Skeletal Muscle Glycogen Synthase Research Articles

Sex differences in insulin regulation of skeletal muscle glycogen synthase and changes during weight loss and exercise in adults.

The authors sought to understand sex differences in muscle metabolism in 73 older men and women. Body composition, VO2max, and insulin sensitivity (M) by 3-hour hyperinsulinemic-euglycemic clamp with vastus lateralis muscle biopsies were measured. Women had lower body weight, VO2max, and fat-free mass than men. Men had lower M, lower change (insulin minus basal) in muscle glycogen synthase (GS) activity, and lower change in AKT protein expression than women. M was associated with the change (insulin-basal) in GS activity and the change in AKT protein expression. Sex differences (n = 60) were tested with 6-month weight loss or 3×/week aerobic exercise training. The postintervention minus preintervention change (insulin-basal) (∆∆) in GS activity (fractional, independent, total) was higher in men than women in the weight loss group and ∆∆ in GS fractional activity was higher in women than men in the aerobic exercise group. In all participants, ∆∆ in GS fractional and independent activities was related to ∆∆ in AKT expression and glycogen content. Sex differences in insulin sensitivity may be explained at the cellular muscle level, and to improve skeletal muscle insulin action in older adults, it may be necessary to recommend different behavioral strategies depending on the individual's sex.

A Dioxidovanadium Complex cis-[VO2 (obz) py] Attenuates Hyperglycemia in Streptozotocin (STZ)-Induced Diabetic Male Sprague-Dawley Rats via Increased GLUT4 and Glycogen Synthase Expression in the Skeletal Muscle.

Vanadium has demonstrated antihyperglycemic effects in diabetes mellitus (DM) but is, however, associated with toxicity. Therefore, new vanadium complexes envisaged to possess heightened therapeutic potency while rendering less toxicity are being explored. Accordingly, the aim of the study was to investigate the effects of a dioxidovanadium (V) complex, cis-[VO2 (obz) py], on selected glucose metabolism markers in streptozotocin (STZ)-induced diabetic rats. STZ-induced diabetic rats were treated orally with cis-[VO2 (obz) py] (10, 20, and 40 mg/kg) twice every 3rd day for 5 weeks. Blood glucose concentrations, body weight, and food and water intake were monitored weekly, for 5 weeks. Rats were then euthanized after which blood, liver, and muscle tissues were collected for biochemical analysis. The administration of dioxidovanadium complex significantly decreased blood glucose concentrations throughout the 5-week period in comparison with the diabetic control (DC). The attenuation of hyperglycemia was accompanied by an increased glycogen concentration in both liver and muscle tissues in the treated groups. Furthermore, a significant increase was observed in the expression of glucose transporter type 4 (GLUT4) in the skeletal muscle tissues and glycogen synthase in the liver tissues. These findings indicate that our vanadium complex cis-[VO2 (obz) py] may exert antihyperglycemic effects through increased glucose uptake, glycogen synthesis, and increased GLUT4 and glycogen synthase expression.

Insulin Resistance in Skeletal Muscle of Chronic Stroke.

A stroke can lead to reduced mobility affecting skeletal muscle mass and fatty infiltration which could lead to systemic insulin resistance, but this has not been examined and the mechanisms are currently unknown. The objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in paretic (P) and nonparetic (NP) skeletal muscle in chronic stroke, and to compare to nonstroke controls. Participants were mild to moderately disabled adults with chronic stroke (n = 30, 60 ± 8 years) and sedentary controls (n = 35, 62 ± 8 years). Insulin sensitivity (M) and bilateral GS activity were determined after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Stroke subjects had lower aerobic capacity than controls, but M was not significantly different. Insulin-stimulated activities of GS (independent, total, fractional), as well as absolute differences (insulin minus basal) and the percent change (insulin minus basal, relative to basal) in GS activities, were all significantly lower in P versus NP muscle. Basal GS fractional activity was 3-fold higher, and the increase in GS fractional activity during the clamp was 2-fold higher in control versus P and NP muscle. Visceral fat and intermuscular fat were associated with lower M. The effect of in vivo insulin to increase GS fractional activity was associated with M in control and P muscle. A reduction in insulin action on GS in paretic muscle likely contributes to skeletal muscle-specific insulin resistance in chronic stroke.

P.67Overexpression of a constitutively active glycogen synthase in the GSL30 mouse leads to a polyglucosan myopathy

Polyglucosan accumulation is a characteristic feature of several glycogen storage diseases of mammals, including eight in humans and one in horses — the latter associated with a gain of function, through a constitutively active mutant glycogen synthase (GYS1). GSL30 mice overexpressing constitutively active rabbit glycogen synthase in skeletal muscle, accumulate 5-6 times more glycogen in skeletal muscle than wild-type (WT) controls. We hypothesised that this excessive glycogen accumulation would also be associated with a myopathic histological phenotype characterised by polyglucosan accumulation, as seen in GYS1 mutant horses. We examined serum CK activity and skeletal muscle histology from 6 week, 3 month and 8 month old GSL30 mice compared with age-matched WT controls (n=5 to 8). Cryosections from the gastrocnemius, a predominantly fast-twitch muscle, were stained with H&E and amylase-periodic acid Schiff and scored (blinded to genotype) for the presence of internalised nuclei and amylase-resistant polyglucosan within myofibres. Whilst there was no difference in serum CK activity between groups (P>0.05), GSL30 mice had significantly greater numbers of internalised myonuclei than WT controls at 8 months (13.2±6% vs. 0.5%±0.4%; mean±SD; P<0.0001). Polyglucosan was not identified in WT control muscle, but was present increasingly in GSL30 mouse muscle fibres as they aged (reaching 70±5% of myofibres at 8 months). In 6 week old GSL30 mice, amylase-resistant polyglucosan was predominantly subsarcolemmal; by 3 months of age, punctate deposits were identified within the sarcoplasm. By 8 months of age, large pools of glycogen and amylase-resistant polyglucosan, often encompassing the entire myofiber, and focal areas of necrosis were evident. This data reveals that overexpression of glycogen leads to a polyglucosan myopathy in GSL30 mice. Future work will examine the influence of diet and exercise on the phenotype.

Acute Hypoglycemia in Healthy Humans Impairs Insulin-Stimulated Glucose Uptake and Glycogen Synthase in Skeletal Muscle: A Randomized Clinical Study.

Hypoglycemia is the leading limiting factor in glycemic management of insulin-treated diabetes. Skeletal muscle is the predominant site of insulin-mediated glucose disposal. Our study used a crossover design to test to what extent insulin-induced hypoglycemia affects glucose uptake in skeletal muscle and whether hypoglycemia counterregulation modulates insulin and catecholamine signaling and glycogen synthase activity in skeletal muscle. Nine healthy volunteers were examined on three randomized study days: 1) hyperinsulinemic hypoglycemia (bolus insulin), 2) hyperinsulinemic euglycemia (bolus insulin and glucose infusion), and 3) saline control with skeletal muscle biopsies taken just before, 30 min after, and 75 min after insulin/saline injection. During hypoglycemia, glucose levels reached a nadir of ∼2.0 mmol/L, and epinephrine rose to ∼900 pg/mL. Hypoglycemia impaired insulin-stimulated glucose disposal and glucose clearance in skeletal muscle, whereas insulin signaling in glucose transport was unaffected by hypoglycemia. Insulin-stimulated glycogen synthase activity was completely ablated during hyperinsulinemic hypoglycemia, and catecholamine signaling via cAMP-dependent protein kinase and phosphorylation of inhibiting sites on glycogen synthase all increased.

Progresses of the Past Decade on Factors Contributing to Skeletal Muscle Glycogen Synthase Regulation

Glycogen is the primary storage form of glucose, which is metabolized actively in the liver and skeletal muscle of mammals. The skeletal muscle glycogen is used intracellularly to provide starting substrate for glycolysis, whereas liver glycogen is used mainly for the maintenance of blood glucose homeostasis. The conversion of glucose to glycogen is an ordered process, in which the enzyme glycogen synthase plays an essential role. The key events in the synthesis of glycogen have been elucidated in both the liver and muscle. The process of understanding the regulatory mechanism of glycogen synthase activity has led to the discoveries of many important players in cellular metabolism. Research in identifying factors modulating muscle glycogen synthesis has been of popular interest within the past decade. The goal of this review is to summarize potential factors affecting skeletal muscle glycogen synthase expression level and activity in literatures published for the past decade.

Effects of LEP, GYS1, MYOD1, and MYF5 polymorphisms on pig economic traits

Abstract In the present study, we examined the effect of single nucleotide polymorphisms (SNPs) of leptin (LEP), skeletal muscle glycogen synthase (GYS1), myogenic differentiation 1 (MYOD1), and myogenic factor 5 (MYF5) genes on economic trait association in pigs. LEP/HindIII, MYOD1/DdeI, MYF5/FokI, and GYS1/FokI genotypes were identified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) from 466 pigs comprised of Duroc, Landrace and Yorkshire breeds. The LEP/HindIII polymorphism differed significantly with respect to average daily gain (ADG) in Duroc pigs (P&lt;0.05). However, the GYS1/FokI polymorphism was not significantly associated with any trait. The MYOD1/DdeI polymorphism was significantly associated with both ADG and meat percentage (MP) in Duroc pigs, and ADG, backfat thickness (BFT) and feed efficiency (FE) in Landrace pigs, whereas the MYOD1/DdeI polymorphism was not significantly associated with any trait in Yorkshire pigs. In addition, the MYF5/FokI polymorphism revealed a close relationship with ADG in Duroc pigs. In conclusion, we believe that the SNPs within LEP, MYOD1 and MYF5 in certain pig breeds play important roles as potential genetic markers for economic traits of pigs.

Dysregulation of muscle glycogen synthase in recovery from exercise in type 2 diabetes.

Insulin and exercise stimulate skeletal muscle glycogen synthase (GS) activity by dephosphorylation and changes in kinetic properties. The aim of this study was to investigate the effects of insulin, exercise and post-exercise insulin stimulation on GS phosphorylation, activity and substrate affinity in obesity and type 2 diabetes. Obese men with type 2 diabetes (n = 13) and weight-matched controls (n = 14) underwent euglycaemic-hyperinsulinaemic clamps in the rested state and 3 h after 60 min of cycling (70% maximal pulmonary oxygen uptake [VO2max]). Biopsies from vastus lateralis muscle were obtained before and after clamps, and before and immediately after exercise. Insulin-stimulated glucose uptake was lower in diabetic patients vs obese controls with or without prior exercise. Post exercise, glucose partitioning shifted away from oxidation and towards storage in both groups. Insulin and, more potently, exercise increased GS activity (fractional velocity [FV]) and substrate affinity in both groups. Both stimuli caused dephosphorylation of GS at sites 3a + 3b, with exercise additionally decreasing phosphorylation at sites 2 + 2a. In both groups, changes in GS activity, substrate affinity and dephosphorylation at sites 3a + 3b by exercise were sustained 3 h post exercise and further enhanced by insulin. Post exercise, reduced GS activity and substrate affinity as well as increased phosphorylation at sites 2 + 2a were found in diabetic patients vs obese controls. Exercise-induced activation of muscle GS in obesity and type 2 diabetes involves dephosphorylation of GS at sites 3a + 3b and 2 + 2a and enhanced substrate affinity, which is likely to facilitate glucose partitioning towards storage. Lower GS activity and increased phosphorylation at sites 2 + 2a in type 2 diabetes in the recovery period imply an impaired response to exercise.

Altered Mitochondrial Oxidative Phosphorylation Capacity in Horses Suffering from Polysaccharide Storage Myopathy

IntroductionExertional myopathies are a common cause of exercise intolerance in the equine athlete, and Polysaccharide Storage Myopathy (PSSM) is a widely described muscular disease. It is characterized by an accumulation of abnormal glycogen in myofibers due to a genetic defect in the skeletal muscle glycogen synthase (GYS1) enzyme. We hypothesized that energy production through oxidative phosphorylation (OXPHOS) in muscular mitochondria might be impaired in type‐1 PSSM‐affected horses.MethodsEight horses with a history of exertional rhabdomyolysis were tested for the GYS1 mutation. Muscle biopsies were collected and used for histological analysis and high resolution respirometry (HRR). HRR values from 3 groups of horses (5 PSSM‐positive horses, 3 horses with a history of myopathy but PSSM‐negative and 16 healthy controls) were compared using a linear mixed model to take into account repeated (2–3 times) measurements made for each horse.ResultsIn 5/8 horses histology revealed an accumulation of abnormal glycogen in myofibers. These 5 horses also tested positive for the GYS1 mutation. A severe depression of maximal OXPHOS capacity was observed by HRR in 7/8 horses with exertional rhabdomyolysis, with lower values in PSSM‐positive cases (4/5).ConclusionsOur study shows a severely decreased OXPHOS capacity in PSSM‐affected horses. PSSM is considered primarily a defect in glycogen synthesis but altered OXPHOS might play a central role in its pathogenesis.Ethical Animal ResearchAll study procedures were approved by the Animal Ethic Commission of the University of Liege. Sources of funding: University of Liège. Competing interests: none.

Expression and Regulation of Glycogen Synthase in Equine Type 1 Polysaccharide Storage Myopathy

IntroductionEquine Type 1 polysaccharide storage myopathy (PSSM1) is associated with a missense, gain‐of‐function mutation in the equine skeletal muscle glycogen synthase gene (GYS1). Affected horses have excessive glycogen storage, amylopectate‐like inclusions in their muscle and higher muscle glycogen synthase activities (GS) than controls. GS is normally allosterically‐regulated by glucose‐6‐phosphate and enzyme activity is reduced by phosphorylation. We hypothesised that the higher GS activity in PSSM1‐affected horses might be due either to 1) increased expression or reduced degradation of the mutant enzyme or 2) due to reduced phosphorylation.MethodsSemimembranosus muscle samples were biopsied from 4 homozygotes (HH), 13 heterozygotes (RH) and 12 control (RR) Belgian Draught horses. Samples were analysed for glycogen content and GS activity. Western immunoblotting was used to examine expression of total and phosphorylated forms of GS, GSK3β and GLUT4. Data was analysed using non‐parametric ANOVA statistical tests with post‐hoc Dunn's multiple comparison tests.ResultsPSSM1 affected heterozygotes and homozygotes had significantly higher muscle glycogen contents than controls (RR vs. RH P = 0.005, RR vs. HH P = 0.041) and GS activity was significantly higher in PSSM1‐affected homozygotes than in heterozygotes and control horses (P = 0.03). Although there was no difference in total GS expression between groups, there was significantly more expression of GS phosphorylated at site 2 + 2a in PSSM1‐affected horses in comparison with controls (P = 0.009).ConclusionsOur data suggest that increased GS activity in PSSM1‐affected horses is not associated with increased expression or degradation of the mutant enzyme and occurs despite increased phosphorylation of GS at site 2 + 2a. Alternative mechanisms, including aberrant ligand binding might account for the increased GS activity in PSSM1‐affected muscle.Ethical Animal ResearchMuscle biopsy samples were collected according to ethical approval from the local Institutional Animal Care and Use Committee. Explicit owner informed consent for participation in this study is not stated. Sources of funding: Petplan Charitable Trust and The Royal Veterinary College. Competing interests: none.

Aerobic exercise plus weight loss improves insulin sensitivity and increases skeletal muscle glycogen synthase activity in older men.

The purpose of this study was to determine the effects of 6-month aerobic exercise training + weight loss (AEX + WL) on basal and insulin activation of glycogen synthase, basal citrate synthase activity, and Akt and AS160 phosphorylation in older, overweight/obese insulin-resistant men (n = 14; 63 ± 2 years; body mass index, 32 ± kg/m(2)). Muscle samples of the vastus lateralis were collected before and during a 3-hour 80 mU/m(2)/min hyperinsulinemic-euglycemic clamp. AEX + WL increased VO2max by 11% (p < .05) and decreased body weight (-9%, p < .001). AEX + WL increased basal citrate synthase activity by 46% (p < .01) and insulin activation of independent (2.9-fold) and fractional (2.3-fold) activities (both p < .001) of glycogen synthase. AEX + WL had no effect on phosphorylation of Akt or AS160. Glucose utilization (M) improved 25% (p < .01), and the change tended to be related to the increase in insulin activation of glycogen synthase fractional activity (r = .50, p = .08) following AEX + WL. In summary, AEX + WL has a robust effect on insulin activation of skeletal muscle glycogen synthase activity that likely contributes to improved glucose utilization in older insulin-resistant men.

Resveratrol ameliorates diabetes‐related metabolic changes via activation of AMP‐activated protein kinase and its downstream targets in db/db mice

This study investigated the effects of resveratrol (RV) on diabetes-related metabolic changes in a spontaneous model of type 2 diabetes, as well as activation of AMP-activated protein kinase (AMPK) and downstream targets. C57BL/KsJ-db/db mice were fed a normal diet with RV (0.005% and 0.02%, w/w) or rosiglitazone (RG, 0.001%, w/w) for 6 weeks. Both doses of RV significantly decreased blood glucose, plasma free fatty acid, triglyceride, apo B/apo AІ levels and increased plasma adiponectin levels. RV activated AMPK and downstream targets leading to decreased blood HbA1c levels, hepatic gluconeogenic enzyme activity, and hepatic glycogen, while plasma insulin levels, pancreatic insulin protein, and skeletal muscle GLUT4 protein were higher after RV supplementation. The high RV dose also significantly increased hepatic glycolytic gene expression and enzyme activity, along with skeletal muscle glycogen synthase protein expression, similar to RG. Furthermore, RV dose dependently decreased hepatic triglyceride content and phosphorylated I kappa B kinase (p-IKK) protein expression, while hepatic uncoupling protein (UCP) and skeletal muscle UCP expression were increased. RV potentiates improving glycemic control, glucose uptake, and dyslipidemia, as well as protecting against pancreatic β-cell failure in a spontaneous type 2 diabetes model. Dietary RV has potential as an antidiabetic agent via activation of AMPK and its downstream targets.

Hyperglycaemia normalises insulin action on glucose metabolism but not the impaired activation of AKT and glycogen synthase in the skeletal muscle of patients with type 2 diabetes

In type 2 diabetes, reduced insulin-stimulated glucose disposal, primarily glycogen synthesis, is associated with defective insulin activation of glycogen synthase (GS) in skeletal muscle. Hyperglycaemia may compensate for these defects, but to what extent it involves improved insulin signalling to glycogen synthesis remains to be clarified. Whole-body glucose metabolism was studied in 12 patients with type 2 diabetes, and 10 lean and 10 obese non-diabetic controls by means of indirect calorimetry and tracers during a euglycaemic-hyperinsulinaemic clamp. The diabetic patients underwent a second isoglycaemic-hyperinsulinaemic clamp maintaining fasting hyperglycaemia. Muscle biopsies from m. vastus lateralis were obtained before and after the clamp for examination of GS and relevant insulin signalling components. During euglycaemia, insulin-stimulated glucose disposal, glucose oxidation and non-oxidative glucose metabolism were reduced in the diabetic group compared with both control groups (p < 0.05). This was associated with impaired insulin-stimulated GS and AKT2 activity, deficient dephosphorylation at GS sites 2 + 2a, and reduced Thr308 and Ser473 phosphorylation of AKT. When studied under hyperglycaemia, all variables of insulin-stimulated glucose metabolism were normalised compared with the weight-matched controls. However, insulin activation and dephosphorylation (site 2 + 2a) of GS as well as activation of AKT2 and phosphorylation at Thr308 and Ser473 remained impaired (p < 0.05). These data confirm that hyperglycaemia compensates for decreased whole-body glucose disposal in type 2 diabetes. In contrast to previous less well-controlled studies, we provide evidence that the compensatory effect of hyperglycaemia in patients with type 2 diabetes does not involve normalisation of insulin action on GS or upstream signalling in skeletal muscle.

Exercise with calorie restriction improves insulin sensitivity and glycogen synthase activity in obese postmenopausal women with impaired glucose tolerance

Our objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in normal (NGT) vs. impaired glucose-tolerant (IGT) obese postmenopausal women and to determine whether an increase in insulin activation of GS is associated with an improvement in insulin sensitivity (M) following calorie restriction (CR) and/or aerobic exercise plus calorie restriction (AEX + CR) in women with NGT and IGT. We did a longitudinal, clinical intervention study of CR compared with AEX + CR. Overweight and obese women, 49-76 yr old, completed 6 mo of CR (n = 46) or AEX + CR (n = 50) with Vo(2 max), body composition, and glucose tolerance testing. Hyperinsulinemic euglycemic (80 mU·m(-2)·min(-1)) clamps (n = 73) and skeletal muscle biopsies (before and during clamp) (n = 58) were performed before and after the interventions (n = 50). After 120 min of hyperinsulinemia during the clamp, GS fractional activity and insulin's effect to increase GS fractional activity (insulin - basal) were significantly lower in IGT vs. NGT (P < 0.01) at baseline. GS total activity increased during the clamp in NGT (P < 0.05), but not IGT, at baseline. CR and AEX + CR resulted in a significant 8% weight loss with reductions in total fat mass, visceral fat, subcutaneous fat, and intramuscular fat. Overall, M increased (P < 0.01), and the change in M (postintervention - preintervention) was associated with the change in insulin-stimulated GS fractional activity (partial r = 0.44, P < 0.005). In IGT, the change (postintervention - preintervention) in insulin-stimulated GS total activity was greater following AEX + CR than CR alone (P < 0.05). In IGT, insulin-stimulated GS-independent (P < 0.005) and fractional activity (P = 0.06) increased following AEX + CR. We conclude that the greatest benefits at the whole body and cellular level (insulin activation of GS) in older women at highest risk for diabetes are derived from a lifestyle intervention that includes exercise and diet.

Estimated prevalence of the GYS-1 mutation in healthy Austrian Haflingers

The aim of this study was to determine the occurrence and frequency of a mutation in the gene coding for skeletal muscle glycogen synthase type 1 (GYS-1), which is the...

Molecular and functional characterization of glycogen synthase in the porcine satellite cells under insulin treatment

Glycogen synthase (GS) catalyzes the key step of glycogen synthesis and plays an important role in glycogen metabolism in liver and muscle. In this study, we cloned the cDNA and promoter sequences of porcine glycogen synthesis genes (GYS1 and GYS2). Expression analysis revealed that porcine GYS1 was highly expressed in the skeletal muscle and heart. GYS2 was expressed specifically in liver and subcutaneous adipose tissue. The expression level of GYS1 was up-regulated from proliferation to differentiation in the porcine satellite cells, and insulin did not significantly affect the transcription of GYS1. Insulin stimulated 72-h-differentiated satellite cells as indicated by decrease in phosphorylation of GS, but did not affect GYS1 transcription and total GS protein level, suggesting that the effect of insulin is primarily mediated via posttranscriptional control rather than regulated at the transcriptional level. Four single-nucleotide polymorphisms (SNPs) were detected in the promoter and cDNA sequences of porcine GYS1. Association analyses revealed that the GYS1 Hin6I and MvaI polymorphisms both had significant associations (P < 0.05) with pH of M. longissimus dorsi (pHLD), M. biceps femoris (pHBF) and M. semipinalis capitis (pHSC) at 45 min postmortem. These results provide useful information for further investigation on the function of glycogen synthase in porcine skeletal muscle.

Epidemiology of Exertional Rhabdomyolysis Susceptibility in Standardbred Horses Reveals Associated Risk Factors and Underlying Enhanced Performance

BackgroundExertional rhabdomyolysis syndrome is recognised in many athletic horse breeds and in recent years specific forms of the syndrome have been identified. However, although Standardbred horses are used worldwide for racing, there is a paucity of information about the epidemiological and performance-related aspects of the syndrome in this breed. The objectives of this study therefore were to determine the incidence, risk factors and performance effects of exertional rhabdomyolysis syndrome in Standardbred trotters and to compare the epidemiology and genetics of the syndrome with that in other breeds.Methodology/Principal FindingsA questionnaire-based case-control study (with analysis of online race records) was conducted following identification of horses that were determined susceptible to exertional rhabdomyolysis (based on serum biochemistry) from a total of 683 horses in 22 yards. Thirty six exertional rhabdomyolysis-susceptible horses were subsequently genotyped for the skeletal muscle glycogen synthase (GYS1) mutation responsible for type 1 polysaccharide storage myopathy. A total of 44 susceptible horses was reported, resulting in an annual incidence of 6.4 (95% CI 4.6–8.2%) per 100 horses. Female horses were at significantly greater risk than males (odds ratio 7.1; 95% CI 2.1–23.4; p = 0.001) and nervous horses were at a greater risk than horses with calm or average temperaments (odds ratio 7.9; 95% CI 2.3–27.0; p = 0.001). Rhabdomyolysis-susceptible cases performed better from standstill starts (p = 0.04) than controls and had a higher percentage of wins (p = 0.006). All exertional rhabdomyolysis-susceptible horses tested were negative for the R309H GYS1 mutation.Conclusions/SignificanceExertional rhabdomyolysis syndrome in Standardbred horses has a similar incidence and risk factors to the syndrome in Thoroughbred horses. If the disorder has a genetic basis in Standardbreds, improved performance in susceptible animals may be responsible for maintenance of the disorder in the population.

Effects of adrenaline on whole-body glucose metabolism and insulin-mediated regulation of glycogen synthase and PKB phosphorylation in human skeletal muscle

In the present study, we investigated the effect of adrenaline on insulin-mediated regulation of glucose and fat metabolism with focus on regulation of skeletal muscle PKB, GSK-3, and glycogen synthase (GS) phosphorylation. Ten healthy subjects (5 men and 5 women) received a 240-minute intravenous infusion of adrenaline (0.05 μg/[kg min]) or saline; after 120 minutes, a hyperinsulinemic-euglycemic clamp was added. Adrenaline infusion increased blood glucose concentration by approximately 50%, but the hyperinsulinemic clamp normalized blood glucose within 30 minutes. Glucose infusion rate during the last hour was approximately 60% lower during adrenaline infusion compared with saline (4.3 ± 0.5 vs 11.2 ± 0.6 mg/kg lean body mass per minute). Insulin increased PKB Ser473, PKB Thr308, and GSK-3β Ser9 phosphorylation in skeletal muscles; coinfusion of adrenaline did not influence insulin-stimulated PKB and GSK-3 phosphorylation. Adrenaline alone did not influence phosphorylation of PKB and GSK-3β. Insulin increased GS fractional activity and decreased GS Ser641 and Ser645,649,653,657 phosphorylation. In the presence of adrenaline, insulin did neither activate GS nor dephosphorylate GS Ser641. Surprisingly, GS Ser7 phosphorylation was not influenced by adrenaline. Adrenaline increased plasma lactate concentration; and muscle glycogen content was reduced in skeletal muscle the day after adrenaline infusion, supporting that insulin does not stimulate glycogen synthesis in skeletal muscles when adrenaline is present. In conclusion, adrenaline did not influence basal or insulin-stimulated PKB and GSK-3β phosphorylation in muscles, but completely blocked insulin-mediated GS activation and Ser641 dephosphorylation. Still, insulin normalized adrenaline-mediated hyperglycemia.

Helpful introduction to signal transduction

![Figure][1] Structure and Function in Cell Signalling By John Nelson Wiley (2008) 410 pages, ISBN 978-0-470-02551-2, £34.95 (paperback) When studying cell signaling, it is easy to get overwhelmed with the complexity of each pathway and with the crosstalk between them. Teaching the

Effects of aging and calorie restriction on rat skeletal muscle glycogen synthase and glycogen phosphorylase

Calorie restriction’s (CR) effects on age-associated changes in glycogen-metabolizing enzymes were studied in rat soleus (SOL) and tibialis anterior (TA) muscles. Old (24 months) compared to young (6 months) rats maintained ad libitum on a standard diet had reduced glycogen synthase (GS) activity, lower muscle GS protein levels, increased phosphorylation of GS at site 3a with less activation in SOL. Age-associated impairments in GS protein and activation-phosphorylation were also shown in TA. There was an age-associated reduction in glycogen phosphorylase (GP) activity level in SOL, while brain/muscle isoforms (B/M) of GP protein levels were higher. GP activity and protein levels were preserved, but GP was inactivated in TA with age. Glycogen content was unchanged in both muscles. CR did not alter GS or GP activity/protein levels in young rats. CR hindered age-related decreases in GS activity/protein, unrelated to GS mRNA levels, and GS inactivation-phosphorylation; not on GP. In older rats, CR enhanced glycogen accumulation in SOL. Short-term fasting did not recapitulate CR effects in old rats. Thus, the predominant age-associated impairments on skeletal muscle GS and GP activities occur in the oxidative SOL muscle of rats, and CR can attenuate the loss of GS activity/activation and stimulate glycogen accumulation.