Effect of exercise, training, and glycogen availability on IL-6 receptor expression in human skeletal muscle.

The cytokine interleukin-6 (IL-6) exerts it actions via the IL-6 receptor (IL-6R) in conjunction with the ubiquitously expressed gp130 receptor. IL-6 is tightly regulated in response to exercise, being affected by factors such as exercise intensity and duration, as well as energy availability. Although the IL-6 response to exercise has been extensively studied, little is known about the regulation of the IL-6R response. In the present study, we aimed to investigate the effect of exercise, training, and glycogen availability, factors known to affect IL-6, on the regulation of gene expression of the IL-6R in human skeletal muscle. Human subjects performed either 10 wk of training with an acute exercise bout before and after the training period, or a low-glycogen vs. normal-glycogen acute exercise trial. The IL-6R mRNA response was evaluated in both trials. In response to acute exercise, an increase in IL-6R mRNA levels was observed. Neither training nor intramuscular glycogen levels had an effect on the IL-6R mRNA response to exercise. However, after 10 wk of training, the skeletal muscle expressed a higher mRNA level of IL-6R compared with before training. The present study demonstrated that the IL-6R gene expression levels in skeletal muscle are increased in response to acute exercise, a response that is very well conserved, being affected by neither training status nor intramuscular glycogen levels, as opposed to IL-6. However, after the training period, IL-6R mRNA production was increased in skeletal muscle, suggesting a sensitization of skeletal muscle to IL-6 at rest.     

Influence of differing macronutrient intakes on muscle glycogen resynthesis after resistance exercise

The provision of additional protein (Pro)to a carbohydrate (CHO) supplement resulted in an enhanced rate ofmuscle glycogen resynthesis after endurance exercise (Zawadzki et al.,J. Appl. Physiol. 72: 1854-1859,1992). A comparison of isoenergetic CHO and CHO/Pro formula drinks onmuscle glycogen resynthesis has not been examined after eitherendurance or resistance exercise. We studied the effect of isoenergeticCHO (1 g/kg) and CHO/Pro/fat (66% CHO, 23% Pro, 11% fat) definedformula drinks and placebo (Pl) given immediately(t = 0 h) and 1 h(t = +1 h) after resistance exercisein 10 healthy young men. They performed a whole body workout (9 exercises/3 sets at 80% 1 repetition maximum) with unilateral kneeextension exercise [exercise (Ex) and control (Con) leg].The CHO/Pro/fat and CHO trials resulted in significantly greater(P < 0.05) plasma insulin andglucose concentration compared with Pl. Muscle glycogen wassignificantly lower (P < 0.05) for the Ex vs. Con leg immediately postexercise for all three conditions. The rate of glycogen resynthesis was significantly greater(P < 0.05) for both CHO/Pro/fat andCHO (23.0 ± 4.5 and 19.3 ± 6.1 mmol · kg drymuscle¹ · h¹,respectively) vs. Pl (Ex = 2.8 ± 2.3 and Con = 1.4 ± 3.6 mmol · kg drymuscle¹ · h¹).These results demonstrated that a bout of resistance exercise resultedin a significant decrease in muscle glycogen and that consumption of anisoenergetic CHO or CHO/Pro/fat formula drink resulted in similar ratesof muscle glycogen resynthesis after resistance exercise. This suggeststhat total energy content and CHO content are important in theresynthesis of muscle glycogen.

     

Cardiotonic steroids stimulate glycogen synthesis in human skeletal muscle cells via a Src- and ERK1/2-dependent mechanism

The cardiotonic steroid, ouabain, a specific inhibitor of Na(+),K(+)-ATPase, initiates protein-protein interactions that lead to an increase in growth and proliferation in different cell types. We explored the effects of ouabain on glucose metabolism in human skeletal muscle cells (HSMC) and clarified the mechanisms of ouabain signal transduction. In HSMC, ouabain increased glycogen synthesis in a concentration-dependent manner reaching the maximum at 100 nM. The effect of ouabain was additive to the effect of insulin and was independent of phosphatidylinositol 3-kinase inhibitor LY294002 but was abolished in the presence of a MEK1/2 inhibitor (PD98059) or a Src inhibitor (PP2). Ouabain increased Src-dependent tyrosine phosphorylation of alpha(1)- and alpha(2)-subunits of Na(+),K(+)-ATPase and promoted interaction of alpha(1)- and alpha(2)-subunits with Src, as assessed by co-immunoprecipitation with Src. Phosphorylation of ERK1/2 and GSK3alpha/beta, as well as p90rsk activity, was increased in response to ouabain in HSMC, and these responses were prevented in the presence of PD98059 and PP2. Incubation of HSMC with 100 nM ouabain increased phosphorylation of the alpha-subunits of the Na-pump at a MAPK-specific Thr-Pro motif. Ouabain treatment decreased the surface abundance of alpha(2)-subunit, whereas abundance of the alpha(1)-subunit was unchanged. Marinobufagenin, an endogenous vertebrate bufadienolide cardiotonic steroid, increased glycogen synthesis in HSMC at 10 nM concentration, similarly to 100 nM ouabain. In conclusion, ouabain and marinobufagenin stimulate glycogen synthesis in skeletal muscle. This effect is mediated by activation of a Src-, ERK1/2-, p90rsk-, and GSK3-dependent signaling pathway.     

耐力运动对大鼠骨骼肌ERK1/2活性的影响

目的:探讨耐力运动对大鼠骨骼肌蛋白总量(t-ERK1/2)及磷酸化ERK1/2(p-ERK1/2)及ERK2mRMA表达的影响。方法:SD大鼠随机分为对照组和运动组。运动组分为1h/d和1.5h/d组,共7周,运动结束后24h和48h取材,测定葡萄糖和胰岛素浓度;Westernblot法检测骨骼肌t-ERK1/2、p-ERK1/2蛋白表达;RT-PCR法分析ERK2mRNA表达。结果:与对照组比较,运动组胰岛素浓度降低;各运动组p-ERK1/2升高;1.5h/d-24h和-48h组t-ERK1/2增高;1h/d-24h组与1.5h/d-24h和-48hERK2mRNA表达增高。结论:耐力运动可能通过增加ERK1/2活性,提高大鼠骨骼肌对胰岛素的敏感性。     

Akt signaling in skeletal muscle: regulation by exercise and passive stretch

Akt/protein kinase B is a serine/threonine kinase that has emerged as a critical signaling component for mediating numerous cellular responses. Contractile activity has recently been demonstrated to stimulate Akt signaling in skeletal muscle. Whether physiological exercise in vivo activates Akt is controversial, and the initiating factors that result in the stimulation of Akt during contractile activity are unknown. In the current study, we demonstrate that treadmill running exercise of rats using two different protocols (intermediate high or high-intensity exhaustive exercise) significantly increases Akt activity and phosphorylation in skeletal muscle composed of various fiber types. To determine if Akt activation during contractile activity is triggered by mechanical forces applied to the skeletal muscle, isolated skeletal muscles were incubated and passively stretched. Passive stretch for 10 min significantly increased Akt activity (2-fold) in the fast-twitch extensor digitorum longus (EDL) muscle. However, stretch had no effect on Akt in the slow-twitch soleus muscle, although there was a robust phosphorylation of the stress-activated protein kinase p38. Similar to contraction, stretch-induced Akt activation in the EDL was fully inhibited in the presence of the phosphatidylinositol 3-kinase inhibitor wortmannin, whereas glycogen synthase kinase-3 (GSK3) phosphorylation was only partially inhibited. Stretch did not cause dephosphorylation of glycogen synthase on GSK3-targeted sites in the absence or presence of wortmannin. We conclude that physiological exercise in vivo activates Akt in multiple skeletal muscle fiber types and that mechanical tension may be a part of the mechanism by which contraction activates Akt in fast-twitch muscles.     

Glucose formation in human skeletal muscle. Influence of glycogen content.

Eight men exercised at 66% of their maximal isometric force to fatigue after prior decrease in the glycogen store in one leg (low-glycogen, LG). The exercise was repeated with the contralateral leg (control) at the same relative intensity and for the same duration. Muscle (quadriceps femoris) glycogen content decreased in the LG leg from 199 +/- 17 (mean +/- S.E.M.) to 163 +/- 16 mmol of glucosyl units/kg dry wt. (P less than 0.05), and in the control leg from 311 +/- 23 to 270 +/- 18 mmol/kg (P less than 0.05). The decrease in glycogen corresponded to a similar accumulation of glycolytic intermediates. Muscle glucose increased in the LG leg during the contraction, from 1.8 +/- 0.1 to 4.3 +/- 0.6 mmol/kg dry wt. (P less than 0.01), whereas no significant increase occurred in the control leg (P greater than 0.05). It is concluded that during exercise glucose is formed from glycogen through the debranching enzyme when muscle glycogen is decreased to values below about 200 mmol/kg dry wt.     

有氧运动对2型糖尿病大鼠骨骼肌ERK1/2活性的影响

目的：观察有氧运动对2型糖尿病大鼠骨骼肌细胞外信号调节激酶（ERK1/2）活性的影响,探讨有氧运动对2型糖尿病的预防和调控机制。方法：将75只SD大鼠随机分为正常对照组（CON）、糖尿病对照1组（DC1）、糖尿病运动1组（DE1）、糖尿病对照2组（DC2）、糖尿病运动2组（DE2）5组（n=15）。正常对照组用普通饲料喂养,糖尿病组用高脂高糖配方饲料喂养。经过8周高脂高糖喂养后,糖尿病2组大鼠腹腔内注射链脲佐菌素（STZ）,诱发2型糖尿病;糖尿病运动1组游泳的最后1周初和糖尿病对照1组同时注射STZ,注射剂量为35 mg/kg,3 d后尾部取血测血糖≥ 16.7 mmol/L为造模成功。运动干预8周后,测定大鼠血清胰岛素、骨骼肌中ERK1/2蛋白表达等指标。结果：①与正常对照组比较,糖尿病各对照组血液中总胆固醇（TC）、甘油三酯（TG）、低密度脂蛋白（LDL-C）、游离脂肪酸（FFA）显著升高（P<0.05,P<0.01）,空腹血糖（FBG）、胰岛素（FIN）含量和胰岛素抵抗指数（HOMA-IR）显著升高（P<0.01）,ERK1/2磷酸化的蛋白表达显著下降（P<0.05）,糖尿病对照2组ERK1/2蛋白含量显著下降（P<0.05）;②8周游泳运动后,与糖尿病对照组比较,糖尿病运动组血液中TC、TG、FFA、LDL-C显著下降（P<0.05）,FBG、FIN、HOMA-IR显著下降（P<0.05,P<0.01）,ERK1/2磷酸化蛋白表达显著升高（P<0.05）。结论：长时间有氧运动,增加了骨骼肌ERK1/2磷酸化水平,改善了2型糖尿病大鼠胰岛素抵抗的状况,降低血糖。这可能是改善糖代谢紊乱,提高胰岛素敏感性的机制之一。     

Influence of skeletal muscle glycogen on passive rewarming after hypothermia

To examine the influence of muscle glycogen on the thermal responses to passive rewarming subsequent to mild hypothermia, eight subjects completed two cold-water immersions (18 degrees C), followed by 75 min of passive rewarming (24 degrees C air, resting in blanket). The experiments followed several days of different exercise-diet regimens eliciting either low (LMG; 141.0 +/- 10.5 mmol.kg.dry wt-1) or normal (NMG; 526.2 +/- 44.2 mmol.kg.dry wt-1) prewarming muscle glycogen levels. Cold-water immersion was performed for 180 min or to a rectal temperature (Tre) of 35.5 degrees C. In four subjects (group A, body fat = 20 +/- 1%), postimmersion Tre was similar to preimmersion Tre for both trials (36.73 +/- 0.18 vs. 37.26 +/- 0.18 degrees C, respectively). Passive rewarming in group A resulted in an increase in Tre of only 0.13 +/- 0.08 degrees C. Conversely, initial rewarming Tre for the other four subjects (group B, body fat = 12 +/- 1%) averaged 35.50 +/- 0.05 degrees C for both trials. Rewarming increased Tre similarly in group B during both LMG (0.76 +/- 0.25 degrees C) and NMG (0.89 +/- 0.13 degrees C). Afterdrop responses, evident only in those individuals whose body core cooled during immersion (group B), were not different between LMG and NMG. These data support the contention that Tre responses during passive rewarming are related to body insulation. Furthermore these results indicate that low muscle glycogen levels do not impair rewarming time nor alter after-drop responses during passive rewarming after mild-to-moderate hypothermia.     

Palmitic acid acutely stimulates glucose uptake via activation of Akt and ERK1/2 in skeletal muscle cells

Chronic exposure to saturated fatty acids can cause insulin resistance. However, the acute effects of fatty acids are not clear and need to be elucidated because plasma fatty acid concentrations fluctuate postprandially. Here, we present the acute effects of palmitate (PA) on skeletal muscle cells and their underlying molecular mechanisms. Immuno-fluorescence results showed that PA rapidly induced GLUT4 translocation and stimulated glucose uptake in rat skeletal muscle cell line L6. Phosphorylation of AMP-activated protein kinase (AMPK), Akt, and extracellular signal-related kinase1/2 (ERK1/2) was enhanced by PA in a time-dependent manner. Cell surface-bound PA was sufficient to stimulate Akt phosphorylation. The inhibitors of PI3 kinase (PI3K), AMPK, Akt, and ERK1/2 could decrease PA-induced glucose uptake, and PI3K inhibitor decreased AMPK, Akt, and ERK1/2 phosphorylation. Weakening AMPK activity reduced phosphorylation of Akt but not ERK1/2, and Akt inhibitor could not affect ERK1/2 activation either. Meanwhile, ERK1/2 inhibitors had no effect on Akt phosphorylation. Taken together, our data suggest that PA-mediated glucose uptake in skeletal muscle cells may be stimulated by the binding of PA to cell surface and followed by PI3K/AMPK/Akt and PI3K/ERK1/2 pathways independently.     

Role of submaximal exercise in promoting creatine and glycogen accumulation in human skeletal muscle.

We examined the effect of glycogen-depleting exercise on subsequent muscle total creatine (TCr) accumulation and glycogen resynthesis during postexercise periods when the diet was supplemented with carbohydrate (CHO) or creatine (Cr) + CHO. Fourteen subjects performed one-legged cycling exercise to exhaustion. Muscle biopsies were taken from the exhausted (Ex) and nonexhausted (Nex) limbs after exercise and after 6 h and 5 days of recovery, during which CHO (CHO group, n = 7) or Cr + CHO (Cr+CHO group, n = 7) supplements were ingested. Muscle TCr concentration ([TCr]) was unchanged in both groups 6 h after supplementation commenced but had increased in the Ex (P < 0.001) and Nex limbs (P < 0.05) of the Cr+CHO group after 5 days. Greater TCr accumulation was achieved in the Ex limbs (P < 0.01) of this group. Glycogen was increased above nonexercised concentrations in the Ex limbs of both groups after 5 days, with the concentration being greater in the Cr+CHO group (P = 0.06). Thus a single bout of exercise enhanced muscle Cr accumulation, and this effect was restricted to the exercised muscle. However, exercise also diminished CHO-mediated insulin release, which may have attenuated insulin-mediated muscle Cr accumulation. Ingesting Cr with CHO also augmented glycogen supercompensation in the exercised muscle.     

Effects of resistance exercise with and without creatine supplementation on gene expression and cell signaling in human skeletal muscle.

To test the hypothesis that creatine supplementation would enhance the anabolic responses of muscle cell signaling and gene expression to exercise, we studied nine subjects who received either creatine or a placebo (maltodextrin) for 5 days in a double-blind fashion before undergoing muscle biopsies: at rest, immediately after exercise (10 x 10 repetitions of one-leg extension at 80% 1 repetition maximum), and 24 and 72 h later (all in the morning after fasting overnight). Creatine supplementation decreased the phosphorylation state of protein kinase B (PKB) on Thr308 at rest by 60% (P < 0.05) and that of eukaryotic initiation factor 4E-binding protein on Thr37/46 (4E-BP1) by 30% 24 h postexercise (P < 0.05). Creatine increased mRNA for collagen 1 (alpha(1)), glucose transporter-4 (GLUT-4), and myosin heavy chain I at rest by 250%, 45%, and 80%, respectively, and myosin heavy chain IIA (MHCIIA) mRNA immediately after exercise by 70% (all P < 0.05). Immediately after exercise, and independent of creatine, mRNA for muscle atrophy F-box (MAFbx), MHCIIA, peroxisome proliferator-activated receptor gamma coactivator-1alpha, and interleukin-6 were upregulated (60-350%; P < 0.05); the phosphorylation state of p38 both in the sarcoplasm and nucleus were increased (12- and 25-fold, respectively; both P < 0.05). Concurrently, the phosphorylation states of PKB (Thr308) and 4E-BP1 (Thr37/46) were decreased by 50% and 75%, respectively (P < 0.05). Twenty-four hours postexercise, MAFbx, myostatin, and GLUT-4 mRNA expression decreased below preexercise values (-35 to -50%; P < 0.05); calpain 1 mRNA increased 70% 72 h postexercise (P < 0.05) and at no other time. In conclusion, 5 days of creatine supplementation do not enhance anabolic signaling but increase the expression of certain targeted genes.     

Anabolic signaling and protein synthesis in human skeletal muscle after dynamic shortening or lengthening exercise

We hypothesized a differential activation of the anabolic signaling proteins protein kinase B (PKB) and p70 S6 kinase (p70(S6K)) and subsequent differential stimulation of human muscle protein synthesis (MPS) after dynamic shortening or lengthening exercise. Eight healthy men [25 +/- 5 yr, BMI 26 +/- 3 kg/m(-2) (means +/- SD)] were studied before and after 12 min of repeated stepping up to knee height, and down again, while carrying 25% of their body weight, i.e., shortening exercise with the "up" leg and lengthening exercise with contralateral "down" leg. Quadriceps biopsies were taken before and 3, 6, and 24 h after exercise. After exercise, over 2 h before the biopsies, the subjects ingested 500 ml of water containing 45 g of essential amino acids and 135 g of sucrose. Rates of muscle protein synthesis were determined via incorporation over time of [1-(13)C]leucine (相似文献   

Insulin and exercise differentially regulate PI3-kinase and glycogen synthase in human skeletal muscle.

The purpose of this study was to determine the separate and combined effects of exercise and insulin on the activation of phosphatidylinositol 3-kinase (PI3-kinase) and glycogen synthase in human skeletal muscle in vivo. Seven healthy men performed three trials in random order. The trials included 1) ingestion of 2 g/kg body wt carbohydrate in a 10% solution (CHO); 2) 75 min of semirecumbent cycling exercise at 75% of peak O(2) consumption; followed by 5 x 1-min maximal sprints (Ex); and 3) Ex, immediately followed by ingestion of the carbohydrate solution (ExCHO). Plasma glucose and insulin were increased (P < 0.05) at 15 and 30 (Post-15 and Post-30) min after the trial during CHO and ExCHO, although insulin was lower for ExCHO. Hyperinsulinemia during recovery in CHO and ExCHO led to an increase (P < 0.001) in PI3-kinase activity at Post-30 compared with basal, although the increase was lower (P < 0. 004) for ExCHO. Furthermore, PI3-kinase activity was suppressed (P < 0.02) immediately after exercise (Post-0) during Ex and ExCHO. Area under the insulin response curve for all trials was positively associated with PI3-kinase activity (r = 0.66, P < 0.001). Glycogen synthase activity did not increase during CHO but was increased (P < 0.05) at Post-0 and Post-30 during Ex and ExCHO. Ingestion of the drink increased (P < 0.05) carbohydrate oxidation during CHO and ExCHO, although the increase after ExCHO was lower (P < 0.05) than CHO. Carbohydrate oxidation was directly correlated with PI3-kinase activity for all trials (r = 0.63, P < 0.001). In conclusion, under resting conditions, ingestion of a carbohydrate solution led to activation of the PI3-kinase pathway and oxidation of the carbohydrate. However, when carbohydrate was ingested after intense exercise, the PI3-kinase response was attenuated and glycogen synthase activity was augmented, thus facilitating nonoxidative metabolism or storage of the carbohydrate. Activation of glycogen synthase was independent of PI3-kinase.     

siRNA-based gene silencing reveals specialized roles of IRS-1/Akt2 and IRS-2/Akt1 in glucose and lipid metabolism in human skeletal muscle

Type 2 diabetes is associated with defects in insulin signaling and the resulting abnormal glucose and lipid metabolism. The complexity of insulin signaling cascades is highlighted by the existence of multiple isoforms of target proteins implicated in metabolic and gene-regulatory events. We utilized siRNA to decipher the specific role of predominant insulin receptor substrates and Akt isoforms expressed in human skeletal muscle. Gene silencing revealed specialized roles of insulin signaling cascades to metabolic endpoints. IRS-1 and Akt2 were required for myoblast differentiation and glucose metabolism, whereas IRS-2 and Akt1 were dispensable. A key role of IRS-2 and Akt1 in lipid metabolism was revealed, highlighting reciprocal relationships between metabolic pathways. Unraveling the isoform-specific regulation of glucose and lipid metabolism by key elements along insulin signaling cascades through siRNA-mediated gene silencing in human tissues will facilitate the discovery of novel targets for the treatment of diabetes and related metabolic disorders.     

Regulation of glycogen phosphorylase and PDH during exercise in human skeletal muscle during hypoxia

The present study examined the acute effects of hypoxia on the regulation of skeletal muscle metabolism at rest and during 15 min of submaximal exercise. Subjects exercised on two occasions for 15 min at 55% of their normoxic maximal oxygen uptake while breathing 11% O(2) (hypoxia) or room air (normoxia). Muscle biopsies were taken at rest and after 1 and 15 min of exercise. At rest, no effects on muscle metabolism were observed in response to hypoxia. In the 1st min of exercise, glycogenolysis was significantly greater in hypoxia compared with normoxia. This small difference in glycogenolysis was associated with a tendency toward a greater concentration of substrate, free P(i), in hypoxia compared with normoxia. Pyruvate dehydrogenase activity (PDH(a)) was lower in hypoxia at 1 min compared with normoxia, resulting in a reduced rate of pyruvate oxidation and a greater lactate accumulation. During the last 14 min of exercise, glycogenolysis was greater in hypoxia despite a lower mole fraction of phosphorylase a. The greater glycogenolytic rate was maintained posttransformationally through significantly higher free [AMP] and [P(i)]. At the end of exercise, PDH(a) was greater in hypoxia compared with normoxia, contributing to a greater rate of pyruvate oxidation. Because of the higher glycogenolytic rate in hypoxia, the rate of pyruvate production continued to exceed the rate of pyruvate oxidation, resulting in significant lactate accumulation in hypoxia compared with no further lactate accumulation in normoxia. Hence, the elevated lactate production associated with hypoxia at the same absolute workload could in part be explained by the effects of hypoxia on the activities of the rate-limiting enzymes, phosphorylase and PDH, which regulate the rates of pyruvate production and pyruvate oxidation, respectively.