Glucose transport and cell surface GLUT-4 protein in skeletal muscle of the obese Zucker rat

Thirty sweet years of GLUT4

A pivotal metabolic function of insulin is the stimulation of glucose uptake into muscle and adipose tissues. The discovery of the insulin-responsive glucose transporter type 4 (GLUT4) protein in 1988 inspired its molecular cloning in the following year. It also spurred numerous cellular mechanistic studies laying the foundations for how insulin regulates glucose uptake by muscle and fat cells. Here, we reflect on the importance of the GLUT4 discovery and chronicle additional key findings made in the past 30 years. That exocytosis of a multispanning membrane protein regulates cellular glucose transport illuminated a novel adaptation of the secretory pathway, which is to transiently modulate the protein composition of the cellular plasma membrane. GLUT4 controls glucose transport into fat and muscle tissues in response to insulin and also into muscle during exercise. Thus, investigation of regulated GLUT4 trafficking provides a major means by which to map the essential signaling components that transmit the effects of insulin and exercise. Manipulation of the expression of GLUT4 or GLUT4-regulating molecules in mice has revealed the impact of glucose uptake on whole-body metabolism. Remaining gaps in our understanding of GLUT4 function and regulation are highlighted here, along with opportunities for future discoveries and for the development of therapeutic approaches to manage metabolic disease.

Effect of Lithium on the Mechanism of Glucose Transport in Skeletal Muscles

While lithium is known to stimulate glucose transport into skeletal muscle, the mechanisms of the increased glucose transport by lithium in skeletal muscle are not well defined yet. We excised epitrochlearis muscles from male Wistar rats and measured the transport rates of a glucose analog into lithium-, insulin-, and muscular contraction-stimulated skeletal muscle tissue and we also analyzed the levels of cell surface glucose transport 4 using a photolabeling and multicolor immunofluorescence method. In addition, we generated a cell line that stably expresses myc-tagged GLUT4 to measure the rates of GLUT4 internalization and externalization. Lithium significantly increased 2-DG glucose transport rate in skeletal muscles; however, it was significantly lower than the stimulation induced by the maximum concentration of insulin or tetanic contraction. But co-treatment of lithium with insulin or tetanic contraction increased glucose transport rate by ∼200% more than lithium alone. When skeletal muscle tissues were treated with lithium, insulin, and muscular contraction, the levels of cell surface GLUT4 protein contents were increased similarly by ∼6-fold compared with the basal levels. When insulin or lithium stimuli were maintained, the rate of GLUT4myc internalization was significantly lower, and lithium was found to suppress the internalization of GLUT4myc more strongly. The lithium-induced increase in glucose uptake of skeletal muscles appears to increase in cell surface GLUT4 levels caused by decreased internalization of GLUT4. It is concluded that co-treatment of lithium with insulin and muscular contraction had a synergistic effect on glucose transport rate in skeletal muscle.

Protein Phosphatase 1-α Regulates AS160 Ser588 and Thr642 Dephosphorylation in Skeletal Muscle

Akt substrate of 160 kDa (AS160) phosphorylation on Thr(642) and Ser(588) by Akt is essential for insulin's full effect on glucose transport. However, protein phosphorylation is determined by the balance of actions by kinases and phosphatases, and the specific phosphatase(s) controlling AS160 dephosphorylation is (are) unknown. Accordingly, we assessed roles of highly expressed skeletal muscle serine/threonine phosphatases (PP1, PP2A, PP2B, and PP2C) on AS160 dephosphorylation. Preliminary screening of candidate phosphatases used an AS160 dephosphorylation assay. Lysates from insulin-stimulated skeletal muscle were treated with pharmacological phosphatase inhibitors and assessed for AS160 Ser(588) and Thr(642) dephosphorylation. AS160 dephosphorylation on both phosphorylation sites was unaltered by PP2B or PP2C inhibitors. Okadaic acid (low dose inhibits PP2A; high dose inhibits PP1) delayed AS160 Ser(588) (both doses) and Thr(642) (high dose only) dephosphorylation concomitant with greater Akt phosphorylation (both doses). AS160 was coimmunoprecipitated with PP1-α but not with PP1-β, PP1-γ1, or PP2A. Recombinant inhibitor-2 protein (a selective PP1 inhibitor) delayed AS160 dephosphorylation on both phosphorylation sites without altering Akt phosphorylation. Furthermore, knockdown of PP1-α but not PP1-β or PP1-γ1 by small interfering RNA caused greater AS160 Ser(588) and Thr(642) phosphorylation concomitant with unaltered Akt phosphorylation. Together, these results identified PP1-α as a regulator of AS160 Thr(642) and Ser(588) dephosphorylation in skeletal muscle.

The Physiological Effects of Dandelion (Taraxacum Officinale) in Type 2 Diabetes

The tremendous rise in the economic burden of type 2 diabetes (T2D) has prompted a search for alternative and less expensive medicines. Dandelion offers a compelling profile of bioactive components with potential anti-diabetic properties. The Taraxacum genus from the Asteraceae family is found in the temperate zone of the Northern hemisphere. It is available in several areas around the world. In many countries, it is used as food and in some countries as therapeutics for the control and treatment of T2D. The anti-diabetic properties of dandelion are attributed to bioactive chemical components; these include chicoric acid, taraxasterol (TS), chlorogenic acid, and sesquiterpene lactones. Studies have outlined the useful pharmacological profile of dandelion for the treatment of an array of diseases, although little attention has been paid to the effects of its bioactive components on T2D to date. This review recapitulates previous work on dandelion and its potential for the treatment and prevention of T2D, highlighting its anti-diabetic properties, the structures of its chemical components, and their potential mechanisms of action in T2D. Although initial research appears promising, data on the cellular impact of dandelion are limited, necessitating further work on clonal β-cell lines (INS-1E), α-cell lines, and human skeletal cell lines for better identification of the active components that could be of use in the control and treatment of T2D. In fact, extensive in-vitro, in-vivo, and clinical research is required to investigate further the pharmacological, physiological, and biochemical mechanisms underlying the effects of dandelion-derived compounds on T2D.

The role of the renin-angiotensin system in the development of insulin resistance in skeletal muscle

The canonical renin-angiotensin system (RAS) involves the initial action of renin to cleave angiotensinogen to angiotensin I (ANG I), which is then converted to ANG II by the angiotensin converting enzyme (ACE). ANG II plays a critical role in numerous physiological functions, and RAS overactivity underlies many conditions of cardiovascular dysregulation. In addition, ANG II, by acting on both endothelial and myocellular AT1 receptors, can induce insulin resistance by increasing cellular oxidative stress, leading to impaired insulin signaling and insulin-stimulated glucose transport activity. This insulin resistance associated with RAS overactivity, when coupled with progressive ß-cell dysfunction, eventually leads to the development of type 2 diabetes. Interventions that target RAS overactivity, including ACE inhibitors, ANG II receptor blockers, and, most recently, renin inhibitors, are effective both in reducing hypertension and in improving whole-body and skeletal muscle insulin action, due at least in part to enhanced Akt-dependent insulin signaling and insulin-dependent glucose transport activity. ANG-(1-7), which is produced from ANG II by the action of ACE2 and acts via Mas receptors, can counterbalance the deleterious actions of the ACE/ANG II/AT1 receptor axis on the insulin-dependent glucose transport system in skeletal muscle. This beneficial effect of the ACE2/ANG-(1-7)/Mas receptor axis appears to depend on the activation of Akt. Collectively, these findings underscore the importance of RAS overactivity in the multifactorial etiology of insulin resistance in skeletal muscle, and provide support for interventions that target the RAS to ameliorate both cardiovascular dysfunctions and insulin resistance in skeletal muscle tissue.

An amino acid mixture is essential to optimize insulin-stimulated glucose uptake and GLUT4 translocation in perfused rodent hindlimb muscle

The purpose of this study was to investigate whether an amino acid mixture increases glucose uptake across perfused rodent hindlimb muscle in the presence and absence of a submaximal insulin concentration, and if the increase in glucose uptake is related to an increase in GLUT4 plasma membrane density. Sprague-Dawley rats were separated into one of four treatment groups: basal, amino acid mixture, submaximal insulin, or amino acid mixture with submaximal insulin. Glucose uptake was greater for both insulin-stimulated treatments compared with the non-insulin-stimulated treatment groups but amino acids only increased glucose uptake in the presence of insulin. Phosphatidylinositol 3-kinase (PI 3-kinase) activity was greater for both insulin-stimulated treatments with amino acids having no additional impact. Akt substrate of 160 kDa (AS160) phosphorylation, however, was increased by the amino acids in the presence of insulin, but not in the absence of insulin. AMPK was unaffected by insulin or amino acids. Plasma membrane GLUT4 protein concentration was greater in the rats treated with insulin compared with no insulin in the perfusate. In the presence of insulin, amino acids increased GLUT4 density in the plasma membrane but had no effect in the absence of insulin. AS160 phosphorylation and plasma membrane GLUT4 density accounted for 76% of the variability in muscle glucose uptake. Collectively, these findings suggest that the beneficial effects of an amino acid mixture on skeletal muscle glucose uptake, in the presence of a submaximal insulin concentration, are due to an increase in AS160 phosphorylation and plasma membrane-associated GLUT4, but independent of PI 3-kinase and AMPK activation.

Munc18c provides stimulus-selective regulation of GLUT4 but not fatty acid transporter trafficking in skeletal muscle

Insulin-, and contraction-induced GLUT4 and fatty acid (FA) transporter translocation may share common trafficking mechanisms. Our objective was to examine the effects of partial Munc18c ablation on muscle glucose and FA transport, FA oxidation, GLUT4 and FA transporter (FAT/CD36, FABPpm, FATP1, FATP4) trafficking to the sarcolemma, and FAT/CD36 to mitochondria. In Munc18c(-/+) mice, insulin-stimulated glucose transport and GLUT4 sarcolemmal appearance were impaired, but were unaffected by contraction. Insulin- and contraction-stimulated FA transport, sarcolemmal FA transporter appearance, and contraction-mediated mitochondrial FAT/CD36 were increased normally in Munc18c(-/+) mice. Hence, Munc18c provides stimulus-specific regulation of GLUT4 trafficking, but not FA transporter trafficking.

A novel method to measure glucose uptake and myosin heavy chain isoform expression of single fibers from rat skeletal muscle

Skeletal muscle includes many individual fibers with diverse phenotypes. A barrier to understanding muscle glucose uptake at the cellular level has been the absence of a method to measure glucose uptake by single fibers from mammalian skeletal muscle. This study's primary objective was to develop a procedure to measure glucose uptake by single fibers from rat skeletal muscle. Rat epitrochlearis muscles were incubated ex vivo with [(3)H]-2-deoxy-d-glucose, with or without insulin or AICAR, before isolation of ~10-30 single fibers from each muscle. Fiber type (myosin heavy chain [MHC] isoform) and glucose uptake were determined for each single fiber. Insulin-stimulated glucose uptake (which was cytochalasin B inhibitable) varied according to MHC isoform expression, with ~2-fold greater values for IIA versus IIB or IIX fibers and ~1.3-fold greater for hybrid (IIB/X) versus IIB fibers. In contrast, AICAR-stimulated glucose uptake was ~1.5-fold greater for IIB versus IIA fibers. A secondary objective was to assess insulin resistance of single fibers from obese versus lean Zucker rats. Genotype differences were observed for insulin-stimulated glucose uptake and inhibitor κB (IκB)-β abundance in single fibers (obese less than lean), with decrements for glucose uptake (44-58%) and IκB-β (25-32%) in each fiber type. This novel method creates a unique opportunity for future research focused on understanding muscle glucose uptake at the cellular level.

Effect of insulin and contraction on glycogen synthase phosphorylation and kinetic properties in epitrochlearis muscles from lean and obese Zucker rats

In the present study, the effects of insulin and contraction on glycogen synthase (GS) kinetic properties and phosphorylation were investigated in epitrochlearis muscles from lean and obese Zucker rats. Total GS activity and protein expression were ~15% lower in epitrochlearis from obese rats compared with lean rats. Insulin-stimulated GS fractional activity and affinity for UDP-glucose were lower (higher K(m)) in muscles from obese rats. GS Ser(641) and Ser(645,649,653,657) phosphorylation was higher in insulin-stimulated muscles from obese rats, which agreed with lower GS activation. Contraction-mediated GS dephosphorylation of Ser(641), Ser(641+645), Ser(645,649,653,657), and Ser(7+10) was normal in muscles from obese Zucker rats, and GS fractional activity increased to similar levels in epitrochlearis muscles from lean and obese rats. GS affinity for UDP glucose was ~0.8, ~0.4, and ~0.1 mM with assay buffers containing 0, 0.17, and 12 mM glucose 6-phosphate, respectively. Contraction increased affinity for UDP-glucose (reduced K(m)) at a physiological concentration of glucose 6-phosphate (0.17 mM) to ~0.2 mM in muscles from both lean and obese rats. Interestingly, in the absence of glucose 6-phosphate in the assay buffer, contraction (and insulin) did not influence GS affinity for UDP-glucose, indicating that affinity is regulated by sensitivity for glucose 6-phosphate. In conclusion, contraction-mediated activation and dephosphorylation of GS were normal in muscles from obese Zucker rats, whereas insulin-mediated GS activation and dephosphorylation were impaired.

The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise

Glycogen is the storage form of carbohydrates in mammals. In humans the majority of glycogen is stored in skeletal muscles (∼500 g) and the liver (∼100 g). Food is supplied in larger meals, but the blood glucose concentration has to be kept within narrow limits to survive and stay healthy. Therefore, the body has to cope with periods of excess carbohydrates and periods without supplementation. Healthy persons remove blood glucose rapidly when glucose is in excess, but insulin-stimulated glucose disposal is reduced in insulin resistant and type 2 diabetic subjects. During a hyperinsulinemic euglycemic clamp, 70-90% of glucose disposal will be stored as muscle glycogen in healthy subjects. The glycogen stores in skeletal muscles are limited because an efficient feedback-mediated inhibition of glycogen synthase prevents accumulation. De novo lipid synthesis can contribute to glucose disposal when glycogen stores are filled. Exercise physiologists normally consider glycogen's main function as energy substrate. Glycogen is the main energy substrate during exercise intensity above 70% of maximal oxygen uptake ([Formula: see text]) and fatigue develops when the glycogen stores are depleted in the active muscles. After exercise, the rate of glycogen synthesis is increased to replete glycogen stores, and blood glucose is the substrate. Indeed insulin-stimulated glucose uptake and glycogen synthesis is elevated after exercise, which, from an evolutional point of view, will favor glycogen repletion and preparation for new "fight or flight" events. In the modern society, the reduced glycogen stores in skeletal muscles after exercise allows carbohydrates to be stored as muscle glycogen and prevents that glucose is channeled to de novo lipid synthesis, which over time will causes ectopic fat accumulation and insulin resistance. The reduction of skeletal muscle glycogen after exercise allows a healthy storage of carbohydrates after meals and prevents development of type 2 diabetes.

The physiological regulation of glucose flux into muscle in vivo

Skeletal muscle glucose uptake increases dramatically in response to physical exercise. Moreover, skeletal muscle comprises the vast majority of insulin-sensitive tissue and is a site of dysregulation in the insulin-resistant state. The biochemical and histological composition of the muscle is well defined in a variety of species. However, the functional consequences of muscle biochemical and histological adaptations to physiological and pathophysiological conditions are not well understood. The physiological regulation of muscle glucose uptake is complex. Sites involved in the regulation of muscle glucose uptake are defined by a three-step process consisting of: (1) delivery of glucose to muscle, (2) transport of glucose into the muscle by GLUT4 and (3) phosphorylation of glucose within the muscle by a hexokinase (HK). Muscle blood flow, capillary recruitment and extracellular matrix characteristics determine glucose movement from the blood to the interstitium. Plasma membrane GLUT4 content determines glucose transport into the cell. Muscle HK activity, cellular HK compartmentalization and the concentration of the HK inhibitor glucose 6-phosphate determine the capacity to phosphorylate glucose. Phosphorylation of glucose is irreversible in muscle; therefore, with this reaction, glucose is trapped and the uptake process is complete. Emphasis has been placed on the role of the glucose transport step for glucose influx into muscle with the past assertion that membrane transport is rate limiting. More recent research definitively shows that the distributed control paradigm more accurately defines the regulation of muscle glucose uptake as each of the three steps that define this process are important sites of flux control.

Increased levels of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) improve lipid utilisation, insulin signalling and glucose transport in skeletal muscle of lean and insulin-resistant obese Zucker rats

AIMS/HYPOTHESIS

Reductions in peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) levels have been associated with the skeletal muscle insulin resistance. However, in vivo, the therapeutic potential of PGC-1alpha has met with failure, as supra-physiological overexpression of PGC-1alpha induced insulin resistance, due to fatty acid translocase (FAT)-mediated lipid accumulation. Based on physiological and metabolic considerations, we hypothesised that a modest increase in PGC-1alpha levels would limit FAT upregulation and improve lipid metabolism and insulin sensitivity, although these effects may differ in lean and insulin-resistant muscle.

METHODS

Pgc-1alpha was transfected into lean and obese Zucker rat muscles. Two weeks later we examined mitochondrial biogenesis, intramuscular lipids (triacylglycerol, diacylglycerol, ceramide), GLUT4 and FAT levels, insulin-stimulated glucose transport and signalling protein phosphorylation (thymoma viral proto-oncogene 2 [Akt2], Akt substrate of 160 kDa [AS160]), and fatty acid oxidation in subsarcolemmal and intermyofibrillar mitochondria.

RESULTS

Electrotransfection yielded physiologically relevant increases in Pgc-1alpha (also known as Ppargc1a) mRNA and protein ( approximately 25%) in lean and obese muscle. This induced mitochondrial biogenesis, and increased FAT and GLUT4 levels, insulin-stimulated glucose transport, and Akt2 and AS160 phosphorylation in lean and obese animals, while bioactive intramuscular lipids were only reduced in obese muscle. Concurrently, PGC-1alpha increased palmitate oxidation in subsarcolemmal, but not in intermyofibrillar mitochondria, in both groups. In obese compared with lean animals, the PGC-1alpha-induced improvement in insulin-stimulated glucose transport was smaller, but intramuscular lipid reduction was greater.

CONCLUSIONS/INTERPRETATIONS

Increases in PGC-1alpha levels, similar to those that can be induced by physiological stimuli, altered intramuscular lipids and improved fatty acid oxidation, insulin signalling and insulin-stimulated glucose transport, albeit to different extents in lean and insulin-resistant muscle. These positive effects are probably attributable to limiting the PGC-1alpha-induced increase in FAT, thereby preventing bioactive lipid accumulation as has occurred in transgenic PGC-1alpha animals.

Insulin resistance for glucose uptake and Akt2 phosphorylation in the soleus, but not epitrochlearis, muscles of old vs. adult rats

The slow-twitch soleus, but not fast-twitch muscle, of old vs. adult rats has previously been demonstrated to become insulin resistant for in vivo glucose uptake. We probed cellular mechanisms for the age effect by assessing whether insulin resistance for glucose uptake was an intrinsic characteristic of the muscle ex vivo and by analyzing key insulin signaling steps. We hypothesized that isolated soleus and epitrochlearis (fast-twitch) muscles from old (25 mo) vs. adult (9 mo) male Fisher-344 x Brown Norway rats would have insulin resistance for Akt2 Thr308 phosphorylation (pAkt2Thr308), AS160 phosphorylation Thr642 (pAS160Thr642), and atypical PKC (aPKCzeta/lambda) activity corresponding in magnitude to the extent of insulin resistance for [3H]-2-deoxyglucose (2-DG) uptake. Epitrochlearis insulin-stimulated 2-DG uptake above basal values was unaltered by age, and epitrochlearis pAkt2Thr308, pAS160Thr642, and aPKCzeta/lambda activity were not significantly different in adult vs. old rats. Conversely, insulin-stimulated 2-DG uptake by the soleus of old vs. adult rats was reduced with 1.2 nM (42%) and 30 nM (28%) insulin concomitant with an age-related decline in pAkt2Thr308 of the insulin-stimulated soleus. There were no age effects on pAS160Thr642 or aPKCzeta/lambda activity or abundance of Akt2, AS160, GLUT4 or Appl1 protein in either muscle. The results suggest the possibility that an age-related decline in pAkt2Thr308, acting by a mechanism other than reduced pAS160Thr642, may play a role in the insulin resistance in the soleus of old rats. Skeletal muscle insulin resistance in old age is distinctive compared with other insulin-resistant rodent models that are not selective for greater insulin resistance in the soleus vs. the epitrochlearis.

Exercise maintains euglycemia in association with decreased activation of c-Jun NH2-terminal kinase and serine phosphorylation of IRS-1 in the liver of ZDF rats

Stress-activated systems and oxidative stress are involved in insulin resistance, which, along with beta-cell failure, contribute to the development of type 2 diabetes mellitus (T2DM). Exercise improves insulin resistance and glucose tolerance, and these adaptations may, in part, be related to reductions in inflammation and oxidative stress. We investigated circulating and tissue-specific markers of inflammation and oxidative stress and insulin-signaling pathways in a rodent model of T2DM, the Zucker diabetic fatty rat, with and without voluntary exercise. At 5 wk of age, Zucker diabetic fatty rats (n = 8-9/group) were divided into basal (B), voluntary exercise (E), and sedentary control (S) groups. B rats were euthanized at 6 wk of age, and S and E rats were euthanized 10 wk later. E rats ran approximately 5 km/day, which improved insulin sensitivity and maintained fed and fasted glucose levels and glucose tolerance. Ten weeks of exercise also decreased whole body markers of inflammation and oxidative stress in plasma and liver, including lowered circulating IL-6, haptoglobin, and malondialdehyde levels, hepatic protein oxidation, and phosphorylated JNK, the latter indicating decreased JNK activity. Hepatic phosphoenolpyruvate carboxykinase levels and Ser(307)-phosphorylated insulin receptor substrate-1 were also reduced in E compared with S rats. In summary, we show that, in a rodent model of T2DM, voluntary exercise decreases circulating markers of inflammation and oxidative stress and lowers hepatic JNK activation and Ser(307)-phosphorylated insulin receptor substrate-1. These changes in oxidative stress markers and inflammation are associated with decreased hyperglycemia and insulin resistance and reduced expression of the main gluconeogenic enzyme phosphoenolpyruvate carboxykinase.

Additive effect of contraction and insulin on glucose uptake and glycogen synthase in muscle with different glycogen contents

Insulin and contraction regulate glucose uptake and glycogen synthase (GS) via distinct mechanisms in skeletal muscles, and an additive effect has been reported. Glycogen content is known to influence both contraction- and insulin-stimulated glucose uptake and GS activity. Our study reports that contraction and insulin additively stimulate glucose uptake in rat epitrochlearis muscles with normal (NG) and high (HG) glycogen contents, but the additive effect was only partial. In muscles with low glycogen (LG) content no additive effect was seen, but glucose uptake was higher in LG than in NG and HG during contraction, insulin stimulation, and when the two stimuli were combined. In LG, contraction-stimulated AMP-activated protein kinase (AMPK) activity and insulin-stimulated PKB phosphorylation were higher than in NG and HG, but phosphorylation of Akt substrate of 160 kDa was not elevated correspondingly. GLUT4 content was 50% increased in LG (rats fasted 24 h), which may explain the increased glucose uptake. Contraction and insulin also additively increased GS fractional activity in NG and HG but not in LG. GS fractional activity correlated most strongly with GS Ser641 phosphorylation (R -0.94, P<0.001). GS fractional activity also correlated with GS Ser7,10 phosphorylation, but insulin did not reduce GS Ser7,10 phosphorylation. In conclusion, an additive effect of contraction and insulin on glucose uptake and GS activity occurs in muscles with normal and high glycogen content but not in muscles with low glycogen content. Furthermore, contraction, insulin, and glycogen content all regulate GS Ser641 phosphorylation and GS fractional activity in concert.

Glycogen content regulates insulin- but not contraction-mediated glycogen synthase activation in the rat slow-twitch soleus muscles

AIM

The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined.

METHODS

Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin.

RESULTS

Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser(641) (R = -0.93, P < 0.001) and Ser(645,649,653,657) (R = -0.85, P < 0.001) phosphorylation, but not with Ser(7) phosphorylation.

CONCLUSION

Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser(641) and Ser(645,649,653,657) seems to regulate GS activity in soleus.

Four grams of glucose

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle.

Roles of insulin signalling and p38 MAPK in the activation by lithium of glucose transport in insulin-resistant rat skeletal muscle

We have demonstrated previously in insulin-sensitive skeletal muscle that lithium, an alkali metal and non-selective inhibitor of glycogen synthase kinase-3 (GSK-3), activates glucose transport by engaging the stress-activated p38 mitogen-activated protein kinase (p38 MAPK). However, it is presently unknown whether this same mechanism underlies lithium action on the glucose transport system in insulin-resistant skeletal muscle. We therefore assessed the effects of lithium on basal and insulin-stimulated glucose transport, glycogen synthesis, insulin signalling (insulin receptor (IR), Akt, and GSK-3), and p38 MAPK in soleus muscle from female obese Zucker rats. Lithium (10 mM LiCl) increased basal glucose transport by 49% (p < 0.05) and net glycogen synthesis by 2.4-fold (p < 0.05). In the absence of insulin, lithium did not induce IR tyrosine phosphorylation, but did enhance (p < 0.05) Akt ser(473) phosphorylation (40%) and GSK-3beta ser(9) phosphorylation (88%). Lithium potentiated (p < 0.05) the stimulatory effects of insulin on glucose transport (74%), glycogen synthesis (2.4-fold), Akt ser(473) phosphorylation (39%), and GSK-3beta ser(9) phosphorylation (36%), and elicited robust increases (p < 0.05) in p38 MAPK phosphorylation both in the absence (100%) or presence (88%) of insulin. The selective p38 MAPK inhibitor A304000 (10 muM) completely blocked basal activation of glucose transport by lithium, and significantly reduced (42%, p < 0.05) the lithium-induced enhancement of insulin-stimulated glucose transport in insulin-resistant muscle. These results indicate that lithium enhances both basal and insulin-stimulated glucose transport and glycogen synthesis in insulin-resistant skeletal muscle of female obese Zucker rats, and that these lithium-dependent effects are associated with enhanced Akt and GSK-3beta serine phosphorylation. As in insulin-sensitive muscle, the lithium-induced activation of glucose transport in insulin-resistant skeletal muscle is dependent on the engagement of p38 MAPK.

Metabolic challenges reveal impaired fatty acid metabolism and translocation of FAT/CD36 but not FABPpm in obese Zucker rat muscle

We examined, in muscle of lean and obese Zucker rats, basal, insulin-induced, and contraction-induced fatty acid transporter translocation and fatty acid uptake, esterification, and oxidation. In lean rats, insulin and contraction induced the translocation of the fatty acid transporter FAT/CD36 (43 and 41%, respectively) and plasma membrane-associated fatty acid binding protein (FABPpm; 19 and 60%) and increased fatty acid uptake (63 and 40%, respectively). Insulin and contraction increased lean muscle palmitate esterification and oxidation 72 and 61%, respectively. In obese rat muscle, basal levels of sarcolemmal FAT/CD36 (+33%) and FABPpm (+14%) and fatty acid uptake (+30%) and esterification (+32%) were increased, whereas fatty acid oxidation was reduced (-28%). Insulin stimulation of obese rat muscle increased plasmalemmal FABPpm (+15%) but not plasmalemmal FAT/CD36, blunted fatty acid uptake and esterification, and failed to reduce fatty acid oxidation. In contracting obese rat muscle, the increases in fatty acid uptake and esterification and FABPpm translocation were normal, but FAT/CD36 translocation was impaired and fatty acid oxidation was blunted. There was no relationship between plasmalemmal fatty acid transporters and palmitate partitioning. In conclusion, fatty acid metabolism is impaired at several levels in muscles of obese Zucker rats; specifically, they are 1) insulin resistant with respect to FAT/CD36 translocation and fatty acid uptake, esterification, and oxidation and 2) contraction resistant with respect to fatty acid oxidation and FAT/CD36 translocation, but, conversely, 3) obese muscles are neither insulin nor contraction resistant at the level of FABPpm. Finally, 4) there is no evidence that plasmalemmal fatty acid transporters contribute to the channeling of fatty acids to specific metabolic destinations within the muscle.

Upregulation of myocellular DGAT1 augments triglyceride synthesis in skeletal muscle and protects against fat-induced insulin resistance

Increased fat deposition in skeletal muscle is associated with insulin resistance. However, exercise increases both intramyocellular fat stores and insulin sensitivity, a phenomenon referred to as "the athlete's paradox". In this study, we provide evidence that augmenting triglyceride synthesis in skeletal muscle is intrinsically connected with increased insulin sensitivity. Exercise increased diacylglycerol (DAG) acyltransferase (DGAT) activity in skeletal muscle. Channeling fatty acid substrates into TG resulted in decreased DAG and ceramide levels. Transgenic overexpression of DGAT1 in mouse skeletal muscle replicated these findings and protected mice against high-fat diet-induced insulin resistance. Moreover, in isolated muscle, DGAT1 deficiency exacerbated insulin resistance caused by fatty acids, whereas DGAT1 overexpression mitigated the detrimental effect of fatty acids. The heightened insulin sensitivity in the transgenic mice was associated with attenuated fat-induced activation of DAG-responsive PKCs and the stress mediator JNK1. Consistent with these changes, serine phosphorylation of insulin receptor substrate 1 was reduced, and Akt activation and glucose 4 membrane translocation were increased. In conclusion, upregulation of DGAT1 in skeletal muscle is sufficient to recreate the athlete's paradox and illustrates a mechanism of exercise-induced enhancement of muscle insulin sensitivity. Thus, increasing muscle DGAT activity may offer a new approach to prevent and treat insulin resistance and type 2 diabetes mellitus.

Chronic selective glycogen synthase kinase-3 inhibition enhances glucose disposal and muscle insulin action in prediabetic obese Zucker rats

Increasing evidence supports a negative role of glycogen synthase kinase-3 (GSK-3) in regulation of skeletal muscle glucose transport. We assessed the effects of chronic treatment of insulin-resistant, prediabetic obese Zucker (fa/fa) rats with a highly selective GSK-3 inhibitor (CT118637) on glucose tolerance, whole body insulin sensitivity, plasma lipids, skeletal muscle insulin signaling, and in vitro skeletal muscle glucose transport activity. Obese Zucker rats were treated with either vehicle or CT118637 (30 mg/kg body wt) twice per day for 10 days. Fasting plasma insulin and free fatty acid levels were reduced by 14 and 23% (P < 0.05), respectively, in GSK-3 inhibitor-treated animals compared with vehicle-treated controls. The glucose response during an oral glucose tolerance test was reduced by 18% (P < 0.05), and whole body insulin sensitivity was increased by 28% (P < 0.05). In vivo insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (50%) and IRS-1-associated phosphatidylinositol-3' kinase (79%) relative to fasting plasma insulin levels were significantly elevated (P < 0.05) in plantaris muscles of GSK-3 inhibitor-treated animals. Whereas basal glucose transport in isolated soleus and epitrochlearis muscles was unaffected by chronic GSK-3 treatments, insulin stimulation of glucose transport above basal was significantly enhanced (32-60%, P < 0.05). In summary, chronic treatment of insulin-resistant, prediabetic obese Zucker rats with a specific GSK-3 inhibitor enhances oral glucose tolerance and whole body insulin sensitivity and is associated with an amelioration of dyslipidemia and an improvement in IRS-1-dependent insulin signaling in skeletal muscle. These results provide further evidence that selective targeting of GSK-3 in muscle may be an effective intervention for the treatment of obesity-associated insulin resistance.

Voltage-gated potassium channel Kv1.3 regulates GLUT4 trafficking to the plasma membrane via a Ca2+-dependent mechanism

Kv1.3 is a voltage-gated K(+) channel expressed in insulin-sensitive tissues. We previously showed that gene inactivation or pharmacological inhibition of Kv1.3 channel activity increased peripheral insulin sensitivity independently of body weight by augmenting the amount of GLUT4 at the plasma membrane. In the present study, we further examined the effect Kv1.3 on GLUT4 trafficking and tested whether it occurred via an insulin-dependent pathway. We found that Kv1.3 inhibition by margatoxin (MgTX) stimulated glucose uptake in adipose tissue and skeletal muscle and that the effect of MgTX on glucose transport was additive to that of insulin. Furthermore, whereas the increase in uptake was wortmannin insensitive, it was completely inhibited by dantrolene, a blocker of Ca(2+) release from intracellular Ca(2+) stores. In white adipocytes in primary culture, channel inhibition by Psora-4 increased GLUT4 translocation to the plasma membrane. In these cells, GLUT4 protein translocation was unaffected by the addition of wortmannin but was significantly inhibited by dantrolene. Channel inhibition depolarized the membrane voltage and led to sustained, dantrolene-sensitive oscillations in intracellular Ca(2+) concentration. These results indicate that the apparent increase in insulin sensitivity observed in association with inhibition of Kv1.3 channel activity is mediated by an increase in GLUT4 protein at the plasma membrane, which occurs largely through a Ca(2+)-dependent process.

Exercise training and the antioxidant alpha-lipoic acid in the treatment of insulin resistance and type 2 diabetes

One hallmark of the insulin-resistant state of prediabetes and overt type 2 diabetes is an impaired ability of insulin to activate glucose transport in skeletal muscle, due to defects in IRS-1-dependent signaling. An emerging body of evidence indicates that one potential factor in the multifactorial etiology of skeletal muscle insulin resistance is oxidative stress, an imbalance between the cellular exposure to an oxidant stress and the cellular antioxidant defenses. Exposure of skeletal muscle to an oxidant stress leads to impaired insulin signaling and subsequently to reduced glucose transport activity. Numerous studies have demonstrated that treatment of insulin-resistant animals and type 2 diabetic humans with antioxidants, including alpha-lipoic acid (ALA), is associated with improvements in skeletal muscle glucose transport activity and whole-body glucose tolerance. An additional intervention that is effective in ameliorating the skeletal muscle insulin resistance of prediabetes and type 2 diabetes is endurance exercise training. Recent investigations have demonstrated that the combination of exercise training and antioxidant treatment using ALA in an animal model of obesity-associated insulin resistance provides a unique interactive effect resulting in a greater improvement in insulin action on skeletal muscle glucose transport than either intervention individually. Moreover, this interactive effect of exercise training and ALA is due in part to improvements in IRS-1-dependent insulin signaling. These studies highlight the effectiveness of combining endurance exercise training and antioxidants in beneficially modulating the molecular defects in insulin action observed in insulin-resistant skeletal muscle.

Contraction activates glucose uptake and glycogen synthase normally in muscles from dexamethasone-treated rats

Glucocorticoids cause insulin resistance in skeletal muscle. The aims of the present study were to investigate the effects of contraction on glucose uptake, insulin signaling, and regulation of glycogen synthesis in skeletal muscles from rats treated with the glucocorticoid analog dexamethasone (1 mg x kg(-1) x day(-1) ip for 12 days). Insulin resistance in dexamethasone-treated rats was confirmed by reduced insulin-stimulated glucose uptake (approximately 35%), glycogen synthesis (approximately 70%), glycogen synthase activation (approximately 80%), and PKB Ser(473) phosphorylation (approximately 40%). Chronic dexamethasone treatment did not impair glucose uptake during contraction in soleus or epitrochlearis muscles. In epitrochlearis (but not in soleus), the presence of insulin during contraction enhanced glucose uptake to similar levels in control and dexamethasone-treated rats. Contraction also increased glycogen synthase fractional activity and dephosphorylated glycogen synthase at Ser(645), Ser(649), Ser(653), and Ser(657) normally in muscles from dexamethasone-treated rats. After contraction, insulin-stimulated glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats. Contraction did not increase insulin-stimulated PKB Ser(473) or glycogen synthase kinase-3 (GSK-3) phosphorylation. Instead, contraction increased GSK-3beta Ser(9) phosphorylation in epitrochlearis (but not in soleus) in muscles from control and dexamethasone-treated rats. In conclusion, contraction stimulates glucose uptake normally in dexamethasone-induced insulin resistant muscles. After contraction, insulin's ability to stimulate glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats.

Decreased contraction-stimulated glucose transport in isolated epitrochlearis muscles of pregnant rats

Late pregnancy is characterized by insulin resistance for glucose transport in skeletal muscle. The main purpose of this study was to investigate the effect of late pregnancy on contraction-stimulated glucose transport in isolated rat skeletal muscle after in vitro electrical stimulation. Isolated epitrochlearis muscles of 19-day pregnant and aged-matched nonpregnant control rats were studied. One muscle from each rat was stimulated to contract, and the contralateral muscle served as a resting control. Tension developed during contractile activity, 3-O-methylglucose (3-MG) transport rate, and glycogen concentration were determined. Epitrochlearis muscles from other rats were used to measure insulin-stimulated 3-MG transport. There was no detectable difference between the nonpregnant and pregnant groups for contractile performance (peak tension, total tension, or fatigue). Pregnancy was not associated with significant changes in muscle glycogen concentration (resting or after contractile activity) or the contraction-stimulated decrement in glycogen concentration. For muscles from pregnant vs. nonpregnant groups, there was a 22% reduction (P < or = 0.05) in contraction-stimulated glucose transport, a 28% decrease (P < or = 0.05) in insulin-stimulated glucose transport, and unchanged basal glucose transport. In conclusion, isolated epitrochlearis muscles from pregnant vs. nonpregnant rats had a relative decrement in contraction-stimulated glucose transport that was similar to the relative decline in insulin-stimulated glucose transport. The decrement in contraction-stimulated glucose transport was not attributable to pregnancy-related changes in tension development or glycogen levels. The similar relative decline in insulin- and contraction-stimulated glucose transport raises the possibility that pregnancy impairs a distal process that is common to mechanisms whereby each stimulus activates glucose transport.

Interactions of exercise training and alpha-lipoic acid on insulin signaling in skeletal muscle of obese Zucker rats

We have shown previously (Saengsirisuwan V, Kinnick TR, Schmit MB, and Henriksen EJ. J Appl Physiol 91: 145-153, 2001) that the antioxidant R-(+)-alpha-lipoic acid (R-ALA), combined with endurance exercise training (ET), increases glucose transport in insulin-resistant skeletal muscle in an additive fashion. The purpose of the present study was to investigate possible cellular mechanisms responsible for this interactive effect. We evaluated the effects of R-ALA alone, ET alone, or R-ALA and ET in combination on insulin-stimulated glucose transport, protein expression, and functionality of specific insulin-signaling factors in soleus muscle of obese Zucker (fa/fa) rats. Obese animals remained sedentary, received R-ALA (30 mg.kg body wt(-1).day(-1)), performed ET (daily treadmill running for < or =60 min), or underwent both R-ALA treatment and ET for 15 days. R-ALA or ET individually increased (P < 0.05) insulin-mediated (5 mU/ml) glucose transport (2-deoxyglucose uptake) in soleus muscle by 45 and 68%, respectively, and this value was increased to the greatest extent (124%) in the combined treatment group. Soleus insulin receptor substrate (IRS)-1 protein was significantly increased by R-ALA alone (30%) or ET alone (31%), and a further enhancement (55%) was observed after the combination treatment in the obese animals. Enhanced levels of IRS-1 protein expression after individual or combined interventions were significantly correlated with insulin action on glucose transport activity (r = 0.597, P = 0.0055). Similarly, insulin-mediated IRS-1 associated with the p85 regulatory subunit of phosphatidylinositol 3-kinase was increased by R-ALA (317%) and ET (319%) and to the greatest extent (435%) (all P < 0.05) by the combination treatment. These results indicate that the improvements of insulin action in insulin-resistant skeletal muscle after R-ALA or ET, alone and in combination, were associated with increases in IRS-1 protein expression and IRS-1 associated with p85.

Effects of insulin, contraction, and phorbol esters on mitogen-activated protein kinase signaling in skeletal muscle from lean and ob/ob mice

Effects of diverse stimuli, including insulin, muscle contraction, and phorbol 12-myristate-13-acetate (PMA), were determined on phosphorylation of mitogen-activated protein kinase (MAPK) signaling modules (c-Jun NH(2)-terminal kinase [JNK], p38 MAPK, and extracellular signal-related kinase [ERK1/2]) in skeletal muscle from lean and ob/ob mice. Insulin increased phosphorylation of JNK, p38 MAPK, and ERK1/2 in isolated extensor digitorum longus (EDL) and soleus muscle from lean mice in a time- and dose-dependent manner. Muscle contraction and PMA also elicited robust effects on these parallel MAPK modules. Insulin action on JNK, p38 MAPK, and ERK1/2 phosphorylation was significantly impaired in EDL and soleus muscle from ob/ob mice. In contrast, muscle contraction-mediated JNK, p38 MAPK, and ERK1/2 phosphorylation was preserved. PMA effects on phosphorylation of JNK and ERK1/2 were normal in ob/ob mice, whereas effects on p38 MAPK were abolished. In conclusion, insulin, contraction, and PMA activate MAPK signaling in skeletal muscle. Insulin-mediated responses on MAPK signaling are impaired in skeletal muscle from ob/ob mice, whereas the effect of contraction is generally well preserved. In addition, PMA-induced phosphorylation of JNK and ERK1/2 are preserved, whereas p38 MAPK pathways are impaired in skeletal muscle from ob/ob mice. Thus, appropriate MAPK responses can be elicited in insulin-resistant skeletal muscle via an insulin-independent mechanism.

Effect of glycogen synthase overexpression on insulin-stimulated muscle glucose uptake and storage

Insulin-stimulated muscle glucose uptake is inversely associated with the muscle glycogen concentration. To investigate whether this association is a cause and effect relationship, we compared insulin-stimulated muscle glucose uptake in noncontracted and postcontracted muscle of GSL3-transgenic and wild-type mice. GSL3-transgenic mice overexpress a constitutively active form of glycogen synthase, which results in an abundant storage of muscle glycogen. Muscle contraction was elicited by in situ electrical stimulation of the sciatic nerve. Right gastrocnemii from GSL3-transgenic and wild-type mice were subjected to 30 min of electrical stimulation followed by hindlimb perfusion of both hindlimbs. Thirty minutes of contraction significantly reduced muscle glycogen concentration in wild-type (49%) and transgenic (27%) mice, although transgenic mice retained 168.8 +/- 20.5 micromol/g glycogen compared with 17.7 +/- 2.6 micromol/g glycogen for wild-type mice. Muscle of transgenic and wild-type mice demonstrated similar pre- (3.6 +/- 0.3 and 3.9 +/- 0.6 micromol.g(-1).h(-1) for transgenic and wild-type, respectively) and postcontraction (7.9 +/- 0.4 and 7.0 +/- 0.4 micromol.g(-1).h(-1) for transgenic and wild-type, respectively) insulin-stimulated glucose uptakes. However, the [14C]glucose incorporated into glycogen was greater in noncontracted (151%) and postcontracted (157%) transgenic muscle vs. muscle of corresponding wild-type mice. These results indicate that glycogen synthase activity is not rate limiting for insulin-stimulated glucose uptake in skeletal muscle and that the inverse relationship between muscle glycogen and insulin-stimulated glucose uptake is an association, not a cause and effect relationship.

Modulation of metabolic control by angiotensin converting enzyme (ACE) inhibition

Angiotensin converting enzyme (ACE) inhibitors are a widely used intervention for blood pressure control, and are particularly beneficial in hypertensive type 2 diabetic subjects with insulin resistance. The hemodynamic effects of ACE inhibitors are associated with enhanced levels of the vasodilator bradykinin and decreased production of the vasoconstrictor and growth factor angiotensin II (ATII). In insulin-resistant conditions, ACE inhibitors can also enhance whole-body glucose disposal and glucose transport activity in skeletal muscle. This review will focus on the metabolic consequences of ACE inhibition in insulin resistance. At the cellular level, ACE inhibitors acutely enhance glucose uptake in insulin-resistant skeletal muscle via two mechanisms. One mechanism involves the action of bradykinin, acting through bradykinin B(2) receptors, to increase nitric oxide (NO) production and ultimately enhance glucose transport. A second mechanism involves diminution of the inhibitory effects of ATII, acting through AT(1) receptors, on the skeletal muscle glucose transport system. The acute actions of ACE inhibitors on skeletal muscle glucose transport are associated with upregulation of insulin signaling, including enhanced IRS-1 tyrosine phosphorylation and phosphatidylinositol-3-kinase activity, and ultimately with increased cell-surface GLUT-4 glucose transporter protein. Chronic administration of ACE inhibitors or AT(1) antagonists to insulin-resistant rodents can increase protein expression of GLUT-4 in skeletal muscle and myocardium. These data support the concept that ACE inhibitors can beneficially modulate glucose control in insulin-resistant states, possibly through a NO-dependent effect of bradykinin and/or antagonism of ATII action on skeletal muscle.

Epinephrine inhibits insulin-stimulated muscle glucose transport

We recently demonstrated that epinephrine could inhibit the activation by insulin of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-kinase) in skeletal muscle (Hunt DG, Zhenping D, and Ivy JL. J Appl Physiol 92: 1285-1292, 2002). Activation of PI3-kinase is recognized as an essential step in the activation of muscle glucose transport by insulin. We therefore investigated the effect of epinephrine on insulin-stimulated glucose transport in both fast-twitch (epitrochlearis) and slow-twitch (soleus) muscle of the rat by using an isolated muscle preparation. Glucose transport was significantly increased in the epitrochlearis and soleus when incubated in 50 and 100 microU/ml insulin, respectively. Activation of glucose transport by 50 microU/ml insulin was inhibited by 24 nM epinephrine in both muscle types. This inhibition of glucose transport by epinephrine was accompanied by suppression of IRS-1-associated PI3-kinase activation. However, when muscles were incubated in 100 microU/ml insulin, 24 nM epinephrine was unable to inhibit IRS-1-associated PI3-kinase activation or glucose transport. Even when epinephrine concentration was increased to 500 nM, no attenuating effect was observed on glucose transport. Results of this study indicate that epinephrine is capable of inhibiting glucose transport activated by a moderate, but not a high, physiological insulin concentration. The inhibition of glucose transport by epinephrine appears to involve the inhibition of IRS-1-associated PI3-kinase activation.

Isoform-specific regulation of 5' AMP-activated protein kinase in skeletal muscle from obese Zucker (fa/fa) rats in response to contraction

Glucose transport can be activated in skeletal muscle in response to insulin via activation of phosphoinositide (PI) 3-kinase and in response to contractions or hypoxia, presumably via activation of 5' AMP-activated protein kinase (AMPK). We determined the effects of insulin and muscle contraction/hypoxia on PI 3-kinase, AMPK, and glucose transport activity in epitrochlearis skeletal muscle from insulin-resistant Zucker (fa/ fa) rats. Insulin-stimulated glucose transport in isolated skeletal muscle was reduced 47% in obese versus lean rats, with a parallel 42% reduction in tyrosine-associated PI 3-kinase activity. Contraction and hypoxia elicited normal responses for glucose transport in skeletal muscle from insulin-resistant obese rats. Isoform-specific AMPK activity was measured in skeletal muscle in response to insulin, contraction, or hypoxia. Contraction increased AMPKalpha1 activity 2.3-fold in lean rats, whereas no effect was noted in obese rats. Hypoxia increased AMPKalpha1 activity to a similar extent (more than sixfold) in lean and obese rats. Regardless of genotype, contraction, and hypoxia, each increased AMPKalpha2 activity more than fivefold, whereas insulin did not alter either AMPKalpha1 or -alpha2 activity in skeletal muscle. In conclusion, obesity-related insulin resistance is associated with an isoform-specific impairment in AMPKalpha1 in response to contraction. However, this impairment does not appear to affect contraction-stimulated glucose transport. Activation of AMPKalpha2 in response to muscle contraction/ exercise is associated with a parallel and normal increase in glucose transport in insulin-resistant skeletal muscle.

Invited review: Effects of acute exercise and exercise training on insulin resistance

Insulin resistance of skeletal muscle glucose transport is a key defect in the development of impaired glucose tolerance and Type 2 diabetes. It is well established that both an acute bout of exercise and chronic endurance exercise training can have beneficial effects on insulin action in insulin-resistant states. This review summarizes the present state of knowledge regarding these effects in the obese Zucker rat, a widely used rodent model of obesity-associated insulin resistance, and in insulin-resistant humans with impaired glucose tolerance or Type 2 diabetes. A single bout of prolonged aerobic exercise (30-60 min at approximately 60-70% of maximal oxygen consumption) can significantly lower plasma glucose levels, owing to normal contraction-induced stimulation of GLUT-4 glucose transporter translocation and glucose transport activity in insulin-resistant skeletal muscle. However, little is currently known about the effects of acute exercise on muscle insulin signaling in the postexercise state in insulin-resistant individuals. A well-established adaptive response to exercise training in conditions of insulin resistance is improved glucose tolerance and enhanced skeletal muscle insulin sensitivity of glucose transport. This training-induced enhancement of insulin action is associated with upregulation of specific components of the glucose transport system in insulin-resistant muscle and includes increased protein expression of GLUT-4 and insulin receptor substrate-1. It is clear that further investigations are needed to further elucidate the specific molecular mechanisms underlying the beneficial effects of acute exercise and exercise training on the glucose transport system in insulin-resistant mammalian skeletal muscle.

Propranolol prevents epinephrine from limiting insulin-stimulated muscle glucose uptake during contraction

Beta-blockade results in rapid glucose clearance and premature fatigue during exercise. To investigate the cause of this increased glucose clearance, we studied the acute effects of propranolol on insulin-stimulated muscle glucose uptake during contraction in the presence of epinephrine with an isolated rat muscle preparation. Glucose uptake increased in both fast- (epitrochlearis) and slow-twitch (soleus) muscle during insulin or contraction stimulation. In the presence of 24 nM epinephrine, glucose uptake during contraction was completely suppressed when insulin was present. This suppression of glucose uptake by epinephrine was accompanied by a decrease in insulin receptor substrate (IRS)-1-phosphatidylinositol 3 (PI3)-kinase activity. Propranolol had no direct effect on insulin-stimulated glucose uptake during contraction. However, epinephrine was ineffective in attenuating insulin-stimulated glucose uptake during contraction in the presence of propranolol. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake during contraction occurred in conjunction with its inability to completely suppress IRS-1-PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake during contraction is severely diminished in muscle exposed to propranolol. Thus the increase in glucose clearance and premature fatigue associated with beta-blockade could result from the inability of epinephrine to attenuate insulin-stimulated muscle glucose uptake.

Clenbuterol prevents epinephrine from antagonizing insulin-stimulated muscle glucose uptake

In the present study, we investigated the effects of chronic clenbuterol treatment on insulin-stimulated glucose uptake in the presence of epinephrine in isolated rat skeletal muscle. Insulin (50 microU/ml) increased glucose uptake in both fast-twitch (epitrochlearis) and slow-twitch (soleus) muscles. In the presence of 24 nM epinephrine, insulin-stimulated glucose uptake was completely suppressed. This suppression of glucose uptake by epinephrine was accompanied by an increase in the intracellular concentration of glucose 6-phosphate and a decrease in insulin-receptor substrate-1-associated phosphatidylinositol 3-kinase (IRS-1/PI3-kinase) activity. Clenbuterol treatment had no direct effect on insulin-stimulated glucose uptake. However, after clenbuterol treatment, epinephrine was ineffective in attenuating insulin-stimulated muscle glucose uptake. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake occurred in conjunction with its inability to increase the intracellular concentration of glucose 6-phosphate and attenuate IRS-1/PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake is severely diminished in muscle from rats pretreated with clenbuterol.

Acyl-CoA binding protein expression is fiber type- specific and elevated in muscles from the obese insulin-resistant Zucker rat

Accumulation of acyl-CoA is hypothesized to be involved in development of insulin resistance. Acyl-CoA binds to acyl-CoA binding protein (ACBP) with high affinity, and therefore knowledge about ACBP concentration is important for interpreting acyl-CoA data. In the present study, we used a sandwich enzyme-linked immunosorbent assay to quantify ACBP concentration in different muscle fiber types. Furthermore, ACBP concentration was compared in muscles from lean and obese Zucker rats. Expression of ACBP was highest in the slow-twitch oxidative soleus muscle and lowest in the fast-twitch glycolytic white gastrocnemius (0.46 +/- 0.02 and 0.16 +/- 0.005 microg/mg protein, respectively). Expression of ACBP was soleus > red gastrocnemius > extensor digitorum longus > white gastrocnemius. Similar fiber type differences were found for carnitine palmitoyl transferase (CPT)-1, and a correlation was observed between ACBP and CPT-1. Muscles from obese Zucker rats had twice the triglyceride content, had approximately twice the long-chain acyl CoA content, and were severely insulin resistant. ACBP concentration was approximately 30% higher in all muscles from obese rats. Activities of CPT-1 and 3-hydroxy-acyl-CoA dehydrogenase were increased in muscles from obese rats, whereas citrate synthase activity was similar. In conclusion, ACBP expression is fiber type-specific with the highest concentration in oxidative muscles and the lowest in glycolytic muscles. The 90% increase in the concentration of acyl-CoA in obese Zucker muscle compared with only a 30% increase in the concentration of ACBP supports the hypothesis that an increased concentration of free acyl-CoA is involved in the development of insulin resistance.

Glucose uptake and metabolic stress in rat muscles stimulated electrically with different protocols

In the present study, the relationship between the pattern of electrical stimulation and glucose uptake was investigated in slow-twitch muscles (soleus) and fast-twitch muscles (epitrochlearis) from Wistar rats. Muscles were stimulated electrically for 30 min in vitro with either single pulses (frequencies varied between 0.8 and 15 Hz) or with 200-ms trains (0.1-2 Hz). Glucose uptake (measured with tracer amount of 2-[(3)H]deoxyglucose) increased with increasing number of impulses whether delivered as single pulses or as short trains. The highest glucose uptake achieved with short tetanic contractions was similar in soleus and epitrochlearis (10.9 +/- 0.7 and 12.0 +/- 0.8 mmol x kg dry wt(-1) x 30 min(-1), respectively). Single pulses, on the other hand, increased contraction-stimulated glucose uptake less in soleus than in epitrochlearis (7.5 +/- 1.1 and 11.7 +/- 0.5 mmol x kg dry wt(-1) x 30 min(-1), respectively; P < 0.02). Glucose uptake correlated with glycogen breakdown in soleus (r = 0.84, P < 0.0001) and (epitrochlearis: r = 0.91, P < 0.0001). Contraction-stimulated glucose uptake also correlated with breakdown of ATP and PCr and with reduction in force. Our data suggest that metabolic stress mediates contraction-stimulated glucose uptake.

Interactions of exercise training and lipoic acid on skeletal muscle glucose transport in obese Zucker rats

Exercise training (ET) or the antioxidant R(+)-alpha-lipoic acid (R-ALA) individually increases insulin action in the insulin-resistant obese Zucker rat. The purpose of the present study was to determine the interactions of ET and R-ALA on insulin action and oxidative stress in skeletal muscle of the obese Zucker rat. Animals either remained sedentary, received R-ALA (30 mg x kg body wt(-1) x day(-1)), performed ET (treadmill running), or underwent both R-ALA treatment and ET for 6 wk. During an oral glucose tolerance test, ET alone or in combination with R-ALA resulted in a significant lowering of the glucose (26-32%) and insulin (29-30%) responses compared with sedentary controls. R-ALA alone decreased (19%) the glucose-insulin index (indicative of increased insulin sensitivity), and this parameter was reduced (48-52%) to the greatest extent in the ET and combined treatment groups. ET or R-ALA individually increased insulin-mediated glucose transport activity in isolated epitrochlearis (44-48%) and soleus (37-57%) muscles. The greatest increases in insulin action in these muscles (80 and 99%, respectively) were observed in the combined treatment group. Whereas the improvement in insulin-mediated glucose transport in soleus due to R-ALA was associated with decreased protein carbonyl levels (an index of oxidative stress), improvement because of ET was associated with decreased protein carbonyls as well as enhanced GLUT-4 protein. However, there was no interactive effect of ET and R-ALA on GLUT-4 protein or protein carbonyl levels. These results indicate that ET and R-ALA interact in an additive fashion to improve insulin action in insulin-resistant skeletal muscle. Because the further improvement in muscle glucose transport in the combined group was not associated with additional upregulation of GLUT-4 protein or a further reduction in oxidative stress, the mechanism for this interaction must be due to additional, as yet unidentified, factors.

Effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats

Activation of AMP-activated protein kinase (AMPK) with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofurano-side (AICAR) increases glucose transport in skeletal muscle via an insulin-independent pathway. To examine the effects of AMPK activation on skeletal muscle glucose transport activity and whole-body carbohydrate and lipid metabolism in an insulin-resistant rat model, awake obese Zuckerfa/fa rats (n = 26) and their lean (n = 23) littermates were infused for 90 min with AICAR, insulin, or saline. The insulin infusion rate (4 mU.kg(-1).min(-1)) was selected to match the glucose requirements during AICAR (bolus, 100 mg/kg; constant, 10 mg.kg(-1).min(-1)) isoglycemic clamps in the lean rats. The effects of these identical AICAR and insulin infusion rates were then examined in the obese Zucker rats. AICAR infusion increased muscle AMPK activity more than fivefold (P < 0.01 vs. control and insulin) in both lean and obese rats. Plasma triglycerides, fatty acid concentrations, and glycerol turnover, as assessed by [2-13C]glycerol, were all decreased in both lean and obese rats infused with AICAR (P < 0.05 vs. basal), whereas insulin had no effect on these parameters in the obese rats. Endogenous glucose production rates, measured by [U-13C]glucose, were suppressed by >50% during AICAR and insulin infusions in both lean and obese rats (P < 0.05 vs. basal). In lean rats, rates of whole-body glucose disposal increased by more than two-fold (P < 0.05 vs. basal) during both AICAR and insulin infusion; [3H]2-deoxy-D-glucose transport activity increased to a similar extent, by >2.2-fold (both P < 0.05 vs. control), in both soleus and red gastrocnemius muscles of lean rats infused with either AICAR or insulin. In the obese Zucker rats, neither AICAR nor insulin stimulated whole-body glucose disposal or soleus muscle glucose transport activity. However, AICAR increased glucose transport activity by approximately 2.4-fold (P < 0.05 vs. control) in the red gastrocnemius from obese rats, whereas insulin had no effect. In summary, acute infusion of AICAR in an insulin-resistant rat model activates skeletal muscle AMPK and increases glucose transport activity in red gastrocnemius muscle while suppressing endogenous glucose production and lipolysis. Because type 2 diabetes is characterized by diminished rates of insulin-stimulated glucose uptake as well as increased basal rates of endogenous glucose production and lipolysis, these results suggest that AICAR-related compounds may represent a new class of antidiabetic agents.

GLUT4 overexpression in db/db mice dose-dependently ameliorates diabetes but is not a lifelong cure

We previously reported that overexpression of GLUT4 in lean, nondiabetic C57BL/KsJ-lepr(db/+) (db/+) mice resulted in improved glucose tolerance associated with increased basal and insulin-stimulated glucose transport in isolated skeletal muscle. We used the diabetic (db/db) litter mates of these mice to examine the effects of GLUT4 overexpression on in vivo glucose utilization and on in vitro glucose transport and GLUT4 translocation in diabetic mice. We examined in vivo glucose disposal by oral glucose challenge and hyperinsulinemic-hyperglycemic clamps. We also evaluated the in vitro relationship between glucose transport activity and cell surface GLUT4 levels as assessed by photolabeling with the membrane-impermeant reagent 2-N-(4-(1-azi-2,2,2-trifluoroethyl)benzoyl)-1,3-bis(D-mannose-4-yloxy)-2-propylamine in extensor digitorum longus (EDL) muscles. All parameters were examined as functions of animal age and the level of GLUT4 overexpression. In young mice (age 10-12 weeks), both lower (two- to threefold) and higher (four- to fivefold) levels of GLUT4 overexpression were associated with improved glucose tolerance compared to age-matched nontransgenic (NTG) mice. However, glucose tolerance deteriorated with age in db/db mice, although less rapidly in transgenic mice expressing the higher level of GLUT4. Glucose infusion rates during hyperinsulinemic-hyperglycemic clamps were increased with GLUT4 overexpression, compared with NTG mice in both lower and higher levels of GLUT4 overexpression, even in the older mice. Surprisingly, isolated EDL muscles from diabetic db/db mice did not exhibit alterations in either basal or insulin-stimulated glucose transport activity or cell surface GLUT4 compared to nondiabetic db/+ mice. Furthermore, both GLUT4 overexpression levels and animal age are associated with increased basal and insulin-stimulated glucose transport activities and cell surface GLUT4. However, the observed increased glucose transport activity in older db/db mice was not accompanied by an equivalent increase in cell surface GLUT4 compared to younger animals. Thus, although in vivo glucose tolerance is improved with GLUT4 overexpression in young animals, it deteriorates with age; in contrast, insulin responsiveness as assessed by the clamp technique remains improved with GLUT4 overexpression, as does in vitro insulin action. In summary, despite an impairment in whole-body glucose tolerance, skeletal muscle of the old transgenic GLUT4 db/db mice is still insulin responsive in vitro and in vivo.

Insulin stimulation of muscle protein synthesis in obese Zucker rats is not via a rapamycin-sensitive pathway

The obese Zucker rat is resistant to insulin for glucose disposal, but it is unknown whether this insulin resistance is accompanied by alterations of insulin-mediated muscle protein synthesis. We examined rates of muscle protein synthesis either with or without insulin in lean and obese Zucker rats with the use of a bilateral hindlimb preparation. Additional experiments examined insulin's effect on protein synthesis with or without rapamycin, an inhibitor of protein synthesis. Protein synthesis in red and white gastrocnemius was stimulated by insulin compared with control (no insulin) in obese (n = 10, P<0.05) but not in lean (n = 10, P>0.05) Zucker rats. In white gastrocnemius, rapamycin significantly reduced rates of protein synthesis compared with control in lean (n = 6) and obese (n = 6) rats; however, in red gastrocnemius, the attenuating effect of rapamycin occurred only in obese rats. The addition of insulin to rapamycin resulted in rates of synthesis that were similar to those for rapamycin alone for lean rats and to those for insulin alone (augmented) for obese rats in both tissues. Our results demonstrate that insulin enhances protein synthesis in muscle that is otherwise characterized as insulin resistant. Furthermore, rapamycin inhibits protein synthesis in muscle of obese Zucker rats; however, stimulation of protein synthesis by insulin is not via a rapamycin-sensitive pathway.

Insulin-sensitive regulation of glucose transport and GLUT4 translocation in skeletal muscle of GLUT1 transgenic mice

Skeletal muscle glucose transport was examined in transgenic mice overexpressing the glucose transporter GLUT1 using both the isolated incubated-muscle preparation and the hind-limb perfusion technique. In the absence of insulin, 2-deoxy-d-glucose uptake was increased approximately 3-8-fold in isolated fast-twitch muscles of GLUT1 transgenic mice compared with non-transgenic siblings. Similarly, basal glucose transport activity was increased approximately 4-14-fold in perfused fast-twitch muscles of transgenic mice. In non-transgenic mice insulin accelerated glucose transport activity approximately 2-3-fold in isolated muscles and to a much greater extent ( approximately 7-20-fold) in perfused hind-limb preparations. The observed effect of insulin on glucose transport in transgenic muscle was similarly dependent upon the technique used for measurement, as insulin had no effect on isolated fast-twitch muscle from transgenic mice, but significantly enhanced glucose transport in perfused fast-twitch muscle from transgenic mice to approximately 50-75% of the magnitude of the increase observed in non-transgenic mice. Cell-surface glucose transporter content was assessed via 2-N-4-(l-azi-2,2,2-trifluoroethyl)benzoyl-1,3-bis-(d -mannos-4-yloxy)-2-propylamine photolabelling methodology in both isolated and perfused extensor digitorum longus (EDL). Cell-surface GLUT1 was enhanced by as much as 70-fold in both isolated and perfused EDL of transgenic mice. Insulin did not alter cell-surface GLUT1 in either transgenic or non-transgenic mice. Basal levels of cell-surface GLUT4, measured in either isolated or perfused EDL, were similar in transgenic and non-transgenic mice. Interestingly, insulin enhanced cell-surface GLUT4 approximately 2-fold in isolated EDL and approximately 6-fold in perfused EDL of both transgenic and non-transgenic mice. In summary, these results reveal differences between isolated muscle and perfused hind-limb techniques, with the latter method showing a more robust responsiveness to insulin. Furthermore, the results demonstrate that muscle overexpressing GLUT1 has normal insulin-induced GLUT4 translocation and the ability to augment glucose-transport activity above the elevated basal rates.

Regulation of GLUT4 protein expression and glycogen storage after prolonged exercise

The purpose of this study was to determine the time course of GLUT4 protein accumulation following an exercise-carbohydrate supplementation regimen, and to evaluate the effect of this regimen on GLUT4 mRNA regulation. Rats were exercised by swimming and intubated with 1 mL of a 50% glucose solution immediately post-exercise. Exercise significantly reduced muscle glycogen by 50%. By 1.5 h of recovery, muscle glycogen was normalized, but continued to increase above the control level during the next 16 h. A faster and larger repletion of glycogen occurred in the fast-twitch red compared with the fast-twitch white muscle during the 16 h of recovery. GLUT4 protein concentration in fast-twitch red muscle was significantly increased above control by 1.5 h of recovery, and progressively increased throughout the recovery period. Fast-twitch white muscle demonstrated a similar trend, but the increase in GLUT4 protein did not reach significance until 5 h of recovery. Fast-twitch red muscle GLUT4 mRNA was increased by 53% above control immediately post-exercise, but returned to the control level by 1.5 h of recovery. GLUT4 mRNA associated with polysomes, however, increased significantly during this time and remained elevated for a minimum of 5 h. The results suggest that the increased GLUT4 protein expression following a regimen of exercise-carbohydrate supplementation occurs sufficiently fast to contribute to the resynthesis of muscle glycogen, and is controlled by both pre-translational and translational mechanisms.

Evidence for altered sensitivity of the nitric oxide/cGMP signalling cascade in insulin-resistant skeletal muscle

Nitric oxide activates guanylate cyclase to form cGMP, comprising a signalling system that is believed to be a distinct mechanism for increasing glucose transport and metabolism in skeletal muscle. The effects of a selective cGMP phosphodiesterase inhibitor, zaprinast, on basal glucose utilization was investigated in incubated rat soleus muscle preparations isolated from both insulin-sensitive (lean Zucker; Fa/?) and insulin-resistant (obese Zucker; fa/fa) rats. Zaprinast at 27 microM significantly increased cGMP levels in incubated soleus muscle isolated from lean, but not obese, Zucker rats. Muscles were incubated with 14C-labelled glucose and various concentrations of zaprinast (3, 27 and 243 microM). Zaprinast (at 27 and 243 microM) significantly increased rates of net and 14C-labelled lactate release and of glycogen synthesis in lean Zucker rat soleus muscle; glucose oxidation was also increased by 27 microM zaprinast. In addition, regardless of concentration, the phosphodiesterase inhibitor failed to increase any aspect of 14C-labelled glucose utilization in soleus muscles isolated from obese Zucker rats. The maximal activity of nitric oxide synthase (NOS) was significantly decreased in insulin-resistant obese Zucker muscles. Thus the lack of effect of zaprinast in insulin-resistant skeletal muscle is consistent with decreased NOS activity. To test whether there is a defect in insulin-resistant skeletal muscle for endogenous activation of guanylate cyclase, soleus muscles were isolated from both insulin-sensitive and insulin-resistant Zucker rats and incubated with various concentrations of the NO donor sodium nitroprusside (SNP; 0.1, 1, 5 and 15 mM). SNP significantly increased rates of net and 14C-labelled lactate release, as well as glucose oxidation in muscles isolated from both insulin-sensitive and insulin-resistant rats. A decreased response to SNP was observed in the dose-dependent generation of cGMP within isolated soleus muscles from insulin-resistant rats. A possible link between impaired NO/cGMP signalling and abnormal glucose utilization by skeletal muscle is discussed.

Regulation of cell surface GLUT4 in skeletal muscle of transgenic mice

Marked overexpression of the glucose transporter GLUT4 in skeletal muscle membrane fractions of GLUT4 transgenic (TG) mice is accompanied by disproportionately small increases in basal and insulin-stimulated glucose transport activity. Thus we have assessed cell surface GLUT4 by photolabelling with the membrane-impermeant reagent 2-N-[4-(1-azi-2,2,2-trifluoroethyl)benzoyl]-1, 3-bis(D-mannos-4-yloxy)-2-propylamine (ATB-BMPA) and measured the corresponding glucose transport activity using 2-deoxyglucose in isolated extensor digitorum longus (EDL) muscles from non-transgenic (NTG) and GLUT4 TG mice in the absence and presence of 13.3 nM (2000 mu units/ml) insulin, without or with hypoxia as a model of muscle contraction. TG mice displayed elevated rates of glucose transport activity under basal and insulin-stimulated conditions, and in the presence of insulin plus hypoxia, compared with NTG mice. Photoaffinity labelling of cell surface GLUT4 indicated corresponding elevations in plasma membrane GLUT4 in the basal and insulin-stimulated states, and with insulin plus hypoxia, but no difference in cell surface GLUT4 during hypoxia stimulation. Subcellular fractionation of hindlimb muscles confirmed the previously observed 3-fold overexpression of GLUT4 in the TG compared with the NTG mice. These results suggest that: (1) alterations in glucose transport activity which occur with GLUT4 overexpression in EDL muscles are directly related to cell surface GLUT4 content, regardless of the levels observed in the corresponding subcellular membrane fractions, (2) while overexpression of GLUT4 influences both basal and insulin-stimulated glucose transport activity, the response to hypoxia/ contraction-stimulated glucose transport is unchanged, and (3) subcellular fractionation provides little insight into the subcellular trafficking of GLUT4, and whatever relationship is demonstrated in EDL muscles from NTG mice is disrupted on GLUT4 overexpression.