’s law for electrical circuits where TSGG and Rg are analogous to voltage gradient and current. TSGG decreases and Rg increases in working muscle, demonstrating that resistance to membrane glucose transport decreases (i.e., TSGG/Rg decreases) precisely as one would expect from accelerated GLUT4 translocation. In contrast, [G]im increases in the presence of the increase in Rg and resistance at intracellular phosphorylation increases. Results obtained using the countertransport method clearly demonstrate a shift in the barriers to MGU during exercise so that phosphorylation is the chief site of resistance.

The second approach used to dissect control of MGU was to genetically increase muscle GLUT4 and HK II. The hypothesis is that HK II overexpression would increase the ability of working muscle to consume glucose, whereas GLUT4 overexpression would have no effect were it tested (5, 15). Mice overexpressing GLUT4 (GLUT4Tg) or HK II (HKTg) were catheterized chronically, and Rg was measured isotopically during treadmill exercise. Consistent with predictions of the countertransport approach, HKTg increased exercise-stimulated Rg, whereas GLUT4Tg did not. A corollary to these findings is that reduced HK II would have a diminished exercise-stimulated MGU. This was the case as the Rg response to exercise in mice with a heterozygous HK II deletion was diminished (6). Figure 1 is a compilation of studies (4, 5, 8, 10) conducted in mice with varying degrees of muscle GLUT4 and HK II expression. Several significant points can be garnered from this figure: 1) HK II overexpression has no effect on Kg in sedentary mice, whereas it increases with GLUT4 overexpression; 2) 50% of normal GLUT4 expression is adequate for the stimulatory effect of exercise on Kg, and GLUT4 overexpression causes no added stimulation, and 3) HK II overexpression, on the other hand, causes a nearly twofold increase in the Vmax for Kg of working muscle.


Figure 1
View larger version (10K):
[in this window]
[in a new window]
  		Fig. 1. Dependence of muscle glucose uptake on glucose transport and phosphorylation. A compilation of studies (3, 4, 7, 9) conducted in sedentary and exercising mice with varying degrees of muscle GLUT4 expression (GLUT4 expression in wild-type (WT) mice is set equal to 1) that have WT expression of hexokinase (HK) II or HK II overexpression. Several significant points can be garnered from this figure: 1) in sedentary mice HK II overexpression has no effect on the rate constant for muscle glucose uptake (Kg), whereas it increases with increased GLUT4 overexpression; 2) 50% of normal GLUT4 expression is completely adequate for the stimulatory effect of exercise on Kg and GLUT4 overexpression causes no further effect, and 3) HK II overexpression causes a nearly twofold increase in the Vmax with which glucose is consumed by muscle.

 
In understanding the control of MGU, it is important to recognize that the same shift in control from transport to phosphorylation seen with exercise is also a characteristic of insulin stimulation determined using countertransport and the same mouse models described above (4, 7, 9, 13–15). Insulin-stimulated GLUT4 translocation to the sarcolemma decreases resistance at the transport step, shifting control to phosphorylation. Does the shift in control evident in rodent exercise models also occur in exercising humans? It is difficult to apply the tests used in rodents to humans, and tools to assess control of MGU in humans (e.g., positron emission tomography) cannot currently be applied to working muscle. Biopsy techniques have been used to show that resting human muscle was 2.1 ± 0.3 mmol/kg and rises to 3.3 ± 0.4 mmol/kg after 10 min of moderate exercise and 11.6 ± 0.7 mmol/kg at exhaustive high-intensity exercise (16). Although these measurements are whole tissue and are not intracellular, the increased concentrations are still consistent with a shift in control from membrane transport to phosphorylation. Factors such as muscle fiber type (12, 24), nutritional state (e.g., glycogen stored; 15, 27), and exercise duration (17) may also influence the control of MGU by transport and phosphorylation due to effects on muscle glucose 6-phosphate and, possibly, fatty acid availability.

REFERENCES

  1. Brozinick JT Jr, Etgen GJ Jr, Yaspelkis BB III, Ivy JL. The effects of muscle contraction and insulin on glucose-transporter translocation in rat skeletal muscle. Biochem J 297: 539–545, 1994.[ISI][Medline]
  2. Cleasby ME, Davey JR, Reinten TA, Graham MW, James DE, Kraegen EW, and Cooney GJ. Acute bidirectional manipulation of muscle glucose uptake by in vivo electrotransfer of constructs targeting glucose transporter genes. Diabetes 54: 2702–2711, 2005.[Abstract/Free Full Text]
  3. Etgen GJ Jr, Farrar RP, and Ivy JL. Effect of chronic electrical stimulation on GLUT-4 protein content in fast-twitch muscle. Am J Physiol Regul Integr Comp Physiol 264: R816–R819, 1993.[Abstract/Free Full Text]
  4. Fueger PT, Bracy DP, Malabanan CM, Pencek RR, Granner DK, and Wasserman DH. Hexokinase II overexpression improves exercise-stimulated but not insulin-stimulated muscle glucose uptake in high-fat-fed C57BL/6J mice. Diabetes 53: 306–314, 2004.[Abstract/Free Full Text]
  5. Fueger PT, Bracy DP, Malabanan CM, Pencek RR, and Wasserman DH. Distributed control of glucose uptake by working muscles of conscious mice: roles of transport and phosphorylation. Am J Physiol Endocrinol Metab 286: E77–R84, 2004.[Abstract/Free Full Text]
  6. Fueger PT, Heikkinen S, Bracy DP, Malabanan CM, Pencek RR, Laakso M, and Wasserman DH. Hexokinase II partial knockout impairs exercise-stimulated glucose uptake in oxidative muscles of mice. Am J Physiol Endocrinol Metab 285: E958–E963, 2003.[Abstract/Free Full Text]
  7. Fueger PT, Hess HS, Bracy DP, Pencek RR, Posey KA, Charron MJ, and Wasserman DH. Regulation of insulin-stimulated muscle glucose uptake in the conscious mouse: role of glucose transport is dependent on glucose phosphorylation capacity. Endocrinology 145: 4912–4916, 2004.[Abstract/Free Full Text]
  8. Fueger PT, Hess HS, Posey KA, Bracy DP, Pencek RR, Charron MJ, and Wasserman DH. Control of exercise-stimulated muscle glucose uptake by GLUT4 is dependent on glucose phosphorylation capacity in the conscious mouse. J Biol Chem 279: 50956–50961, 2004.[Abstract/Free Full Text]
  9. Fueger PT, Shearer J, Bracy DP, Posey KA, Pencek RR, McGuinness OP, and Wasserman DH. Control of muscle glucose uptake: test of the rate-limiting step paradigm in conscious, unrestrained mice. J Physiol 562: 925–935, 2005.[Abstract/Free Full Text]
  10. Fueger PT, Shearer J, Krueger TM, Posey KA, Bracy DP, Heikkinen S, Laakso M, Rottman JN, and Wasserman DH. Hexokinase II protein content is a determinant of exercise endurance capacity in the mouse. J Physiol 566: 533–541, 2005.[Abstract/Free Full Text]
  11. Goodyear LJ and Kahn BB. Exercise, glucose transport, and insulin sensitivity. Annu Rev Med 49: 235–261, 1998.[CrossRef][ISI][Medline]
  12. Halseth AE, Bracy DP, and Wasserman DH. Functional limitations to glucose uptake in muscles comprised of different fiber types. Am J Physiol Endocrinol Metab 280: E994–E999, 2001.[Abstract/Free Full Text]
  13. Halseth AE, Bracy DP, and Wasserman DH. Limitations to basal and insulin-stimulated skeletal muscle glucose uptake in the high-fat-fed rat. Am J Physiol Endocrinol Metab 279: E1064–E1071, 2000.[Abstract/Free Full Text]
  14. Halseth AE, Bracy DP, and Wasserman DH. Limitations to exercise- and maximal insulin-stimulated muscle glucose uptake. J Appl Physiol 85: 2305–2313, 1998.[Abstract/Free Full Text]
  15. Halseth AE, Bracy DP, and Wasserman DH. Overexpression of hexokinase II increases insulin and exercise-stimulated muscle glucose uptake in vivo. Am J Physiol Endocrinol Metab 276: E70–E77, 1999.[Abstract/Free Full Text]
  16. Katz A, Broberg S, Sahlin K, and Wahren J. Leg glucose uptake during maximal dynamic exercise in humans. Am J Physiol Endocrinol Metab 251: E65–E70, 1986.[Abstract/Free Full Text]
  17. Katz A, Sahlin K, and Broberg S. Regulation of glucose utilization in human skeletal muscle during moderate dynamic exercise. Am J Physiol Endocrinol Metab 260: E411–E415, 1991.[Abstract/Free Full Text]
  18. Koistinen HA and Zierath JR. Regulation of glucose transport in human skeletal muscle. Ann Med 34: 410–418, 2002.[CrossRef][ISI][Medline]
  19. Koval JA, DeFronzo RA, O’Doherty RM, Printz R, Ardehali H, Granner DK, and Mandarino LJ. Regulation of hexokinase II activity and expression in human muscle by moderate exercise. Am J Physiol Endocrinol Metab 274: E304–E308, 1998.[Abstract/Free Full Text]
  20. Marshall BA, Hansen PA, Ensor NJ, Ogden MA, and Mueckler M. GLUT-1 or GLUT-4 transgenes in obese mice improve glucose tolerance but do not prevent insulin resistance. Am J Physiol Endocrinol Metab 276: E390–E400, 1999.[Abstract/Free Full Text]
  21. Marshall BA and Mueckler MM. Differential effects of GLUT-1 or GLUT-4 overexpression on insulin responsiveness in transgenic mice. Am J Physiol Endocrinol Metab 267: E738–E744, 1994.[Abstract/Free Full Text]
  22. Morgan HE, Regen DM, and Park CR. Identification of a mobile carrier-mediated sugar transport system in muscle. J Biol Chem 239: 369–374, 1964.[Free Full Text]
  23. O’Doherty RM, Halseth AE, Granner DK, Bracy DP, and Wasserman DH. Analysis of insulin-stimulated skeletal muscle glucose uptake in conscious rat using isotopic glucose analogs. Am J Physiol Endocrinol Metab 274: E287–E296, 1998.[Abstract/Free Full Text]
  24. Petersen HA, Fueger PT, Bracy DP, Wasserman DH, and Halseth AE. Fiber type-specific determinants of Vmax for insulin-stimulated muscle glucose uptake in vivo. Am J Physiol Endocrinol Metab 284: E541–E548, 2003.[Abstract/Free Full Text]
  25. Petersen KF and Shulman GI. Pathogenesis of skeletal muscle insulin resistance in type 2 diabetes mellitus. Am J Cardiol 90: 11G–18G, 2002.[ISI][Medline]
  26. Ploug T, Galbo H, Ohkuwa T, Tranum-Jensen J, and Vinten J. Kinetics of glucose transport in rat skeletal muscle membrane vesicles: effects of insulin and contractions. Am J Physiol Endocrinol Metab 262: E700-E711, 1992.[Abstract/Free Full Text]
  27. Ren JM, Marshall BA, Mueckler MM, McCaleb M, Amatruda JM, and Shulman GI. Overexpression of Glut4 protein in muscle increases basal and insulin-stimulated whole body glucose disposal in conscious mice. J Clin Invest 95: 429–432, 1995.[ISI][Medline]
  28. Treadway JL, Hargrove DM, Nardone NA, McPherson RK, Russo JF, Milici AJ, Stukenbrok HA, Gibbs EM, Stevenson RW, and Pessin JE. Enhanced peripheral glucose utilization in transgenic mice expressing the human GLUT4 gene. J Biol Chem 269: 29956–29961, 1994.[Abstract/Free Full Text]
  29. Tsao TS, Burcelin R, Katz EB, Huang L, and Charron MJ. Enhanced insulin action due to targeted GLUT4 overexpression exclusively in muscle. Diabetes 45: 28–36, 1996.[Abstract]
  30. Wallberg-Henriksson H and Zierath JR. GLUT4: a key player regulating glucose homeostasis? Insights from transgenic and knockout mice (review). Mol Membr Biol 18: 205–211, 2001.[CrossRef][ISI][Medline]



This article has been cited by other articles:


Home page
 Am. J. Physiol. Regul. Integr. Comp. Physiol.Home page
G. Raja, L. Brau, T. N. Palmer, and P. A. Fournier
Fiber-specific responses of muscle glycogen repletion in fasted rats physically active during recovery from high-intensity physical exertion
Am J Physiol Regulatory Integrative Comp Physiol, August 1, 2008; 295(2): R633 - R641.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Full Text (PDF) Free
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Wasserman, D. H.
Right arrow Articles by Fueger, P. T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Wasserman, D. H.
Right arrow Articles by Fueger, P. T.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2006 by the American Physiological Society.