Pre- and posttranslational upregulation of muscle-specific glycogen synthase in athletes

Effect of Moderate-Intense Training and Detraining on Glucose Metabolism, Lipid Profile, and Liver Enzymes in Male Wistar Rats: A Preclinical Randomized Study

Exercise training positively regulates glucose metabolism. This study investigated the impact of training and detraining on glucose metabolism, lipid profiles, and liver enzymes. Twenty-six rats completed an initial 4-week moderate-intense training (T0-T4). Then, the animals were randomly assigned to two groups at the end of week 4: AT4: detraining for 8 weeks; AT8: training for 8 weeks and 4-week detraining. Six animals were sacrificed at T0 and T4, four animals/group at T8, and three/group at T12. The study continued for 12 weeks, and all parameters were assessed at T0, T4, T8, and T12. IPGTT significantly improved after 4 weeks of training (p < 0.01) and was further reduced in AT8 at T8. In AT8, 8-week training significantly reduced total cholesterol at T4 and T12 vs. T0 (p < 0.05), LDL at T4, T8, and T12 vs. T0 (p < 0.01), ALP at T8, T12 vs. T0 (p < 0.01), and increased HDL at T8 and ALT at T8 and T12 vs. T0 (p < 0.05). Triglycerides and hexokinase activity increased significantly at T4 and T8 (p < 0.05) and then decreased at T12 in AT8. Pyruvate and glycogen increased at T12 in AT8 vs. AT4. Eight-week training improved LPL and ATGL expressions. Training positively modulated insulin, glucose metabolism, and lipid profiles, but detraining reduced the benefits associated with the initial training.

Exercise training and work task induced metabolic and stress-related mRNA and protein responses in myalgic muscles

The aim was to assess mRNA and/or protein levels of heat shock proteins, cytokines, growth regulating, and metabolic proteins in myalgic muscle at rest and in response to work tasks and prolonged exercise training. A randomized controlled trial included 28 females with trapezius myalgia and 16 healthy controls. Those with myalgia performed ~7 hrs repetitive stressful work and were subsequently randomized to 10 weeks of specific strength training, general fitness training, or reference intervention. Muscles biopsies were taken from the trapezius muscle at baseline, after work and after 10 weeks intervention. The main findings are that the capacity of carbohydrate oxidation was reduced in myalgic compared with healthy muscle. Repetitive stressful work increased mRNA content for heat shock proteins and decreased levels of key regulators for growth and oxidative metabolism. In contrast, prolonged general fitness as well as specific strength training decreased mRNA content of heat shock protein while the capacity of carbohydrate oxidation was increased only after specific strength training.

Effects of acute hypoxia tests on blood markers in high-level endurance athletes

The aim of this study was to determine the response of blood markers to acute hypoxia in high-level endurance athletes before training based on "living high-training low" model. Thirty endurance athletes performed a hypoxic cycling test and spent 3 h at rest in a simulated altitude of 3,000 m. At the end of the hypoxic cycling test, the quantity of the natural antisense transcript of HIF-1alpha mRNA (aHIF) transcript increased significantly (+37%, P = 0.024). After 3-h exposure, at a simulated altitude of 3,000 m, the amount of HIF-1alpha mRNA increased significantly (+57%, P = 0.012). Moreover, a large inter-subject range was observed in response to the hypoxic cycling test and to the prolonged hypoxic exposure: -133%/+79% and -82%/+653% for HIF-1alpha mRNA, 69%/+324% and -76%/+229% for aHIF. This study shows a large inter-variability of blood markers in elite athletes in response to acute hypoxic exposure corroborating previous observations made in other populations.

Effects of endurance exercise training on insulin signaling in human skeletal muscle: interactions at the level of phosphatidylinositol 3-kinase, Akt, and AS160

The purpose of this study was to investigate the mechanisms explaining improved insulin-stimulated glucose uptake after exercise training in human skeletal muscle. Eight healthy men performed 3 weeks of one-legged knee extensor endurance exercise training. Fifteen hours after the last exercise bout, insulin-stimulated glucose uptake was approximately 60% higher (P < 0.01) in the trained compared with the untrained leg during a hyperinsulinemic-euglycemic clamp. Muscle biopsies were obtained before and after training as well as after 10 and 120 min of insulin stimulation in both legs. Protein content of Akt1/2 (55 +/- 17%, P < 0.05), AS160 (25 +/- 8%, P = 0.08), GLUT4 (52 +/- 19%, P < 0.001), hexokinase 2 (HK2) (197 +/- 40%, P < 0.001), and insulin-responsive aminopeptidase (65 +/- 15%, P < 0.001) increased in muscle in response to training. During hyperinsulinemia, activities of insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3-kinase (PI3-K) (P < 0.005), Akt1 (P < 0.05), Akt2 (P < 0.005), and glycogen synthase (GS) (percent I-form, P < 0.05) increased similarly in both trained and untrained muscle, consistent with increased phosphorylation of Akt Thr(308), Akt Ser(473), AS160, glycogen synthase kinase (GSK)-3alpha Ser(21), and GSK-3beta Ser(9) and decreased phosphorylation of GS site 3a+b (all P < 0.005). Interestingly, training improved insulin action on thigh blood flow, and, furthermore, in both basal and insulin-stimulated muscle tissue, activities of Akt1 and GS and phosphorylation of AS160 increased with training (all P < 0.05). In contrast, training reduced IRS-1-associated PI3-K activity (P < 0.05) in both basal and insulin-stimulated muscle tissue. Our findings do not support generally improved insulin signaling after endurance training; rather it seems that improved insulin-stimulated glucose uptake may result from hemodynamic adaptations as well as increased cellular protein content of individual insulin signaling components and molecules involved in glucose transport and metabolism.

Impact of genetic versus environmental factors on the control of muscle glycogen synthase activation in twins

Storage of glucose as glycogen accounts for the largest proportion of muscle glucose metabolism during insulin infusion in normal and insulin-resistant subjects. Studies in first-degree relatives have indicated a genetic origin of the defective insulin activation of muscle glycogen synthase (GS) in type 2 diabetes. The aim of this study was to evaluate the relative impact of genetic versus nongenetic factors on muscle GS activation and regulation in young and elderly twins examined with a 2-h euglycemic-hyperinsulinemic (40 mU x m(-2) x min(-1)) clamp combined with indirect calorimetry and excision of muscle biopsies. The etiological components were determined using structural equation modeling. Fractional GS activity; GS phosphorylation at sites 2, 2 + 2a, and 3a + 3b corrected for total GS protein; and GS kinase 3 (GSK3) activity were similar in both age groups, whereas total GS activity and protein were lower in elderly compared with younger twins. GS fractional activity increased significantly during insulin stimulation in both young and elderly twins. Conversely, there was a significant decrease in GS phosphorylation at site 3a + 3b and GSK3 activity during insulin stimulation in both age groups, whereas GS phosphorylation at site 2 and 2 + 2a only decreased on insulin stimulation in the younger twins. The increment in whole-body glucose disposal (Rd) and nonoxidative glucose metabolism (insulin - basal) correlated significantly with the increment in GS fractional activity. Fractional GS activity had a major environmental component in both age groups. GSK3 activity exhibited a genetic component in young (basal: a2 = 0.42; insulin: a2 = 0.58) and elderly (insulin: a2 = 0.56) twins. Furthermore, GS phosphorylation at site 2 (insulin: a2 = 0.69) in the elderly and at site 3a + 3b (insulin: a2 = 0.50) in the young twins had a genetic component. In conclusion, GSK3 activity and GS phosphorylation, particularly at sites 2 and 3a + 3b, had major genetic components. Total and fractional GS activities per se were, on the other hand, predominantly controlled by environmental factors. Moreover, GS activity was intact with increasing age, despite a significant reduction in nonoxidative glucose metabolism.

Strength training increases insulin-mediated glucose uptake, GLUT4 content, and insulin signaling in skeletal muscle in patients with type 2 diabetes

Strength training represents an alternative to endurance training for patients with type 2 diabetes. Little is known about the effect on insulin action and key proteins in skeletal muscle, and the necessary volume of strength training is unknown. A total of 10 type 2 diabetic subjects and 7 healthy men (control subjects) strength-trained one leg three times per week for 6 weeks while the other leg remained untrained. Each session lasted no more than 30 min. After strength training, muscle biopsies were obtained, and an isoglycemic-hyperinsulinemic clamp combined with arterio-femoral venous catheterization of both legs was carried out. In general, qualitatively similar responses were obtained in both groups. During the clamp, leg blood flow was higher (P < 0.05) in trained versus untrained legs, but despite this, arterio-venous extraction glucose did not decrease in trained legs. Thus, leg glucose clearance was increased in trained legs (P < 0.05) and more than explained by increases in muscle mass. Strength training increased protein content of GLUT4, insulin receptor, protein kinase B-alpha/beta, glycogen synthase (GS), and GS total activity. In conclusion, we found that strength training for 30 min three times per week increases insulin action in skeletal muscle in both groups. The adaptation is attributable to local contraction-mediated mechanisms involving key proteins in the insulin signaling cascade.

Muscle- and fibre type-specific expression of glucose transporter 4, glycogen synthase and glycogen phosphorylase proteins in human skeletal muscle

The muscle- and fibre type-specific expression of skeletal muscle glucose transporter 4 (GLUT4), glycogen synthase (GS) and glycogen phosphorylase (GP) was investigated in six young male subjects. Single muscle fibres were dissected from vastus lateralis (VL), soleus (SO) and triceps brachii (TB) muscle biopsy samples. On the basis of myosin heavy chain (MHC) expression, fibres were pooled into three groups (MHC I, MHC IIA and MHC IIX) and the GLUT4, GS and GP content of 15-40 pooled fibres determined using SDS-PAGE and immunological detection. In VL, the GLUT4 content in the pooled muscle fibres expressing MHC I was approximately 33% higher ( P<0.05) than in fibres expressing MHC IIA or IIX. There was no difference in GLUT4 content between fibres expressing MHC IIA or IIX, nor were there any differences in GS and GP content between any of the fibre types. In SO, there was no difference in GLUT4, GS and GP between fibres expressing MHC I or IIA. No fibres expressing type IIX were detected. In TB, fibres expressing MHC IIA and IIX had significantly ( P<0.05) more GP (66% and 55 % in MHC IIA and MHCIIX, respectively) than those expressing MHC I, whilst there was no difference in GP between MHC IIA and MHC IIX fibres. The GLUT4 and the GS content was similar in fibres expressing MHC I, IIA and IIX in the TB. Our data directly demonstrate that some proteins, like GLUT4 and GP, are expressed in a fibre type-specific manner in some, but not all, muscles, whilst other proteins, like GS, are not. In human skeletal muscle the GLUT4, GS and GP content thus seems to be related primarily to factors other than the fibre type as defined by the expression of contractile protein. These findings imply that it is not possible to generalize fibre type-dependent protein expression on the basis of biopsies from only one muscle.

Resistance training and insulin action in humans: effects of de-training

Aerobic endurance training increases insulin action in skeletal muscle, but the effect of resistance training has not been well described. Controversy exists about whether the effect of resistance training is merely due to an increase in muscle mass. We studied the effect of cessation of resistance training in young, healthy subjects by taking muscle biopsies and measuring insulin-mediated whole body and leg glucose uptake rates after 90 days of heavy resistance training (T) and again after 90 days of de-training (dT). Data on leg glucose uptake were expressed relative to accurate measures of leg muscle mass by MRI scanning. Muscle strength (239 +/- 43 vs. 208 +/- 33 N m), quadriceps area (8463 +/- 453 vs. 7763 +/- 329 mm2) and glycogen content (458 +/- 22 vs. 400 +/- 26 mmol (kg dry weight muscle)(-1)) decreased, while myosin heavy chain isoform IIX increased 4-fold in dT vs. T, respectively (all P < 0.05). GLUT4 mRNA levels and enzyme activities and mRNA levels of glycolytic, lipolytic and glyconeogenic enzymes did not change with de-training. Likewise, capillary density did not change. Whole body glucose uptake decreased 11 % and leg glucose uptake decreased from 75 +/- 11 (T) to 50 +/- 6 (dT) nmol min(-1) (mm muscle)(-2) (P < 0.05) at maximal insulin, the latter decrease being due to decreased arterio-femoral venous glucose extraction. The decrease was mainly due to reduced non-oxidative glucose disposal. We have thus shown that 90 days after the termination of heavy resistance training, insulin-mediated glucose uptake rates per unit of skeletal muscle have decreased significantly.

Effects of low-resistance/high-repetition strength training in hypoxia on muscle structure and gene expression

To test the hypothesis that severe hypoxia during low-resistance/high-repetition strength training promotes muscle hypertrophy, 19 untrained males were assigned randomly to 4 weeks of low-resistance/high-repetition knee extension exercise in either normoxia or in normobaric hypoxia ( FiO(2) 0.12) with recovery in normoxia. Before and after the training period, isokinetic strength tests were performed, muscle cross-sectional area (MCSA) measured (magnetic resonance imaging) and muscle biopsies taken. The significant increase in strength endurance capacity observed in both training groups was not matched by changes in MCSA, fibre type distribution or fibre cross-sectional area. RT-PCR revealed considerable inter-individual variations with no significant differences in the mRNA levels of hypoxia markers, glycolytic enzymes and myosin heavy chain isoforms. We found significant correlations, in the hypoxia group only, for those hypoxia marker and glycolytic enzyme mRNAs that have previously been linked to hypoxia-specific muscle adaptations. This is interpreted as a small, otherwise undetectable adaptation to the hypoxia training condition. In terms of strength parameters, there were, however, no indications that low-resistance/high-repetition training in severe hypoxia is superior to equivalent normoxic training.

Effect of high-intensity training on exercise-induced gene expression specific to ion homeostasis and metabolism

Changes in gene expression during recovery from high-intensity, intermittent, one-legged exercise were studied before and after 5.5 wk of training. Genes related to metabolism, as well as Na+, K+, and pH homeostasis, were selected for analyses. After the same work was performed before and after the training period, several muscle biopsies were obtained from vastus lateralis muscle. In the untrained state, the Na+-K+-ATPase alpha1-subunit mRNA level was approximately threefold higher (P < 0.01) at 0, 1, and 3 h after exercise, relative to the preexercise resting level. After 3-5 h of recovery in the untrained state, pyruvate dehydrogenase kinase 4 and hexokinase II mRNA levels were elevated 13-fold (P < 0.001) and 6-fold (P < 0.01), respectively. However, after the training period, only pyruvate dehydrogenase kinase 4 mRNA levels were elevated (P < 0.05) during the recovery period. No changes in resting mRNA levels were observed as a result of training. In conclusion, cellular adaptations to high-intensity exercise training may, in part, be induced by transcriptional regulation. After training, the transcriptional response to an exercise bout at a given workload is diminished.

Glycogen-dependent effects of 5-aminoimidazole-4-carboxamide (AICA)-riboside on AMP-activated protein kinase and glycogen synthase activities in rat skeletal muscle

5'-AMP-activated protein kinase (AMPK) functions as a metabolic switch in mammalian cells and can be artificially activated by 5-aminoimidazole-4-carboxamide (AICA)-riboside. AMPK activation during muscle contraction is dependent on muscle glycogen concentrations, but whether glycogen also modifies the activation of AMPK and its possible downstream effectors (glycogen synthase and glucose transport) by AICA-riboside in resting muscle is not known. Thus, we have altered muscle glycogen levels in rats by a combination of swimming exercise and diet and investigated the effects of AICA-riboside in the perfused rat hindlimb muscle. Two groups of rats, one with super-compensated muscle glycogen content (approximately 200-300% of normal; high glycogen [HG]) and one with moderately lowered muscle glycogen content (approximately 80% of normal; low glycogen [LG]), were generated. In both groups, the degree of activation of the alpha2 isoform of AMPK by AICA-riboside depended on muscle type (white gastrocnemius >> red gastrocnemius > soleus). Basal and AICA-riboside-induced alpha2-AMPK activity were markedly lowered in the HG group (approximately 50%) compared with the LG group. Muscle 2-deoxyglucose uptake was also increased and glycogen synthase activity decreased by AICA-riboside. Especially in white gastrocnemius, these effects, as well as the absolute activity levels of AMPK-alpha2, were markedly reduced in the HG group compared with the LG group. The inactivation of glycogen synthase by AICA-riboside was accompanied by decreased gel mobility and was eliminated by protein phosphatase treatment. We conclude that acute AICA-riboside treatment leads to phosphorylation and deactivation of glycogen synthase in skeletal muscle. Although the data do not exclude a role of other kinases/phosphatases, they suggest that glycogen synthase may be a target for AMPK in vivo. Both basal and AICA-riboside-induced AMPK-alpha2 and glycogen synthase activities, as well as glucose transport, are depressed when the glycogen stores are plentiful. Because the glycogen level did not affect adenine nucleotide concentrations, our data suggest that glycogen may directly affect the activation state of AMPK in skeletal muscle.

Free radicals in the physiological control of cell function

At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, however, nitric oxide (NO), superoxide anion, and related reactive oxygen species (ROS) play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and reestablish "redox homeostasis." Higher organisms, however, have evolved the use of NO and ROS also as signaling molecules for other physiological functions. These include regulation of vascular tone, monitoring of oxygen tension in the control of ventilation and erythropoietin production, and signal transduction from membrane receptors in various physiological processes. NO and ROS are typically generated in these cases by tightly regulated enzymes such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. In a given signaling protein, oxidative attack induces either a loss of function, a gain of function, or a switch to a different function. Excessive amounts of ROS may arise either from excessive stimulation of NAD(P)H oxidases or from less well-regulated sources such as the mitochondrial electron-transport chain. In mitochondria, ROS are generated as undesirable side products of the oxidative energy metabolism. An excessive and/or sustained increase in ROS production has been implicated in the pathogenesis of cancer, diabetes mellitus, atherosclerosis, neurodegenerative diseases, rheumatoid arthritis, ischemia/reperfusion injury, obstructive sleep apnea, and other diseases. In addition, free radicals have been implicated in the mechanism of senescence. That the process of aging may result, at least in part, from radical-mediated oxidative damage was proposed more than 40 years ago by Harman (J Gerontol 11: 298-300, 1956). There is growing evidence that aging involves, in addition, progressive changes in free radical-mediated regulatory processes that result in altered gene expression.

The effect of exercise training on hormone-sensitive lipase in rat intra-abdominal adipose tissue and muscle

1. Adrenaline-stimulated lipolysis in adipose tissue may increase with training. The rate-limiting step in adipose tissue lipolysis is catalysed by the enzyme hormone-sensitive lipase (HSL). We studied the effect of exercise training on the activity of the total and the activated form of HSL, referred to as HSL (DG) and HSL (TG), respectively, and on the concentration of HSL protein in retroperitoneal (RE) and mesenteric (ME) adipose tissue, and in the extensor digitorum longus (EDL) and soleus muscles in rats. 2. Rats (weighing 96 +/- 1 g, mean +/- S.E.M.) were either swim trained (T, 18 weeks, n = 12) or sedentary (S, n = 12). Then RE and ME adipose tissue and the EDL and soleus muscles were incubated for 20 min with 4.4 microM adrenaline. 3. HSL enzyme activities in adipose tissue were higher in T compared with S rats. Furthermore, in RE adipose tissue, training also doubled HSL protein concentration (P < 0.05). In ME adipose tissue, the HSL protein levels did not differ significantly between T and S rats. In muscle, HSL (TG) activity as well as HSL (TG)/HSL (DG) were lower in T rats, whereas HSL (DG) activity did not differ between groups. Furthermore, HSL protein concentration in muscle did not differ between T and S rats (P > 0.05). 4. In conclusion, training increased the amount of HSL and the sensitivity of HSL to stimulation by adrenaline in intra-abdominal adipose tissue, the extent of the change differing between anatomical locations. In contrast, in skeletal muscle the amount of HSL was unchanged and its sensitivity to stimulation by adrenaline reduced after training.

Exercise-induced overexpression of key regulatory proteins involved in glucose uptake and metabolism in tetraplegic persons: molecular mechanism for improved glucose homeostasis

Complete spinal cord lesion leads to profound metabolic abnormalities and striking changes in muscle morphology. Here we assess the effects of electrically stimulated leg cycling (ESLC) on whole body insulin sensitivity, skeletal muscle glucose metabolism, and muscle fiber morphology in five tetraplegic subjects with complete C5-C7 lesions. Physical training (seven ESLC sessions/wk for 8 wk) increased whole body insulin-stimulated glucose uptake by 33+/-13%, concomitant with a 2.1-fold increase in insulin-stimulated (100 microU/ml) 3-O-methylglucose transport in isolated vastus lateralis muscle. Physical training led to a marked increase in protein expression of GLUT4 (378+/-85%), glycogen synthase (526+/-146%), and hexokinase II (204+/-47%) in vastus lateralis muscle, whereas phosphofructokinase expression (282+/-97%) was not significantly changed. Hexokinase II activity was significantly increased, whereas activity of phosphofructokinase, glycogen synthase, and citrate synthase was not changed after training. Muscle fiber type distribution and fiber area were markedly altered compared to able-bodied subjects before ESLC training, with no change noted in either parameter after ECSL training. In conclusion, muscle contraction improves insulin action on whole body and cellular glucose uptake in cervical cord-injured persons through a major increase in protein expression of key genes involved in the regulation of glucose metabolism. Furthermore, improvements in insulin action on glucose metabolism are independent of changes in muscle fiber type distribution.