Glycogen Synthase Fractional Activity 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.

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.

Effect of postexercise temperature elevation on postexercise glycogen metabolism of isolated mouse soleus muscle.

The effects of temperature elevation after intense repeated contractions on glycogen and energy metabolism as well as contractile function of isolated mouse soleus muscle (slow twitch, oxidative) were investigated. Muscles were stimulated electrically to perform repeated tetanic contractions for 10 min at 25°C, which reduced tetanic force by ~85% and glycogen by 50%. After 120-min recovery at 25°C glycogen was fully restored (~125% of basal), whereas after recovery at 35°C glycogen decreased further (~25% of basal). Glycogen synthase fractional activity averaged 31.8 ± 3.1% (baseline = 33.8 ± 3.4%) after 120-min recovery at 25°C but was increased after recovery at 35°C (63.8 ± 4.8%; P < 0.001 vs. 25°C). Phosphorylase fractional and total activities were not affected by the higher temperature. However, recovery at 35°C resulted in a significantly higher content of the phosphorylase substrate inorganic phosphate (~20%; P < 0.01 vs. 25°C). Finally, fatigue development during a subsequent bout of repeated contractions at 25°C was similar after 120-min recovery at 25°C and 35°C. These data demonstrate that after intense contractions elevated temperature inhibits glycogen accumulation, likely by increasing the availability of the phosphorylase substrate inorganic phosphate, but has no effect on fatigue development. Thus after heat exposure phosphorylase plays a significant role in glycogen accumulation, and glycogen does not limit muscle performance in isolated mouse soleus muscle after recovery from elevated temperature. NEW & NOTEWORTHY Whether elevated temperature affects glycogen biogenesis and contractile performance of isolated slow-twitch muscle is not known. Here we show that after a bout of repeated contractions in isolated mouse soleus muscle at 25°C, increasing muscle temperature during recovery to 35°C blocked glycogen accumulation compared with recovery at 25°C. Surprisingly, during a subsequent bout of repeated contractions at 25°C, the rate of fatigue was not different between groups after recovery at the two temperatures.

Heating after intense repeated contractions inhibits glycogen accumulation in mouse EDL muscle: role of phosphorylase in postexercise glycogen metabolism.

The effects of heating on glycogen synthesis (incorporation of [14C]glucose into glycogen) and accumulation after intense repeated contractions were investigated. Isolated mouse extensor digitorum longus muscle (type II) was stimulated electrically to perform intense tetanic contractions at 25°C. After 120 min recovery at 25°C, glycogen accumulated to almost 80% of basal, whereas after recovery at 35°C, glycogen remained low (~25% of basal). Glycogen synthesis averaged 0.97 ± 0.07 µmol·30 min-1·g wet wt-1 during recovery at 25°C and 1.48 ± 0.08 during recovery at 35°C ( P < 0.001). There were no differences in phosphorylase and glycogen synthase total activities nor in phosphorylase fractional activity, whereas glycogen synthase fractional activity was increased by ~50% after recovery at 35°C vs. 25°C. Inorganic phosphate (Pi, substrate for phosphorylase) was markedly increased (~300% of basal) following contraction but returned to control levels after 120 min recovery at 25°C. In contrast, Pi remained elevated after recovery at 35°C (>2-fold higher than recovery at 25°C). Estimates of glycogen breakdown indicated that phosphorylase activity (either via inhibition at 25°C or activation at 35°C) was responsible for ~60% of glycogen accumulation during recovery at 25°C and ~45% during recovery at 35°C. These data demonstrate that despite the enhancing effect of heating on glycogen synthesis during recovery from intense contractions, glycogen accumulation is inhibited owing to Pi-mediated activation of phosphorylase. Thus phosphorylase can play a quantitatively important role in glycogen biogenesis during recovery from repeated contractions in isolated type II muscle.

Abstract T P114: Insulin Resistance in Skeletal Muscle of Chronic Stroke

Risk of glucose intolerance and diabetes increases in chronic stroke. The purpose of this study was to assess insulin sensitivity and glycogen synthase (GS), a known benchmark index of insulin action in skeletal muscle, and to compare the activity of this important regulatory enzyme between paretic (P) and non-paretic (NP) skeletal muscle in chronic stroke. We measured insulin sensitivity (M) and bilateral GS fractional activity (ratio of independent to total activity), in lyophilized microdissected muscle samples obtained after an overnight fast and 2 hrs into a 3-hr 80 mU . m -2. min -1 hyperinsulinemic-euglycemic clamp in 21 stroke survivors (n=15 men, n=6 women) (age: 59±2 yrs, BMI: 31±2 kg/m 2 , X±SEM). All had hemiparetic gait after ischemic stroke (&gt;6 months), low aerobic capacity (VO 2 peak, 19.7±1.3 ml/kg/min), and wide range of %body fat (11-48%). Leg lean mass was lower in P than NP (9.3±0.5 vs. 10.0±0.5 kg, P&lt;0.001). Subjects had either normal glucose tolerance (n=7), impaired glucose tolerance (n=7), or diabetes (n=7) and insulin resistance (M: 38.5±2.6 umol/kgFFM/min). Insulin robustly increased GS fractional activity (basal vs. insulin) in P (2.8±0.4 vs.7.5±0.8%, P&lt;0.00001) and NP (2.7±0.4 vs. 9.1±1.1%, P&lt;0.00001) muscle. The %change was greater in NP than P (213±32 vs. 296±36%, P=0.04). The effect of in vivo insulin to increase GS fractional activity was associated with M in P and NP muscle (r=0.59 and r=0.49, P&lt;0.05). In conclusion, muscle atrophy and a reduction in insulin action in paretic muscle likely contribute to whole body insulin resistance in chronic stroke.

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.

Effect of acute exercise on glycogen synthase in muscle from obese and diabetic subjects.

Insulin stimulates glycogen synthase (GS) through dephosphorylation of serine residues, and this effect is impaired in skeletal muscle from insulin-resistant [obese and type 2 diabetic (T2DM)] subjects. Exercise also increases GS activity, yet it is not known whether the ability of exercise to affect GS is impaired in insulin-resistant subjects. The objective of this study was to examine the effect of acute exercise on GS phosphorylation and enzyme kinetic properties in muscle from insulin-resistant individuals. Lean normal glucose-tolerant (NGT), obese NGT, and obese T2DM subjects performed 40 min of moderate-intensity cycle exercise (70% of Vo(2max)). GS kinetic properties and phosphorylation were measured in vastus lateralis muscle before exercise, immediately after exercise, and 3.5 h postexercise. In lean subjects, GS fractional activity increased twofold after 40 min of exercise, and it remained elevated after the 3.5-h rest period. Importantly, exercise also decreased GS K(m) for UDP-glucose from ≈0.5 to ≈0.2 mM. In lean subjects, exercise caused significant dephosphorylation of GS by 50-70% (Ser(641), Ser(645), and Ser(645,649,653,657)), and phosphorylation of these sites remained decreased after 3.5 h; Ser⁷ phosphorylation was not regulated by exercise. In obese NGT and T2DM subjects, exercise increased GS fractional activity, decreased K(m) for UDP-glucose, and decreased GS phosphorylation as effectively as in lean NGT subjects. We conclude that the molecular regulatory process by which exercise promotes glycogen synthesis in muscle is preserved in insulin-resistant subjects.

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

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

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.

Insulin-stimulated glycogen synthesis and glycogen synthase activation after electrical stimulation of epitrochlearis muscles with different initial glycogen contents

Glycogen synthesis increases after muscle contraction and during insulin stimulation, and insulin-stimulated glycogen synthesis is enhanced after contraction. We hypothesized that the initial glycogen content influences the magnitude of additive effect of contraction on insulin-stimulated glycogen synthesis. Contraction and insulin had full additive effect on rate of glycogen synthesis measured after contraction in muscles with normal and high glycogen content. In muscles with low glycogen, contraction increased insulin-stimulated glycogen synthesis nearly as much as in muscles with normal glycogen, but not to the sum of the two stimuli studied separately; still glycogen synthesis was generally highest in muscles with low glycogen. Glycogen synthase fractional activity inversely correlated with glycogen content and contraction increased glycogen synthase fractional activity. Contraction and insulin additively increased glycogen synthase fractional activity at all glycogen contents. In conclusion, after contraction insulin-stimulated glycogen synthesis was increased by rather similar magnitude at all glycogen contents in concert with increased glycogen synthase activation.

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

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

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.

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

The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined. Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin. Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser(641) (R = -0.93, P < 0.001) and Ser(645,649,653,657) (R = -0.85, P < 0.001) phosphorylation, but not with Ser(7) phosphorylation. Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser(641) and Ser(645,649,653,657) seems to regulate GS activity in soleus.

Insulin action and signalling in fat and muscle from dexamethasone-treated rats

Glucocorticoids initiate whole body insulin resistance and the aim of the present study was to investigate effects of dexamethasone on protein expression and insulin signalling in muscle and fat tissue. Rats were injected with dexamethasone (1mg/kg/day, i.p.) or placebo for 11 days before insulin sensitivity was evaluated in vitro in soleus and epitrochlearis muscles and in isolated epididymal adipocytes. Dexamethasone treatment reduced insulin-stimulated glucose uptake and glycogen synthesis by 30–70% in epitrochlearis and soleus, and insulin-stimulated glucose uptake by ∼40% in adipocytes. 8-bromo-cAMP-stimulated lipolysis was ∼2-fold higher in adipocytes from dexamethasone-treated rats and insulin was less effective to inhibit cAMP-stimulated lipolysis. A main finding was that dexamethasone decreased expression of PKB and insulin-stimulated Ser473 and Thr308 phosphorylation in both muscles and adipocytes. Expression of GSK-3 was not influenced by dexamethasone treatment in muscles or adipocytes and insulin-stimulated GSK-3β Ser9 phosphorylation was reduced in muscles only. A novel finding was that glycogen synthase (GS) Ser7 phosphorylation was higher in both muscles from dexamethasone-treated rats. GS expression decreased (by 50%) in adipocytes only. Basal and insulin-stimulated GS Ser641 and GS Ser645,649,653,657 phosphorylation was elevated in epitrochlearis and soleus muscles and GS fractional activity was reduced correspondingly. In conclusion, dexamethasone treatment (1) decreases PKB expression and insulin-stimulated phosphorylation in both muscles and adipocytes, and (2) increases GS phosphorylation (reduces GS fractional activity) in muscles and decreases GS expression in adipocytes. We suggest PKB and GS as major targets for dexamethasone-induced insulin resistance.

Palmitate action to inhibit glycogen synthase and stimulate protein phosphatase 2A increases with risk factors for type 2 diabetes

Recent studies have suggested that abnormal regulation of protein phosphatase 2A (PP2A) is associated with Type 2 diabetes in rodent and human tissues. Results with cultured mouse myotubes support a mechanism for palmitate activation of PP2A, leading to activation of glycogen synthase kinase 3. Phosphorylation and inactivation of glycogen synthase by glycogen synthase kinase 3 could be the mechanism for long-chain fatty acid inhibition of insulin-mediated carbohydrate storage in insulin-resistant subjects. Here, we test the effects of palmitic acid on cultured muscle glycogen synthase and PP2A activities. Palmitate inhibition of glycogen synthase fractional activity is increased in subjects with high body mass index compared with subjects with lower body mass index (r = -0.43, P = 0.03). Palmitate action on PP2A varies from inhibition in subjects with decreased 2-h plasma glucose concentration to activation in subjects with increased 2-h plasma glucose concentration (r = 0.45, P < 0.03) during oral glucose tolerance tests. The results do not show an association between palmitate effects on PP2A and glycogen synthase fractional activity. We conclude that subjects at risk for Type 2 diabetes have intrinsic differences in palmitate regulation of at least two enzymes (PP2A and glycogen synthase), contributing to abnormal insulin regulation of glucose metabolism.

Glycogen content and contraction regulate glycogen synthase phosphorylation and affinity for UDP-glucose in rat skeletal muscles

Glycogen content and contraction strongly regulate glycogen synthase (GS) activity, and the aim of the present study was to explore their effects and interaction on GS phosphorylation and kinetic properties. Glycogen content in rat epitrochlearis muscles was manipulated in vivo. After manipulation, incubated muscles with normal glycogen [NG; 210.9 +/- 7.1 mmol/kg dry weight (dw)], low glycogen (LG; 108.1 +/- 4.5 mmol/ kg dw), and high glycogen (HG; 482.7 +/- 42.1 mmol/kg dw) were contracted or rested before the studies of GS kinetic properties and GS phosphorylation (using phospho-specific antibodies). LG decreased and HG increased GS K(m) for UDP-glucose (LG: 0.27 +/- 0.02 < NG: 0.71 +/- 0.06 < HG: 1.11 +/- 0.12 mM; P < 0.001). In addition, GS fractional activity inversely correlated with glycogen content (R = -0.70; P < 0.001; n = 44). Contraction decreased K(m) for UDP-glucose (LG: 0.14 +/- 0.01 = NG: 0.16 +/- 0.01 < HG: 0.33 +/- 0.03 mM; P < 0.001) and increased GS fractional activity, and these effects were observed independently of glycogen content. In rested muscles, GS Ser(641) and Ser(7) phosphorylation was decreased in LG and increased in HG compared with NG. GSK-3beta Ser(9) and AMPKalpha Thr(172) phosphorylation was not modulated by glycogen content in rested muscles. Contraction decreased phosphorylation of GS Ser(641) at all glycogen contents. However, contraction increased GS Ser(7) phosphorylation even though GS was strongly activated. In conclusion, glycogen content regulates GS affinity for UDP-glucose and low affinity for UDP-glucose in muscles with high glycogen content may reduce glycogen accumulation. Contraction increases GS affinity for UDP-glucose independently of glycogen content and creates a unique phosphorylation pattern.

Low Birth Weight and Zygosity Status Is Associated With Defective Muscle Glycogen and Glycogen Synthase Regulation in Elderly Twins

An adverse intrauterine environment indicated by both low birth weight and monozygosity is associated with an age- or time-dependent reduction in glucose disposal and nonoxidative glucose metabolism in twins, suggesting impaired regulation of muscle glycogen synthesis. We measured the activities of glycogen synthase (GS), GS kinase (GSK)3 alpha, GS phosphorylation, and glycogen levels in muscle biopsies obtained from 184 young and elderly twins before and after a euglycemic-hyperinsulinemic clamp. Elderly monozygotic twins had significantly lower fractional GS activity amidst higher glycogen and GS protein levels compared with dizygotic twins. In addition, we demonstrated strong nongenetic associations between birth weight and defect muscle glycogen metabolism in elderly--but not in younger--twins. Thus, for every 100 g increase in birth weight within pairs, GS fractional activity, GS protein level, and glycogen content was increased by 4.2, 8.7, and 4.5%, respectively, in elderly twins. Similarly, for every 100 g increase in birth weight, GSK3 alpha activity and GS phosphorylation at the sites 2, 2+2a, and 3a+3b were decreased by 3.1, 9.0, 10.1, and 9.5%, respectively. The age- or time-dependent nongenetic impact of birth weight on insulin action in twins may partly be explained by reduced insulin activation of muscle GS, mediated through increased GSK3 alpha activity and GS phosphorylation. Reduced GS activity and negative feedback inhibition of glycogen metabolism by glycogen per se may contribute to the insulin resistance in elderly monozygotic compared with dizygotic twins.

Contraction Activates Glycogen Synthase Independently of Initial Glycogen Concentration

Muscle contraction activates glycogen synthesis, but the mechanism for activation of glycogen synthase after contractile activity is poorly understood. Glycogen concentration is powerful regulator of glycogen synthase and it has been suggested that reduced glycogen content determines activation of glycogen synthase after contractile activity. PURPOSE: The purpose of the present study was to investigate effects of muscle contraction on glycogen synthesis, glycogen synthase activation and glycogen synthase phosphorylation in muscle with different glycogen concentrations. METHODS: We manipulated the muscle glycogen content to low (94.8 ±5.0 mmol-kg−1; 24-hours fasted rats; n=36), normal (203.8±10.2 mmol-kg1; Free access to chow; n=28), and high (459.3±30.9 mmol-kg−1; 24-hours fasted rats were refed for 24 hours to supercompensate glycogen; n=32). After manipulation, muscles were dissected for in vitro studies, mounted on contraction apparatus, and stimulated electrically for 30 min. Subsequently, glycogen synthesis, glycogen synthase activity, and glycogen synthase Ser645, Ser649, Ser653, Ser657 phosphorylation were assessed. RESULTS: Contraction decreased glycogen content by 120.6±4.7 (n=28) and 131.5±8.4 mmol-kg−1 (n=32) in muscles with normal and high glycogen, respectively. In muscles with low glycogen, glycogen breakdown was reduced to 67.4±5.0 mmol-kg−1(p<0.001; n=28). Rate of glycogen synthesis after contraction was inversely correlated to initial glycogen content. Contraction increased GS activity in all groups (p < 0.001). Interestingly, contraction activated glycogen synthase in muscles with high glycogen (p < 0.01) without dephosphorylation of the sites phosphorylated by GSK-3. Contraction dephosphorylated these sites in muscles with low and normal glycogen. When insulin was present after contraction, an additive effect of insulin on glycogen synthesis and glycogen synthase activation. Interestingly, rate of glycogen synthesis reach the same level in muscles with low and normal glycogen when insulin was present after contraction; so did glycogen synthase fractional activity and glycogen synthase Ser645, Ser649, Ser653, Ser657 phosphorylation. CONCLUSION: Contractile activity is a powerful activator of GS and glycogen synthesis even in muscles high glycogen content. In muscles with high glycogen, glycogen synthase fractional activity was increased without dephosphorylation of the sites phosphorylated by GSK-3, indicating that contraction activates glycogen synthase by dephosphorylation of other sites. Supported by the Research Council of Norway and The Novo Nordisk Foundation.

Effects of Aerobic and Resistive Exercise Training on Glucose Disposal and Skeletal Muscle Metabolism in Older Men

Aging is associated with insulin resistance, primarily as a result of physical inactivity and increased abdominal obesity. We hypothesized that aerobic (AEX) or resistive (RT) exercise training would result in comparable improvements in glucose disposal in older men, but that there would be different metabolic adaptations in skeletal muscle. Thirty-nine older (63+/-1 years, mean+/-standard error of the mean), overweight and obese (body mass index=30.3+/-0.4 kg/m2) men were assigned to AEX (treadmill walking and/or jogging, n=19) or RT (upper and lower body, n=20) programs 3 d/wk for 6 months, with 9 completing AEX and 13 completing RT. Testing before and after the exercise programs included body composition, euglycemic-hyperinsulinemic clamps, and vastus lateralis muscle biopsies. Maximal oxygen consumption (VO2max) increased by 16% after AEX (p<.01), while leg and arm muscle strength increased by 45+/-5% and 27+/-5% after RT (p<.0001). Although participants were monitored to maintain their body weight during the exercise program, body weight decreased by 2% after AEX (p<.05), and increased by 2% after RT (p<.05). Whole-body glucose disposal, determined during the last 30 minutes of a 2-hour 480 pmol/m2/min euglycemic-hyperinsulinemic clamp, increased comparably by 20%-25% after AEX (51+/-5 to 61+/-5 microM/kgfat-free mass/min, p<.05) and RT (49+/-3 to 58+/-3 microM/kgfat-free mass/min, p<.05). The increase in vastus lateralis muscle glycogen synthase fractional activity in response to insulin stimulation was significantly higher after AEX compared to after RT (279+/-59% compared to 100+/-28% change, p<.05). Neither AEX nor RT altered muscle glycogen synthase total activity, glycogen content, or levels of phosphotidylinositol 3-kinase. These results suggest that AEX and RT result in comparable improvements in glucose metabolism in older men, whereas an increase in insulin activation of glycogen synthase occurred only with AEX. These improvements in insulin sensitivity could reduce the risk of metabolic syndrome and type 2 diabetes and attenuate the development of cardiovascular disease.

Glucocorticoid-induced insulin resistance in skeletal muscles: defects in insulin signalling and the effects of a selective glycogen synthase kinase-3 inhibitor

Treatment with glucocorticoids, especially at high doses, induces insulin resistance. The aims of the present study were to identify the potential defects in insulin signalling that contribute to dexamethasone-induced insulin resistance in skeletal muscles, and to investigate whether the glycogen synthase-3 (GSK-3) inhibitor CHIR-637 could restore insulin-stimulated glucose metabolism. Skeletal muscles were made insulin-resistant by treating male Wistar rats with dexamethasone, a glucocorticoid analogue, for 12 days. Insulin-stimulated glucose uptake, glycogen synthesis and insulin signalling were studied in skeletal muscles in vitro. Dexamethasone treatment decreased the ability of insulin to stimulate glucose uptake, glycogen synthesis and glycogen synthase fractional activity. In addition, the dephosphorylation of glycogen synthase by insulin was blocked. These defects were paralleled by reduced insulin-stimulated protein kinase B (PKB) and GSK-3 phosphorylation. While expression of PKB, GSK-3 and glycogen synthase was not reduced by dexamethasone treatment, expression of the p85alpha subunit of phosphatidylinositol 3-kinase (PI 3-kinase) was increased. Inhibition of GSK-3 by CHIR-637 increased glycogen synthase fractional activity in soleus muscle from normal and dexamethasone-treated rats, although the effect was more pronounced in control rats. CHIR-637 did not improve insulin-stimulated glucose uptake in muscles from dexamethasone-treated rats. We demonstrated that chronic dexamethasone treatment impairs insulin-stimulated PKB and GSK-3 phosphorylation, which may contribute to insulin resistance in skeletal muscles. Acute pharmacological inhibition of GSK-3 activated glycogen synthase in muscles from dexamethasone-treated rats, but GSK-3 inhibition did not restore insulin-stimulated glucose uptake.