Exercise induces recruitment of the “insulin-responsive glucose transporter”. Evidence for distinct intracellular insulin- and exercise-recruitable transporter pools in skeletal muscle

Effect of different carbohydrate drinks on whole body carbohydrate storage after exhaustive exercise

Seven untrained male subjects participated in a double-blind, crossover study conducted to determine the efficacy of different carbohydrate drinks in promoting carbohydrate storage in the whole body and skeletal muscle during recovery from exhaustive exercise. The postabsorptive subjects first completed an exercise protocol designed to deplete muscle fibers of glycogen, then consumed 330 ml of one of three carbohydrate drinks (18.5% glucose polymer, 18.5% sucrose, or 12% sucrose; wt/vol) and also received a primed constant infusion of [1-(13)C]glucose for 2 h. Nonoxidative glucose disposal (3.51 +/- 0.28, 18.5% glucose polymer; 2.96 +/- 0.32, 18.5% sucrose; 2.97 +/- 0.16, 12% sucrose; all mmol. kg(-1). h(-1)) and storage of muscle glycogen (5.31 +/- 1.11, 18.5% glucose polymer; 4.07 +/- 1.05, 18.5% sucrose; 3.45 +/- 0.85, 12% sucrose; all mmol. kg wet wt(-1). h(-1); P < 0.05) were greater after consumption of the glucose polymer drink than after either sucrose drink. The results suggest that the consumption of a glucose polymer drink (containing 61 g carbohydrate) promotes a more rapid storage of carbohydrate in the whole body, skeletal muscle in particular, than an isoenergetic sucrose drink.

Exercise-induced changes in expression and activity of proteins involved in insulin signal transduction in skeletal muscle: differential effects on insulin-receptor substrates 1 and 2

Level of physical activity is linked to improved glucose homeostasis. We determined whether exercise alters the expression and/or activity of proteins involved in insulin-signal transduction in skeletal muscle. Wistar rats swam 6 h per day for 1 or 5 days. Epitrochlearis muscles were excised 16 h after the last exercise bout, and were incubated with or without insulin (120 nM). Insulin-stimulated glucose transport increased 30% and 50% after 1 and 5 days of exercise, respectively. Glycogen content increased 2- and 4-fold after 1 and 5 days of exercise, with no change in glycogen synthase expression. Protein expression of the glucose transporter GLUT4 and the insulin receptor increased 2-fold after 1 day, with no further change after 5 days of exercise. Insulin-stimulated receptor tyrosine phosphorylation increased 2-fold after 5 days of exercise. Insulin-stimulated tyrosine phosphorylation of insulin-receptor substrate (IRS) 1 and associated phosphatidylinositol (PI) 3-kinase activity increased 2.5- and 3. 5-fold after 1 and 5 days of exercise, despite reduced (50%) IRS-1 protein content after 5 days of exercise. After 1 day of exercise, IRS-2 protein expression increased 2.6-fold and basal and insulin-stimulated IRS-2 associated PI 3-kinase activity increased 2. 8-fold and 9-fold, respectively. In contrast to IRS-1, IRS-2 expression and associated PI 3-kinase activity normalized to sedentary levels after 5 days of exercise. Insulin-stimulated Akt phosphorylation increased 5-fold after 5 days of exercise. In conclusion, increased insulin-stimulated glucose transport after exercise is not limited to increased GLUT4 expression. Exercise leads to increased expression and function of several proteins involved in insulin-signal transduction. Furthermore, the differential response of IRS-1 and IRS-2 to exercise suggests that these molecules have specialized, rather than redundant, roles in insulin signaling in skeletal muscle.

Endocytosis in skeletal muscle fibers

Defining the organization of endocytic pathway in multinucleated skeletal myofibers is crucial to understand the routing of membrane proteins, such as receptors and glucose transporters, through this system. Here we analyzed the organization of the endocytic trafficking pathways in isolated rat myofibers. We found that sarcolemmal-coated pits and transferrin receptors were concentrated in the I band areas. Fluid phase markers were taken up into vesicles in the same areas along the whole length of the fibers and were then delivered into structures around and between the nuclei. These markers also accumulated beneath the neuromuscular and myotendinous junctions. The recycling compartment, labeled with transferrin, appeared as perinuclear and interfibrillar dots that partially colocalized with the GLUT4 compartment. Low-density lipoprotein, a marker of the lysosome-directed pathway, was transported into sparsely distributed perinuclear and interfibrillar dots that contacted microtubules. A majority of these dots did not colocalize with internalized transferrin, indicating that the recycling and the lysosome-directed pathways were distinct. In conclusion, the I band areas were active in endocytosis along the whole length of the multinucleated myofibers. The sorting endosomes distributed in a cross-striated fashion while the recycling and late endosomal compartments showed perinuclear and interfibrillar localizations and followed the course of microtubules.

Biochemical and functional evidences for a GLUT-4 homologous protein in avian skeletal muscle

The characteristics and modulation of glucose transport were investigated in skeletal muscles of 5-wk-old Muscovy ducklings (Cairina moschata). Glucose uptake by sarcolemmal vesicles isolated from gastrocnemius muscle followed typical Michaelis-Menten kinetics with a K(m) value (17 mM) similar to that described in equivalent mammalian preparations. Western blot analysis of duckling sarcolemma using antibodies directed against rat GLUT-4 transporter revealed an immunoreactive protein of similar molecular mass (45 kDa) to that present in rats. When ducklings were killed in the postabsorptive state, GLUT-4 homologous protein was located predominantly (80%) in intracellular membranes. Insulin stimulation of a perfused leg muscle preparation in vitro led to the translocation of GLUT-4 homologous proteins from intracellular pools to the sarcolemma, with a subsequent increase in glucose uptake by sarcolemmal vesicles and perfused muscles. Glucose transport was positively controlled by the metabolic needs of skeletal muscle as reflected by the increased glucose uptake of sarcolemmal vesicles isolated from cold-acclimated ducklings. Present results, therefore, demonstrate, for the first time in an avian species, the existence in skeletal muscle of a glucose transporter showing molecular and functional homologies with the mammalian GLUT-4 transporter.

Postexercise glucose uptake and glycogen synthesis in skeletal muscle from GLUT4-deficient mice

To determine the role of GLUT4 on postexercise glucose transport and glycogen resynthesis in skeletal muscle, GLUT4-deficient and wild-type mice were studied after a 3 h swim exercise. In wild-type mice, insulin and swimming each increased 2-deoxyglucose uptake by twofold in extensor digitorum longus muscle. In contrast, insulin did not increase 2-deoxyglucose glucose uptake in muscle from GLUT4-null mice. Swimming increased glucose transport twofold in muscle from fed GLUT4-null mice, with no effect noted in fasted GLUT4-null mice. This exercise-associated 2-deoxyglucose glucose uptake was not accompanied by increased cell surface GLUT1 content. Glucose transport in GLUT4-null muscle was increased 1.6-fold over basal levels after electrical stimulation. Contraction-induced glucose transport activity was fourfold greater in wild-type vs. GLUT4-null muscle. Glycogen content in gastrocnemius muscle was similar between wild-type and GLUT4-null mice and was reduced approximately 50% after exercise. After 5 h carbohydrate refeeding, muscle glycogen content was fully restored in wild-type, with no change in GLUT4-null mice. After 24 h carbohydrate refeeding, muscle glycogen in GLUT4-null mice was restored to fed levels. In conclusion, GLUT4 is the major transporter responsible for exercise-induced glucose transport. Also, postexercise glycogen resynthesis in muscle was greatly delayed; unlike wild-type mice, glycogen supercompensation was not found. GLUT4 it is not essential for glycogen repletion since muscle glycogen levels in previously exercised GLUT4-null mice were totally restored after 24 h carbohydrate refeeding.-Ryder, J. W., Kawano, Y., Galuska, D., Fahlman, R., Wallberg-Henriksson, H., Charron, M. J., Zierath, J. R. Postexercise glucose uptake and glycogen synthesis in skeletal muscle from GLUT4-deficient mice.

NMR of glycogen in exercise

Natural-abundance 13C NMR spectroscopy is a non-invasive technique that enables in vivo assessments of muscle and/or liver glycogen concentrations. Over the last several years, 13C NMR has been developed and used to obtain information about human glycogen metabolism with diet and exercise. Since NMR is non-invasive, more data points are available over a specified time course, dramatically improving the time resolution. This improved time resolution has enabled the documentation of subtleties of muscle glycogen re-synthesis following severe glycogen depletion that were not previously observed. Muscle and liver glycogen concentrations have been tracked in several different human populations under conditions that include: (1) muscle glycogen recovery from intense localized exercise with normal insulin and with insulin suppressed; (2) muscle glycogen recovery in an insulin-resistant population; (3) muscle glycogen depletion during prolonged low-intensity exercise; (4) effect of a mixed meal on postprandial muscle and liver glycogen synthesis. The present review focuses on basic 13C NMR and gives results from selected studies.

Interaction of physical activity and diet: implications for insulin-glucose dynamics

In Western countries 25-35% of the population have insulin resistance syndrome characteristics. The defects most likely to explain the insulin resistance of the insulin resistance syndrome include: 1) the glucose transport system of skeletal muscle (GLUT-4) and its different signalling proteins and enzymes; 2) glucose phosphorylation by hexokinase; 3) glycogen synthase activity and 4) competition between glucose and fatty acid oxidation (glucose-fatty acid cycle). High carbohydrate/low fat diets deteriorate insulin sensitivity on the short term. However, on the long term, high fat/low carbohydrate diets have a lower satiating power, induce low leptin levels and eventually lead to higher energy consumption, obesity and more insulin resistance. Moderately high-carbohydrate (45-55% of the daily calories)/low-fat diets seem to be a good choice with regard to the prevention of diabetes and cardiovascular risk factors as far as the carbohydrates are rich in fibers. Long-term interventions with regular exercise programs show a 1/3 decrease in the appearance of overt diabetes in glucose intolerant subjects. Furthermore, diet and exercise interventions "normalise" the mortality rate of patients with impared glucose tolerance. Therefore, moderately high carbohydrate/low fat diets are most likely to prevent obesity and type 2 diabetes. Triglycerides should be monitored and, in some cases, a part of the carbohydrates could be replaced by fat rich in monounsaturated fatty acids. However, total caloric intake is of utmost importance, as weight gain is the major determinant for the onset of insulin resistance and glucose intolerance. Regular (when possible daily) exercise, decreases cardiovascular risk. With regard to insulin resistance, resistance training seems to offer some advantages over aerobic endurance activities.

GLUT-4myc ectopic expression in L6 myoblasts generates a GLUT-4-specific pool conferring insulin sensitivity

Insulin stimulates glucose uptake into muscle and fat cells via recruitment of the glucose transporter 4 (GLUT-4) from intracellular store(s) to the cell surface. Robust stimulation of glucose uptake by insulin coincides with the expression of GLUT-4 during differentiation of muscle and fat cells, but it is not known if GLUT-4 expression suffices to confer insulin sensitivity to glucose uptake. We have therefore examined the effect of expression of a myc epitope-tagged GLUT-4 (GLUT-4myc) into L6 myoblasts, which do not express endogenous GLUT-4 until differentiated into myotubes. Ectopic expression of GLUT-4myc markedly improved insulin sensitivity of glucose uptake in L6 myoblasts. The GLUT-4myc protein distributed equally to the cell surface and intracellular compartments in myoblasts, and the intracellular fraction of GLUT-4myc further increased in myotubes. In myoblasts, the intracellular GLUT-4myc compartment contained the majority of the insulin-regulatable amino peptidase (IRAP) but less than half of the GLUT-1, suggesting segregation of GLUT-4myc and IRAP to a specific cellular locus. Insulin stimulation of phosphatidylinositol 3-kinase and protein kinase B-alpha activities was similar for L6-GLUT-4myc myoblasts and myotubes. At both stages, GLUT-4myc responded to insulin by translocating to the cell surface. These results suggest that GLUT-4myc segregates into a specific compartment in L6 myoblasts and confers insulin sensitivity to these cells. L6-GLUT-4myc myoblasts, which are easily transfectable with various constructs, are a useful resource to study insulin action.

Calphostin C is an inhibitor of contraction, but not insulin-stimulated glucose transport, in skeletal muscle

Wortmannin selectively impairs insulin-stimulated glucose transport in skeletal muscle. To search for an inhibitor specific for contraction-stimulated glucose transport, we screened a number of calmodulin and PKC inhibitors for their ability to impair contraction- and insulin-stimulated 2-deoxyglucose uptake in incubated rat soleus muscles. In concentrations that did not reduce contraction-induced force output, among calmodulin inhibitors W-7 inhibited both contraction- and insulin-stimulated glucose transport by up to 50% (P < 0.05), while Calmidazolium impaired only insulin-stimulated glucose transport (P < 0.05), and Trifluoperazine and Phenoxybenzamine did not influence glucose transport. In concentrations that did not reduce force generation, among PKC inhibitors Calphostin C specifically inhibited contraction-stimulated glucose transport (P < 0.05), whereas insulin-stimulated transport was impaired by Rottlerin and Bisindolylmaleimide I (P < 0.05), and both contraction- and insulin-stimulated glucose transport were inhibited by RO-31-8220 (P < 0.05). Calphostin C did not reduce contraction-induced increase in AMP-activated protein kinase (AMPK) activity. In conclusion, we have identified specific inhibitors of both contraction- and insulin-stimulated glucose transport. Both calmodulin and different isoenzymes of the PKC family may be involved in contraction- and insulin-stimulated glucose transport. Calphostin C does not influence glucose transport during contractions via stimulation of AMPK. Calphostin C may be used to unravel signal transduction in contraction-stimulated glucose transport.

Effect of tension on contraction-induced glucose transport in rat skeletal muscle

We questioned the general view that contraction-induced muscle glucose transport only depends on stimulation frequency and not on workload. Incubated soleus muscles were electrically stimulated at a given pattern for 5 min. Resting length was adjusted to achieve either no force (0% P), maximum force (100% P), or 50% of maximum force (50% P). Glucose transport (2-deoxy-D-glucose uptake) increased directly with force development (P < 0.05) [27 +/- 2 (basal), 45 +/- 2 (0% P), 68 +/- 3 (50% P), and 94 +/- 3 (100% P) nmol. g(-1). 5 min(-1)]. Glycogen decreased at 0% P but did not change further with force development (P > 0.05). Lactate, AMP, and IMP concentrations were higher (P < 0.05) and ATP concentrations lower (P < 0.05) when force was produced than when it was not. 5'-AMP-activated protein kinase (AMPK) activity increased directly with force [20 +/- 2 (basal), 60 +/- 11 (0% P), 91 +/- 12 (50% P), and 109 +/- 12 (100% P) pmol. mg(-1). min(-1)]. Passive stretch (approximately 86% P) doubled glucose transport without altering metabolism. In conclusion, contraction-induced muscle glucose transport varies directly with force development and is not solely determined by stimulation frequency. AMPK activity is probably an essential determinant of contraction-induced glucose transport. In contrast, glycogen concentrations per se do not play a major role. Finally, passive stretch per se increases glucose transport in muscle.

Hormonal and metabolic responses to maintained hyperglycemia during prolonged exercise

We studied the effects of maintained hyperglycemia (12 mmol/l) on endurance exercise to determine the hormonal and metabolic responses, the maximal rate of glucose infusion (i.e., utilization), and the effects on muscle glycogen stores. Eight men undertook two trials during which they exercised on a cycle ergometer at an intensity of approximately 70% peak O(2) uptake for 120 min. In the first trial (trial A), subjects had their blood glucose concentration clamped at 12 mmol/l 30 min before exercise and throughout exercise. The same rate and volume of infusion of saline as had occurred for trial A were used in a placebo trial (trial B). Maintained hyperglycemia resulted in significantly lowered plasma concentrations of nonesterified fatty acid, glycerol, 3-hydroxybutyrate, epinephrine, norepinephrine, and growth hormone (P < 0.001) during exercise, whereas concentrations of plasma insulin were significantly elevated (P < 0.001). Calculations of the rates of total carbohydrate oxidation showed that trial A resulted in significantly higher values when compared with trial B (P < 0.01) and that the maximal rates of glucose infusion varied between 1.33 and 2.78 g/min at 100-120 min. Muscle glycogen concentrations were significantly depleted (P < 0.01) after both trials (trial A, 170.3 micromol/g dry wt decrease; trial B, 206 micromol/g dry wt decrease), although this apparent difference may be accounted for by storage of 22.6 g glucose during the 30-min prime infusion. The results from this study confirm that maintained hyperglycemia attenuates the hormonal response and promotes carbohydrate oxidation and utilization and that muscle glycogen may not be spared.

Regulation of myocardial glucose uptake and transport during ischemia and energetic stress

Myocardial glucose utilization increases in response to the energetic stress imposed on the heart by exercise, pressure overload, and myocardial ischemia. Recruitment of glucose transport proteins is the cellular mechanism by which the heart increases glucose transport for subsequent metabolism. Moderate regional ischemia leads to the translocation of both glucose transporters, GLUT4 and GLUT1, to the sarcolemma in vivo. Myocardial ischemia also stimulates 5'-adenosine monophosphate-activated protein kinase, which may be a fuel gauge in the heart and other tissues signaling the need to turn on energy-generating metabolic pathways. Pharmacologic stimulation of this kinase increases cardiac glucose uptake and transporter translocation, suggesting that it may play an important role in augmenting glucose entry in the setting of ischemic or energetic stress. Thus, recent work has provided insight into the cellular and molecular mechanisms responsible for glucose uptake during energetic stress, which may lead to new approaches to the treatment of patients with coronary artery disease.

Protein kinase B/Akt participates in GLUT4 translocation by insulin in L6 myoblasts

L6 myoblasts stably transfected with a GLUT4 cDNA harboring an exofacial myc epitope tag (L6-GLUT4myc myoblasts) were used to study the role of protein kinase B alpha (PKBalpha)/Akt1 in the insulin-induced translocation of GLUT4 to the cell surface. Surface GLUT4myc was detected by immunofluorescent labeling of the myc epitope in nonpermeabilized cells. Insulin induced a marked translocation of GLUT4myc to the plasma membrane within 20 min. This was prevented by transient transfection of a dominant inhibitory construct of phosphatidylinositol (PI) 3-kinase (Deltap85alpha). Transiently transfected cells were identified by cotransfection of green fluorescent protein. A constitutively active PKBalpha, created by fusion of a viral Gag protein at its N terminus (GagPKB), increased the cell surface density of GLUT4myc compared to that of neighboring nontransfected cells. A kinase-inactive, phosphorylation-deficient PKBalpha/Akt1 construct with the mutations K179A (substitution of alanine for the lysine at position 179), T308A, and S473A (AAA-PKB) behaved as a dominant-negative inhibitor of insulin-dependent activation of cotransfected wild-type hemagglutinin (HA)-tagged PKB. Furthermore, AAA-PKB markedly inhibited the insulin-induced phosphorylation of cotransfected BAD, demonstrating inhibition of the endogenous PKB/Akt. Under the same conditions, AAA-PKB almost entirely blocked the insulin-dependent increase in surface GLUT4myc. PKBalpha with alanine substitutions T308A and S473A (AA-PKB) or K179A (A-PKB) alone was a less potent inhibitor of insulin-dependent activation of wild-type HA-PKB or GLUT4myc translocation than was AAA-PKB. Cotransfection of AAA-PKB with a fourfold DNA excess of HA-PKB rescued insulin-stimulated GLUT4myc translocation. AAA-PKB did not prevent actin bundling (membrane ruffling), though this response was PI 3-kinase dependent. Therefore, it is unlikely that AAA-PKB acted by inhibiting PI 3-kinase signaling. These results outline an important role for PKBalpha/Akt1 in the stimulation of glucose transport by insulin in muscle cells in culture.

GLUT4 trafficking in insulin-sensitive cells. A morphological review

In recent years, there have been major advances in the understanding of both the cell biology of vesicle trafficking between intracellular compartments and the molecular targeting signals intrinsic to the trafficking proteins themselves. One system to which these advances have been profitably applied is the regulation of the trafficking of a glucose transporter, GLUT4, from intracellular compartment(s) to the cell surface in response to insulin. The unique nature of the trafficking of GLUT4 and its expression in highly differentiated cells makes this a question of considerable interest to cell biologists. Unraveling the tangled web of molecular events coordinating GLUT4 trafficking will eventually lead to a greater understanding of mammalian glucose metabolism, as well as fundamental cell biological principles related to organelle biogenesis and protein trafficking.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

GLUT-1 or GLUT-4 transgenes in obese mice improve glucose tolerance but do not prevent insulin resistance

Insulin-stimulated glucose uptake is defective in patients with type 2 diabetes. To determine whether transgenic glucose transporter overexpression in muscle can prevent diabetes induced by a high-fat, high-sugar diet, singly (GLUT-1, GLUT-4) and doubly (GLUT-1 and -4) transgenic mice were placed on a high-fat, high-sugar diet or a standard chow diet. On the high-fat, high-sugar diet, wild-type but not transgenic mice developed fasting hyperglycemia and glucose intolerance (peak glucose of 337 +/- 19 vs. 185-209 mg/dl in the same groups on the high-fat, high-sugar diet and 293 +/- 13 vs. 166-194 mg/dl on standard chow). Hyperinsulinemic clamps showed that transporter overexpression elevated insulin-stimulated glucose utilization on standard chow (49 +/- 4 mg. kg-1. min-1 in wild-type vs. 61 +/- 4, 67 +/- 5, and 63 +/- 6 mg. kg-1. min-1 in GLUT-1, GLUT-4, and GLUT-1 and -4 transgenic mice given 20 mU. kg-1. min-1 insulin, and 54 +/- 7, 85 +/- 4, and 98 +/- 11 in wild-type, GLUT-1, and GLUT-4 mice given 60-80 mU. kg-1. min-1 insulin). On the high-fat, high-sugar diet, wild-type and GLUT-1 mice developed marked insulin resistance, but GLUT-4 and GLUT-1 and -4 mice were somewhat protected (glucose utilization during hyperinsulinemic clamp of 28.5 +/- 3.4 vs. 42.4 +/- 5.9, 51.2 +/- 8.1, and 55.9 +/- 4. 9 mg. kg-1. min-1 in wild type, GLUT-1, GLUT-4, GLUT-1 and -4 mice). These data demonstrate that overexpression of GLUT-1 and/or GLUT-4 enhances whole body glucose utilization and prevents the development of fasting hyperglycemia and glucose intolerance induced by a high-fat, high-sugar diet. GLUT-4 overexpression improves the insulin resistance induced by the diet. We conclude that upregulation of glucose transporters in skeletal muscle may be an effective therapeutic approach to the treatment of human type 2 diabetes.

Age-associated alterations in cardiac and skeletal muscle glucose transporters, insulin and IGF-1 receptors, and PI3-kinase protein contents in the C57BL/6 mouse

We investigated potential age-related changes in cardiac and skeletal muscle protein contents of glut-4 and glut-1 transporters, insulin and insulin-like growth factor-1 (IGF-1) receptors, and phosphatidylinositol 3-kinase (PI3-kinase) in the C57B1/6 mouse. Myocardial glut-4 content increased four- to five-fold between mid- to late-adulthood with no further age-related changes. Increases in myocardial glut-1 preceded the increase in glut-4 and was of a much smaller magnitude (25-40%). Skeletal muscle glut-4 was also increased (38-49%) and no further changes were noted between adulthood and old age. Cardiac insulin receptor and the p85 alpha subunit of PI3-kinase both declined by about 40%, whereas the skeletal muscle content of these two proteins were unaffected by aging. Cardiac (-23 to -24%) and skeletal muscle (-40 to -62%) IGF-1 receptor levels were decreased in adult and old animals with senescence being associated with a further decrease in cardiac IGF-1 receptor levels to 20% of controls. A two- to three-fold increase in both basal and maximal in vitro autophosphorylation of the cardiac insulin and IGF-1 receptors by their respective ligands was observed with senescence. It appears that cardiac and skeletal muscle demonstrate differential responses in terms of the magnitude and temporal responses of age-associated alterations in glucose transport related protein contents in the C57B1/6 mouse.

Rapid stimulation of glucose transport by mitochondrial uncoupling depends in part on cytosolic Ca2+ and cPKC

2,4-Dinitrophenol (DNP) uncouples the mitochondrial oxidative chain from ATP production, preventing oxidative metabolism. The consequent increase in energy demand is, however, contested by cells increasing glucose uptake to produce ATP via glycolysis. In L6 skeletal muscle cells, DNP rapidly doubles glucose transport, reminiscent of the effect of insulin. However, glucose transport stimulation by DNP does not require insulin receptor substrate-1 phosphorylation and is wortmannin insensitive. We report here that, unlike insulin, DNP does not activate phosphatidylinositol 3-kinase, protein kinase B/Akt, or p70 S6 kinase. However, chelation of intra- and extracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid-AM in conjunction with EGTA inhibited DNP-stimulated glucose uptake by 78.9 +/- 3.5%. Because Ca2+-sensitive, conventional protein kinase C (cPKC) can activate glucose transport in L6 muscle cells, we examined whether cPKC may be translocated and activated in response to DNP in L6 myotubes. Acute DNP treatment led to translocation of cPKCs to plasma membrane. cPKC immunoprecipitated from plasma membranes exhibited a twofold increase in kinase activity in response to DNP. Overnight treatment with 4-phorbol 12-myristate 13-acetate downregulated cPKC isoforms alpha, beta, and gamma and partially inhibited (45.0 +/- 3.6%) DNP- but not insulin-stimulated glucose uptake. Consistent with this, the PKC inhibitor bisindolylmaleimide I blocked PKC enzyme activity at the plasma membrane (100%) and inhibited DNP-stimulated 2-[3H]deoxyglucose uptake (61.2 +/- 2.4%) with no effect on the stimulation of glucose transport by insulin. Finally, the selective PKC-beta inhibitor LY-379196 partially inhibited DNP effects on glucose uptake (66.7 +/- 1.6%). The results suggest interfering with mitochondrial ATP production acts on a signal transduction pathway independent from that of insulin and partly mediated by Ca2+ and cPKCs, of which PKC-beta likely plays a significant role.

Calorie restriction increases cell surface GLUT-4 in insulin-stimulated skeletal muscle

Reduced calorie intake [calorie restriction (CR); 60% of ad libitum (AL)] leads to enhanced glucose transport without altering total GLUT-4 glucose transporter abundance in skeletal muscle. Therefore, we tested the hypothesis that CR (20 days) alters the subcellular distribution of GLUT-4. Cell surface GLUT-4 content was higher in insulin-stimulated epitrochlearis muscles from CR vs. AL rats. The magnitude of this increase was similar to the CR-induced increase in glucose transport, and GLUT-4 activity (glucose transport rate divided by cell surface GLUT-4) was unaffected by diet. The CR effect was specific to the insulin-mediated pathway, as evidenced by the observations that basal glucose transport and cell surface GLUT-4 content, as well as hypoxia-stimulated glucose transport, were unchanged by diet. CR did not alter insulin's stimulation of insulin receptor substrate (IRS)-1-associated phosphatidylinositol 3-kinase (PI3K) activity. Muscle abundance of IRS-2 and p85 subunit of PI3K were unaltered by diet, but IRS-1 content was lower in CR vs. AL. These data demonstrate that, despite IRS-1-PI3K activity similar to AL, CR specifically increases insulin's activation of glucose transport by enhancing the steady-state proportion of GLUT-4 residing on the cell surface.

Molecular mechanisms involved in GLUT4 translocation in muscle during insulin and contraction stimulation

Studies in mammalian cells have established the existence of numerous intracellular signaling cascades that are critical intermediates in the regulation of various biological functions. Over the past few years considerable research has shown that many of these signaling proteins are expressed in skeletal muscle. However, the detailed mechanisms involved in the regulation of glucose transporter (GLUT4) translocation from intracellular compartments to the cell surface membrane in response to insulin and contractions in skeletal muscle are not well understood. In the present essay we report three different approaches to unravel the GLUT4 translocation mechanism: 1. specific pertubation of the insulin and/or contraction signaling pathways; 2. characterization of the protein composition of GLUT4-containing vesicles with the expectation that knowledge of the constituent proteins of the vesicles may help in understanding their trafficking; 3. degree of co-immunolocalization of the GLUT4 glucose transporters with other membrane marker proteins assessed by immunofluorescense and electron microscopy.

Insulin signaling and glucose transport in insulin resistant skeletal muscle. Special reference to GLUT4 transgenic and GLUT4 knockout mice

Glucose homeostasis is impaired in patients with non-insulin dependent diabetes mellitus (NIDDM) and this defect in due in part, to defects in glucose transport in skeletal muscle. Intense interest is now focused on whether reduced insulin-mediated glucose transport in muscle from NIDDM patients results from alterations in the insulin signal transduction pathway or from alterations in traffic and/or translocation of GLUT4 to the plasma membrane. Recently, potential targets for impaired traffic/translocation of GLUT4 have been reported to include defective phosphorylation of IRS-1 and reduced PI-3 kinase activity. In addition to insulin signaling defects, impaired glucose transport may result from a defect(s) in the activation or functional capacity of GLUT4. Because GLUT4 is dysregulated in skeletal muscle from NIDDM patients, it is an attractive target for gene therapy. Overexpression of GLUT4 in muscle results in increased glucose uptake and metabolism, and protects against the development of insulin resistance in transgenic mice. Genetic ablation of GLUT4 results in impaired insulin tolerance and defects in glucose metabolism in skeletal muscle. Because impaired muscle glucose transport leads to reduced whole body glucose uptake and hyperglycemia, understanding the molecular regulation of glucose transport in skeletal muscle is necessary to develop effective strategies to prevent or reduce the incidence of NIDDM.

An overview of muscle glucose uptake during exercise. Sites of regulation

The uptake of blood glucose by skeletal muscle is a complex process. In order to be metabolized, glucose must travel the path from blood to interstitium to intracellular space and then be phosphorylated to glucose 6-phosphate (G6P). Movement of glucose from blood to interstitium is determined by skeletal muscle blood flow, capillary recruitment and the endothelial permeability to glucose. The influx of glucose from the interstitium to intracellular space is determined by the number of glucose transporters in the sarcolemma and the glucose gradient across the sarcolemma. The capacity to phosphorylate glucose is determined by the amount of skeletal muscle hexokinase II, hexokinase II compartmentalization within the cell, and the concentration of the hexokinase II inhibitor G6P. Any change in glucose uptake occurs due to an alteration in one or more of these steps. Based on the low calculated intracellular glucose levels and the higher affinity of glucose for phosphorylation relative to transport, glucose transport is generally considered rate-determining for basal muscle glucose uptake. Exercise increases both the movement of glucose from blood to sarcolemma and the permeability of the sarcolemma to glucose. Whether the ability to phosphorylate glucose is increased in the working muscle remains to be clearly shown. It is possible that the accelerated glucose delivery and transport rates during exercise bias regulation so that muscle glucose phosphorylation exerts more control on muscle glucose uptake. Conditions that alter glucose uptake during exercise, such as increased NEFA concentrations, decreased oxygen availability and adrenergic stimulation, must work by altering one or more of the three steps involved in glucose uptake. This review describes the regulation of glucose uptake during exercise at each of these sites under a number of conditions, as well as describing muscle glucose uptake in the post-exercise state.

GLUT5 expression and fructose transport in human skeletal muscle

Biochemical and immunocytochemical studies have revealed that, in addition to GLUT1 and GLUT4, human skeletal muscle also expresses the GLUT5 hexose transporter. The subcellular distribution of GLUT5 is distinct from that of GLUT4, being localised exclusively in the sarcolemmal membrane. The substrate selectivity of GLUT5 is also considered to be different to that of GLUT1 and GLUT4 in that it operates primarily as a fructose transporter. Consistent with this suggestion studies in isolated human sarcolemmal vesicles have shown that fructose transport obeys saturable kinetics with a Vmax of 477 +/- 37 pmol.mg protein-1 min-1 and a Km of 8.3 +/- 1.2 mM. Unlike glucose uptake, fructose transport in sarcolemmal vesicles was not inhibited by cytochalasin B suggesting that glucose and fructose are unlikely to share a common route of entry into human muscle. Muscle exercise, which stimulates glucose uptake through the increased translocation of GLUT4 to the plasma membrane, does not increase fructose transport or sarcolemmal GLUT5 content. In contrast, muscle inactivity, induced as a result of limb immobilisation, caused a significant reduction in muscle GLUT4 expression with no detectable effects on GLUT5. The presence of a fructose transporter in human muscle is compatible with studies showing that this tissue can utilise fructose for both glycolysis and glycogenesis. However, the full extent to which provision of fructose via GLUT5 is important in meeting the energy requirements of human muscle during both physiological and pathophysiological circumstances remains an issue requiring further investigation.

Snareing GLUT4 at the plasma membrane in muscle and fat

Explosive advances in the understanding of vesicle trafficking between intracellular compartments have occurred in recent years. These investigations inspired an attractive model for intracellular membrane transport, referred as the SNARE hypothesis. These advances have been profitably applied to one system in muscle and fat; the regulation of intracellular trafficking of the insulin-regulatable facilitative glucose transporter (GLUT4). Investigations in insulin-sensitive cell types revealed a remarkable conservation in the mechanism of vesicular transport between synaptic vesicles in the presynaptic nerve terminal and GLUT4-containing vesicles in muscle and fat. On the other hand, unique players in insulin-regulatable GLUT4 movement have also been clarified during this process. Thus, unveiling the molecular mechanisms regulating insulin-stimulated GLUT4 trafficking will significantly contribute to our understanding of whole body glucose homeostasis as well as the cell biology of protein trafficking, membrane dynamics, and organelle biogenesis.

Analysis of GLUT4 distribution in whole skeletal muscle fibers: identification of distinct storage compartments that are recruited by insulin and muscle contractions

The effects of insulin stimulation and muscle contractions on the subcellular distribution of GLUT4 in skeletal muscle have been studied on a preparation of single whole fibers from the rat soleus. The fibers were labeled for GLUT4 by a preembedding technique and observed as whole mounts by immunofluorescence microscopy, or after sectioning, by immunogold electron microscopy. The advantage of this preparation for cells of the size of muscle fibers is that it provides global views of the staining from one end of a fiber to the other and from one side to the other through the core of the fiber. In addition, the labeling efficiency is much higher than can be obtained with ultracryosections. In nonstimulated fibers, GLUT4 is excluded from the plasma membrane and T tubules. It is distributed throughout the muscle fibers with approximately 23% associated with large structures including multivesicular endosomes located in the TGN region, and 77% with small tubulovesicular structures. The two stimuli cause translocation of GLUT4 to both plasma membrane and T tubules. Quantitation of the immunogold electron microscopy shows that the effects of insulin and contraction are additive and that each stimulus recruits GLUT4 from both large and small depots. Immunofluorescence double labeling for GLUT4 and transferrin receptor (TfR) shows that the small depots can be further subdivided into TfR-positive and TfR-negative elements. Interestingly, we observe that colocalization of TfR and GLUT4 is increased by insulin and decreased by contractions. These results, supported by subcellular fractionation experiments, suggest that TfR-positive depots are only recruited by contractions. We do not find evidence for stimulation-induced unmasking of resident surface membrane GLUT4 transporters or for dilation of the T tubule system (Wang, W., P.A. Hansen, B.A. Marshall, J.O. Holloszy, and M. Mueckler. 1996. J. Cell Biol. 135:415-430).

Structural and physiologic determinants of human erythrocyte sugar transport regulation by adenosine triphosphate

Human erythrocyte sugar transport is mediated by the integral membrane protein GLUT1 and is regulated by cytosolic ATP [Carruthers, A., and Helgerson, A. L. (1989) Biochemistry 28, 8337-8346]. This study asks the following questions. (1) Where is the GLUT1 ATP binding site? (2) Is ATP-GLUT1 interaction sufficient for sugar transport regulation? (3) Is ATP modulation of transport subject to metabolic control? GLUT1 residues 301-364 were identified as one element of the GLUT1 ATP binding domain by peptide mapping and N-terminal sequence analysis of proteolytic fragments of azidoATP-photolabeled GLUT1. Nucleotide binding and sugar transport experiments undertaken with dimeric and tetrameric forms of GLUT1 indicate that only tetrameric GLUT1 binds and is subject to modulation by ATP. Reconstitution experiments indicate that nucleotide and tetrameric GLUT1 are sufficient for ATP modulation of sugar transport. Feedback control of GLUT1 regulation by ATP was investigated by measuring sugar uptake into erythrocyte ghosts containing or lacking ATP and glycolytic intermediates. Only AMP and ADP modulate ATP regulation of transport. Reduced cytosolic pH inhibits ATP modulation of GLUT1-mediated 3OMG uptake and increases Kd(app) for ATP interaction with GLUT1. We conclude that tetrameric but not dimeric GLUT1 is subject to direct regulation by cytosolic ATP and that this regulation is antagonized by intracellular AMP and acidification.

Restoration of hypoxia-stimulated glucose uptake in GLUT4-deficient muscles by muscle-specific GLUT4 transgenic complementation

To investigate whether GLUT4 is required for exercise/hypoxia-induced glucose uptake, we assessed glucose uptake under hypoxia and normoxia in extensor digitorum longus (EDL) and soleus muscles from GLUT4-deficient mice. In EDL and soleus from wild type control mice, hypoxia increased 2-deoxyglucose uptake 2-3-fold. Conversely, hypoxia did not alter 2-deoxyglucose uptake in either EDL or soleus from either male or female GLUT4-null mice. Next we introduced the fast-twitch skeletal muscle-specific MLC-GLUT4 transgene into GLUT4-null mice to determine whether changes in the metabolic milieu accounted for the lack of hypoxia-mediated glucose transport. Transgenic complementation of GLUT4 in EDL was sufficient to restore hypoxia-mediated glucose uptake. Soleus muscles from MLC-GLUT4-null mice were transgene-negative, and hypoxia-stimulated 2-deoxyglucose uptake was not restored. Although ablation of GLUT4 in EDL did not affect normoxic glycogen levels, restoration of GLUT4 to EDL led to an increase in glycogen under hypoxic conditions. Male GLUT4-null soleus displayed reduced normoxic glycogen stores, but female null soleus contained significantly more glycogen under normoxia and hypoxia. Reduced normoxic levels of ATP and phosphocreatine were measured in male GLUT4-null soleus but not in EDL. However, transgenic complementation of GLUT4 prevented the decrease in hypoxic ATP and phosphocreatine levels noted in male GLUT4-null and control EDL. In conclusion, we have demonstrated that GLUT4 plays an essential role in the regulation of muscle glucose uptake in response to hypoxia. Because hypoxia is a useful model for exercise, our results suggest that stimulation of glucose transport in response to exercise in skeletal muscle is totally dependent upon GLUT4. Furthermore, the compensatory glucose transport system that exists in GLUT4-null soleus muscle is not sensitive to hypoxia/muscle contraction.

Binary interactions of the SNARE proteins syntaxin-4, SNAP23, and VAMP-2 and their regulation by phosphorylation

The SNARE hypothesis proposes that synaptic vesicles dock at presynaptic membranes via interactions among the vesicular, integral membrane proteins VAMP (vesicle-associated membrane protein) and synaptotagmin and the target membrane proteins SNAP25 (synaptosome-associated protein with an Mr of 25 kDa) and syntaxin-1. Non-neuronal cells express isoforms of these proteins, believed to mediate secretory vesicle docking and/or fusion. Secretion in neuronal and non-neuronal systems differs in time course, Ca2+ dependence, and regulatory input. It is not known whether the non-neuronal protein isoforms form complexes akin to those of their neuronal counterparts. In this study, we defined the binding characteristics of three SNARE proteins: SNAP23, VAMP-2, and syntaxin-4. Binary, saturable interactions among all three partners (VAMP-2-syntaxin-4, VAMP-2-SNAP23, and SNAP23-syntaxin-4) were measured in vitro. Unlike its neuronal counterpart, SNAP23 did not potentiate VAMP-2 binding to its putative t-SNARE partner, syntaxin-4. The susceptibility of SNARE proteins to phosphorylation by exogenous kinases and their impact on binary interactions were explored. Syntaxin-4 was efficiently phosphorylated by casein kinase II (CKII) and cAMP-dependent protein kinase (PKA) (incorporating 0.8 and 3.9 mol of phosphate/mol of syntaxin-4, respectively), while syntaxin-1 was only strongly phosphorylated by CKII. Each of the syntaxin isoforms was weakly phosphorylated by protein kinase C (PKC) (<0.05 mol of phosphate/mol of syntaxin-4). Importantly, PKA but not casein kinase II phosphorylation of syntaxin-4 disrupted its binding to SNAP23. We hypothesize that PKA may modulate syntaxin-4-dependent SNARE complex formation to regulate exocytosis in non-neuronal cells.

Dissociation of GLUT4 translocation and insulin-stimulated glucose transport in transgenic mice overexpressing GLUT1 in skeletal muscle

Overexpression of the human GLUT1 glucose transporter protein in skeletal muscle of transgenic mice results in large increases in basal glucose transport and metabolism, but impaired stimulation of glucose transport by insulin, contractions, or hypoxia (Gulve, E. A., Ren, J.-M., Marshall, B. A., Gao, J., Hansen, P. A., Holloszy, J. O. , and Mueckler, M. (1994) J. Biol. Chem. 269, 18366-18370). This study examined the relationship between glucose transport and cell-surface glucose transporter content in isolated skeletal muscle from wild-type and GLUT1-overexpressing mice using 2-deoxyglucose, 3-O-methylglucose, and the 2-N-[4-(1-azi-2,2, 2-trifluoroethyl)benzoyl]-1,3-bis(D-mannos-4-yloxy)-2-propyl amine exofacial photolabeling technique. Insulin (2 milliunits/ml) stimulated a 3-fold increase in 2-deoxyglucose uptake in extensor digitorum longus muscles of control mice (0.47 +/- 0.07 micromol/ml/20 min in basal muscle versus 1.44 micromol/ml/20 min in insulin-stimulated muscle; mean +/- S.E.). Insulin failed to increase 2-deoxyglucose uptake above basal rates in muscles overexpressing GLUT1 (4.00 +/- 0.40 micromol/ml/20 min in basal muscle versus 3.96 +/- 0.37 micromol/ml/20 min in insulin-stimulated muscle). A similar lack of insulin stimulation in muscles overexpressing GLUT1 was observed using 3-O-methylglucose. However, the magnitude of the insulin-stimulated increase in cell-surface GLUT4 photolabeling was nearly identical (approximately 3-fold) in wild-type and GLUT1-overexpressing muscles. This apparently normal insulin-stimulated translocation of GLUT4 in GLUT1-overexpressing muscle was confirmed by immunoelectron microscopy. Our findings suggest that GLUT4 activity at the plasma membrane can be dissociated from the plasma membrane content of GLUT4 molecules and thus suggest that the intrinsic activity of GLUT4 is subject to regulation.

Insulin, but not contraction, activates Akt/PKB in isolated rat skeletal muscle

Insulin and muscle contraction potently stimulate glucose uptake in mammalian skeletal muscle. Studies in muscle and adipose tissue have shown that insulin induces its receptor-dependent phosphorylation of insulin receptor substrates 1 and 2, which leads to activation of polyphosphatidylinositol (PI) 3'-kinase. In contrast, muscle contraction stimulates glucose transport via a mechanism that is independent of insulin, but the two pathways may converge downstream at the level of stimulation of GLUT4 translocation. In the present study, we have examined the role of Akt, an insulin-activated serine threonine kinase that has previously been shown to increase glucose transport in adipocytes. Either insulin or in vitro muscle contraction significantly elevated glucose transport in isolated rat epitrochlearis and soleus muscles. However, Akt kinase activity was significantly stimulated by insulin and not contraction. Moreover, wortmannin, an inhibitor of PI 3'-kinase, completely blocked the insulin-stimulated increase in Akt activity and glucose transport but did not alter either of these parameters in contracting muscles. The increases in Akt activity were paralleled by a decrease in the electrophoretic mobility of Akt, indicative of phosphorylation of Akt by an upstream kinase. These changes in Akt mobility appeared to be at least partially because of phosphorylation of Akt on serine 473. A putative downstream target of Akt, p70 S6 kinase, showed similar changes in mobility in response to insulin but not contraction. These data support the view that Akt is a downstream target of PI 3'-kinase and is involved in the signaling pathways involved in insulin but not contraction stimulation of glucose transport in skeletal muscle. These data provide further evidence that two distinct pathways exist for the stimulation of glucose transport in mammalian skeletal muscle.

Effects of high-intensity intermittent swimming on glucose transport in rat epitrochlearis muscle

Recently (K. Kawanaka, I. Tabata, and M. Higuchi. J. Appl. Physiol. 83: 429-433, 1997), we demonstrated that glucose transport activity after repeated 10-s-long in vitro tetani in rat epitrochlearis (Epi) muscle was negatively correlated with the postcontraction muscle glycogen concentration. Therefore, we examined whether high-intensity intermittent swimming, which depletes muscle glycogen to a lower level than that observed after ten 10-s-long in vitro tetani, elicits higher glucose transport than that observed after ten 10-s-long in vitro tetani, which has been regarded as the exercise-induced maximal stimulus for glucose transport. In male rats, 2-deoxy-D-glucose transport rate in Epi muscle after eight bouts of high-intensity intermittent swimming with a weight equal to 18% of body mass (exercise duration: 20 s, rest duration between exercise bouts: 40 s) was higher than that observed after the ten 10-s-long tetani (2.25 +/- 0.08 vs. 1.02 +/- 0.16 micromol . ml intracellular water-1 . 20 min-1). Muscle glycogen concentration in Epi after eight bouts of high-intensity intermittent swimming was significantly lower than that observed after ten 10-s-long in vitro tetani (7.6 +/- 0.5 vs. 14.8 +/- 1.4 micromol glucose/g muscle). These observations show that the high-intensity intermittent swimming increases glucose transport in rat Epi to a much higher level than that induced by ten 10-s-long in vitro tetani, which has been regarded as the exercise-related maximal stimulus for glucose transport. Furthermore, this finding suggests that the lower muscle glycogen level after high-intensity intermittent swimming than after in vitro tetani may play a role, because there was a significant negative correlation between glucose transport and muscle glycogen concentration in Epi after high-intensity swimming and in vitro tetani.

Epinephrine translocates GLUT-4 but inhibits insulin-stimulated glucose transport in rat muscle

We examined the effects of epinephrine (25, 50, and 150 nM) on 1) basal and insulin-stimulated 3-O-methylglucose (3-MG) transport in perfused rat muscles and 2) GLUT-4 in skeletal muscle plasma membranes. Insulin increased glucose transport 330-600% in three types of skeletal muscle [white (WG) and (RG) gastrocnemius and soleus (SOL)]. Glucose transport was also increased by epinephrine (22-48%) in these muscles (P < 0.05). In contrast, the insulin-stimulated 3-MG transport was reduced by epinephrine in all three types of muscles; maximal reductions were observed at 25 nM epinephrine in WG (-25%) and RG (-32.5%). A dose-dependent decrease occurred in SOL (-27% at 25 nM; -55% at 150 nM, P < 0.05). Insulin (20 mU/ml) and epinephrine (150 nM) each translocated GLUT-4 to the plasma membrane, and no differences in translocation were observed between insulin and epinephrine (P > 0.05). In addition, epinephrine did not inhibit insulin-stimulated GLUT-4 translocation, and the combined epinephrine and insulin effects on GLUT-4 translocation were not additive. The increase in surface GLUT-4 was associated with increases in muscle cAMP concentrations, but only when epinephrine alone was present. No relationship was evident between muscle cAMP concentrations and surface GLUT-4 in the combined epinephrine and insulin-stimulated muscles. These studies indicate that epinephrine can translocate GLUT-4 while at the same time increasing glucose transport when insulin is absent, or can inhibit glucose transport when insulin is present.

Insulin increases the association of Akt-2 with Glut4-containing vesicles

Expression of a constitutively active, membrane-associated Akt-1 (PKB alpha) construct in 3T3L1 adipocytes was shown to induce glucose uptake in the absence of insulin by stimulating Glut4 translocation to the plasma membrane (Kohn, A. D., Summers, S. A., Birnbaum, M. J., and Roth, R. A. (1996) J. Biol. Chem. 271, 31372-31378). However, in rat fat cell the vast majority of Akt-1 is cytosolic and shows no re-distribution to the plasma membrane in response to insulin. On the other hand, little work has been done with other Akt family members such as Akt-2 (PKB beta) or Akt-3 (PKB gamma). In this report, an analysis of the subcellular distribution of Akt-2 in rat adipocytes shows that Akt-2 is present in significant amounts in various membrane compartments, as well as in the cytosol, and the former include the light microsomes where Glut4 is present in the basal state. The distribution of Akt-2 in resting adipocytes was found to substantially overlap with that of Glut4 when light microsomes were subfractionated by a sucrose velocity gradient indicating possible co-localization. We confirmed co-localization of Akt-2 and Glut4 in the basal state by immunopurification of Glut4 vesicles, which exhibited a 5.5-fold increase in Akt-2 in response to insulin relative to the amount of Glut4. These results are consistent with the possibility that Akt-2 may be involved in Glut4 vesicle translocation.

Exercise, glucose transport, and insulin sensitivity

Physical exercise can be an important adjunct in the treatment of both non-insulin-dependent diabetes mellitus and insulin-dependent diabetes mellitus. Over the past several years, considerable progress has been made in understanding the molecular basis for these clinically important effects of physical exercise. Similarly to insulin, a single bout of exercise increases the rate of glucose uptake into the contracting skeletal muscles, a process that is regulated by the translocation of GLUT4 glucose transporters to the plasma membrane and transverse tubules. Exercise and insulin utilize different signaling pathways, both of which lead to the activation of glucose transport, which perhaps explains why humans with insulin resistance can increase muscle glucose transport in response to an acute bout of exercise. Exercise training in humans results in numerous beneficial adaptations in skeletal muscles, including an increase in GLUT4 expression. The increase in muscle GLUT4 in trained individuals contributes to an increase in the responsiveness of muscle glucose uptake to insulin, although not all studies show that exercise training in patients with diabetes improves overall glucose control. However, there is now extensive epidemiological evidence demonstrating that long-term regular physical exercise can significantly reduce the risk of developing non-insulin-dependent diabetes mellitus.

Differential targeting of vesicular stomatitis virus G protein and influenza virus hemagglutinin appears during myogenesis of L6 muscle cells

Exocytic organelles undergo profound reorganization during myoblast differentiation and fusion. Here, we analyzed whether glycoprotein processing and targeting changed during this process by using vesicular stomatitis virus (VSV) G protein and influenza virus hemagglutinin (HA) as models. After the induction of differentiation, the maturation and transport of the VSV G protein changed dramatically. Thus, only half of the G protein was processed and traveled through the Golgi, whereas the other half remained unprocessed. Experiments with the VSV tsO45 mutant indicated that the unprocessed form folded and trimerized normally and then exited the ER. It did not, however, travel through the Golgi since brefeldin A recalled it back to the ER. Influenza virus HA glycoprotein, on the contrary, acquired resistance to endoglycosidase H and insolubility in Triton X-100, indicating passage through the Golgi. Biochemical and morphological assays indicated that the HA appeared at the myotube surface. A major fraction of the Golgi-processed VSV G protein, however, did not appear at the myotube surface, but was found in intracellular vesicles that partially colocalized with the regulatable glucose transporter. Taken together, the results suggest that, during early myogenic differentiation, the VSV G protein was rerouted into developing, muscle-specific membrane compartments. Influenza virus HA, on the contrary, was targeted to the myotube surface.

Rapid reversal of adaptive increases in muscle GLUT-4 and glucose transport capacity after training cessation

Previous studies have shown that when exercise is stopped there is a rapid reversal of the training-induced adaptive increase in muscle glucose transport capacity. Endurance exercise training brings about an increase in GLUT-4 in skeletal muscle. The primary purpose of this study was to determine whether the rapid reversal of the increase in maximally insulin-stimulated glucose transport after cessation of training can be explained by a similarly rapid decrease in GLUT-4. A second purpose was to evaluate the possibility, suggested by previous studies, that the magnitude of the adaptive increase in muscle GLUT-4 decreases when exercise training is extended beyond a few days. We found that both GLUT-4 and maximally insulin-stimulated glucose transport were increased approximately twofold in epitrochlearis muscles of rats trained by swimming for 6 h/day for 5 days or 5 wk. GLUT-4 was 90% higher, citrate synthase activity was 23% higher, and hexokinase activity was 28% higher in triceps muscle of the 5-day trained animals compared with the controls. The increases in GLUT-4 protein and in insulin-stimulated glucose transport were completely reversed within 40 h after the last exercise bout, after both 5 days and 5 wk of training. In contrast, the increases in citrate synthase and hexokinase activities were unchanged 40 h after 5 days of exercise. These results support the conclusion that the rapid reversal of the increase in the insulin responsiveness of muscle glucose transport after cessation of training is explained by the short half-life of the GLUT-4 protein.

Regulation of GLUT4 protein and glycogen synthase during muscle glycogen synthesis after exercise

The pattern of muscle glycogen synthesis following its depletion by exercise is biphasic. Initially, there is a rapid, insulin independent increase in the muscle glycogen stores. This is then followed by a slower insulin dependent rate of synthesis. Contributing to the rapid phase of glycogen synthesis is an increase in muscle cell membrane permeability to glucose, which serves to increase the intracellular concentration of glucose-6-phosphate (G6P) and activate glycogen synthase. Stimulation of glucose transport by muscle contraction as well as insulin is largely mediated by translocation of the glucose transporter isoform GLUT4 from intracellular sites to the plasma membrane. Thus, the increase in membrane permeability to glucose following exercise most likely reflects an increase in GLUT4 protein associated with the plasma membrane. This insulin-like effect on muscle glucose transport induced by muscle contraction, however, reverses rapidly after exercise is stopped. As this direct effect on transport is lost, it is replaced by a marked increase in the sensitivity of muscle glucose transport and glycogen synthesis to insulin. Thus, the second phase of glycogen synthesis appears to be related to an increased muscle insulin sensitivity. Although the cellular modifications responsible for the increase in insulin sensitivity are unknown, it apparently helps maintain an increased number of GLUT4 transporters associated with the plasma membrane once the contraction-stimulated effect on translocation has reversed. It is also possible that an increase in GLUT4 protein expression plays a role during the insulin dependent phase.

Glucose transporter levels in a male spontaneous non-insulin-dependent diabetes mellitus rat of the Otsuka Long-Evans Tokushima Fatty strain

Otsuka Long-Evans Tokushima Fatty (OLETF) rats are a new strain of spontaneous non-insulin-dependent diabetes mellitus (NIDDM) models. To evaluate the role of glucose transporters (GLUT) in the development of diabetes in this model, we examined the action of insulin on the translocation of GLUT4 and GLUT1 in isolated adipocytes, and the GLUT4 protein levels in muscles. Long-Evans Tokushima Otsuka (LETO) rats were used as a control strain. In adipocytes, the GLUT4 protein levels in OLETF rats at 30 weeks of age (diabetic stage) were considerably lower than those in LETO rats at the same age. At a pre-diabetic stage (7 weeks), there were no significant differences in GLUT4 protein levels in adipocytes between LETO and OLETF rats. However, the degree of GLUT4 translocation in OLETF rats was lower than that in LETO rats at 7 weeks of age. There were no differences in GLUT1 levels in adipocytes between the two strains. In muscles, the decrease in GLUT4 protein was observed in OLETF rats at 30 weeks of age. Whether such a difference is under the influence of hyperglycemia was also examined using rats rendered diabetic by 70% pancreatectomy. OLETF rats aged 7 weeks were subjected to partial pancreatectomy (Px) and sham pancreatectomy (sham). At 4 weeks after surgery, GLUT4 protein levels in adipose tissues and skeletal muscles were determined. GLUT4 decrease was observed for both tissues of hyperglycemic Px rats compared with euglycemic sham. Moreover, we examined the direct effect of glucose on GLUT4 protein using primary cultured adipocytes of OLETF rats at 5 weeks of age. After 7-day culture with normal (5.6 mmol/l) or high (25 mmol/l) concentrations of glucose, the GLUT4 protein levels in adipocytes decreased at 25 mmol/l glucose compared with 5.6 mmol/l glucose. These findings suggest an early defect in the insulin resistance of OLETF rats probably reflects impaired GLUT4 translocation. The GLUT4 decrease, which occurs later in the process appears to be a consequence, rather than a cause of diabetes in OLETF rats.

Exercise regulation of glucose transport in skeletal muscle

Exercise increases the rate of glucose uptake into the contracting skeletal muscles. This effect of exercise is similar to the action of insulin on glucose uptake, and the mechanism through which both stimuli increase skeletal muscle glucose uptake involves the translocation of GLUT-4 glucose transporters to the plasma membrane and transverse tubules. Most studies suggest that exercise and insulin recruit distinct GLUT-4-containing vesicles and/or mobilize different "pools" of GLUT-4 proteins originating from unique intracellular locations. There are different intracellular signaling pathways that lead to insulin- and exercise-stimulated GLUT-4 translocation. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum leading to the activation of other signaling intermediaries, and there is also evidence for autocrine- or paracrine-mediated activation of transport. The period after exercise is characterized by increased sensitivity of muscle glucose uptake to insulin, which can be substantially prolonged in the face of carbohydrate deprivation. The ability of exercise to utilize insulin-independent mechanisms to increase glucose uptake in skeletal muscle has important clinical implications, especially for patients with diseases that are associated with peripheral insulin resistance, such as non-insulin-dependent diabetes mellitus.

Glucose transporter content and enzymes of metabolism in nerve-repair grafted muscle of aging Fischer 344 rats

Aging and grafting are associated with decreased ability of muscle to sustain power, likely reflecting diminished fuel availability. To assess mechanisms that may contribute to availability of glucose, we studied GLUT-1 and GLUT-4 protein as well as mRNA contents and enzymes of glucose metabolism in grafted and control medial gastrocnemius (MG) muscles of 6-, 12-, and 24-mo-old male Fischer 344 rats. There was no effect of age or grafting on MG GLUT-4 content. There was both an age- and graft-associated increase in GLUT-1 content (P = 0.0044 and 0.0063, respectively). There was no effect of aging or grafting on hexokinase and phosphofructokinase activity or on protein and glycogen content. Muscle mass and citrate synthase activity were significantly diminished with grafting. Citrate synthase activity was significantly greater in the 12-mo-old compared with the 6- and 24-mo-old animals. Grafting in combination with aging had no impact on any of the parameters measured. We conclude that diminished glucose transporter expression cannot explain the decreased ability of aged muscle to sustain power. In addition, we conclude that the diminished ability of the grafted MG muscle to sustain power may be explained, in part, by a decrease in energy available from oxidative metabolism.

Exogenous carbohydrate utilisation: effects on metabolism and exercise performance

It is generally recognized that a decrease in carbohydrate availability can lead to the development of fatigue during prolonged exercise in humans. Administration of glucose or other carbohydrates before or during exercise has been shown to postpone fatigue, conserve muscle glycogen and improve performance. Carbohydrates can be categorised according to their ability to increase blood glucose concentration (known as glycaemic index) and by the extent they stimulate the release of insulin. The glycaemic index is reflected in the rate at which consumed carbohydrate is made available in the blood. Glucose is the only type of carbohydrate that can readily be oxidised by skeletal muscle for energy production. Gastric emptying is the primary factor limiting the rate of carbohydrate delivery to the blood and therefore influences the utilisation of exogenous carbohydrate ingested before or during exercise. Various methods have been used to assess the oxidation of exogenous carbohydrates during exercise. Peak rates of CHO oxidation during exercise have been reported between 0.4 and 1.0 g/min, and the rates of oxidation do not appear to be influenced to a major extent by the use of multiple drinking schedule in comparison with a single bolus schedule. Previous studies also suggest that the ingestion of fructose during exercise does not offer any additional benefits over ingestion of glucose or glucose polymer solutions of similar concentration. The hormones insulin, glucagon and adrenaline together with cortisol and growth hormone play key roles in the regulation of carbohydrate metabolism during exercise. Ingestion of moderately concentrated carbohydrate solutions (4-8%) enhances prolonged exercise performance and is appropriate for optimising energy and fluid delivery without causing adverse effects. The ergogenic effects of carbohydrate ingestion on performance during intermittent exercise such as competitive sports are less well established, although the evidence to date suggests diminished performance when carbohydrate are limiting.

Electrical stimulation induces fiber type-specific translocation of GLUT-4 to T tubules in skeletal muscle

Insulin and contraction independently stimulate glucose transport in skeletal muscle. Whereas insulin activates glucose transport more in muscles composed of type I and IIa fibers, electrical stimulation increases glucose transport at least as much in type IIb fiber-enriched muscles despite the fact that the latter fiber type contains less GLUT-4 glucose transporters. The aim of the present study was to test the hypothesis that a greater GLUT-4 translocation to the cell surface may underlie the higher contraction-stimulated glucose transport in type IIb myofibers. Leg muscles from rats were stimulated in situ at 100 Hz (200 ms) each 2 s via the sciatic nerve over a period of 20 min while the contralateral leg was kept at rest. Muscle 2-[3H]deoxy-D-glucose uptake (2-DG) was measured in separated red gastrocnemius (RG, type I and IIa fibers) and white gastrocnemius (WG, type IIb fibers) muscles. Resting 2-DG uptake was greater in RG than WG. Electrical stimulation increased 2-DG uptake over resting values similarly in WG and RG. Fractions enriched with either plasma membranes, transverse (T) tubules, triads, or GLUT-4-enriched intracellular membranes were isolated from RG and WG using a recently developed subcellular fractionation procedure. Electrical stimulation similarly increased GLUT-4 protein content in plasma membranes of RG and WG, whereas it stimulated GLUT-4 translocation more (approximately 50%) in T tubules of WG than in RG. GLUT-4 content was not changed in triads of both muscle types. The increments in cell surface GLUT-4 protein levels were paralleled by significant reductions in the amount of the transporter in the intracellular membrane fractions of both muscle types (by 60% in RG and 56% in WG). It is concluded that electrically induced contraction stimulates GLUT-4 translocation more in T tubules of WG than RG. The physiological implications of this finding for glucose uptake by contracting RG and WG muscles is discussed.

More tetanic contractions are required for activating glucose transport maximally in trained muscle

Exercise training increases contraction-stimulated maximal glucose transport and muscle glycogen level in skeletal muscle. However, there is a possibility that more muscle contractions are required to maximally activate glucose transport in trained than in untrained muscle, because increased glycogen level after training may inhibit glucose transport. Therefore, the purpose of this study was to investigate the relationship between the increase in glucose transport and the number of tetanic contractions in trained and untrained muscle. Male rats swam 2 h/day for 15 days. In untrained epitrochlearis muscle, resting glycogen was 26.6 micromol glucose/g muscle. Ten, 10-s-long tetani at a rate of 1 contraction/min decreased glycogen level to 15.4 micromol glucose/g muscle and maximally increased 2-deoxy-D-glucose (2-DG) transport. Training increased contraction-stimulated maximal 2-DG transport (+71%; P < 0.01), GLUT-4 protein content (+78%; P < 0.01), and resting glycogen level (to 39.3 micromol glucose/g muscle; P < 0.01) on the next day after the training ended, although this training effect might be due, at least in part, to last bout of exercise. In trained muscle, 20 tetani were necessary to maximally activate glucose transport. Twenty tetani decreased muscle glycogen to a lower level than 10 tetani (18.9 vs. 24.0 micromol glucose/g muscle; P < 0.01). Contraction-stimulated 2-DG transport was negatively correlated with postcontraction muscle glycogen level in trained (r = -0.60; P < 0.01) and untrained muscle (r = -0.57; P < 0.01).

Characterization of two distinct intracellular GLUT4 membrane populations in muscle fiber. Differential protein composition and sensitivity to insulin

A major objective for the understanding of muscle glucose disposal is the elucidation of the intracellular trafficking pathway of GLUT4 glucose carriers in the muscle fiber. In this report, we provide functional and biochemical characterization of two distinct intracellular GLUT4 vesicle pools obtained from rat skeletal muscle. The two pools showed a differential response to insulin; thus, one showed a marked decrease in GLUT4 levels but the other did not. They also showed a markedly different protein composition as detected by quantitative vesicle immunoisolation analysis. The GLUT4 pool showing no response to insulin contained SCAMP proteins and the vSNARE proteins VAMP2 and cellubrevin, whereas only VAMP2 was found in the insulin-recruitable GLUT4 pool. SDS-PAGE and further silver staining of the immunoprecipitates revealed discrete polypeptide bands associated to the insulin-sensitive pool, and all these polypeptide bands were found in the insulin-insensitive population. Furthermore, some polypeptide bands were exclusive to the insulin-insensitive population. The presence of cellubrevin and SCAMP proteins, endosomal markers, suggest that the insulin-insensitive GLUT4 membrane population belongs to an endosomal compartment. In addition, we favor the view that the insulin-sensitive GLUT4 membrane pool is segregated from the endosomal GLUT4 population and is undergoes exocytosis to the cell surface in response to insulin. Intracellular GLUT4 membranes obtained from skeletal muscle contain cellubrevin, and VAMP2 and GLUT4-vesicles from cardiomyocytes also contain cellubrevin. This suggests that vSNARE proteins are key constituents of GLUT4 vesicles. The presence of the tSNARE protein SNAP25 in skeletal muscle membranes and SNAP25 and syntaxin 1A and syntaxin 1B in cardiomyocyte plasma membranes further suggest a role of the SNAREs in GLUT4 trafficking in muscle.

Insulin-induced recruitment of glucose transporter 4 (GLUT4) and GLUT1 in isolated rat cardiac myocytes. Evidence of the existence of different intracellular GLUT4 vesicle populations

Using isolated rat cardiomyocytes we have examined: 1) the effect of insulin on the cellular distribution of glucose transporter 4 (GLUT4) and GLUT1, 2) the total amount of these transporters, and 3) the co-localization of GLUT4, GLUT1, and secretory carrier membrane proteins (SCAMPs) in intracellular membranes. Insulin induced 5.7- and 2.7-fold increases in GLUT4 and GLUT1 at the cell surface, respectively, as determined by the nonpermeant photoaffinity label [3H]2-N-[4(1-azi-2,2,2-trifluoroethyl)benzoyl]-1, 3-bis-(D-mannos-4-yloxy)propyl-2-amine. The total amount of GLUT1, as determined by quantitative Western blot analysis of cell homogenates, was found to represent a substantial fraction ( approximately 30%) of the total glucose transporter content. Intracellular GLUT4-containing vesicles were immunoisolated from low density microsomes by using monoclonal anti-GLUT4 (1F8) or anti-SCAMP antibodies (3F8) coupled to either agarose or acrylamide. With these different immunoisolation conditions two GLUT4 membrane pools were found in nonstimulated cells: one pool with a high proportion of GLUT4 and a low content in GLUT1 and SCAMP 39 (pool 1) and a second GLUT4 pool with a high content of GLUT1 and SCAMP 39 (pool 2). The existence of pool 1 was confirmed by immunotitration of intracellular GLUT4 membranes with 1F8-acrylamide. Acute insulin treatment caused the depletion of GLUT4 in both pools and of GLUT1 and SCAMP 39 in pool 2.

IN CONCLUSION

1) GLUT4 is the major glucose transporter to be recruited to the surface of cardiomyocytes in response to insulin; 2) these cells express a high level of GLUT1; and 3) intracellular GLUT4-containing vesicles consist of at least two populations, which is compatible with recently proposed models of GLUT4 trafficking in adipocytes.

Nuclear magnetic resonance studies of glucose metabolism in non-insulin-dependent diabetes mellitus subjects

In this review, the results of a series of NMR experiments investigating glucose storage and synthesis in NIDDM patients and normal controls have been summarized. These have shown: 1. The deficit in nonoxidative glucose disposal in NIDDM subjects results from a defect in the muscle glycogen synthesis pathway. 2. Reduced activity of glucose transporter/hexokinase step in this pathway accounts for the reduced rate of glycogen synthesis in NIDDM patients. 3. This reduced activity of GT/Hk is a genetic defect present before the clinical onset of disease in prediabetic descendants of diabetic parents. 4. In muscle from normal, healthy subjects the rate of glycogen synthesis is controlled by the glucose transport/hexokinase activity step and not by the activity of the muscle glycogen synthase enzyme. 5. Hepatic gluconeogenesis is responsible for most hepatic glucose production during an overnight fast in both normal and NIDDM subjects, and increases in gluconeogenic flux are responsible for the increased rate of hepatic glucose production in NIDDM subjects. 6. In contrast to human muscle, where glycogenesis ceases at rest, in the liver gluconeogenesis and glycogenolysis are always active. Numerous previous studies were considered prior to embarking in each of these NMR experiments. In the original research articles we published, the earlier studies were discussed in terms of the relevant literature. Here, however, I have chosen to present the NMR data as simply as possible, in the hope of exposing the significance of these studies by disentangling the results from the complexities of NMR methodology.

Voluntary exercise training enhances glucose transport in muscle stimulated by insulin-like growth factor I

Skeletal muscle glucose transport can be regulated by hormonal factors such as insulin and insulin-like growth factor I (IGF-I). Although it is well established that exercise training increases insulin action on muscle glucose transport, it is currently unknown whether exercise training leads to an enhancement of IGF-I-stimulated glucose transport in skeletal muscle. Therefore, we measured glucose transport activity [by using 2-deoxy-D-glucose glucose (2-DG) uptake] in the isolated rat epitrochlearis muscle stimulated by submaximally and maximally effective concentrations of insulin (0.2 and 13.3 nM) or IGF-I (5 and 50 nM) after 1, 2, and 3 wk of voluntary wheel running (WR). After 1 wk of WR, both submaximal and maximal insulin-stimulated 2-DG uptake rates were significantly (P < 0.05) enhanced (43 and 31%) compared with those of sedentary controls, and these variables were further increased after 2 (86 and 57%) and 3 wk (71 and 70%) of WR. Submaximal and maximal IGF-I-stimulated 2-DG uptake rates were significantly enhanced after 1 wk of WR (82 and 61%, and these increases did not expand substantially after 2 (71 and 58%) and 3 wk (96 and 70%) of WR. This enhancement of hormone-stimulated 2-DG uptake in WR muscles preceded any alteration in glucose transporter (GLUT-4) protein level, which increased only after 2 (24%) and 3 wk (54%) of WR. Increases in GLUT-4 protein were significantly correlated (r = 0.844) with increases in citrate synthase. These results indicate that exercise training can enhance both insulin-stimulated and IGF-I-stimulated muscle glucose transport activity and that these improvements can develop without an increase in GLUT-4 protein.

Effect of maternal diabetes upon fetal rat myocardial and skeletal muscle glucose transporters

We investigated the effect of streptozotocin-induced short-term maternal diabetes upon fetal rat myocardial and skeletal muscle glucose transporter Glut 1 (basal form) and Glut 4 (insulin-responsive form) protein concentrations by Western blot analysis. In the severely diabetic group (SEVERE-D, n = 17), a 3-fold increase in maternal and fetal glucose concentrations (p < 0.01) was associated with a 3-fold decline in maternal (p < 0.01) with no change in fetal insulin levels when compared with the streptozotocin-treated nondiabetic (n = 10) and vehicle-treated control (control, n = 14) groups. These changes in the SEVERE-D group when compared with controls were associated with a 30 and 65% decline, respectively, in fetal myocardial and skeletal muscle (forelimb and hind limb) Glut 1 protein concentrations. The fetal myocardium also demonstrated a 45% decline in Glut 4 protein levels. Fetal skeletal muscle Glut 4 protein, which was expressed only at very low levels in controls showed no change in SEVERE-D. Immunohistochemical analysis revealed a myocyte-plasma membrane association of Glut 1 and an intracellular Glut 4 distribution in the fetal myocardium and skeletal muscle. No Glut 1 immunoreactivity was noted in either the fetal myocardial or skeletal muscle perineural sheaths, blood vessels, or the entrapped fetal red blood cells. This subcellular localization pattern was unaltered in all three treatment groups. We conclude that maternal diabetes causing fetal hyperglycemia with normoinsulinemia suppresses fetal myocardial Glut 1 and Glut 4 and fetal skeletal muscle Glut 1. The decline in the plasma membrane associated Glut 1 concentrations may serve a protective function by reducing the glucose transport rate into fetal myocardial and skeletal muscle cells, which otherwise could be vulnerable to high circulating glucose. The in-utero maternal diabetes induced decrease in fetal myocardial intracellular-Glut 4 concentration could herald the emergence of insulin resistance.