Carbohydrate Metabolism In Skeletal Muscle Research Articles

Stable suppression of skeletal muscle fructose-1,6-bisphosphatase during ground squirrel hibernation: Potential implications of reversible acetylation as a regulatory mechanism

Mammalian hibernation is a period that involves substantial metabolic change in order to promote survival in harsh conditions, with animals typically relying on non-carbohydrate fuel stores during long bouts of torpor. However, the use and maintenance of carbohydrate fuel stores remains important during periods of arousal from torpor as well as when exiting hibernation. Gluconeogenesis plays a key role in maintaining glucose stores; however, little is known about this process within the muscles of hibernating mammals. Here, we used 13-lined ground squirrels (Ictidomys tridecemlineatus) as our model for mammalian hibernation, and showed that skeletal muscle fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11), the rate-limiting enzyme for the gluconeogenic pathway, was suppressed during torpor as compared to the euthermic control. A physical assessment of partially purified FBPase via exposure to increasing concentrations of the denaturant urea indicated that FBPase from the two conditions were structurally distinct. Western blot analysis suggests that the kinetic and physical differences between euthermic and torpid FBPase may be derived from differential acetylation, whereby increased acetylation of the torpid enzyme makes FBPase more rigid and less active. This study increases our understanding of skeletal muscle carbohydrate metabolism during mammalian hibernation and sets forth a potentially novel mechanism for the regulation of FBPase during environmental stress.

Specific shifts in the endocannabinoid system in hibernating brown bears

In small hibernators, global downregulation of the endocannabinoid system (ECS), which is involved in modulating neuronal signaling, feeding behavior, energy metabolism, and circannual rhythms, has been reported to possibly drive physiological adaptation to the hibernating state. In hibernating brown bears (Ursus arctos), we hypothesized that beyond an overall suppression of the ECS, seasonal shift in endocannabinoids compounds could be linked to bear’s peculiar features that include hibernation without arousal episodes and capacity to react to external disturbance. We explored circulating lipids in serum and the ECS in plasma and metabolically active tissues in free-ranging subadult Scandinavian brown bears when both active and hibernating. In winter bear serum, in addition to a 2-fold increase in total fatty acid concentration, we found significant changes in relative proportions of circulating fatty acids, such as a 2-fold increase in docosahexaenoic acid C22:6 n-3 and a decrease in arachidonic acid C20:4 n-6. In adipose and muscle tissues of hibernating bears, we found significant lower concentrations of 2-arachidonoylglycerol (2-AG), a major ligand of cannabinoid receptors 1 (CB1) and 2 (CB2). Lower mRNA level for genes encoding CB1 and CB2 were also found in winter muscle and adipose tissue, respectively. The observed reduction in ECS tone may promote fatty acid mobilization from body fat stores, and favor carbohydrate metabolism in skeletal muscle of hibernating bears. Additionally, high circulating level of the endocannabinoid-like compound N-oleoylethanolamide (OEA) in winter could favor lipolysis and fatty acid oxidation in peripheral tissues. We also speculated on a role of OEA in the conservation of an anorexigenic signal and in the maintenance of torpor during hibernation, while sustaining the capacity of bears to sense stimuli from the environment.

Irradiation impairs mitochondrial function and skeletal muscle oxidative capacity: significance for metabolic complications in cancer survivors

BackgroundMetabolic complications are highly prevalent in cancer survivors treated with irradiation but the underlying mechanisms remain unknown. MethodsChow or high fat-fed C57Bl/6J mice were irradiated (6Gy) before investigating the impact on whole-body or skeletal muscle metabolism and profiling their lipidomic signature. Using a transgenic mouse model (Tg:Pax7-nGFP), we isolated muscle progenitor cells (satellite cells) and characterised their metabolic functions. We recruited childhood cancer survivors, grouped them based on the use of total body irradiation during their treatment and established their lipidomic profile. ResultsIn mice, irradiation delayed body weight gain and impaired fat pads and muscle weights. These changes were associated with impaired whole-body fat oxidation in chow-fed mice and altered ex vivo skeletal muscle fatty acid oxidation, potentially due to a reduction in oxidative fibres and reduced mitochondrial enzyme activity. Irradiation led to fasting hyperglycaemia and impaired glucose uptake in isolated skeletal muscles. Cultured satellite cells from irradiated mice showed decreased fatty acid oxidation and reduced glucose uptake, recapitulating the host metabolic phenotype. Irradiation resulted in a remodelling of lipid species in skeletal muscles, with the extensor digitorum longus muscle being particularly affected. A large number of lipid species were reduced, with several of these species showing a positive correlation with mitochondrial enzymes activity. In cancer survivors exposed to irradiation, we found a similar decrease in systemic levels of most lipid species, and lipid species that increased were positively correlated with insulin resistance (HOMA-IR). ConclusionIrradiation leads to long-term alterations in body composition, and lipid and carbohydrate metabolism in skeletal muscle, and affects muscle progenitor cells. Such changes result in persistent impairment of metabolic functions, providing a new mechanism for the increased prevalence of metabolic diseases reported in irradiated individuals. In this context, changes in the lipidomic signature in response to irradiation could be of diagnostic value.

Effects of Taurine Administration on Carbohydrate Metabolism in Skeletal Muscle during the Post-Exercise Phase.

We previously reported that taurine (2-aminoethanesulfonic acid; dose: 0.5 mg/g body weight) administration after treadmill running at 25 m/min for 90 min increased the glycogen concentration in the skeletal muscle of ICR mice at 120 min after the exercise (Takahashi et al. 2014). In the current study, we further investigated the effects of taurine administration on glycogen repletion and carbohydrate metabolism in the tibialis anterior muscle after endurance exercise. The metabolomic profiles of the tibialis anterior muscle at 120 min after the exercise were analyzed by a capillary electrophoresis-time-of-flight mass spectrometry (n=6). Fructose-1,6-bisphosphate (F1,6P), a glycogenolytic/glycolytic intermediate produced by phosphofructokinase, was significantly lower in the taurine-treated group than that in the control group (p<0.01). Dihydroxyacetonephosphate (DHAP), a downstream product of F1,6P was lower (p=0.05) and glycerol 3-phosphate, a downstream product of F1,6P and DHAP, tended to be lower (p=0.09) in the taurine-treated group than in the controls. At that time, phosphorylated Ser293 on the E1α subunit of pyruvate dehydrogenase (PDH) tended to be higher in the taurine-treated mice than in the controls (p=0.09, n=5). There was a positive correlation between phosphorylation of the PDH E1α subunit at Ser293 and glycogen concentration (r=0.73, p<0.05). Our results showed that the enhanced glycogen repletion in skeletal muscle by taurine treatment during the post-exercise phase was accompanied by the lower levels of glycogenolytic/glycolytic intermediates.

Sodium valproate increases the brain isoform of glycogen phosphorylase: looking for a compensation mechanism in McArdle disease using a mouse primary skeletal-muscle culture in vitro.

ABSTRACTMcArdle disease, also termed ‘glycogen storage disease type V’, is a disorder of skeletal muscle carbohydrate metabolism caused by inherited deficiency of the muscle-specific isoform of glycogen phosphorylase (GP-MM). It is an autosomic recessive disorder that is caused by mutations in the PYGM gene and typically presents with exercise intolerance, i.e. episodes of early exertional fatigue frequently accompanied by rhabdomyolysis and myoglobinuria. Muscle biopsies from affected individuals contain subsarcolemmal deposits of glycogen. Besides GP-MM, two other GP isoforms have been described: the liver (GP-LL) and brain (GP-BB) isoforms, which are encoded by the PYGL and PYGB genes, respectively; GP-BB is the main GP isoform found in human and rat foetal tissues, including the muscle, although its postnatal expression is dramatically reduced in the vast majority of differentiated tissues with the exception of brain and heart, where it remains as the major isoform. We developed a cell culture model from knock-in McArdle mice that mimics the glycogen accumulation and GP-MM deficiency observed in skeletal muscle from individuals with McArdle disease. We treated mouse primary skeletal muscle cultures in vitro with sodium valproate (VPA), a histone deacetylase inhibitor. After VPA treatment, myotubes expressed GP-BB and a dose-dependent decrease in glycogen accumulation was also observed. Thus, this in vitro model could be useful for high-throughput screening of new drugs to treat this disease. The immortalization of these primary skeletal muscle cultures could provide a never-ending source of cells for this experimental model. Furthermore, VPA could be considered as a gene-expression modulator, allowing compensatory expression of GP-BB and decreased glycogen accumulation in skeletal muscle of individuals with McArdle disease.

Exercise and Regulation of Carbohydrate Metabolism.

Carbohydrates are the preferred substrate for contracting skeletal muscles during high-intensity exercise and are also readily utilized during moderate intensity exercise. This use of carbohydrates during physical activity likely played an important role during the survival of early Homo sapiens, and genes and traits regulating physical activity, carbohydrate metabolism, and energy storage have undoubtedly been selected throughout evolution. In contrast to the life of early H. sapiens, modern lifestyles are predominantly sedentary. As a result, intake of excessive amounts of carbohydrates due to the easy and continuous accessibility to modern high-energy food and drinks has not only become unnecessary but also led to metabolic diseases in the face of physical inactivity. A resulting metabolic disease is type 2 diabetes, a complex endocrine disorder characterized by abnormally high concentrations of circulating glucose. This disease now affects millions of people worldwide. Exercise has beneficial effects to help control impaired glucose homeostasis with metabolic disease, and is a well-established tool to prevent and combat type 2 diabetes. This chapter focuses on the effects of exercise on carbohydrate metabolism in skeletal muscle and systemic glucose homeostasis. We will also focus on the molecular mechanisms that mediate the effects of exercise to increase glucose uptake in skeletal muscle. It is now well established that there are different proximal signaling pathways that mediate the effects of exercise and insulin on glucose uptake, and these distinct mechanisms are consistent with the ability of exercise to increase glucose uptake in the face of insulin resistance in people with type 2 diabetes. Ongoing research in this area is aimed at defining the precise mechanism by which exercise increases glucose uptake and insulin sensitivity and the types of exercise necessary for these important health benefits.

Circulating branched‐chain amino acid concentrations are associated with obesity and future insulin resistance in children and adolescents

What is already known about this subject Circulating concentrations of branched-chain amino acids (BCAAs) can affect carbohydrate metabolism in skeletal muscle, and therefore may alter insulin sensitivity. BCAAs are elevated in adults with diet-induced obesity, and are associated with their future risk of type 2 diabetes even after accounting for baseline clinical risk factors. What this study adds Increased concentrations of BCAAs are already present in young obese children and their metabolomic profiles are consistent with increased BCAA catabolism. Elevations in BCAAs in children are positively associated with insulin resistance measured 18 months later, independent of their initial body mass index. Branched-chain amino acid (BCAA) concentrations are elevated in response to overnutrition, and can affect both insulin sensitivity and secretion. Alterations in their metabolism may therefore play a role in the early pathogenesis of type 2 diabetes in overweight children. To determine whether paediatric obesity is associated with elevations in fasting circulating concentrations of BCAAs (isoleucine, leucine and valine), and whether these elevations predict future insulin resistance. Sixty-nine healthy subjects, ages 8-18 years, were enrolled as a cross-sectional cohort. A subset of subjects who were pre- or early-pubertal, ages 8-13 years, were enrolled in a prospective longitudinal cohort for 18 months (n = 17 with complete data). Elevations in the concentrations of BCAAs were significantly associated with body mass index (BMI) Z-score (Spearman's Rho 0.27, P = 0.03) in the cross-sectional cohort. In the subset of subjects that followed longitudinally, baseline BCAA concentrations were positively associated with homeostasis model assessment for insulin resistance measured 18 months later after controlling for baseline clinical factors including BMI Z-score, sex and pubertal stage (P = 0.046). Elevations in the concentrations of circulating BCAAs are significantly associated with obesity in children and adolescents, and may independently predict future insulin resistance.

Direct involvement of tumor necrosis factor-α in the regulation of glucose uptake in rainbow trout muscle cells

The proinflammatory cytokine TNF-α is known to have a direct action on skeletal muscle in mammals. However, little is known regarding the potential effects of cytokines on nonimmune tissues, particularly in skeletal muscle, in fish. The aim of this study was to investigate the effects of recombinant trout TNF-α (rtTNF-α) on skeletal muscle carbohydrate metabolism in rainbow trout (Oncorhynchus mykiss). We used a primary cell culture of muscle cells from rainbow trout to show that rtTNF-α stimulates glucose uptake in myoblasts and myotubes at concentrations that do not affect the viability of the cells, requiring de novo protein synthesis as shown by the impairment of rtTNF-α-stimulated glucose uptake by cycloheximide. With the use of specific inhibitors, we show that rtTNF-α-stimulated glucose uptake is mediated by the p38MAPK, NF-κB, and JNK pathways. Additionally, we provide evidence that the stimulatory effects of rtTNF-α on glucose uptake in trout skeletal muscle cells may be caused, at least in part, by an increase in the amount of GLUT4 at the plasma membrane. Incubation of trout muscle cells with conditioned medium from LPS-stimulated trout macrophages, enriched in TNF-α, increased glucose uptake. Our results indicate that recombinant, as well as native trout TNF-α, directly stimulates glucose uptake in trout muscle cells and provide evidence, for the first time in nonmammalian vertebrates, for a potential regulatory role of TNF-α in skeletal muscle metabolism.

Naturally occurring R225W mutation of the gene encoding AMP-activated protein kinase (AMPK)γ3 results in increased oxidative capacity and glucose uptake in human primary myotubes

AMP-activated protein kinase (AMPK) has a broad role in the regulation of glucose and lipid metabolism making it a promising target in the treatment of type 2 diabetes mellitus. We therefore sought to characterise for the first time the effects of chronic AMPK activation on skeletal muscle carbohydrate metabolism in carriers of the rare gain-of-function mutation of the gene encoding AMPKgamma(3) subunit, PRKAG3 R225W. Aspects of fuel metabolism were studied in vitro in myocytes isolated from vastus lateralis of PRKAG3 R225W carriers and matched control participants. In vivo, muscular strength and fatigue were evaluated by isokinetic dynamometer and surface electromyography, respectively. Glucose uptake in exercising quadriceps was determined using [(18)F]fluorodeoxyglucose and positron emission tomography. Myotubes from PRKAG3 R225W carriers had threefold higher mitochondrial content (p < 0.01) and oxidative capacity, higher leak-dependent respiration (1.6-fold, p < 0.05), higher basal glucose uptake (twofold, p < 0.01) and higher glycogen synthesis rates (twofold, p < 0.05) than control myotubes. They also had higher levels of intracellular glycogen (p < 0.01) and a trend for lower intramuscular triacylglycerol stores. R225W carriers showed remarkable resistance to muscular fatigue and a trend for increased glucose uptake in exercising muscle in vivo. Through the enhancement of skeletal muscle glucose uptake and increased mitochondrial content, the R225W mutation may significantly enhance exercise performance. These findings are also consistent with the hypothesis that the gamma(3) subunit of AMPK is a promising tissue-specific target for the treatment of type 2 diabetes mellitus, a condition in which glucose uptake and mitochondrial function are impaired.

Exercise, sex, menstrual cycle phase, and 17β-estradiol influence metabolism-related genes in human skeletal muscle

Higher fat and lower carbohydrate and amino acid oxidation are observed in women compared with men during endurance exercise. We hypothesized that the observed sex difference is due to estrogen and that menstrual cycle phase or supplementation of men with 17beta-estradiol (E(2)) would coordinately influence the mRNA content of genes involved in lipid and/or carbohydrate metabolism in skeletal muscle. Twelve men and twelve women had muscle biopsies taken before and immediately after 90 min of cycling at 65% peak oxygen consumption (Vo(2peak)). Women were studied in the midfollicular (Fol) and midluteal (Lut) phases, and men were studied after 8 days of E(2) or placebo supplementation. Targeted RT-PCR was used to compare mRNA content for genes involved in transcriptional regulation and lipid, carbohydrate, and amino acid metabolism. Sex was the greatest predictor of substrate metabolism gene content. Sex affected the mRNA content of FATm, FABPc, SREBP-1c, mtGPAT, PPARdelta, PPARalpha, CPTI, TFP-alpha, GLUT4, HKII, PFK, and BCOADK (P < 0.05). E(2) administration significantly (P < 0.05) affected the mRNA content of PGC-1alpha, PPARalpha, PPARdelta, TFP-alpha, CPTI, SREBP-1c, mtGPAT, GLUT4, GS-1, and AST. Acute exercise increased the mRNA abundance for PGC-1alpha, HSL, FABPc, CPTI, GLUT4, HKII, and AST (P < 0.05). Menstrual cycle had a small effect on PPARdelta, GP, and glycogenin mRNA content. Overall, women have greater mRNA content for several genes involved in lipid metabolism, which is partially due to an effect of E(2).

Skeletal muscle type comparison of pyruvate dehydrogenase phosphatase activity and isoform expression: effects of obesity and endurance training

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate metabolism in skeletal muscle. PDH is activated by PDH phosphatase (PDP) and deactivated by PDH kinase (PDK). Obesity has a large negative impact on skeletal muscle carbohydrate metabolism, whereas endurance training has been shown to improve regulatory control of skeletal muscle carbohydrate metabolism, more so when coupled with obesity. A majority of this literature has focused on PDK, with little information available on PDP. To determine the relative role of PDP in regulating skeletal muscle PDH activity with obesity and endurance training, obese and lean Zucker rats remained sedentary or were endurance trained (1 h/day, 5 days/wk) for a period of 8 wk. Soleus, red gastrocnemius, (RG), and white gastrocnemius (WG) muscles were sampled after the training period. The main findings were 1) obesity resulted in a 46% decrease in PDP activity expressed per milligram extracted mitochondrial protein only in RG, while PDP isoform content was unchanged; 2) 8 wk of endurance training led to a significant 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; 3) 8 wk of endurance training led to a trending 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; and 4) PDP2 protein content was not affected by obesity or training. These results suggest that decreased PDP activity in oxidative skeletal muscles may play a role in the impairment of carbohydrate metabolism in obese rats, which is reversible with endurance training.

Specificity of training adaptation: time for a rethink?

The key components of any training programme are the volume (how much), intensity (how hard) and frequency (how often) of exercise sessions. These ‘training impulses’ determine the magnitude of adaptive responses that either enhance (fitness) or decrease (fatigue) exercise capacity (Hawley, 2002). A long held view is that the training response/adaptation is directly related to the volume of exercise undertaken (Fitts et al. 1975). However, there is obviously a threshold volume/duration beyond which additional stimuli do not induce further increases in functional capacity. This ‘biological ceiling’ is important because it implies that the regulatory control mechanisms signalling adaptive responses are ultimately titrated by exercise duration (Booth & Watson, 1985). Competitive athletes are all too aware of this phenomenon: many elite performers walk a tightrope between chronic intensive training and inadequate recovery that can culminate in decrements in performance and the ‘overtraining syndrome.’ Biological scientists are also mindful that training volume and adaptation can be dissociated. Over 35 years ago Dudley et al. (1982) demonstrated that rats undertaking intense workbouts for shorter time induced similar increases in the maximal activities of several oxidative enzymes (i.e. cytochrome c) to those observed after more prolonged submaximal exercise training.

Levels of plasma insulin, leptin and adiponectin, and activities of key enzymes in carbohydrate metabolism in skeletal muscle and liver in fasted ICR mice fed dietary n-3 polyunsaturated fatty acids

The aim of this study was to clarify the mechanisms related to plasma glucose concentration in mice fed a diet rich in n-3 polyunsaturated fatty acids (n-3 PUFAs). Male Crlj:CD-1 (ICR) mice were fed experimental diets containing 6% lard (LD), 6% fish oil (FO) or 4.1% lard plus 1.5% docosahexaenoic acid ethyl ester and 0.4% eicosapentaenoic acid ethyl ester (DE) for 12 weeks. There were no marked differences in plasma glucose and insulin concentration changes on glucose tolerance test between the three dietary groups. At the end of the feeding trial, plasma glucose concentration was significantly lower in fasted mice in the FO group than in those in the LD group ( P<.005). Plasma adiponectin concentration was significantly higher in the FO group than in the LD group ( P<.05). Hexokinase, phosphofructokinase, glucose-6-phosphate dehydrogenase and glycerophosphate dehydrogenase activities in skeletal muscle tended to be lower in the FO group than in the LD group, while there were no differences in glucokinase and phosphofructokinase activities in liver between the three dietary groups. However, hepatic glycerophosphate dehydrogenase activity was 53-fold and 4.2-fold higher in the FO group than in the LD and DE groups, respectively ( P<.0005 and P<.05, respectively). These results suggest that the reduction in plasma glucose concentration in mice fed n-3 PUFAs is mainly caused by acceleration of glucose uptake and glycerol synthesis in the liver rather than in the skeletal muscle.

Hyperoxia Decreases Muscle Glycogenolysis, Pyruvate and Lactate Production during Steady State Exercise

Breathing hyperoxic air decreases muscle glycogenolysis, which leads to decreased lactate contents and increases in exercise performance in humans. These findings suggest alterations in skeletal muscle carbohydrate metabolism. PURPOSE: The study had two aims 1) to determine if the decreased muscle and blood lactate found during steady state exercise with hyperoxic vs. normoxic breathing is due to decreased muscle glycogenolysis, leading to decreased pyruvate and lactate accumulations and efflux and 2) to elucidate the mechanisms behind the decreased glycogenolysis. We hypothesized that 60% O2 would decrease muscle glycogenolysis, from decreases in free ADP and AMP, resulting in decreased pyruvate and lactate contents and efflux with no change in pyruvate dehydrogenase (PDH) activity (pyruvate oxidation). METHODS: Seven active male subjects cycled for 40 min at 70% VO2peak, on two occasions while breathing 21 or 60% O2. Arterial and venous blood and blood flow measurements were obtained throughout each trial, and muscle biopsies were taken at rest and following 10, 20 and 40 min of exercise. RESULTS: During exercise, muscle glycogenolysis was reduced by 16% with hyperoxia (267 ± 19 vs. 317 ± 21 mmol glycosyl units · kg−1 dw), but pyruvate accumulation and pyruvate oxidation via PDH activity were unaffected (60%: 3.02 ±0.50 vs. 21%: 3.03 ± 0.39 mmol · kg−1·min−1). However, the reduced glycogenolysis with hyperoxia did result in a 22% reduction (p<0.05) in muscle lactate accumulation (60%: 22.6 ± 6.4 vs. 21%: 31.3 ± 8.7 mmol · kg−1 dw) and decreased blood lactate efflux and total lactate production over 40 min of cycling. We also found that hyperoxia significantly decreased PCr degradation (60%: 36.4 ± 4.0 vs. 21 %: 29.9 ± 5.1 mmol · kg−1 dw), attenuated (p<0.05) the accumulations of Cr and calculated ADPf and AMPf (AMPf: 60%: 3.55 ± 1.14 vs. 21%: 5.67 ± 1.60 umol · kg−1 dw) and reduced (p<0.05) blood epinephrine concentration by ∼44% throughout 40 min of cycling. CONCLUSION: Hyperoxia decreased muscle glycogenolysis, with no change in pyruvate oxidation via PDH, resulting in significantly less muscle and blood lactate accumulation during 40 min of cycling - a tighter metabolic match between pyruvate production and oxidation. The decreased glycogenolysis during exercise with hyperoxia appeared to be the result of attenuated accumulations of the potent allosteric effectors of glycogen phosphorylase, specfically free ADP and AMP and a decreased epinephrine concentration during exercise. Lastly, the significantly diminished rate of PCr utilization during hyperoxia suggests an increase energy provision via oxidative phosphorylation during aerobic exercise.

Effect of short-term sprint interval training on human skeletal muscle carbohydrate metabolism during exercise and time-trial performance

Our laboratory recently showed that six sessions of sprint interval training (SIT) over 2 wk increased muscle oxidative potential and cycle endurance capacity (Burgomaster KA, Hughes SC, Heigenhauser GJF, Bradwell SN, and Gibala MJ. J Appl Physiol 98: 1895-1900, 2005). The present study tested the hypothesis that short-term SIT would reduce skeletal muscle glycogenolysis and lactate accumulation during exercise and increase the capacity for pyruvate oxidation via pyruvate dehydrogenase (PDH). Eight men [peak oxygen uptake (VO2 peak)=3.8+/-0.2 l/min] performed six sessions of SIT (4-7x30-s "all-out" cycling with 4 min of recovery) over 2 wk. Before and after SIT, biopsies (vastus lateralis) were obtained at rest and after each stage of a two-stage cycling test that consisted of 10 min at approximately 60% followed by 10 min at approximately 90% of VO2 peak. Subjects also performed a 250-kJ time trial (TT) before and after SIT to assess changes in cycling performance. SIT increased muscle glycogen content by approximately 50% (main effect, P=0.04) and the maximal activity of citrate synthase (posttraining: 7.8+/-0.4 vs. pretraining: 7.0+/-0.4 mol.kg protein -1.h-1; P=0.04), but the maximal activity of 3-hydroxyacyl-CoA dehydrogenase was unchanged (posttraining: 5.1+/-0.7 vs. pretraining: 4.9+/-0.6 mol.kg protein -1.h-1; P=0.76). The active form of PDH was higher after training (main effect, P=0.04), and net muscle glycogenolysis (posttraining: 100+/-16 vs. pretraining: 139+/-11 mmol/kg dry wt; P=0.03) and lactate accumulation (posttraining: 55+/-2 vs. pretraining: 63+/-1 mmol/kg dry wt; P=0.03) during exercise were reduced. TT performance improved by 9.6% after training (posttraining: 15.5+/-0.5 vs. pretraining: 17.2+/-1.0 min; P=0.006), and a control group (n=8, VO2 peak=3.9+/-0.2 l/min) showed no change in performance when tested 2 wk apart without SIT (posttraining: 18.8+/-1.2 vs. pretraining: 18.9+/-1.2 min; P=0.74). We conclude that short-term SIT improved cycling TT performance and resulted in a closer matching of glycogenolytic flux and pyruvate oxidation during submaximal exercise.

The putative roles of adenosine in insulin- and exercise-mediated regulation of glucose transport and glycogen metabolism in skeletal muscle.

Skeletal muscle is the primary site of whole-body glucose disposal and is vital in determining the overall insulin sensitivity and carbohydrate management. Insulin and physical exercise are important stimuli for muscle glucose transport and glycogen metabolism. While it is known that both insulin and contraction stimulate muscle glucose uptake and glycogen metabolism, the post-receptor mechanisms are not completely understood. Local metabolic factors, such as adenosine, have been suggested to play a role in insulin and contraction regulation of carbohydrate metabolism in skeletal muscle. While adenosine has clearly been shown to potentiate insulin-stimulated glucose transport in adipocytes and heart muscle, its role in carbohydrate metabolism in skeletal muscle is less clear, with numerous diverging findings published to date. This review article summarizes findings on the putative roles of adenosine in insulin and exercise-mediated regulation of carbohydrate metabolism and the signalling pathways proposed to be central to these metabolic stimuli in skeletal muscle.

Role of nitric oxide and nitric oxide synthases in experimental models of denervation and reinnervation.

Nitric oxide (NO) is a short-living free molecule synthesized by three different isoforms of nitric oxide synthases (NOS)-neuronal NOS, endothelial NOS, and inducible NOS-associated with neuromuscular transmission, muscle contractility, mitochondrial respiration, and carbohydrate metabolism in skeletal muscle. Neuronal NOS is constitutively expressed at the muscle fiber sarcolemma linked to the dystrophin-glycoprotein complex and concentrated at the neuromuscular endplate. There is increasing evidence that altered expression of neuronal NOS plays a role in muscle fiber damage in neuromuscular diseases such as dystrophinopathies and denervating disorders. Although there have been some previous conflicting results on the neuronal NOS expression pattern in denervated muscle fibers, it is now well established that denervation is associated with a down-regulation and disappearance of sarcolemmal neuronal NOS at synaptic/extrasynaptic or both sites. As NO has been shown to induce collapse and growth arrest on neuronal growth cones, down-regulation of sarcolemmal neuronal NOS may contribute to axonal regeneration and attraction to muscle fibers aiming at the formation of new motor endplates providing reinnervation and reconstitution of NOS expression. As NO serves as a retrograde messenger, it may trigger structural downstream events responsible for neuromuscular synaptogenesis and preventing polyneural innervation. Nevertheless, decreased NO production in denervation reduces the cytoprotective scavenger function of NO for superoxide anions promoting oxidative stress that is likely to be involved in muscle fiber damage and death. However, the multifaced role of NOS and NO under physiological and pathological conditions remains poorly understood on the basis of the current knowledge.

Effects of S-Nitroso-N-acetyl-penicillamine Administration on Glucose Tolerance and Plasma Levels of Insulin and Glucagon in the Dog

It has been suggested that nitric oxide (NO, nitrogen monoxide) is a regulator of carbohydrate metabolism in skeletal muscle. The present study was undertaken to investigate the acute effects of the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP) on blood glucose levels and on the gluco-regulatory hormones insulin and glucagon in healthy dogs. The acute effects of SNAP on mean arterial pressure and heart rate were also investigated. The drug was administered intravenously and the pre- and postprandial blood glucose, plasma insulin, and glucagon concentrations were determined at half-hour time intervals postadministration after a glucose challenge. The plasma nitrate and nitrite concentrations were measured and taken as the biochemical markers of in vivo NO formation. The oral glucose tolerance test revealed an impaired glucose tolerance in SNAP-treated dogs as reflected by the area under the glucose curve, 1150.50 ± 63.00 mmol × 150 min and 1355.25 ± 102.01 mmol/L × 150 min in dogs treated with 10 and 20 mg/kg of SNAP, respectively, compared with 860.25 ± 60.68 mmol/L × 150 min in captopril-treated controls (P < 0.05). The 2-h blood glucose concentration in dogs treated with 20 mg/kg body wt of SNAP was 9.17 ± 1.10 mmol/L compared with 5.59 ± 0.26 mmol/L for captopril-treated controls (P = 0.015). The oral glucose tolerance test also confirmed an impaired insulin secretion in the SNAP-treated dogs. While the plasma insulin concentration increased gradually in the captopril-treated controls to a peak value of 39.50 ± 2.55 μIU/ml, 1.5 h after a glucose challenge there was a decrease in the plasma insulin concentration in SNAP-treated dogs to a low value of 20.67 ± 0.88 μIU/ml (P = 0.006). In contrast, there were no significant differences in plasma glucagon concentration in SNAP-treated dogs and captopril-treated dogs at any time points. Using the Griess reaction, we found that there was a 27–95% increase in plasma nitrate/nitrite concentration on administration of SNAP. The sustained hyperglycemic effect observed in SNAP-treated dogs was accompanied by a marginal decrease in the mean arterial blood pressure and a significant increase in heart rate (P < 0.05). We conclude that acute administration of SNAP in the oral glucose tolerance test releases NO that modulates the parameters of carbohydrate metabolism.

Redox regulation of skeletal muscle glucose transport.

Although the control of carbohydrate metabolism may be regulated by numerous factors, the redox state of the cell is of primary importance. The redox state may be influenced by a number of different factors, including different reactive oxygen species (ROS) and reactive nitrogen species (RNS) collectively, called reactive oxygen/nitrogen species (RONS). This review attempts to summarize the importance of redox regulation in relation to glucose transport and regulation of carbohydrate metabolism in skeletal muscle. In addition, prior studies implicating the role of different RONS in the control of glucose transport in skeletal muscle will be presented. Finally, the possible involvement of the cGMP, p21ras, and mean arterial pressure (MAP) kinase signal transduction cascades, which have been implicated with redox-sensitive alterations in glucose transport, will also be discussed.

Effects of Chronic Nω-Nitro-[formula omitted]-arginine Methyl Ester Administration on Glucose Tolerance and Skeletal Muscle Glucose Transport in the Rat

It has been suggested that nitric oxide (NO) is a key regulator of carbohydrate metabolism in skeletal muscle. The present study was undertaken to examine the effects of chronic in vivo competitive antagonism of NO synthase (NOS) by the administration of Nω-nitro-l-arginine methyl ester (L-NAME) in the drinking water (1 mg/ml) for 14 days on glucose tolerance and skeletal muscle glucose transport in rats. Oral glucose tolerance tests (OGTT) revealed an impaired glucose tolerance in the L-NAME-treated rats as reflected by the area under the glucose curve (4675 ± 514 mg% × 120 min (control) vs 6653 ± 571 mg% × 120 min (L-NAME treated); P < 0.03). While a large rise in plasma insulin concentration was present in the control rats (0.87 ± 0.34 ng/ml, P < 0.001) during the first 15 min of the OGTT, rises in plasma insulin concentration were absent in the L-NAME-treated rats (0.18 ± 0.13 ng/ml, P = NS). Intravenous glucose tolerance tests confirmed an impaired insulin secretion in the L-NAME-treated rats. In contrast, insulin-stimulated 2-deoxyglucose transport was enhanced (P < 0.03) by chronic NOS inhibition (5.29 ± 0.83 nmol/g/min) compared to control rats (2.21 ± 0.90 nmol/g/min). Plasma sodium concentrations were lower and plasma potassium concentrations were higher in the L-NAME-treated group, indicating an impaired electrolyte status. We conclude that chronic in vivo administration of a NOS inhibitor, while not impairing basal parameters of carbohydrate metabolism, may manifest different responses than acute exposure to the same agent in vitro.