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Published ahead of print on March 2, 2006, doi:10.1164/rccm.200512-1957OC
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American Journal of Respiratory and Critical Care Medicine Vol 173. pp. 1122-1129, (2006)
© 2006 American Thoracic Society
doi: 10.1164/rccm.200512-1957OC

Original Article

Supplemental Oxygen Prevents Exercise-induced Oxidative Stress in Muscle-wasted Patients with Chronic Obstructive Pulmonary Disease

Hanneke A. C. van Helvoort, Yvonne F. Heijdra, Leo M. A. Heunks, Patty L. M. Meijer, Wim Ruitenbeek, Hub M. H. Thijs and P. N. Richard Dekhuijzen

Department of Pulmonary Diseases; Institute for Fundamental and Clinical Human Movement Sciences; and Laboratory of Pediatrics and Neurology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands

Correspondence and requests for reprints should be addressed to P.N. Richard Dekhuijzen, M.D., Ph.D., Radboud University Nijmegen Medical Centre, Department of Pulmonary Diseases (454), P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. E-mail: R.Dekhuijzen{at}long.umcn.nl


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Rationale: Although oxygen therapy is of clear benefit in patients with severe chronic obstructive pulmonary disease (COPD), recent studies have shown that short-term supplementary oxygen may increase oxidative stress and inflammation within the airways.

Objective: We investigated whether systemic inflammation and oxidative stress at rest and during exercise in patients with COPD are influenced by supplemental oxygen.

Methods: Nine normoxemic, muscle-wasted patients with moderate to very severe COPD were studied. Plasma markers of systemic inflammation (leukocyte counts, interleukin 6 [IL-6]) and oxidative stress (lipid peroxidation, protein oxidation, antioxidant capacity) were measured after treatment with either supplemental oxygen (nasal, 4 L · min–1) or compressed air, both at rest (1 h treatment) and after submaximal exercise (40 W, constant work rate). In addition, free-radical production by neutrophils and ATP-degradation products were determined before and after exercise.

Results: Short-term oxygen breathing at rest did not influence systemic low-grade inflammation and oxidative stress. The IL-6 response to exercise was attenuated during cycling with supplemental oxygen. Exercise-induced lipid and protein oxidation were prevented by treatment with supplemental oxygen. This was associated with both decreased free-radical production by neutrophils and reduced formation of (hypo)xanthine and uric acid.

Conclusion: Short-term supplementary oxygen does not affect basal systemic inflammation and oxidative stress but prevents exercise-induced oxidative stress in normoxemic, muscle-wasted patients with COPD, and attenuates plasma IL-6 response. Inhibition of neutrophil activation and ATP degradation appears to be involved in this effect.

Key Words: chronic obstructive pulmonary disease • exercise • oxidative stress • supplemental oxygen • systemic inflammation

Long-term oxygen therapy is one of the few treatments with clear benefits for hypoxemic patients with chronic obstructive pulmonary disease (COPD). It prolongs survival, reduces the frequency of hospitalization and development of pulmonary hypertension, and improves exercise performance and quality of life (1–3). Although benefits are less pronounced, supplemental oxygen has been shown to reduce ventilation, dynamic hyperinflation, and dyspnea during exercise in normoxemic patients with COPD and in those with mild hypoxemia and COPD (4, 5).

Despite the proven benefits of oxygen therapy in COPD, recently published data may shed another light on the effects of this therapy. Philips and coworkers (6) reported that breathing of 28% oxygen at 2.0 L · min–1 via nasal prongs for 30 min while resting resulted in an increase of breath methylated alkane contour in healthy subjects, suggesting increased oxidative stress in exhaled breath. In addition, Carpagnano and coworkers (7) investigated the effects of short-term supplementary oxygen on markers of oxidative stress and inflammation in exhaled breath condensate in both healthy subjects and patients with COPD. Exposure to increased inspiratory oxygen fraction (FIO2, 0.28) for 1 h resulted in enhanced concentrations of interleukin 6 (IL-6) and 8-isoprostane in exhaled breath condensate of patients with COPD and healthy subjects. It is unknown if supplemental oxygen as used in clinical practice alters systemic markers of oxidative stress and inflammation. In healthy subjects, there is some evidence that markers of oxidative stress (i.e., lipid peroxidation and superoxide dismutase activity) in plasma are increased after hyperbaric oxygen therapy (8, 9). Data on the effects of increasing FIO2 on these markers in patients with COPD are not known to the best of our knowledge.

Markers of oxidative stress and inflammation are known to be increased by intense exercise in healthy subjects. Oxidative stress and inflammation are now recognized to play an important role in the pathogenesis of COPD (10, 11), and an increased response to exercise has also been described in patients with COPD (12, 13). The effect of oxygen on exercise-induced systemic oxidative stress and inflammation in patients with COPD is largely unknown. Vina and colleagues (14) showed in five patients with very severe COPD (FEV1, 0.79 ± 0.07 L) with advanced hypoxemia (PaO2, 7.5 ± 0.1 kPa) that exercise-induced (constant work rate, 40 W) blood glutathione oxidation was partially reduced by supplemental oxygen (2–3 L · min–1), indicating that supplemental oxygen decreased exercise-induced oxidative stress. Mechanistically, it is unknown which of the three major sources of intracellular free-radical generation (e.g., neutrophils, mitochondrial electron transport chain, and the xanthine oxidase pathway [12, 15, 16]) are affected by supplemental oxygen.

We investigated the effect of short-term supplemental oxygen at rest and during exercise on markers of systemic inflammation and oxidative stress in normoxemic patients with moderate to very severe COPD. In a crossover design, plasma markers of inflammation (circulating leukocytes, concentration of IL-6) and oxidative stress (lipid peroxidation and oxidation of proteins) were measured in nine patients with COPD during treatment with either compressed air (nasal, 4 L · min–1) or supplemental oxygen (nasal, 4 L · min–1), both at rest (after 1 h treatment) and after a bout of cycle exercise (40 W, constant work rate). To improve insight in the possible mechanisms involved in these systemic effects, both ATP-degradation products (12, 17) and neutrophil activation (15, 16) were measured. In the present study, we selected muscle-wasted patients with COPD to evaluate the systemic effects of supplemental oxygen, because of the increasing evidence for a relation between systemic inflammation, oxidative stress, and muscle wasting in COPD (11, 18, 19), and the importance of any intervention on these features in this specific subgroup of patients with COPD.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Nine (five males) muscle-wasted patients with COPD (fat-free mass index: males < 16 kg · m–2, females < 15 kg · m–2) (20) participated in this study. The patients were recruited from our outpatient clinic and had moderate to very severe COPD according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) classification (21). All had been free of exacerbations for at least 2 mo before the study, and had stopped smoking at least 6 mo before inclusion. Exclusion criteria were the use of oral corticosteroids, long-term oxygen therapy, respiratory insufficiency (PaO2 < 8 kPa, PaCO2 > 6.7 kPa), and other chronic or exercise-limiting diseases. The use of inhaled corticosteroids (n = 4), antioxidants (n = 1), and supplemental vitamins was discontinued 1 wk before exercise testing. All patients were on bronchodilator therapy; none used theophylline. The study was conducted according to the Declaration of Helsinki and was approved by the medical ethical committee of our hospital. Written, informed consent was obtained from all subjects.

Study Design
Pulmonary function, body composition, and a maximal, symptom-limited, incremental bicycle test were assessed as part of the characterization procedure. In two subsequent visits (separated by 1 wk), the systemic effects of supplemental oxygen were evaluated in a double-blind, randomized, and placebo-controlled crossover design (Figure 1). At rest, subjects breathed either supplemental oxygen or compressed air via nasal prongs for 1 h at a flow rate of 4 L · min–1. After a wash-out period of 2 h breathing ambient air, a submaximal constant work rate bicycle test at 40 W was performed with either supplemental oxygen or compressed air (nasal, 4 L · min–1, again randomized and double-blind). Oxygen supplementation of 4 L · min–1 was chosen because this flow rate is commonly used in training programs for patients with COPD (3, 22). A work rate of 40 W was used to produce an energy expenditure of approximately 3 metabolic equivalents, which is equivalent to the power output required to walk in usual activities during daily life (23). Patients were instructed to cycle as long as possible but for a maximum of 30 min. One week later, the protocol was repeated with the other interventions (crossover). At this second visit, patients were instructed to cycle exactly as long as at the previous visit, if possible. Because of the nasal prongs for supplementation of oxygen or compressed air, we were not able to measure ventilatory parameters breath by breath during cycling. At both visits, five arterial blood samples (20 ml/sample) were taken. Samples A1 and A2, and A4 and A5, were used to evaluate the effects of supplemental oxygen compared with compressed air at rest and during exercise, respectively. Sample A3 was used to control wash-out values with baseline values. Measurements after exercise were corrected for plasma volume shifts according to Dill and Costill (24). Changes in systemic inflammation and oxidative stress in response to supplemental oxygen compared with compressed air were measured after 1 h at rest and after constant work rate bicycle exercise.


Figure 1
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  		Figure 1. Study design. A1 = arterial blood sample at baseline; A2 = arterial blood sample after 60-min intervention at rest; A3 = arterial blood sample after wash-out period (control); A4 = arterial blood sample after 3 min unloaded cycling with intervention; A5 = arterial blood sample after constant work rate cycling with intervention.

 
Blood Gas and Lactate Analysis
Arterial blood gasses (PaO2, PaCO2) and lactate concentrations were measured using Gas analyzer Chiron 860 (Bayer, Tarrytown, NY).

Systemic Inflammation
Plasma IL-6 concentrations were measured using quantitative sensitivity and high-sensitivity sandwich ELISAs in kit form (R&D Systems, Minneapolis, MN) according to the supplier's instructions (detection limit, 0.039 pg · ml–1). Leukocyte counts were determined following standard laboratory assays.

Oxidative Stress
Concentrations of thiobarbituric acid–reactive substances (TBARs) were determined fluorometrically (25). Levels of protein carbonyls were measured by means of an ELISA (26, 27).

To investigate the role of free-radical generation by neutrophils, production of reactive oxygen species by isolated neutrophils (stimulated with phorbol myristate acetate) was measured by chemiluminescence (28, 29). To evaluate the role for ATP degradation, plasma levels of xanthine, hypoxanthine, and uric acid were measured by a standard HPLC method with photodiode array detection, essentially as described previously (30). Total antioxidant capacity was assayed spectrophotometrically by measuring the ferric-reducing ability of plasma (31).

Statistics
Results are presented as means ± SE if the variables were normally distributed. Otherwise, median values were presented. Responses were expressed as absolute values and/or as changes compared with baseline. Paired t tests and Wilcoxon signed-rank tests were used to evaluate the responses to 1 h supplemental oxygen and compressed air, and also to compare the exercise-induced effects between supplemental oxygen and compressed air. Statistical significance was taken at the p < 0.05 level. Data were analyzed with SPSS/PC+, version 12.0 (SPSS, Inc., Chicago, IL).


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Subjects showed moderate to very severe airflow obstruction (Table 1). During the incremental bicycle test, none of the patients reached their predicted VO2max, but all achieved their maximal voluntary ventilation (VE/MVV = 98 ± 6%) as a consequence of their ventilatory limitation, whereas heart rate reserve was preserved.


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  		TABLE 1. SUBJECT CHARACTERISTICS

 
Effects of Oxygen Breathing at Rest
The effects of treatment with either compressed air or supplemental oxygen on blood gas tensions, systemic inflammation, and oxidative stress at rest are shown in Table 2. As expected, arterial blood gas tensions were not affected after 1 h treatment with compressed air. Treatment with supplemental oxygen, however, induced a significant increase of PaO2, whereas PaCO2 was not affected. No differences in markers of systemic inflammation and oxidative stress were observed after 1 h breathing supplemental oxygen or compressed air at rest (Table 2).


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  		TABLE 2. BLOOD GAS TENSIONS, SYSTEMIC INFLAMMATION, AND OXIDATIVE STRESS PRE- AND POST-TREATMENT WITH SUPPLEMENTAL OXYGEN OR COMPRESSED AIR AT REST

 
Supplemental Oxygen during Constant Work Rate Cycling
After the wash-out period, an arterial blood sample was taken as control (A3 in Figure 1). Blood gas tensions as well as markers of inflammation and oxidative stress after the wash-out were similar to baseline values (data not shown).

Median cycle time at 40 W was 14 min with supplemental oxygen (range, 4–20 min) and 15 min with compressed air (4–20 min; p = 0.71). Individual exercise durations of both exercise tests are shown in Table 3. At the second visit, four of the nine patients were exhausted before the aimed endurance was reached. Two of them (Subjects 6 and 9) received supplemental oxygen during cycling at the first visit and compressed air at the second visit. The other two patients (Subjects 3 and 5) received compressed air during the first cycle test and could not reach the aimed cycle time with supplemental oxygen during exercise. Physiologic responses to exercise are presented in Table 4. Similar duration and intensity of the exercise resulted in comparable increases of heart rate and arterial lactate concentrations. PaO2, however, changed differently during cycling with compressed air and supplemental oxygen. Although PaO2 slightly decreased to levels less than 9 kPa (mild hypoxemia) during cycling with compressed air, it decreased from hyperoxemic to normoxemic tensions during the exercise with supplemental oxygen. In contrast, PaCO2 similarly increased during exercise with compressed air and supplemental oxygen.


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  		TABLE 3. INDIVIDUAL EXERCISE DURATION OF THE TWO EXERCISE TRIALS

 

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  		TABLE 4. PHYSIOLOGIC RESPONSES TO SUBMAXIMAL EXERCISE WITH SUPPLEMENTAL OXYGEN OR COMPRESSED AIR

 
Exercise-induced leukocytosis did not differ between the two treatments. With compressed air, the number of leukocytes increased from 8.1 ± 0.6 to 9.4 ± 0.9 x 109 L–1 (p < 0.01), and with supplemental oxygen, the number of cells rose from 8.2 ± 0.8 to 9.2 ± 0.9 x 109 L–1 (p < 0.01). These increases were caused by similar elevations of the differential leukocyte subsets (i.e., neutrophils, lymphocytes, monocytes; data not shown) after treatment with supplemental oxygen and compressed air. Plasma levels of IL-6 significantly increased after exercise with both interventions (p < 0.01 with compressed air, p < 0.05 with supplemental oxygen; Figure 2). The increase in IL-6 during cycling with supplemental oxygen ({Delta}IL-6 = 0.2 ± 0.1 pg/ml), however, was significantly lower (p < 0.05) compared with the increase after cycling with compressed air ({Delta}IL-6 = 0.9 ± 0.1 pg/ml). Postexercise concentrations of IL-6 were significantly higher after cycling with compressed air compared with cycling with supplemental oxygen (p < 0.05).


Figure 2
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  		Figure 2. Plasma interleukin (IL)-6 concentrations before and after cycle exercise (40 W) with compressed air (A) or supplemental oxygen (nasal, 4 L · min–1; B) in muscle-wasted patients with chronic obstructive pulmonary disease (COPD). p < 0.01, p < 0.05 for differences between mean IL-6 concentrations (horizontal bars) between pre- and post-exercise.

 
Constant work rate exercise resulted in free-radical–mediated tissue damage, as indicated by increased lipid peroxidation (Figure 3A). Oxygen supplementation prevented the exercise-induced lipid peroxidation (Figure 3B). Changes in TBARs were significantly different between cycling with compressed air ({Delta}TBARs = 0.8 ± 0.2 µM) and cycling with supplemental oxygen ({Delta}TBARs = –0.3 ± 0.1 µM; p < 0.01). Formation of protein carbonyls was significantly increased after exercise with compressed air (p < 0.05; Figure 3C). When the exercise was performed with supplemental oxygen, however, protein oxidation remained unaffected (Figure 3D). Changes in plasma carbonyls were significantly different between exercise with compressed air ({Delta}carbonyls = 0.6 ± 0.2 nM) and supplemental oxygen ({Delta}carbonyls = 0.0 ± 0.1 nM; p < 0.05).


Figure 3
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  		Figure 3. Plasma levels of thiobarbituric acid–reactive substances (TBARs) and protein carbonyls before and after cycle exercise (40 W) with compressed air (A, C), or supplemental oxygen (nasal, 4 L · min–1; B, D) in muscle-wasted patients with COPD. p < 0.01, p < 0.05 for differences between mean carbonyls and TBARs (horizontal bars) between pre- and post-exercise. not sign. = not significant.

 
In response to exercise with compressed air, production of reactive oxygen species by neutrophils significantly increased (p < 0.001; Figure 4A). When the exercise was performed with supplemental oxygen, however, production of reactive oxygen species did not increase (Figure 4B). The increase in radical production ({Delta}respiratory burst) was significantly higher after exercise with compressed air than after exercise with supplemental oxygen (p < 0.01). Elevated levels of (hypo)xanthine (p < 0.05) and uric acid (p < 0.01) after exercise with compressed air suggest ATP degradation. The absence of an increase of these purines after exercise with supplemental oxygen pointed to attenuated ATP degradation (Figure 5). Changes of xanthine, hypoxanthine, and uric acid were significantly different between the two exercise protocols (p < 0.01, p < 0.01, and p < 0.001, respectively).


Figure 4
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  		Figure 4. Production of reactive oxygen species by isolated neutrophils before and after cycle exercise (40 W) with compressed air (A) or supplemental oxygen (nasal, 4 L · min–1; B) in muscle-wasted patients with COPD. p < 0.001 for differences between mean respiratory burst (horizontal bars) between pre- and post-exercise. not sign. = not significant. RLU = relative light units.

 

Figure 5
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  		Figure 5. ATP degradation. Plasma levels of xanthine (A), hypoxanthine (B), and uric acid (C) before and after exercise (40 W) with compressed air or supplemental oxygen in muscle-wasted patients with COPD. *p < 0.05, **p < 0.01 versus pre-exercise.

 
Plasma antioxidant capacity decreased during exercise (p < 0.05) without changes (p = 0.10) between breathing compressed air and supplemental oxygen (Figure 6).


Figure 6
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  		Figure 6. Plasma antioxidant capacity before and after exercise (40 W) with compressed air (A) or supplemental oxygen (nasal, 4 L · min–1; B) in muscle-wasted patients with COPD. p < 0.05 for differences between mean antioxidant capacity (horizontal bars) between pre- and post-exercise.

 
The effect of cycle time on the oxidative stress response was evaluated to exclude any possible effect of a shorter exercise endurance (not significant). No correlation with difference in cycle time between compressed air and supplemental oxygen and the difference in oxidative stress between the two exercise trials was found. Furthermore, even the two patients with a longer cycle time during intervention with supplemental oxygen showed a decreased oxidative stress response compared with compressed air.


   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Our results demonstrate that short-term oxygen breathing (nasal, 4 L/min for 1 h) does not affect basal systemic inflammation and oxidative stress in normoxemic, muscle-wasted patients with moderate to very severe COPD. Second, supplemental oxygen prevents exercise-induced systemic oxidative stress, and attenuates the response of IL-6 in these patients. Free-radical generation by neutrophils and also formation of purines are reduced after exercise with supplemental oxygen, suggesting a mechanistic role for both neutrophils and ATP degradation in the oxidative response to exercise.

Oxygen Therapy at Rest in Normoxemic Patients with COPD
Oxygen therapy prolongs survival, reduces the frequency of hospitalization and development of pulmonary hypertension in hypoxemic patients with COPD (1, 2), and has been shown to improve exercise performance in both hypoxemic and normoxemic patients (3–5). Recently, interesting data about the effects of hyperoxia on markers of oxidative stress and inflammation in exhaled breath condensate of patients with mild to severe hypoxemic COPD and healthy subjects were presented (6, 7). Short-term (30–60 min) breathing of supplemental oxygen (FIO2, 0.28) increased 8-isoprostane, breath methylated alkane contour, and IL-6 concentrations in exhaled breath condensate. One of the aspects important for the clinical relevance of these findings (32) is whether oxygen administration exerted similar systemic effects in these patients. Within healthy subjects, 2 to 3.5 h exposure to very high FIO2 (FIO2, 1.0, and PO2 of 120 kPa, respectively) was associated with increased oxidative stress in both the airways and plasma, as measured by lipid peroxidation, and superoxide dismutase activity (8, 9). In our study, increasing PaO2 to approximately 17 kPa did not affect markers for systemic inflammation or oxidative stress in normoxemic patients with moderate to very severe COPD at rest. This suggests that the increase of markers of inflammation and oxidative stress after short-term supplemental oxygen (7) in a concentration applied to patients with COPD is confined to the airways. The markedly higher fractions and longer exposure time in the studies with healthy subjects (8, 9) compared with our study may have contributed to the different findings.

Supplemental Oxygen during Exercise
During exercise training in both normoxemic and hypoxemic patients with COPD, supplemental oxygen can enhance training intensity (3, 4) and relieve dyspnea. Only a few studies about the systemic effects of supplemental oxygen during exercise have been published. In hypoxemic patients with COPD, oxygen inhalation partially improves the impaired muscle oxidative metabolism during exercise, as indicated by less intracellular acidosis, lower PI/PCr ratio, and an increased PCr resynthesis compared with breathing air (33). In the same study, no effects of supplemental oxygen on muscle oxidative metabolism during exercise were found in healthy subjects.

Markers of oxidative stress and inflammation, both systemically and locally (e.g., in muscle), are known to be increased by exercise in both healthy subjects (reviewed in Reference 34) and patients with COPD (12, 14, 35). Very recently, the effects of supplemental oxygen on oxidative stress during training in healthy subjects were reported for the first time (36). In the latter study, supplemental oxygen used in conjunction with high-intensity interval training at altitude resulted in improvement in exercise performance without inducing additional oxidative stress. Our study shows that exercise-induced oxidative stress in normoxemic patients with COPD was prevented by supplemental oxygen. These findings in normoxemic patients are in line with the results of Vina and colleagues (14), who reported partial prevention of exercise-induced glutathione oxidation by oxygen therapy in five hypoxemic patients with COPD. Besides prevention of systemic oxidative stress, the present study shows that the exercise-induced increase of plasma IL-6 in normoxemic patients with COPD was attenuated by supplemental oxygen. Plasma cytokines are known to increase during exercise (37). The appearance of IL-6 in the circulation is by far the most marked, and its appearance precedes that of the other cytokines (38). IL-6 is produced by many different cells, but the main sources in vivo are stimulated monocytes/macrophages, fibroblasts, and vascular endothelial cells (39), indicative of its role in the modulation of the immune system. Furthermore, other cells have the ability to express IL-6 in response to proinflammatory stimuli, including keratinocytes, osteoblasts, T cells, B cells, neutrophils, eosinophils, mast cells, smooth muscle cells, and skeletal muscle cells (40). Hypoxia induces IL-6 in cultured endothelial cells (41), and hypoxia in vivo elevates serum IL-6 in humans (42). Thus, prevention of low PaO2 using supplemental oxygen during exercise is a potential explanation for the diminished IL-6 response in the present study. Furthermore, cytokines (i.e., IL-6) can cause priming of neutrophils (43–45). Priming of these cells results in an augmented response of these cells to produce reactive oxygen species without direct stimulatory actions on the cells themselves. In response to cell stimulation, these primed cells can become activated to release reactive oxygen species and lysosomal enzymes, which in turn may destroy cellular membranes, induce DNA damage, or affect protein functions by modifying the structures (43–45). Within the present study, neutrophils produced increased reactive oxygen species after exercise with compressed air, but not after exercise with supplemental oxygen. After exercise, circulating neutrophils have a higher potential of producing highly toxic oxidants due to activation of the myeloperoxidase pathway (15). Without influencing the number of cells, supplemental oxygen seems to reduce the activation of neutrophils and thereby prevents exercise-induced free-radical production. According to the above-mentioned theory, the diminished IL-6 response, and the resulting lower postexercise IL-6 concentrations after cycling with supplemental oxygen, might have been the absent primer for the neutrophils. On the other hand, oxidative stress has also been shown to be a major stimulus for exercise-induced cytokine production (46). It has been suggested that oxidative stress precedes the acute inflammatory response to exercise (47). In this way, the prevention of exercise-induced oxidative stress by supplemental oxygen might also have been the cause rather than the result of diminished IL-6 production.

Preventing the production of reactive oxygen species by neutrophils with supplemental oxygen may have contributed to the absence of exercise-induced oxidative stress as was indicated by the absence of oxidation of proteins and lipids. Besides neutrophils, the mitochondrial electron transport chain and the xanthine oxidase pathway have been identified as major sources of intracellular free-radical generation during exercise (12, 15, 16). The elevated hypoxanthine, xanthine, and uric acid levels observed in the present study after cycling with compressed air indeed suggest a role for the latter mechanism. Exercise increases the demand for energy production. The ATP requirements of skeletal muscles must be met by the metabolic processes that are available to regenerate ATP from ADP. When energy requirements exceed the ability of the cell to (re)synthesize ATP, net ATP degradation occurs. Degradation of ATP leads to the release of purine metabolic intermediates (i.e., adenosine, inosine, hypoxanthine, and xanthine). In the presence of xanthine oxidase activity, hypoxanthine and xanthine are converted to superoxide and uric acid. It has been proposed that generation of superoxide by xanthine oxidase is substrate (ATP-degradation products) and not enzyme activity limited. (48) Supplemental oxygen is likely to stimulate ATP formation from ADP, resulting in reduced ATP degradation. The decreased levels of ATP-degradation products and uric acid found after cycling with supplemental oxygen strongly suggest the involvement of this pathway in the prevention of exercise-induced oxidative stress.

Regarding a possible role for hypoxia in the exercise-induced inflammatory and oxidative response, it seems interesting that oxygen supplementation reduced oxidative stress after exercise even in the absence of marked decline of PaO2. We did not find a correlation between the magnitude of oxygen desaturation and the reduction in exercise-induced oxidative stress with supplemental oxygen. This may at least be partially explained by the small number of patients included in the study and by the small differences of PaO2 decline between the subjects. In addition to our study, it would be useful to investigate whether the effects of supplemental oxygen are comparable or even more pronounced in a group of hypoxemic patients with COPD.

Clinical Implications and Future Directions
Repetitive exercise during daily life, or more specifically during exercise training, in patients with COPD may cause frequent exposure to systemic oxidative stress and inflammation, which may result in functional changes (i.e., modification of contractile amount and/or protein function). Indeed, we have recently shown evidence for impaired diaphragm contractile protein function in patients with COPD (49). An interesting hypothesis to be tested is that exercise-induced systemic inflammation and oxidative stress, as shown in the present study, contribute to impaired (respiratory) muscle function in these patients.

Furthermore, the current study has indicated two mechanisms that are involved in exercise-induced systemic oxidative stress, which both can be influenced by treatment with supplemental oxygen. Better understanding of the mechanisms of the systemic effects in COPD may improve insight into the consequences of these effects and may offer new targets for therapies. Diminishing or preventing bursts of systemic inflammation and oxidative stress after physical activities in patients with COPD might be beneficial, but data showing a causal relation among reductions in inflammation and/or oxidative stress and improvement of exercise tolerance, outcomes of rehabilitation, or quality of life have not yet been published. Before we can recommend oxygen supplementation during exercise, it needs to be evaluated whether (1) the effects of supplemental oxygen as shown here are specific for normoxemic, muscle-wasted patients with COPD; (2) the effects remain when patients exercise frequently with oxygen; and (3) there are clinical consequences of oxygen supplementation.

In conclusion, this study demonstrates that supplemental oxygen during exercise diminishes the increase of plasma IL-6 and prevents systemic oxidative stress in normoxemic, muscle-wasted patients with COPD. Neutrophil activation and ATP degradation are both associated with these findings. Oxygen breathing at rest, however, has no effect on the low-grade systemic inflammation and oxidative stress in muscle-wasted patients with COPD. These data suggest that normoxemic, muscle-wasted COPD patients, a subgroup with clear systemic consequences of COPD, can be prevented from exposure to bursts of oxidative stress and inflammation by supplemental oxygen during exercise. Whether this will also result in improvement of exercise tolerance, outcomes of rehabilitation, or quality of life needs to be evaluated.


   Acknowledgments
 
The authors thank the laboratories of the departments of Clinical Chemistry and Gastroenterology and Hepatology from our hospital for their technical assistance and the Laboratory of Pediatrics and Neurology (Ing. M. lutje Berenbroek) for performing the HPLC measurements of purine metabolites.


   FOOTNOTES
 
Supported by an unrestricted educational grant from AstraZeneca, The Netherlands.

Originally Published in Press as DOI: 10.1164/rccm.200512-1957OC on March 2, 2006

Conflict of Interest Statement: None of the authors have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Received in original form December 23, 2005; accepted in final form February 27, 2006


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
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