Insulin signaling and glucose transport in insulin resistant human skeletal muscle

Insulin increases glucose uptake and metabolism in skeletal muscle by signal transduction via protein phosphorylation cascades. Insulin action on signal transduction is impaired in skeletal muscle from Type 2 diabetic subjects, underscoring the contribution of molecular defects to the insulin resistant phenotype. This review summarizes recent work to identify downstream intermediates in the insulin signaling pathways governing glucose homeostasis, in an attempt to characterize the molecular mechanism accounting for skeletal muscle insulin resistance in Type 2 diabetes. Furthermore, the effects of pharmaceutical treatment of Type 2 diabetic patients on insulin signaling and glucose uptake are discussed. The identification and characterization of pathways governing insulin action on glucose metabolism will facilitate the development of strategies to improve insulin sensitivity in an effort to prevent and treat Type 2 diabetes mellitus.     

Regulation of glucose transport in skeletal muscle.

The entry of glucose into muscle cells is achieved primarily via a carrier-mediated system consisting of protein transport molecules. GLUT-1 transporter isoform is normally found in the sarcolemmal (SL) membrane and is thought to be involved in glucose transport under basal conditions. With insulin stimulation, glucose transport is accelerated by translocating GLUT-4 transporters from an intracellular pool out to the T-tubule and SL membranes. Activation of transporters to increase the turnover number may also be involved, but the evidence is far from conclusive. When insulin binds to its receptor, it autophosphorylates tyrosine and serine residues on the beta-subunit of the receptor. The tyrosine residues are thought to activate tyrosine kinases, which in turn phosphorylate/activate as yet unknown second messengers. Insulin receptor antibodies, however, have been reported to increase glucose transport without increasing kinase activity. Insulin resistance in skeletal muscle is a major characteristic of obesity and diabetes mellitus, especially NIDDM. A decrease in the number of insulin receptors and the ability of insulin to activate receptor tyrosine kinase has been documented in muscle from NIDDM patients. Most studies report no change in the intracellular pool of GLUT-4 transporters available for translocation to the SL. Both the quality and quantity of food consumed can regulate insulin sensitivity. A high-fat, refined sugar diet, similar to the typical U.S. diet, causes insulin resistance when compared with a low-fat, complex-carbohydrate diet. On the other hand, exercise increases insulin sensitivity. After an acute bout of exercise, glucose transport in muscle increases to the same level as with maximum insulin stimulation. Although the number of GLUT-4 transporters in the sarcolemma increases with exercise, neither insulin or its receptor is involved. After an initial acute phase, which may involve calcium as the activator, a secondary phase of increased insulin sensitivity can last for up to a day after exercise. The mechanism responsible for the increased insulin sensitivity with exercise is unknown. Regular exercise training also increases insulin sensitivity, which can be documented several days after the final bout of exercise, and again the mechanism is unknown. An increase in the muscle content of GLUT-4 transporters with training has recently been reported. Even though significant progress has been made in the past few years in understanding glucose transport in skeletal muscle, the mechanisms involved in regulating transport are far from being understood.     

Neuregulin signaling on glucose transport in muscle cells

Neuregulin-1, a growth factor that potentiates myogenesis induces glucose transport through translocation of glucose transporters, in an additive manner to insulin, in muscle cells. In this study, we examined the signaling pathway required for a recombinant active neuregulin-1 isoform (rhHeregulin-beta(1), 177-244, HRG) to stimulate glucose uptake in L6E9 myotubes. The stimulatory effect of HRG required binding to ErbB3 in L6E9 myotubes. PI3K activity is required for HRG action in both muscle cells and tissue. In L6E9 myotubes, HRG stimulated PKBalpha, PKBgamma, and PKCzeta activities. TPCK, an inhibitor of PDK1, abolished both HRG- and insulin-induced glucose transport. To assess whether PKB was necessary for the effects of HRG on glucose uptake, cells were infected with adenoviruses encoding dominant negative mutants of PKBalpha. Dominant negative PKB reduced PKB activity and insulin-stimulated glucose transport but not HRG-induced glucose transport. In contrast, transduction of L6E9 myotubes with adenoviruses encoding a dominant negative kinase-inactive PKCzeta abolished both HRG- and insulin-stimulated glucose uptake. In soleus muscle, HRG induced PKCzeta, but not PKB phosphorylation. HRG also stimulated the activity of p70S6K, p38MAPK, and p42/p44MAPK and inhibition of p42/p44MAPK partially repressed HRG action on glucose uptake. HRG did not affect AMPKalpha(1) or AMPKalpha(2) activities. In all, HRG stimulated glucose transport in muscle cells by activation of a pathway that requires PI3K, PDK1, and PKCzeta, but not PKB, and that shows cross-talk with the MAPK pathway. The PI3K, PDK1, and PKCzeta pathway can be considered as an alternative mechanism, independent of insulin, to induce glucose uptake.     

Acute and chronic signals controlling glucose transport in skeletal muscle.

Glucose transport into muscle cells occurs through facilitated diffusion mediated primarily by the GLUT1 and GLUT4 glucose transporters. These transporter proteins are controlled by acute and chronic exposure to insulin, glucose, muscle contraction, and hypoxia. We propose that acute responses occur through recruitment of pre-formed glucose transporters from an intracellular storage site to the plasma membrane. In contrast, chronic control is achieved by changes in transporter biosynthesis and protein stability. Using subcellular fractionation of rat skeletal muscle, recruitment of GLUT4 glucose transporters to the plasma membrane is demonstrated by acute exposure to insulin in vivo. The intracellular pool appears to arise from a unique organelle depleted of transverse tubule, plasma membrane, or sarcoplasmic reticulum markers. In diabetic rats, GLUT4 content in the plasma membranes and in the intracellular pool is reduced, and incomplete insulin-dependent GLUT4 recruitment is observed, possibly through a defective incorporation of transporters to the plasma membrane. The lower content of GLUT4 transporters in the muscle plasma membranes is reversed by restoration of normoglycemia with phlorizin treatment. In some muscle cells in culture, GLUT1 is the only transporter expressed yet they respond to insulin, suggesting that this transporter can also be regulated by acute mechanisms. In the L6 muscle cell line, GLUT1 transporter content diminishes during myogenesis and GLUT4 appears after cell fusion, reaching a molar ratio of about 1:1 in the plasma membrane. Prolonged exposure to high glucose diminishes the amount of GLUT1 protein in the plasma membrane by both endocytosis and reduced biosynthesis, and lowers GLUT4 protein content in the absence of changes in GLUT4 mRNA possibly through increased protein degradation. These studies suggest that the relative contribution of each transporter to transport activity, and the mechanisms by which glucose exerts control of the glucose transporters, will be key subjects of future investigations.     

Insulin-like growth factor II stimulates glucose transport in human skeletal muscle.

We investigated the effect of insulin-like growth factor II (IGF-II) and insulin-like growth factor binding protein-1 (IGFBP-1) on 3-O-methylglucose transport in incubated human skeletal muscle strips. Increasing physiological concentrations of IGF-II stimulated glucose transport in a dose-dependent manner. Glucose transport was maximally stimulated in the presence of 100 ng/ml (13.4 nM) of IGF-II, which corresponded to the effect obtained by 100 microU/ml (0.6 nM) of insulin. Exposure of muscle strips to IGFBP-1 (500 ng/ml) inhibited the maximal effect of IGF-II on glucose transport by 40%. Thus, it is conceivable that IGF-II and IGFBP-1 are physiological regulators of the glucose transport process in human skeletal muscle.     

Modulation of glucose transport in skeletal muscle by reactive oxygen species.

Glucose transport is an essential physiological process that is characteristic of all eukaryotic cells, including skeletal muscle. In skeletal muscle, glucose transport is mediated by the GLUT-4 protein under conditions of increased carbohydrate utilization. The three major physiological stimuli of glucose transport in muscle are insulin, exercise/contraction, and hypoxia. Here, the role of reactive oxygen species (ROS) in modulating glucose transport in skeletal muscle is reviewed. Convincing evidence for ROS involvement in insulin- and hypoxia-mediated transport in muscle is lacking. Recent experiments, based on pharmacological and genetic approaches, support a role for ROS in contraction-mediated glucose transport. During contraction, endogenously produced ROS appear to mediate their effects on glucose transport via AMP-activated protein kinase.     

Contraction regulation of Akt in rat skeletal muscle.

The protein serine/threonine kinase Akt/protein kinase B has been recognized as a critical signaling mediator for multiple cell systems. The function of Akt in skeletal muscle is not well understood, and whether contractile activity stimulates Akt activity has been controversial. In the current study, contraction in situ, induced via sciatic nerve stimulation, significantly increased Akt Ser(473) phosphorylation in multiple muscle types including the extensor digitorum longus (13-fold over basal), plantaris (5.8-fold), red gastrocnemius (4.7-fold), white gastrocnemius (3.3-fold), and soleus (1.6-fold). In addition to increasing phosphorylation, contraction in situ significantly increased the activity of all three Akt isoforms (Akt1 > Akt2 > Akt3) with maximal activation occurring at 2.5 min and returning to base line with 15 min of contraction. Akt phosphorylation and activity were also increased when isolated muscles were contracted in vitro in the absence of systemic factors, although to a much lesser extent. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 fully inhibited contraction-stimulated Akt phosphorylation and activity but did not diminish contraction-stimulated glycogen synthase kinase-3 phosphorylation and glycogen synthase activity. These results demonstrate that contraction increases Akt phosphorylation and activity in skeletal muscle and that this stimulation is rapid, transient, muscle fiber type-specific, and wortmannin- and LY294002-inhibitable. Akt signaling is not necessary for the regulation of glycogen synthase activity in contracting skeletal muscle.     

Dose-responsive insulin regulation of glucose transport in human skeletal muscle

Glucose transport is regarded as the principal rate control step governing insulin-stimulated glucose utilization by skeletal muscle. To assess this step in human skeletal muscle, quantitative PET imaging of skeletal muscle was performed using 3-O-methyl-[11C]glucose (3-[11C]OMG) in healthy volunteers during a two-step insulin infusion [n = 8; 30 and 120 mU.min(-1).m(-2), low (LO) and high (HI)] and during basal conditions (n = 8). Positron emission tomography images were coregistered with MRI to assess 3-[11C]OMG activity in regions of interest placed on oxidative (soleus) compared with glycolytic (tibialis anterior) muscle. Insulin dose-responsive increases of 3-[11C]OMG activity in muscle were observed (P < 0.01). Tissue activity was greater in soleus than in tibialis anterior (P < 0.05). Spectral analysis identified that two mathematical components interacted to shape tissue activity curves. These two components were interpreted physiologically as likely representing the kinetics of 3-[11C]OMG delivery from plasma to tissue and the kinetics of bidirectional glucose transport. During low compared with basal, there was a sixfold increase in k3, the rate constant attributed to inward glucose transport, and another threefold increase during HI (0.012 +/- 0.003, 0.070 +/- 0.014, 0.272 +/- 0.059 min(-1), P < 0.001). Values for k3 were similar in soleus and tibialis anterior, suggesting similar kinetics for transport, but compartmental modeling indicated a higher value in soleus for k1, denoting higher rates of 3-[11C]OMG delivery to soleus than to tibialis anterior. In summary, in healthy volunteers there is robust dose-responsive insulin stimulation of glucose transport in skeletal muscle.     

Oxidant stress and skeletal muscle glucose transport: roles of insulin signaling and p38 MAPK

Oxidative stress can impact the regulation of glucose transport activity in a variety of cell lines. In the present study, we assessed the direct effects of an oxidant stress on the glucose transport system in intact mammalian skeletal muscle preparations. Type IIb (epitrochlearis) and type I (soleus) muscles from insulin-sensitive lean Zucker rats were incubated in 8 mM glucose for 2 h in the absence or presence of 100 mU/ml glucose oxidase to produce the oxidant hydrogen peroxide (H(2)O(2)) (60-90 microM). Glucose transport, glycogen synthase activity, and metabolic signaling factors were then assessed. H(2)O(2) significantly (p < 0.05) activated basal glucose transport and glycogen synthase activities and increased insulin receptor tyrosine phosphorylation, insulin receptor substrate-1 associated with the p85 subunit of phosphatidylinositol-3' kinase (PI3-kinase), and Ser(473) phosphorylation of Akt in both muscle types. This induction of glucose transport by the oxidant stress was prevented by the PI3-kinase inhibitor wortmannin. The oxidant stress also significantly increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) and 5'-AMP-dependent protein kinase. Interestingly, selective inhibition of p38 MAPK using A304000 substantially reduced the activation of glucose transport induced by the oxidant stress. These results support a direct role for oxidative stress in the activation of the glucose transport system in mammalian skeletal muscle and indicate that this process involves engagement of and possible interactions between the PI3-kinase-dependent signaling pathway and activation of p38 MAPK.     

Testosterone and DHEA activate the glucose metabolism-related signaling pathway in skeletal muscle

Circulating dehydroepiandrosterone (DHEA) is converted to testosterone or estrogen in the target tissues. Recently, we demonstrated that skeletal muscles are capable of locally synthesizing circulating DHEA to testosterone and estrogen. Furthermore, testosterone is converted to 5alpha-dihydrotestosterone (DHT) by 5alpha-reductase and exerts biophysiological actions through binding to androgen receptors. However, it remains unclear whether skeletal muscle can synthesize DHT from testosterone and/or DHEA and whether these hormones affect glucose metabolism-related signaling pathway in skeletal muscles. We hypothesized that locally synthesized DHT from testosterone and/or DHEA activates glucose transporter-4 (GLUT-4)-regulating pathway in skeletal muscles. The aim of the present study was to clarify whether DHT is synthesized from testosterone and/or DHEA in cultured skeletal muscle cells and whether these hormones affect the GLUT-4-related signaling pathway in skeletal muscles. In the present study, the expression of 5alpha-reductase mRNA was detected in rat cultured skeletal muscle cells, and the addition of testosterone or DHEA increased intramuscular DHT concentrations. Addition of testosterone or DHEA increased GLUT-4 protein expression and its translocation. Furthermore, Akt and protein kinase C-zeta/lambda (PKC-zeta/lambda) phosphorylations, which are critical in GLUT-4-regulated signaling pathways, were enhanced by testosterone or DHEA addition. Testosterone- and DHEA-induced increases in both GLUT-4 expression and Akt and PKC-zeta/lambda phosphorylations were blocked by a DHT inhibitor. Finally, the activities of phosphofructokinase and hexokinase, main glycolytic enzymes, were enhanced by testosterone or DHEA addition. These findings suggest that skeletal muscle is capable of synthesizing DHT from testosterone, and that DHT activates the glucose metabolism-related signaling pathway in skeletal muscle cells.     

Metformin treatment enhances insulin-stimulated glucose transport in skeletal muscle of Sprague-Dawley rats

Although the glucose-lowering properties of metformin are well-established, its effects on glucose metabolism in skeletal muscle have not been clearly defined. We tested the effects of metformin in young adult male Sprague-Dawley rats, which have a documented reduced response to insulin in skeletal muscle. Rats were treated with metformin for 20 days (320 mg/kg/day) in the drinking water. During this period, metformin completely prevented the increase in food intake and decreased adiposity by 30%. Metformin also reduced insulin secretion by 37% following an intra-peritoneal injection of glucose. Finally, metformin enhanced transport of [3H]-2-deoxyglucose in isolated strips of soleus muscle. Metformin substantially increased insulin-stimulated transport, while having no effect on basal transport. In control rats, a maximal concentration of insulin stimulated transport 77% above basal. In metformin-treated rats, insulin stimulated transport 206% above basal. We conclude that in the Sprague-Dawley rat model, metformin causes a significant increase in insulin-responsiveness.     

Phorbol esters affect skeletal muscle glucose transport in a fiber type-specific manner

Recent evidence has shown that activation of lipid-sensitive protein kinase C (PKC) isoforms leads to skeletal muscle insulin resistance. However, earlier studies demonstrated that phorbol esters increase glucose transport in skeletal muscle. The purpose of the present study was to try to resolve this discrepancy. Treatment with the phorbol ester 12-deoxyphorbol-13-phenylacetate 20-acetate (dPPA) led to an approximately 3.5-fold increase in glucose transport in isolated fast-twitch epitrochlearis and flexor digitorum brevis muscles. Phorbol ester treatment was additive to a maximally effective concentration of insulin in fast-twitch skeletal muscles. Treatment with dPPA did not affect insulin signaling in the epitrochlearis. In contrast, phorbol esters had no effect on basal glucose transport and inhibited maximally insulin-stimulated glucose transport approximately 50% in isolated slow-twitch soleus muscle. Furthermore, dPPA treatment inhibited the insulin-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and the threonine and serine phosphorylation of PKB by approximately 50% in the soleus. dPPA treatment also caused serine phosphorylation of IRS-1 in the slow-twitch soleus muscle. In conclusion, our results show that phorbol esters stimulate glucose transport in fast-twitch skeletal muscles and inhibit insulin signaling in slow-twitch soleus muscle of rats. These findings suggest that mechanisms other than PKC activation mediate lipotoxicity-induced whole body insulin resistance.     

Stimulation of glucose transport in skeletal muscle by the sodium ionophore monensin

The tissue/medium distribution of the nonmetabolized glucose analog 3-O-methyl-D-glucose was measured in mouse diaphragm muscle and related to changes in 45Ca influx, Na+ content and Na+-pump activity. In the presence of external Ca2+ the sodium ionophore monensin greatly increased cellular Na+ content (and decreased K+ content) although 86Rb uptake, reflecting Na+-pump activity was increased. Concomitantly, 45Ca influx was stimulated, presumably through activation of Na+-Ca2+ exchange. In parallel to the rise in Ca2+ influx sugar transport was also increased. Sugar transport was also increased by monensin in the nominal absence of external Ca2+, when Ca2+ influx was minimal. To test if monensin releases Ca2+ from intracellular storage sites in the absence of external Ca2+, the ionophore was added to medium perfusing rat hind limb preparations and the total Ca content of muscle mitochondria was determined. When Ca2+ was present in the perfusate, monensin increased the mitochondrial Ca content. In the absence of Ca2+, the mitochondrial Ca content was lower and was further depressed by monensin, suggesting that elevation of internal Na+ by monensin may increase mitochondrial Ca2+ loss via activation of Na+-Ca2+ exchange across the mitochondrial membrane. The above results are consistent with the effect of monensin on sugar transport being due to alterations in Ca2+ distribution. They support the earlier conclusion that regulation of sugar transport in muscle is Ca2+ dependent.     

Effect of insulin and contraction up on glucose transport in skeletal muscle

The major glucose transporter protein expressed in skeletal muscle is GLUT4. Both muscle contraction and insulin induce translocation of GLUT4 from the intracellular pool to the plasma membrane. The intracellular pathways that lead to contraction- and insulin-stimulated GLUT4 translocation seem to be different, allowing the attainment of a maximal effect when acting together. Insulin utilizes a phosphatidylinositol 3-kinase-dependent mechanism, whereas the exercise signal may be initiated by calcium release from the sarcoplasmic reticulum or from autocrine- or paracrine-mediated activation of glucose transport. During exercise skeletal muscle utilizes more glucose than when at rest. However, endurance training leads to decreased glucose utilization during sub-maximal exercise, in spite of a large increase in the total GLUT4 content associated with training. The mechanisms involved in this reduction have not been totally elucidated, but appear to cause the decrease of the amount of GLUT4 translocated to the plasma membrane by altering the exercise-induced enhancement of glucose transport capacity. On the other hand, the effect of resistance training is controversial. Recent studies, however, demonstrated the improvement in insulin sensitivity correlated with increasing muscle mass. New studies should be designed to define the molecular basis for these important adaptations to skeletal muscle. Since during exercise the muscle may utilize insulin-independent mechanisms to increase glucose uptake, the mechanisms involved should provide important knowledge to the understanding and managing peripheral insulin resistance.     

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

Previous studies have indicated that frequency of stimulation is a major determinant of glucose transport in contracting muscle. We have now studied whether this is so also when total force development or metabolic rate is kept constant. Incubated soleus muscles were electrically stimulated to perform repeated tetanic contractions at four different frequencies (0.25, 0.5, 1, and 2 Hz) for 10 min. Resting length was adjusted to achieve identical total force development or metabolic rate (glycogen depletion and lactate accumulation). Overall, at constant total force development, glucose transport (2-deoxyglucose uptake) increased with stimulation frequency (P < 0.05; basal: 25 +/- 2, 0.25 Hz: 50 +/- 4, 0.5 Hz: 50 +/- 3, 1 Hz: 81 +/- 5, 2 Hz: 79 +/- 3 nmol. g(-1). 5 min(-1)). However, glucose transport was identical (P > 0.05) at the two lower (0.25 and 0.5 Hz) as well as at the two higher (1 and 2 Hz) frequencies. Glycogen decreased (P < 0.05; basal: 19 +/- 1, 0.25 Hz: 13 +/- 1, 0.5 Hz: 12 +/- 2, 1 Hz: 7 +/- 1, 2 Hz: 7 +/- 1 mmol/kg) and 5'-AMP-activated protein kinase (AMPK) activity increased (P < 0. 05; basal: 1.7 +/- 0.4, 0.25 Hz: 32.4 +/- 7.0, 0.5 Hz: 36.5 +/- 2.1, 1 Hz: 63.4 +/- 8.0, 2 Hz: 67.0 +/- 13.4 pmol. mg(-1). min(-1)) when glucose transport increased. Experiments with constant metabolic rate were carried out in soleus, flexor digitorum brevis, and epitrochlearis muscles. In all muscles, glucose transport was identical at 0.5 and 2 Hz (P > 0.05); also, AMPK activity did not increase with stimulation frequency. In conclusion, muscle glucose transport increases with stimulation frequency but only in the face of energy depletion and increase in AMPK activity. This indicates that contraction-induced glucose transport is elicited by metabolic demands rather than by events occurring early during the excitation-contraction coupling.     

Mechanisms of insulin-dependent glucose transport into porcine and bovine skeletal muscle

Euglycemic, hyperinsulinemic clamp tests have shown that adult ruminants are less insulin-sensitive than monogastric omnivores. The present study was carried out to elucidate possible cellular mechanisms contributing to this impaired insulin sensitivity of ruminants. Western blotting was used to measure glucose transporters 1 and 4 (GLUT1, GLUT4) in oxidative (musculus masseter and diaphragm) and glycolytic (musculus longissimus dorsi and semitendinosus) skeletal muscle in the crude membranes of pigs and cows. Muscles were characterized biochemically. To determine insulin-stimulated 3-O-D-[(3)H]-methylglucose (3-O-MG) uptake and GLUT4 translocation, porcine and bovine musculus semitendinosus strips were removed by open muscle biopsy and incubated without and with 0.1 or 20 mIU insulin/ml. GLUT4 translocation was analyzed using subcellular fractionation techniques to isolate partially purified plasma membranes and cytoplasmic vesicles and using Western blotting. GLUT4 protein contents were significantly higher in oxidative than in glycolytic muscles in pigs and cows. GLUT1 protein contents were significantly higher in glycolytic than in oxidative muscles in bovines but not in porcines. The 3-O-MG uptake into musculus semitendinosus was similar in both species. Maximum insulin-induced GLUT4 translocation into musculus semitendinosus plasma membrane was significantly lower in bovines than in porcines. These results indicate that GLUT1 is the predominant glucose transporter in bovine glycolytic muscles and that a reinforced insulin-independent glucose uptake via GLUT1 may compensate for the impaired insulin-stimulated GLUT4 translocation, resulting in a similar 3-O-MG uptake in bovine and porcine musculus semitendinosus. These findings may explain at least in part the impaired in vivo insulin sensitivity of adult ruminants compared with that of omnivorous monogastric animals.     

Substrate regulation of the glucose transport system in rat skeletal muscle. Characterization and kinetic analysis in isolated soleus muscle and skeletal muscle cells in culture

A self-regulatory mechanism of the glucose transport in rat skeletal muscle cells is described. In isolated rat soleus muscles and rat skeletal myocytes and myotubes in culture, pre-exposure to varying glucose concentrations modulated the rate of 2-deoxyglucose uptake. Maximal uptake was observed at glucose concentrations below 3 mM. Between 2.5 and 4.0 mM glucose it was reduced by 25-35%; further elevation of the glucose concentration resulted in a gradual decrease of the transport rate by approximately 2% for each millimolar glucose. The effect of glucose was time-dependent and fully reversible. Insulin rapidly increased the 2-deoxyglucose uptake in the soleus muscle; however, the insulin effect depended on the glucose concentration of the preincubation. Insulin was totally ineffective in muscles pre-exposed to 1.0-3.0 mM glucose, whereas its stimulatory action increased with increasing glucose concentrations above 4 mM. The effect of low glucose and insulin were not additive, and the maximal 2-deoxyglucose uptake rates induced by both conditions were of identical magnitude. It is postulated that glucose may up- and down-regulate its transport by affecting the number of active glucose transporters in the plasma membrane, and that insulin exerts its stimulatory effect only when the extracellular glucose reaches a threshold concentration.     

Impaired insulin signaling in endothelial cells reduces insulin-induced glucose uptake by skeletal muscle

In obese patients with type 2 diabetes, insulin delivery to and insulin-dependent glucose uptake by skeletal muscle are delayed and impaired. The mechanisms underlying the delay and impairment are unclear. We demonstrate that impaired insulin signaling in endothelial cells, due to reduced Irs2 expression and insulin-induced eNOS phosphorylation, causes attenuation of insulin-induced capillary recruitment and insulin delivery, which in turn reduces glucose uptake by skeletal muscle. Moreover, restoration of insulin-induced eNOS phosphorylation in endothelial cells completely reverses the reduction in capillary recruitment and insulin delivery in tissue-specific knockout mice lacking Irs2 in endothelial cells and fed a high-fat diet. As a result, glucose uptake by skeletal muscle is restored in these mice. Taken together, our results show that insulin signaling in endothelial cells plays a pivotal role in the regulation of glucose uptake by skeletal muscle. Furthermore, improving endothelial insulin signaling may serve as a therapeutic strategy for ameliorating skeletal muscle insulin resistance.     

Effects of exercise training and antioxidant R-ALA on glucose transport in insulin-sensitive rat skeletal muscle.

We have recently demonstrated (Saengsirisuwan V, Kinnick TR, Schmit MB, and Henriksen EJ, J Appl Physiol 91: 145-153, 2001) that exercise training (ET) and the antioxidant R-(+)-alpha-lipoic acid (R-ALA) interact in an additive fashion to improve insulin action in insulin-resistant obese Zucker (fa/fa) rats. The purpose of the present study was to assess the interactions of ET and R-ALA on insulin action and oxidative stress in a model of normal insulin sensitivity, the lean Zucker (fa/-) rat. For 6 wk, animals either remained sedentary, received R-ALA (30 mg. kg body wt(-1). day(-1)), performed ET (treadmill running), or underwent both R-ALA treatment and ET. ET alone or in combination with R-ALA significantly increased (P < 0.05) peak oxygen consumption (28-31%) and maximum run time (52-63%). During an oral glucose tolerance test, ET alone or in combination with R-ALA resulted in a significant lowering of the glucose response (17-36%) at 15 min relative to R-ALA alone and of the insulin response (19-36%) at 15 min compared with sedentary controls. Insulin-mediated glucose transport activity was increased by ET alone in isolated epitrochlearis (30%) and soleus (50%) muscles, and this was associated with increased GLUT-4 protein levels. Insulin action was not improved by R-ALA alone, and ET-associated improvements in these variables were not further enhanced with combined ET and R-ALA. Although ET and R-ALA caused reductions in soleus protein carbonyls (an index of oxidative stress), these alterations were not significantly correlated with insulin-mediated soleus glucose transport. These results indicate that the beneficial interactive effects of ET and R-ALA on skeletal muscle insulin action observed previously in insulin-resistant obese Zucker rats are not apparent in insulin-sensitive lean Zucker rats.