More than a store: regulatory roles for glycogen in skeletal muscle adaptation to exercise

The glycogen content of muscle determines not only our capacity for exercise but also the signaling events that occur in response to exercise. The result of the shift in signaling is that frequent training in a low-glycogen state results in improved fat oxidation during steady-state submaximal exercise. This review will discuss how the amount or localization of glycogen particles can directly or indirectly result in this differential response to training. The key direct effect discussed is carbohydrate binding, whereas the indirect effects include the metabolic shift toward fat oxidation, the increase in catecholamines, and osmotic stress. Although our understanding of the role of glycogen in response to training has expanded exponentially over the past 5 years, there are still many questions remaining as to how stored carbohydrate affects the muscular adaptation to exercise.     

Glucose transport rate and glycogen synthase activity both limit skeletal muscle glycogen accumulation

We varied rates of glucose transport and glycogen synthase I (GS-I) activity (%GS-I) in isolated rat epitrochlearis muscle to examine the role of each process in determining the rate of glycogen accumulation. %GS-I was maintained at or above the fasting basal range during 3 h of incubation with 36 mM glucose and 60 microU/ml insulin. Lithium (2 mM LiCl) added to insulin increased glucose transport rate and muscle glycogen content compared with insulin alone. The glycogen synthase kinase-3beta inhibitor GF-109203 x (GF; 10 microM) maintained %GS-I about twofold higher than insulin with or without lithium but did not increase glycogen accumulation. When %GS-I was lowered below the fasting range by prolonged incubation with 36 mM glucose and 2 mU/ml insulin, raising rates of glucose transport with bpV(phen) or of %GS-I with GF produced additive increases in glycogen concentration. Phosphorylase activity was unaffected by GF or bpV(phen). In muscles of fed animals, %GS-I was approximately 30% lower than in those of fasted rats, and insulin-stimulated glycogen accumulation did not occur unless %GS-I was raised with GF. We conclude that the rate of glucose transport is rate limiting for glycogen accumulation unless %GS-I is below the fasting range, in which case both glucose transport rate and GS activity can limit glycogen accumulation.     

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.     

A new glycogen synthase activity ratio in skeletal muscle: effects of exercise and insulin

It was recently reported that MnSO4 stimulates glycogen synthase-dependent glucose transfer from UDPglucose into trichloroacetic acid precipitable endogenous glycoproteins (GSMn(T)) in human muscle extracts. To determine the physiologic significance of this reaction, we compared a new GS activity ratio, GSMn(T)/GSH(E) (where GSH(E) represents the usual glucose transfer to ethanol precipitable exogenous glycogen by GS at 7.2 mM glucose 6-phosphate), with the generally used GSL(E)/GSH(E) ratio (where GSL(E) represents glucose transfer at 0.17 mM glucose 6-P concentration). Biopsies were obtained from the quadriceps femoris muscle of healthy subjects at rest, after 40 min of bicycle exercise at approximately 65% of maximal oxygen uptake and after isometric contraction at 2/3 maximal force to fatigue (approximately 1 min). GSMn(T)/GSH(E) increased from 0.012+/-0.002 at rest to 0.054+/-0.008 (P<0.01) after 40 min of bicycle exercise and the increase in GSMn(T) activity was strongly related to the decrease in endogenous glycogen (i.e.. increase in short-chain endogenous glycoproteins) (r=0.90; P<0.05). On the other hand, GSL(E)/GSH(E) did not change significantly after bicycle exercise (rest = 0.49+/-0.04; exercise = 0.58+/-0.08, P>0.05). GSMn(T)/GSH(E) increased from 0.010+/-0.001 at rest to 0.016+/-0.002 (P<0.05) after isometric exercise, whereas GSL(E)/GSH(E) decreased from 0.27+/-0.04 to 0.20+/-0.02 (P<0.05) under corresponding conditions. Last, insulin, which stimulates glycogen synthesis, also increased GSMn(T)/GSH(E) (1.8-fold, P<0.05), as well as GSL(E)/GSH(E) (1.4-fold, P<0.05), in isolated rat soleus muscle. These data indicate that GSMn(T)/GSH(E) is influenced by endogenous substrate availability and covalent modification. Therefore, GSMn(T)/GSH(E) ratio may prove to be a useful alternative to other GS activity ratios that only reflect changes in the phosphorylation state of GS.     

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.     

有氧运动对衰老大鼠骨骼肌线粒体能量代谢的影响

目的：探讨有氧运动对衰老大鼠骨骼肌线粒体能量代谢的影响。方法：将20只12月龄的雌性Wistar大鼠随机分为老年安静组（AC,n=10）及老年运动组（AE,n=10）,另取10只2月龄的雌性Wistar大鼠为青年安静组（YC,n=10）;安静组大鼠进行正常饲养,运动组大鼠进行坡度为5°,速度为15.2 m/min,第1天运动15 min、第2天运动30 min、从第3天开始每天运动45 min,每周6 d,共12周。12周后所有大鼠断头处死,取腓肠肌样本,差速离心法提取线粒体,测定SOD和GSH-Px活性、MDA含量、三羧酸循环限速酶（CS、ICD和α-KGDHC）活性及呼吸链酶复合体（RCCⅠ~Ⅳ）活性。结果：①与YC组相比,AC组骨骼肌线粒体SOD活性和MDA含量显著增加（P<0.05）,CS和α-KGDHC活性均显著降低（P<0.05）,RCCⅠ、RCCⅡ和RCCⅣ活性均显著下降（P<0.05）,RCCⅢ活性显著升高（P<0.05）;AE组骨骼肌线粒体SOD、GSH-Px活性和MDA含量均显著增加（P<0.01）,CS、ICD和α-KGDHC活性均显著升高（P<0.01）,RCCⅠ~Ⅳ活性均显著升高（P<0.01）。②与AC组相比,AE组骨骼肌线粒体SOD、GSH-Px活性均显著升高（P<0.05）,MDA含量显著下降（P<0.05）,CS、ICD、α-KGDHC和RCCⅠ~Ⅳ活性均显著升高（P<0.01）。结论：有氧运动可以提高老年大鼠骨骼肌线粒体抗氧化能力,降低脂质过氧化水平,提高三羧酸循环及呼吸链功能,促进线粒体能量代谢,延缓衰老过程中线粒体的退行性变化。     

Role of fatty acids in the transition from anaerobic to aerobic metabolism in skeletal muscle during exercise

In moderate physical exercise, the transition from predominantly anaerobic towards predominantly aerobic metabolism is a key step to improve performance. Increase in the supply of oxygen and nutrients, such as free fatty acids (FFA) and glucose, which accompanies high blood flow, is required for this transition. The mechanisms involved in the vasodilation in skeletal muscle during physical activity are not completely known yet. In this article, we postulate a role of FFA and heat production in this process. The presence of uncoupling protein-2 and -3 (UCP-2 and -3) in skeletal muscle, whose activity is dependent on FFA, suggests that these metabolites can act as mitochondrial uncouplers in this tissue. Evidence indicates however that UCPs act as uncouplers only when coenzyme Q is predominantly in the reduced state (i.e. under nonphosphorylation conditions or state 4 respiration) as is observed in resting muscles and in the beginning of physical activity (predominantly anaerobic metabolism). The increase in the lipolytic activity in adipose tissue in the beginning of physical activity results in elevated plasma FFA levels. The FFA can then act on the UCPs, increasing the local heat production. We propose that this calorigenic effect of FFA is important to activate nitric oxide synthase, resulting in nitric oxide production and consequent vasodilation. Therefore, FFA would be important mediators for the changes that occur in muscle metabolism during prolonged physical activity, ensuring the appropriate supply of oxygen and nutrients by increasing blood flow at the beginning of exercise in the contracting skeletal muscles.     

Effect of induced metabolic alkalosis on human skeletal muscle metabolism during exercise

The purpose of the study was to examine the roles of active pyruvate dehydrogenase (PDH(a)), glycogen phosphorylase (Phos), and their regulators in lactate (Lac(-)) metabolism during incremental exercise after ingestion of 0.3 g/kg of either NaHCO(3) [metabolic alkalosis (ALK)] or CaCO(3) [control (CON)]. Subjects (n = 8) were studied at rest, rest postingestion, and during constant rate cycling at three stages (15 min each): 30, 60, 75% of maximal O(2) uptake (VO(2 max)). Radial artery and femoral venous blood samples, leg blood flow, and biopsies of the vastus lateralis were obtained during each power output. ALK resulted in significantly (P < 0.05) higher intramuscular Lac(-) concentration ([Lac(-)]; ALK 72.8 vs. CON 65.2 mmol/kg dry wt), arterial whole blood [Lac(-)] (ALK 8.7 vs. CON 7.0 mmol/l), and leg Lac(-) efflux (ALK 10.0 vs. CON 4.2 mmol/min) at 75% VO(2 max). The increased intramuscular [Lac(-)] resulted from increased pyruvate production due to stimulation of glycogenolysis at the level of Phos a and phosphofructokinase due to allosteric regulation mediated by increased free ADP (ADP(f)), free AMP (AMP(f)), and free P(i) concentrations. PDH(a) increased with ALK at 60% VO(2 max) but was similar to CON at 75% VO(2 max). The increased PDH(a) may have resulted from alterations in the acetyl-CoA, ADP(f), pyruvate, NADH, and H(+) concentrations leading to a lower relative activity of PDH kinase, whereas the similar values at 75% VO(2 max) may have reflected maximal activation. The results demonstrate that imposed metabolic alkalosis in skeletal muscle results in acceleration of glycogenolysis at the level of Phos relative to maximal PDH activation, resulting in a mismatch between the rates of pyruvate production and oxidation resulting in an increase in Lac(-) production.     

Ammonia and amino acid metabolism in human skeletal muscle during exercise.

This review focuses on the ammonia and amino acid metabolic responses of active human skeletal muscle, with a particular emphasis on steady-state exercise. Ammonia production in skeletal muscle involves the purine nucleotide cycle and the amino acids glutamate, glutamine, and alanine and probably also includes the branched chain amino acids as well as aspartate. Ammonia production is greatest during prolonged, steady state exercise that requires 60-80% VO2max and is associated with glutamine and alanine metabolism. Under these circumstances it is unresolved whether the purine nucleotide cycle (AMP deamination) is active; if so, it must be cycling with no IMP accumulation. It is proposed that under these circumstances the ammonia is produced from slow twitch fibers by the deamination of the branched chain amino acids. The ammonia response can be suppressed by increasing the carbohydrate availability and this may be mediated by altering the availability of the branched chain amino acids. The fate of the ammonia released into the circulation is unresolved, but there is indirect evidence that a considerable portion may be excreted by the lung in expired air.     

Purification of lipoamide dehydrogenase from Ascaris muscle mitochondria and its relationship to NADH:NAD+ transhydrogenase activity

Lipoamide dehydrogenase (NADH:lipoamide oxidoreductase EC 1.6.4.3) has been isolated from Ascaris suum muscle mitochondria. This activity has been purified to apparent homogeneity from both the pyruvate dehydrogenase complex and from 150,000g mitochondrial supernatants which were devoid of pyruvate dehydrogenase complex activity. The enzymes from both sources exhibited similar kinetic, catalytic, and regulatory properties and appear to be identical as judged by polyacrylamide gel electrophoresis. The native enzyme acts as a dimer, containing 2 mol of FAD, and has a subunit molecular weight of 54,000, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel chromatography. The enzyme also possesses substantial NADH:NAD⁺ transhydrogenase activity. Heat denaturation and differential solubilization experiments imply that the transhydrogenase activity previously reported is, in fact, associated with the lipoamide dehydrogenase moiety of the Ascaris pyruvate dehydrogenase complex. Whether or not this activity functions physiologically in hydride ion translocation, as previously suggested, remains to be demonstrated.     

Altitude acclimatization and energy metabolic adaptations in skeletal muscle during exercise.

To determine whether the working muscle is able to sustain ATP homeostasis during a hypoxic insult and the mechanisms associated with energy metabolic adaptations during the acclimatization process, seven male subjects [23 +/- 2 (SE) yr, 72.2 +/- 1.6 kg] were given a prolonged exercise challenge (45 min) at sea level (SL), within 4 h after ascent to an altitude of 4,300 m (acute hypoxia, AH), and after 3 wk of sustained residence at 4,300 m (chronic hypoxia, CH). The prolonged cycle test conducted at the same absolute intensity and representing 51 +/- 1% of SL maximal aerobic power (VO2 max) and between 64 +/- 2 (AH) and 66 +/- 1% (CH) at altitude was performed without a reduction in ATP concentration in the working vastus lateralis regardless of condition. Compared with rest, exercise performed during AH resulted in a greater increase (P < 0.05) in muscle lactate concentration (5.11 +/- 0.68 to 22.3 +/- 6.1 mmol/kg dry wt) than exercise performed either at SL (5.88 +/- 0.85 to 11.5 +/- 3.1) or CH (5.99 +/- 0.88 to 12.4 +/- 2.1). These differences in lactate concentration have been shown to reflect differences in arterial lactate concentration and glycolysis (Brooks et al. J. Appl. Physiol. 71: 333-341, 1991). The reduction in glycolysis at least between AH and CH appears to be accompanied by a tighter metabolic control. During CH, free ADP was lower and the ATP-to-free ADP ratio was increased (P < 0.05) compared with AH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diffusional constraints on energy metabolism in skeletal muscle

Aerobic metabolic flux depends on the diffusion of high-energy phosphate molecules (e.g., ATP and phosphocreatine) from the mitochondria to cellular ATPases, as well as the diffusion of other molecules (e.g., ADP, Pi) back to the mitochondria. Here, we develop an approach for evaluating the influence of intracellular metabolite diffusion on skeletal muscle aerobic metabolism through the application of the effectiveness factor (η). This parameter provides an intuitive and informative means of quantifying the extent to which diffusion limits metabolic flux. We start with the classical approach assuming an infinite supply of substrate at the fiber boundary, and we expand this model to ultimately include nonlinear boundary and homogeneous reactions. Comparison of the model with experimental data from a wide range of skeletal muscle types reveals that most muscle fibers are not substantially limited by diffusion (η close to unity), but many are on the brink of rather substantial diffusion limitation. This implies that intracellular metabolite diffusion does not dramatically limit aerobic metabolic flux in most fibers, but it likely plays a role in limiting the evolution of muscle fiber design and function.