Implication of xanthine oxidase in muscle oxidative stress in COPD patients

Objective: The aim of this study was to determine the implication of xanthine oxidase (XO) in the exercise-induced muscle oxidative stress and muscle dysfunction of these patients.

Methods: A randomized, crossover and double-blind study was conducted in nine severe COPD patients, who performed a localized quadriceps endurance test after oral treatment with allopurinol, a XO inhibitor or placebo. Redox status was studied in arterial and venous femoral blood before and after the endurance test.

Results: In placebo condition, muscle exercise resulted in a significant increase in AOPP and isoprostanes, with a significant increase in the venoarterial difference (v-a) in isoprostanes after exercise as compared with before (p<0.05). In contrast, allopurinol treatment prevented the elevation in AOPP levels and v-a isoprostanes after exercise. However, no significant improvement in quadriceps muscle endurance was observed, but allopurinol treatment seemed to preserve muscle strength properties.

Conclusion: This study demonstrates that XO is implicated in the exercise-induced muscle oxidative stress of COPD patients. Allopurinol administration seemed to improve only some muscle properties. Therefore other sources of muscle oxidative stress should be implicated in muscle dysfunction observed in these patients.     

Accessory muscle activation during the superimposed burst technique

Quadriceps muscle activation is assessed using the superimposed burst technique. This technique involves percutaneous muscle stimulation superimposed during maximal isometric volitional knee extension. It is unknown whether accessory muscle activation during maximal knee extension influences estimates of quadriceps muscle activation. Our aim was to compare accessory muscle activation while performing the superimposed burst technique using investigator delivered verbal instruction to constrain the system (CS) and a participant preferred (PP) technique. Twenty five healthy, active individuals (13M/12F, age=23.8 ± 3.35, height=72.73 ± 14.51 cm, and weight=175.29 ± 9.59 kg) were recruited for this study. All participants performed superimposed burst testing with (CS) and without (PP) verbal instruction to encourage isolated quadriceps activation during maximal isometric knee extension. The main outcome variables measured were knee extension torque, quadriceps central activation ratio and mean EMG of vastus lateralis, biceps femoris, and lumbar paraspinal muscles. There were significant differences in knee extension torque (CS=2.87 ± 0.93 Nm/kg, PP=3.40 ± 1.12 Nm/kg, p<0.001), superimposed burst torque (CS=3.40 ±0.98 Nm/kg, PP=3.75 ± 1.11 Nm/kg, p=0.002) and quadriceps CAR (CS=84.1 ± 12.0%, PP=90.2 ± 9.9%, p<0.001) between the techniques. There was also a significant difference in lumbar paraspinal EMG (CS=6.40 ± 8.52%, PP=11.86 ± 14.89%, p=0.043) between the techniques however vastus lateralis EMG was not significantly different. Patient instruction via verbal instruction to constrain proximal structures may help patient minimize confounders to knee extension torque generation while maximizing quadriceps activation.     

Organization of metabolic pathways in vastus lateralis of patients with chronic obstructive pulmonary disease

The objective of this study was to determine whether patients with chronic obstructive lung disease (COPD) display differences in organization of the metabolic pathways and segments involved in energy supply compared with healthy control subjects. Metabolic pathway potential, based on the measurement of the maximal activity (V(max)) of representative enzymes, was assessed in tissue extracted from the vastus lateralis in seven patients with COPD (age 67 +/- 4 yr; FEV(1)/FVC = 44 +/- 3%, where FEV(1) is forced expiratory volume in 1 s and FVC is forced vital capacity; means +/- SE) and nine healthy age-matched controls (age 68 +/- 2 yr; FEV(1)/FVC = 75 +/- 2%). Compared with control, the COPD patients displayed lower (P < 0.05) V(max) (mol.kg protein(-1).h(-1)) for cytochrome c oxidase (COX; 21.2 +/- 2.0 vs. 28.7 +/- 2.2) and 3-hydroxyacyl-CoA dehydrogenase (HADH; 2.54 +/- 0.14 vs. 3.74 +/- 0.12) but not citrate synthase (CS; 2.20 +/- 0.16 vs. 3.19 +/- 0.5). While no differences between groups were observed in V(max) for creatine phosphokinase, phosphorylase (PHOSPH), phosphofructokinase (PFK), pyruvate kinase, and lactate dehydrogenase, hexokinase (HEX) was elevated in COPD (P < 0.05). Enzyme activity ratios were higher (P < 0.05) for HEX/CS, HEX/COX, PHOSPH/HADH and PFK/HADH in COPD compared with control. It is concluded that COPD patients exhibit a reduced potential for both the electron transport system and fat oxidation and an increased potential for glucose phosphorylation while the potential for glycogenolysis and glycolysis remains normal. A comparison of enzyme ratios indicated greater potentials for glucose phosphorylation relative to the citric acid cycle and the electron transport chain and glycogenolysis and glycolysis relative to beta-oxidation.     

Redox balance following magnetic stimulation training in the quadriceps of patients with severe COPD

In severe COPD patients, oxidative stress, which is involved in their peripheral muscle dysfunction, increases in response to exercise. In this study, muscle oxidative stress was explored after quadriceps magnetic stimulation training. A randomized controlled study was conducted on very severe COPD patients, who underwent quadriceps magnetic stimulation training for 8 weeks. A control group was also studied. In both groups, vastus lateralis specimens were obtained before and after the 8-week period. Muscle protein carbonylation and nitration and antioxidant enzymes were determined using immunoblotting and proportions and sizes of type I and II fibres using immunohistochemistry. Compared to controls, magnetic stimulation muscle training did not modify redox balance, whilst inducing a significant increase in type I fibre sizes. In severe COPD patients, it is concluded that quadriceps magnetic stimulation training was a well-tolerated therapeutic intervention, which did not enhance muscle oxidative stress, while increasing the size of slow-twitch fibres.     

Inspiratory muscle work in acute hypoxia influences locomotor muscle fatigue and exercise performance of healthy humans

Our aim was to isolate the independent effects of 1) inspiratory muscle work (W(b)) and 2) arterial hypoxemia during heavy-intensity exercise in acute hypoxia on locomotor muscle fatigue. Eight cyclists exercised to exhaustion in hypoxia [inspired O(2) fraction (Fi(O(2))) = 0.15, arterial hemoglobin saturation (Sa(O(2))) = 81 +/- 1%; 8.6 +/- 0.5 min, 273 +/- 6 W; Hypoxia-control (Ctrl)] and at the same work rate and duration in normoxia (Sa(O(2)) = 95 +/- 1%; Normoxia-Ctrl). These trials were repeated, but with a 35-80% reduction in W(b) achieved via proportional assist ventilation (PAV). Quadriceps twitch force was assessed via magnetic femoral nerve stimulation before and 2 min after exercise. The isolated effects of W(b) in hypoxia on quadriceps fatigue, independent of reductions in Sa(O(2)), were revealed by comparing Hypoxia-Ctrl and Hypoxia-PAV at equal levels of Sa(O(2)) (P = 0.10). Immediately after hypoxic exercise potentiated twitch force of the quadriceps (Q(tw,pot)) decreased by 30 +/- 3% below preexercise baseline, and this reduction was attenuated by about one-third after PAV exercise (21 +/- 4%; P = 0.0007). This effect of W(b) on quadriceps fatigue occurred at exercise work rates during which, in normoxia, reducing W(b) had no significant effect on fatigue. The isolated effects of reduced Sa(O(2)) on quadriceps fatigue, independent of changes in W(b), were revealed by comparing Hypoxia-PAV and Normoxia-PAV at equal levels of W(b). Q(tw,pot) decreased by 15 +/- 2% below preexercise baseline after Normoxia-PAV, and this reduction was exacerbated by about one-third after Hypoxia-PAV (-22 +/- 3%; P = 0.034). We conclude that both arterial hypoxemia and W(b) contribute significantly to the rate of development of locomotor muscle fatigue during exercise in acute hypoxia; this occurs at work rates during which, in normoxia, W(b) has no effect on peripheral fatigue.     

Effects of arterial oxygen content on peripheral locomotor muscle fatigue.

The effect of arterial O2 content (Ca(O2)) on quadriceps fatigue was assessed in healthy, trained male athletes. On separate days, eight participants completed three constant-workload trials on a bicycle ergometer at fixed workloads (314 +/- 13 W). The first trial was performed while the subjects breathed a hypoxic gas mixture [inspired O2 fraction (Fi(O2)) = 0.15, Hb saturation = 81.6%, Ca(O2) = 18.2 ml O2/dl blood; Hypo] until exhaustion (4.5 +/- 0.4 min). The remaining two trials were randomized and time matched with Hypo. The second and third trials were performed while the subjects breathed a normoxic (Fi(O2) = 0.21, Hb saturation = 95.0%, Ca(O2) = 21.3 ml O2/dl blood; Norm) and a hyperoxic (Fi(O2) = 1.0, Hb saturation = 100%, Ca(O2) = 23.8 ml O2/dl blood; Hyper) gas mixture, respectively. Quadriceps muscle fatigue was assessed via magnetic femoral nerve stimulation (1-100 Hz) before and 2.5 min after exercise. Myoelectrical activity of the vastus lateralis was obtained from surface electrodes throughout exercise. Immediately after exercise, the mean force response across 1-100 Hz decreased from preexercise values (P < 0.01) by -26 +/- 2, -17 +/- 2, and -13 +/- 2% for Hypo, Norm, and Hyper, respectively; each of the decrements differed significantly (P < 0.05). Integrated electromyogram increased significantly throughout exercise (P < 0.01) by 23 +/- 3, 10 +/- 1, and 6 +/- 1% for Hypo, Norm, and Hyper, respectively; each of the increments differed significantly (P < 0.05). Mean power frequency fell more (P < 0.05) during Hypo (-15 +/- 2%); the difference between Norm (-7 +/- 1%) and Hyper (-6 +/- 1%) was not significant (P = 0.32). We conclude that deltaCa(O2) during strenuous systemic exercise at equal workloads and durations affects the rate of locomotor muscle fatigue development.     

Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans

The effect of exercise-induced arterial hypoxemia (EIAH) on quadriceps muscle fatigue was assessed in 11 male endurance-trained subjects [peak O2 uptake (VO2 peak) = 56.4 +/- 2.8 ml x kg(-1) x min(-1); mean +/- SE]. Subjects exercised on a cycle ergometer at >or=90% VO2 peak) to exhaustion (13.2 +/- 0.8 min), during which time arterial O2 saturation (Sa(O2)) fell from 97.7 +/- 0.1% at rest to 91.9 +/- 0.9% (range 84-94%) at end exercise, primarily because of changes in blood pH (7.183 +/- 0.017) and body temperature (38.9 +/- 0.2 degrees C). On a separate occasion, subjects repeated the exercise, for the same duration and at the same power output as before, but breathed gas mixtures [inspired O2 fraction (Fi(O2)) = 0.25-0.31] that prevented EIAH (Sa(O2) = 97-99%). Quadriceps muscle fatigue was assessed via supramaximal paired magnetic stimuli of the femoral nerve (1-100 Hz). Immediately after exercise at Fi(O2) 0.21, the mean force response across 1-100 Hz decreased 33 +/- 5% compared with only 15 +/- 5% when EIAH was prevented (P < 0.05). In a subgroup of four less fit subjects, who showed minimal EIAH at Fi(O2) 0.21 (Sa(O2) = 95.3 +/- 0.7%), the decrease in evoked force was exacerbated by 35% (P < 0.05) in response to further desaturation induced via Fi(O2) 0.17 (Sa(O2) = 87.8 +/- 0.5%) for the same duration and intensity of exercise. We conclude that the arterial O2 desaturation that occurs in fit subjects during high-intensity exercise in normoxia (-6 +/- 1% DeltaSa(O2) from rest) contributes significantly toward quadriceps muscle fatigue via a peripheral mechanism.     

Quadriceps metabolism during constant workrate cycling exercise in chronic obstructive pulmonary disease

Impaired resting metabolism in peripheral muscles potentially contributes to exercise intolerance in chronic obstructive pulmonary disease (COPD). This study investigated the cytosolic energy metabolism of the quadriceps, from glycogen degradation to lactate accumulation, in exercising patients with COPD, in comparison to healthy controls. We measured, in 12 patients with COPD and 10 control subjects, resting and post-cycling exercise quadriceps levels of 1) energy substrates and end products of glycolysis (glycogen, glucose, pyruvate, and lactate) and intermediate markers of glycolysis (glucose-6-phosphate, glucose-1-phosphate, fructose-6-phosphate) and 2) the activity of key enzymes involved in the regulation of glycolysis (phosphofructokinase, lactate dehydrogenase). Exercise intensity (P < 0.01), duration (P = 0.049), and total work (P < 0.01) were reduced in patients with COPD. The variations in energy substrates and end products of glycolysis after cycling exercise were of similar magnitude in patients with COPD and controls. Glucose-6-phosphate (P = 0.036) and fructose-6-phosphate (P = 0.042) were significantly elevated in patients with COPD after exercise. Phosphofructokinase (P < 0.01) and lactate dehydrogenase (P = 0.02) activities were greater in COPD. Muscle glycogen utilization (P = 0.022) and lactate accumulation (P = 0.025) per unit of work were greater in COPD. We conclude that cycling exercise induced changes in quadriceps metabolism in patients with COPD that were of similar magnitude to those of healthy controls. These intramuscular events required a much lower exercise work load and time to occur in COPD. Our data suggest a greater reliance on glycolysis during exercise in COPD, which may contribute to exercise intolerance in COPD.     

Local exercise and oxidative stress in chronic obstructive broncho-pneumopathies: preliminary results

The role of altered peripheral muscle function in exercise intolerance of chronic obstructive pulmonary disease (COPD) is now well established. However, the mechanisms underlying this phenomen, have not been determined. One hypothesis is that the oxidative stress, that leads to tissue injury may be involved. A recent study has shown that general exercise caused systemic oxidative stress in COPD patients. However, the origin of this stress was not absolutely clear: airways, muscle, both, or other? The aim of this study was first to determine with a systemic approach, whether systemic oxidative stress occur in patients who perform local exercise and then with a muscular needle biopsy approach, to confirm the muscular origin of this oxidative stress. METHODS: In each approach, 7 COPD patients moderate to severe and 7 age-matched subjects performed an endurance test consisting of dynamic strength of the quadriceps against 40% (systemic approach) or 30% (biopsy approach) of maximal voluntary strength at an imposed regular pace until exhaustion. RESULTS: The results showed in each approach, that endurance test duration was significantly decreased in the COPD patients (p < 0.05). In systemic approach, the results showed that blood vitamin E at rest was significantly decreased in the COPD (p < 0.001), with a significant increase in superoxide anion release by stimulated phagocytes (p < 0.001). Local exercise induced, only in COPD, a significant increase in serum MDA (p < 0.05), which is an index of oxidative stress. In the biopsy approach, the results showed that local exercise induced in COPD an increase in muscular levels of MDA. A significant increase in muscular peroxidase glutathion activity (antioxidant) occurred after exercise only in normal subjects (p < 0.05). In conclusion, this study in COPD, confirms the altered peripheral muscle function, reveals a deficit in blood vitamin E and suggest that local muscular exercise causes a muscular oxidative stress in these patients. Further studies are needed to confirm these results and evaluate the implication of this oxidative stress in the myopathy of COPD.     

耐力训练对大鼠肝脏,股四头肌谷胱甘肽抗氧化系统酶活性的影响

目的和方法：本文通过检测大鼠肝脏、股四头肌中ＧＳＨＰＸ、谷胱甘肽转硫酶（ＧＳＴ）、谷胱甘肽还原酶（ＧＲ）活性及脂质过氧化（ＬＰＯ）产物丙二醛（ＭＤＡ）的含量变化,观察耐力训练对大鼠机体产生内源性自由基及谷胱甘肽抗氧化系统酶活性的影响。结果：ＳＤ雄性大鼠经１１周跑台训练后,安静状态时肝脏中ＭＤＡ含量下降,ＧＳＨＰＸ、ＧＳＨ活性下降,股四头肌中ＧＳＨＰＸ、ＧＳＴ活性升高;９０ｍｉｎ定量负荷运动使大鼠肝脏中ＭＤＡ含量升高,ＧＳＨＰＸ、ＧＳＴ、ＧＲ活性均下降,但训练组ＧＳＨＰＸ、ＧＳＴ活性恢复较快。结论：大鼠经耐力训练后提高了谷胱甘肽抗氧化系统酶的抗氧化功能,表现了良好的运动适应性,且恢复较快。值得注意的是训练组大鼠ＧＲ活性在运动后恢复期存在下降趋势,其机理有待进一步研究。     

Quadriceps and Respiratory Muscle Fatigue Following High-Intensity Cycling in COPD Patients

Exercise intolerance in COPD seems to combine abnormal ventilatory mechanics, impaired O₂ transport and skeletal muscle dysfunction. However their relatie contribution and their influence on symptoms reported by patients remain to be clarified. In order to clarify the complex interaction between ventilatory and neuromuscular exercise limiting factors and symptoms, we evaluated respiratory muscles and quadriceps contractile fatigue, dynamic hyperinflation and symptoms induced by exhaustive high-intensity cycling in COPD patients. Fifteen gold II-III COPD patients (age = 67±6 yr; BMI = 26.6±4.2 kg.m^-2) performed constant-load cycling test at 80% of their peak workload until exhaustion (9.3±2.4 min). Before exercise and at exhaustion, potentiated twitch quadriceps strength (Q_tw), transdiaphragmatic (P_di,tw) and gastric (P_ga,tw) pressures were evoked by femoral nerve, cervical and thoracic magnetic stimulation, respectively. Changes in operational lung volumes during exercise were assessed via repetitive inspiratory capacity (IC) measurements. Dyspnoea and leg discomfort were measured on visual analog scale. At exhaustion, Q_tw (-33±15%, >15% reduction observed in all patients but two) and P_di,tw (-20±15%, >15% reduction in 6 patients) were significantly reduced (P<0.05) but not P_ga,tw (-6±10%, >15% reduction in 3 patients). Percentage reduction in Q_tw correlated with the percentage reduction in P_di,tw (r=0.66; P<0.05). Percentage reductions in P_di,tw and P_ga,tw negatively correlated with the reduction in IC at exhaustion (r=-0.56 and r=-0.62, respectively; P<0.05). Neither dyspnea nor leg discomfort correlated with the amount of muscle fatigue. In conclusion, high-intensity exercise induces quadriceps, diaphragm and less frequently abdominal contractile fatigue in this group of COPD patients. In addition, the rise in end-expiratory lung volume and diaphragm flattening associated with dynamic hyperinflation in COPD might limit the development of abdominal and diaphragm muscle fatigue. This study underlines that both respiratory and quadriceps fatigue should be considered to understand the complex interplay of factors leading to exercise intolerance in COPD patients.     

MAPK signaling in the quadriceps of patients with chronic obstructive pulmonary disease

Muscle atrophy in chronic obstructive pulmonary disease (COPD) is associated with reduced exercise tolerance, muscle strength, and survival. The molecular mechanisms leading to muscle atrophy in COPD remain elusive. The mitogen-activated protein kinases (MAPKs) such as p38 MAPK and ERK 1/2 can increase levels of MAFbx/Atrogin and MuRF1, which are specifically involved in muscle protein degradation and atrophy. Our aim was to investigate the level of activation of p38 MAPK, ERK 1/2, and JNK in the quadriceps of patients with COPD. A biopsy of the quadriceps was obtained in 18 patients with COPD as well as in 9 healthy controls. We evaluated the phosphorylated as well as total protein levels of p38 MAPK, ERK 1/2, and JNK as well as MAFbx/Atrogin and MuRF1 in these muscle samples. The corresponding mRNA expression was also assessed by RT-PCR. Ratios of phosphorylated to total level of p38 MAPK (P = 0.02) and ERK 1/2 (P = 0.01) were significantly elevated in patients with COPD compared with controls. Moreover, protein levels of MAFbx/Atrogin showed a tendency to be greater in patients with COPD (P = 0.08). mRNA expression of p38 MAPK (P = 0.03), ERK 1/2 (P = 0.02), and MAFbx/Atrogin (P = 0.04) were significantly elevated in patients with COPD. In addition, phosphorylated-to-total p38 MAPK ratio (Pearson's r = -0.45; P < 0.05) and phosphorylated-to-total ERK 1/2 ratio (Pearson's r = -0.47; P < 0.05) were negatively associated with the mid-thigh muscle cross-sectional area. These data support the hypothesis that the MAPKs might play a role in the development of muscle atrophy in COPD.     

Development of various types of muscle fibers in the quadriceps femoris and the soleus during human ontogenesis

By means of histological methods for revealing adenosine triphosphatase of myosin (pH 4.6) and succinate dehydrogenase activity, using postmortem material, development of various muscle fibers of the femoral m. quadriceps and m. soleus has been studied in human ontogenesis. The first stage of rearrangements lasts from the 5th-6th month of the uterine development up to 2 years of age and is characterized by formation (from non-differentiated) of oxidative, glycolytic and oxidative-glycolytic fibers. During the period from 2 up to 7-8 years of age the ratio in the types changes slightly, but transversal section size of the muscle fiber increases intensively. Then from 11 up to 17 years of age, together with maximal increment of the fibers transversal section, there is an essential change in the type relation. By the 17th years of age, in the femoral m. quadriceps the part of the fibers with glycolytic type of energy supply increases, while in the m. soleus the oxidative fibers become more numerous. By the 70th years of age in the femoral m. quadriceps relative amount of intermediate fibers increases.     

Weight loss-induced rise in plasma pollutant is associated with reduced skeletal muscle oxidative capacity

In this study, we examined whether weight loss-induced changes in plasma organochlorine compounds (OC) were associated with those in skeletal muscle markers of glycolytic and oxidative metabolism. Vastus lateralis skeletal muscle enzyme activities and plasma OC (Aroclor 1260, polychlorinated biphenyl 153, p,p'-DDE, beta-hexachlorocyclohexane, and hexachlorobenzene) were measured before and after a weight loss program in 17 men and 20 women. Both sexes showed a similar reduction in body weight (approximately 11 kg) in response to treatment, although men lost significantly more fat mass than women (P < 0.05). Enzymatic markers of glycolysis, phosphofructokinase (PFK) activity, and oxidative metabolism, beta-hydroxyacyl-CoA dehydrogenase (HADH), citrate synthase (CS), and cytochrome c oxidase (COX) activities, remained unchanged after weight loss. A significant increase in plasma OC levels was observed in response to weight loss, an effect that was more pronounced in men. No relationship was observed between changes in OC and those in PFK activity in either sex [-0.31 < r < 0.12, not significant (NS)]. However, the greater the increase in plasma OC levels, the greater the reduction in oxidative enzyme (HADH, CS, COX) activities was in response to weight loss in men (-0.75 < r < -0.50, P < 0.05) but not in women (-0.33 < r < 0.33, NS). These results suggest that the weight loss-induced increase in plasma pollutant levels is likely to be associated with reduced skeletal muscle oxidative metabolism in men but not in women.     

Effect of one-legged exercise on the strength, power and endurance of the contralateral leg. A randomized, controlled study using isometric and concentric isokinetic training.

The purpose of this investigation was to study the effect of one-legged exercise on the strength, power and endurance of the contralateral leg. The performance of the knee extensor and flexor muscle of 20 healthy young adults (10 men and 10 women) was first tested by Cybex II+ and 340 dynamometers. Then 10 subjects were chosen at random to train using one leg three times a week for 7 weeks whilst the other 10 served as controls. During the 8th week, the tests were repeated. Both quadriceps and hamstring muscles of the trained subjects showed a cross-transfer effect from the trained limb to the untrained side. This concerned the strength and power, as well as endurance characteristics of these muscles. The average change in peak torque of the quadriceps muscle was +19% (P less than 0.001) in the trained limb, +11% (P less than 0.01) in the untrained limb and 0% in the control limbs. In hamstring muscles the changes were +14% (P less than 0.01), +5% and -1%, respectively. Concerning muscle endurance (work performed during the last 5 contractions in the 25-repetition test) the corresponding changes were +15% (P less than 0.01), +7% (P less than 0.01), and -1% in quadriceps muscle, and +17% (P less than 0.05), +7%, and -3% in hamstring muscles. The average strength benefit in the untrained limb was +36% (hamstring muscles) and +58% (quadriceps muscle) of that achieved in the trained limb. Untrained hamstring muscle showed better benefits in the endurance parameters than in strength or power parameters, while in the quadriceps muscle this effect was reversed. A positive relationship was observed between the changes (greater improvement in the trained limb resulted in greater improvement in the untrained limb) (hamstring muscles: r = 0.83, P less than 0.001, quadriceps muscle: r = 0.53, P less than 0.001). In endurance parameters, this relationship was almost linear while in the strength and power parameters the results were more in favour of a curvilinear relationship with limited benefit.     

Effect of acute severe hypoxia on peripheral fatigue and endurance capacity in healthy humans

We hypothesized that severe hypoxia limits exercise performance via decreased contractility of limb locomotor muscles. Nine male subjects [mean +/- SE maximum O(2) uptake (Vo(2 max)) = 56.5 +/- 2.7 ml x kg(-1) x min(-1)] cycled at > or =90% Vo(2 max) to exhaustion in normoxia [NORM-EXH; inspired O(2) fraction (Fi(O(2))) = 0.21, arterial O(2) saturation (Sp(O(2))) = 93 +/- 1%] and hypoxia (HYPOX-EXH; Fi(O(2)) = 0.13, Sp(O(2)) = 76 +/- 1%). The subjects also exercised in normoxia for a time equal to that achieved in hypoxia (NORM-CTRL; Sp(O(2)) = 96 +/- 1%). Quadriceps twitch force, in response to supramaximal single (nonpotentiated and potentiated 1 Hz) and paired magnetic stimuli of the femoral nerve (10-100 Hz), was assessed pre- and at 2.5, 35, and 70 min postexercise. Hypoxia exacerbated exercise-induced peripheral fatigue, as evidenced by a greater decrease in potentiated twitch force in HYPOX-EXH vs. NORM-CTRL (-39 +/- 4 vs. -24 +/- 3%, P < 0.01). Time to exhaustion was reduced by more than two-thirds in HYPOX-EXH vs. NORM-EXH (4.2 +/- 0.5 vs. 13.4 +/- 0.8 min, P < 0.01); however, peripheral fatigue was not different in HYPOX-EXH vs. NORM-EXH (-34 +/- 4 vs. -39 +/- 4%, P > 0.05). Blood lactate concentration and perceptions of limb discomfort were higher throughout HYPOX-EXH vs. NORM-CTRL but were not different at end-exercise in HYPOX-EXH vs. NORM-EXH. We conclude that severe hypoxia exacerbates peripheral fatigue of limb locomotor muscles and that this effect may contribute, in part, to the early termination of exercise.     

Inositol 1,4-bisphosphate is an allosteric activator of muscle-type 6-phosphofructo-1-kinase.

The allosteric effects of various inositol biphosphate (InsP2) isomers and other inositol phosphates, of glycerophosphoinositol phosphates (GroPInsPx) and of phosphoinositides (PtdInsPx) on muscle-type 6-phosphofructo-1-kinase (PFK) were investigated. The binding of these substances to PFK was indirectly estimated by their ability to stabilize the tetrameric enzyme. At near-physiological concentrations of other allosteric effectors, muscle PFK was activated AMP-dependently by Ins(1,4)P2 (Ka = 43 microM), Ins(2,4)P2 (Ka = 70 microM) and GroPIns4P (Ka = 20 microM). These compounds activated PFK by a mechanism similar to that established for activating hexose bisphosphates. Indirect binding experiments indicated minimal Kd,app. values of about 5 microM for the binding of Ins(1,4)P2 in the presence of 0.1 mM-AMP at pH 7.4. This apparent affinity was comparable with that of fructose 1,6-bisphosphate and glucose 1,6-bisphosphate at identical conditions. The enzyme was also found to interact specifically with PtdIns4P (Kd,app. = 37 microM), the inositol phospholipid carrying Ins(1,4)P2 as its head group. The regulatory behaviour of muscle-type PFK in vitro and the concentrations of Ins(1,4)P2 in vivo (between 4 and greater than 50 nmol/g wet wt. of tissue) are consistent with the hypothesis that there is a functional interaction in vivo. Furthermore, a role of PtdIns4P in membrane compartmentation of PFK is suggested. Comparative experiments with liver PFK indicate that these regulatory properties may be relatively specific for the muscle isoform. Unlike muscle PFK, the liver isoform was slightly activated by sub-micromolar concentrations of Ins(1,4,5)P3.     

Effects of high-intensity training on MCT1, MCT4, and NBC expressions in rat skeletal muscles: influence of chronic metabolic alkalosis

This study investigated the effects of high-intensity training, with or without induced metabolic alkalosis, on lactate transporter (MCT1 and MCT4) and sodium bicarbonate cotransporter (NBC) content in rat skeletal muscles. Male Wistar rats performed high-intensity training on a treadmill 5 times/wk for 5 wk, receiving either sodium bicarbonate (ALK-T) or a placebo (PLA-T) prior to each training session, and were compared with a group of control rats (CON). MCT1, MCT4, and NBC content was measured by Western blotting in soleus and extensor digitorum longus (EDL) skeletal muscles. Citrate synthase (CS) and phosphofructokinase (PFK) activities and muscle buffer capacity (betam) were also evaluated. Following training, CS and PFK activities were significantly higher in the soleus only (P < 0.05), whereas betam was significantly higher in both soleus and EDL (P < 0.05). MCT1 (PLA-T: 30%; ALK-T: 23%) and NBC contents (PLA-T: 85%; ALK-T: 60%) increased significantly only in the soleus following training (P < 0.01). MCT4 content in the soleus was significantly greater in ALK-T (115%) but not PLA-T compared with CON. There was no significant change in protein content in the EDL. Finally, NBC content was related only to MCT1 content in soleus (r = 0.50, P < 0.01). In conclusion, these results suggest that MCT1, MCT4, and NBC undergo fiber-specific adaptive changes in response to high-intensity training and that induced alkalosis has a positive effect on training-induced changes in MCT4 content. The correlation between MCT1 and NBC expression suggests that lactate transport may be facilitated by NBC in oxidative skeletal muscle, which may in turn favor better muscle pH regulation.     

Aging and respiratory muscle metabolic plasticity: effects of endurance training.

We examined the oxidative and antioxidant enzyme activities in respiratory and locomotor muscles in response to endurance training in young and aging rats. Young adult (4-mo-old) and old (24-mo-old) female Fischer 344 rats were divided into four groups: 1) young trained (n = 12), 2) young untrained (n = 12), 3) old trained (n = 10), and 4) old untrained (n = 6). Both young and old endurance-trained animals performed the same training protocol during 10 wk of continuous treadmill exercise (60 min/day, 5 days/wk). Compared with young untrained animals, the young trained group had significantly elevated (P less than 0.05) activities of 3-hydroxyacyl-CoA dehydrogenase (HADH), glutathione peroxidase (GPX), and citrate synthase (CS) in both the costal diaphragm and the plantaris muscle. In contrast, training had no influence (P greater than 0.05) on the activity of lactate dehydrogenase within the costal diaphragm in young animals. In the aging animals, training did not alter (P greater than 0.05) activities of CS, HADH, GPX, or lactate dehydrogenase in the costal diaphragm but significantly (P less than 0.05) increased CS, HADH, and GPX activities in the plantaris muscle. Furthermore, training resulted in higher activities of CS and HADH in the intercostal muscles in the old trained than in the old untrained animals. Finally, activities of CS, HADH, and GPX were significantly (P less than 0.05) lower in the plantaris in the old untrained than in the young untrained animals; however, CS, HADH, and GPX activities were greater (P less than 0.05) in the costal diaphragm in the old sedentary than in the young untrained animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Moderate endurance training prevents doxorubicin-induced in vivo mitochondriopathy and reduces the development of cardiac apoptosis

The objective of this work was to test the hypothesis that endurance training may be protective against in vivo doxorubicin (DOX)-induced cardiomyopathy through mitochondria-mediated mechanisms. Forty adult (6-8 wk old) male Wistar rats were randomly divided into four groups (n = 10/group): nontrained, nontrained + DOX treatment (20 mg/kg), trained (14 wk of endurance treadmill running, 60-90 min/day), and trained + DOX treatment. Mitochondrial respiration, calcium tolerance, oxidative damage, heat shock proteins (HSPs), antioxidant enzyme activity, and apoptosis markers were evaluated. DOX induces mitochondrial respiratory dysfunction, oxidative damage, and histopathological lesions and triggers apoptosis (P < 0.05, n = 10). However, training limited the decrease in state 3 respiration, respiratory control ratio (RCR), uncoupled respiration, aconitase activity, and protein sulfhydryl content caused by DOX treatment and prevented the increased sensitivity to calcium in nontrained + DOX-treated rats (P < 0.05, n = 10). Moreover, training inhibited the DOX-induced increase in mitochondrial protein carbonyl groups, malondialdehyde, Bax, Bax-to-Bcl-2 ratio, and tissue caspase-3 activity (P < 0.05, n = 10). Training also increased by approximately 2-fold the expression of mitochondrial HSP-60 and tissue HSP-70 (P < 0.05, n = 10) and by approximately 1.5-fold the activity of mitochondrial and cytosolic forms of SOD (P < 0.05, n = 10). We conclude that endurance training protects heart mitochondrial respiratory function from the toxic effects of DOX, probably by improving mitochondrial and cell defense systems and reducing cell oxidative stress. In addition, endurance training limited the DOX-triggered apoptosis.