Regulation of PGC-1α expression by a GSK-3ß-TFEB signaling axis in skeletal muscle.

OBJECTIVE: In muscle cells, the peroxisome proliferator-activated receptor γ co-activator 1 (PGC-1) signaling network, which has been shown to be disturbed in the skeletal muscle in several chronic diseases, tightly controls mitochondrial biogenesis and oxidative substrate metabolism. Previously, we showed that inactivation of glycogen synthase kinase (GSK)-3ß potently increased Pgc-1α abundance and oxidative metabolism in skeletal muscle cells. The current study aims to unravel the molecular mechanism driving the increase in Pgc-1α mediated by GSK-3ß inactivation. METHODS: GSK-3ß was inactivated genetically or pharmacologically in C2C12 myotubes and the requirement of transcription factors known to be involved in Pgc-1α transcription for increases in Pgc-1α abundance mediated by inactivation of GSK-3ß was examined. RESULTS: Enhanced PGC-1α promoter activation after GSK-3ß inhibition suggested a transcriptionally-controlled mechanism. While myocyte enhancer factor (MEF)2 transcriptional activity was unaltered, GSK-3ß inactivation increased the abundance and activity of the transcription factors estrogen-related receptor (ERR)α and ERRγ. Pharmacological inhibition or knock-down of ERRα and ERRγ however failed to prevent increases in Pgc-1α mRNA mediated by GSK-3ß inactivation. Interestingly, GSK-3ß inactivation activated transcription factor EB (TFEB), evidenced by decreased phosphorylation and enhanced nuclear localization of the TFEB protein. Moreover, knock-down of TFEB completely prevented increases in Pgc-1α gene expression, PGC-1α promoter activity and PGC-1α protein abundance induced by GSK-3ß inactivation. Furthermore, mutation of a specific TFEB binding site on the PGC-1α promoter blocked promoter activation upon inhibition of GSK-3ß. CONCLUSIONS: In skeletal muscle, GSK-3ß inactivation causes dephosphorylation and nuclear translocation of TFEB resulting in TFEB-dependent induction of Pgc-1α expression.

Molecular signalling towards mitochondrial breakdown is enhanced in skeletal muscle of patients with chronic obstructive pulmonary disease (COPD).

Loss of skeletal muscle mitochondrial oxidative capacity is well-established in patients with COPD, but the role of mitochondrial breakdown herein is largely unexplored. Currently, we studied if mitochondrial breakdown signalling is increased in skeletal muscle of COPD patients and associates with the loss of mitochondrial content, and whether it is affected in patients with iron deficiency (ID) or systemic inflammation. Therefore, mitophagy, autophagy, mitochondrial dynamics and content markers were analysed in vastus lateralis biopsies of COPD patients (N = 95, FEV1% predicted: 39.0 [31.0-53.6]) and healthy controls (N = 15, FEV1% predicted: 112.8 [107.5-125.5]). Sub-analyses were performed on patients stratified by ID or C-reactive protein (CRP). Compared with controls, COPD patients had lower muscle mitochondrial content, higher BNIP3L and lower FUNDC1 protein, and higher Parkin protein and gene-expression. BNIP3L and Parkin protein levels inversely correlated with mtDNA/gDNA ratio and FEV1% predicted. ID-COPD patients had lower BNIP3L protein and higher BNIP3 gene-expression, while high CRP patients had higher BNIP3 and autophagy-related protein levels. In conclusion, our data indicates that mitochondrial breakdown signalling is increased in skeletal muscle of COPD patients, and is related to disease severity and loss of mitochondrial content. Moreover, systemic inflammation is associated with higher BNIP3 and autophagy-related protein levels.

Effect of chronic hypoxia on RAGE and its soluble forms in lungs and plasma of mice.

The receptor for advanced glycation end products (RAGE) is a multi-ligand receptor. Alternative splicing and enzymatic shedding produce soluble forms that protect against damage by ligands including Advanced Glycation End products (AGEs). A link between RAGE and oxygen levels is evident from studies showing RAGE-mediated injury following hyperoxia. The effect of hypoxia on pulmonary RAGE expression and circulating sRAGE levels is however unknown. Therefore mice were exposed to chronic hypoxia for 21 d and expression of RAGE, sheddases in lungs and circulating sRAGE were determined. In addition, accumulation of AGEs in lungs and expression of the AGE detoxifying enzyme GLO1 and receptors were evaluated. In lung tissue gene expression of total RAGE, variants 1 and 3 were elevated in mice exposed to hypoxia, whereas mRAGE and sRAGE protein levels were decreased. In the hypoxic group plasma sRAGE levels were enhanced. Although the levels of pro-ADAM10 were elevated in lungs of hypoxia exposed mice, the relative amount of the active form was decreased and gelatinase activity unaffected. In the lungs, the RAGE ligand HMGB1 was decreased and of the AGEs, only LW-1 was increased by chronic hypoxia. Gene expression of AGE receptors 2 and 3 was significantly upregulated. Chronic hypoxia is associated with downregulation of pulmonary RAGE protein levels, but a relative increase in sRAGE. These alterations might be part of the adaptive and protective response mechanism to chronic hypoxia and are not associated with AGE formation except for the fluorophore LW-1 which emerges as a novel marker of tissue hypoxia.

TNF-α-induced NF-κB activation stimulates skeletal muscle glycolytic metabolism through activation of HIF-1α.

A shift in quadriceps muscle metabolic profile toward decreased oxidative metabolism and increased glycolysis is a consistent finding in chronic obstructive pulmonary disease (COPD). Chronic inflammation has been proposed as a trigger of this pathological metabolic adaptation. Indeed, the proinflammatory cytokine TNF-α impairs muscle oxidative metabolism through activation of the nuclear factor-κB (NF-κB) pathway. Putative effects on muscle glycolysis, however, are unclear. We hypothesized that TNF-α-induced NF-κB signaling stimulates muscle glycolytic metabolism through activation of the glycolytic regulator hypoxia-inducible factor-1α (HIF-1α). Wild-type C2C12 and C2C12-IκBα-SR (blocked NF-κB signaling) myotubes were stimulated with TNF-α, and its effects on glycolytic metabolism and involvement of the HIF pathway herein were investigated. As proof of principle, expression of HIF signaling constituents was investigated in quadriceps muscle biopsies of a previously well-characterized cohort of clinically stable patients with severe COPD and healthy matched controls. TNF-α increased myotube glucose uptake and lactate production and enhanced the activity and expression levels of multiple effectors of muscle glycolytic metabolism in a NF-κB-dependent manner. In addition, TNF-α activated HIF signaling, which required classical NF-κB activation. Moreover, the knockdown of HIF-1α largely attenuated TNF-α-induced increases in glycolytic metabolism. Accordingly, the mRNA levels of HIF-1α and the HIF-1α target gene, vascular endothelial growth factor (VEGF), were increased in muscle biopsies of COPD patients compared with controls, which was most pronounced in the patients with high levels of muscle TNF-α. In conclusion, these data show that TNF-α-induced classical NF-κB activation enhances muscle glycolytic metabolism in a HIF-1α-dependent manner.

Activation of alternative NF-κB signaling during recovery of disuse-induced loss of muscle oxidative phenotype.

Physical inactivity-induced loss of skeletal muscle oxidative phenotype (OXPHEN), often observed in chronic disease, adversely affects physical functioning and quality of life. Potential therapeutic targets remain to be identified, since the molecular mechanisms involved in reloading-induced recovery of muscle OXPHEN remain incompletely understood. We hypothesized a role for alternative NF-κB, as a recently identified positive regulator of muscle OXPHEN, in reloading-induced alterations in muscle OXPHEN. Markers and regulators (including alternative NF-κB signaling) of muscle OXPHEN were investigated in gastrocnemius muscle of mice subjected to a hindlimb suspension/reloading (HLS/RL) protocol. Expression levels of oxidative phosphorylation subunits and slow myosin heavy chain isoforms I and IIA increased rapidly upon RL. After an initial decrease upon HLS, mRNA levels of peroxisome proliferator-activated receptor (PPAR)-γ coactivator (PGC) molecules PGC-1α and PGC-1ß and mRNA levels of mitochondrial transcription factor A (Tfam) and estrogen-related receptor α increased upon RL. PPAR-δ, nuclear respiratory factor 1 (NRF-1), NRF-2α, and sirtuin 1 mRNA levels increased during RL although expression levels were unaltered upon HLS. In addition, both Tfam and NRF-1 protein levels increased significantly during the RL period. Moreover, upon RL, IKK-α mRNA and protein levels increased, and phosphorylation of P100 and subsequent processing to P52 were elevated, reflecting alternative NF-κB activation. We conclude that RL-induced recovery of muscle OXPHEN is associated with activation of alternative NF-κB signaling.

Classical NF-κB activation impairs skeletal muscle oxidative phenotype by reducing IKK-α expression.

BACKGROUND: Loss of quadriceps muscle oxidative phenotype (OXPHEN) is an evident and debilitating feature of chronic obstructive pulmonary disease (COPD). We recently demonstrated involvement of the inflammatory classical NF-κB pathway in inflammation-induced impairments in muscle OXPHEN. The exact underlying mechanisms however are unclear. Interestingly, IκB kinase α (IKK-α: a key kinase in the alternative NF-κB pathway) was recently identified as a novel positive regulator of skeletal muscle OXPHEN. We hypothesised that inflammation-induced classical NF-κB activation contributes to loss of muscle OXPHEN in COPD by reducing IKK-α expression. METHODS: Classical NF-κB signalling was activated (molecularly or by tumour necrosis factor α: TNF-α) in cultured myotubes and the impact on muscle OXPHEN and IKK-α levels was investigated. Moreover, the alternative NF-κB pathway was modulated to investigate the impact on muscle OXPHEN in absence or presence of an inflammatory stimulus. As a proof of concept, quadriceps muscle biopsies of COPD patients and healthy controls were analysed for expression levels of IKK-α, OXPHEN markers and TNF-α. RESULTS: IKK-α knock-down in cultured myotubes decreased expression of OXPHEN markers and key OXPHEN regulators. Moreover, classical NF-κB activation (both by TNF-α and IKK-ß over-expression) reduced IKK-α levels and IKK-α over-expression prevented TNF-α-induced impairments in muscle OXPHEN. Importantly, muscle IKK-α protein abundance and OXPHEN was reduced in COPD patients compared to controls, which was more pronounced in patients with increased muscle TNF-α mRNA levels. CONCLUSION: Classical NF-κB activation impairs skeletal muscle OXPHEN by reducing IKK-α expression. TNF-α-induced reductions in muscle IKK-α may accelerate muscle OXPHEN deterioration in COPD.

Triggers and mechanisms of skeletal muscle wasting in chronic obstructive pulmonary disease.

Skeletal muscle wasting contributes to impaired exercise capacity, reduced health-related quality of life and is an independent determinant of mortality in chronic obstructive pulmonary disease. An imbalance between protein synthesis and myogenesis on the one hand, and muscle proteolysis and apoptosis on the other hand, has been proposed to underlie muscle wasting in this disease. In this review, the current understanding of the state and regulation of these processes governing muscle mass in this condition is presented. In addition, a conceptual mode of action of disease-related determinants of muscle wasting including disuse, hypoxemia, malnutrition, inflammation and glucocorticoids is provided by overlaying the available associative clinical data with causal evidence, mostly derived from experimental models. Significant progression has been made in understanding and managing muscle wasting in chronic obstructive pulmonary disease. Further examination of the time course of muscle wasting and specific disease phenotypes, as well as the application of systems biology and omics approaches in future research will allow the development of tailored strategies to prevent or reverse muscle wasting in chronic obstructive pulmonary disease. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Regulation of skeletal muscle oxidative phenotype by classical NF-κB signalling.

BACKGROUND: Impairments in skeletal muscle oxidative phenotype (OXPHEN) have been linked to the development of insulin resistance, metabolic inflexibility and progression of the metabolic syndrome and have been associated with progressive disability in diseases associated with chronic systemic inflammation. We previously showed that the inflammatory cytokine tumour necrosis factor-α (TNF-α) directly impairs muscle OXPHEN but underlying molecular mechanisms remained unknown. Interestingly, the inflammatory signalling pathway classical nuclear factor-κB (NF-κB) is activated in muscle in abovementioned disorders. Therefore, we hypothesised that muscle activation of classical NF-κB signalling is sufficient and required for inflammation-induced impairment of muscle OXPHEN. METHODS: Myotubes from mouse and human muscle cell lines were subjected to activation or blockade of the classical NF-κB pathway. In addition, wild-type and MISR (muscle-specific inhibition of classical NF-κB) mice were injected intra-muscularly with TNF-α. Markers and key regulators of muscle OXPHEN were investigated. RESULTS: Classical NF-κB activation diminished expression of oxidative phosphorylation (OXPHOS) sub-units, slow myosin heavy chain expression, activity of mitochondrial enzymes and potently reduced intra-cellular ATP levels. Accordingly, PGC-1/PPAR/NRF-1/Tfam signalling, the main pathway controlling muscle OXPHEN, was impaired upon classical NF-κB activation which required intact p65 trans-activation domains and depended on de novo gene transcription. Unlike wild-type myotubes, IκBα-SR myotubes (blocked classical NF-κB signalling) were refractory to TNF-α-induced impairments in OXPHEN and its regulation by the PGC-1/PPAR/NRF-1/Tfam cascade. In line with in vitro data, NF-κB blockade in vivo abrogated TNF-α-induced reductions in PGC-1α expression. CONCLUSION: Classical NF-κB activation impairs skeletal muscle OXPHEN.

The mechanisms of cachexia underlying muscle dysfunction in COPD.

Pulmonary cachexia is a prevalent, debilitating, and well-recognized feature of COPD associated with increased mortality and loss of peripheral and respiratory muscle function. The exact cause and underlying mechanisms of cachexia in COPD are still poorly understood. Increasing evidence, however, shows that pathological changes in intracellular mechanisms of muscle mass maintenance (i.e., protein turnover and myonuclear turnover) are likely involved. Potential factors triggering alterations in these mechanisms in COPD include oxidative stress, myostatin, and inflammation. In addition to muscle wasting, peripheral muscle in COPD is characterized by a fiber-type shift toward a more type II, glycolytic phenotype and an impaired oxidative capacity (collectively referred to as an impaired oxidative phenotype). Atrophied diaphragm muscle in COPD, however, displays an enhanced oxidative phenotype. Interestingly, intrinsic abnormalities in (lower limb) peripheral muscle seem more pronounced in either cachectic patients or weight loss-susceptible emphysema patients, suggesting that muscle wasting and intrinsic changes in peripheral muscle's oxidative phenotype are somehow intertwined. In this manuscript, we will review alterations in mechanisms of muscle mass maintenance in COPD and discuss the involvement of oxidative stress, inflammation, and myostatin as potential triggers of cachexia. Moreover, we postulate that an impaired muscle oxidative phenotype in COPD can accelerate the process of cachexia, as it renders muscle in COPD less energy efficient, thereby contributing to an energy deficit and weight loss when not dietary compensated. Furthermore, loss of peripheral muscle oxidative phenotype may increase the muscle's susceptibility to inflammation- and oxidative stress-induced muscle damage and wasting.

TNF-alpha impairs regulation of muscle oxidative phenotype: implications for cachexia?

Chronic obstructive pulmonary disease (COPD) is characterized by weight loss, muscle wasting (in advanced disease ultimately resulting in cachexia), and loss of muscle oxidative phenotype (oxphen). This study investigates the effect of inflammation (as a determinant of muscle wasting) on muscle oxphen by using cell studies combined with analyses of muscle biopsies of patients with COPD and control participants. We analyzed markers (citrate synthase, ß-hydroxyacyl-CoA dehydrogenase, and cytochrome c oxidase IV) and regulators (PGC-1α, PPAR-α, and Tfam) of oxphen in vastus lateralis muscle biopsies of patients with advanced COPD and healthy smoking control participants. Here 17 of 73 patients exhibited elevated muscle TNF-α mRNA levels. In these patients, significantly lower mRNA levels of all oxidative markers/regulators were found. Interestingly, these patients also had a significantly lower body mass index and tended to have less muscle mass. In cultured muscle cells, mitochondrial protein content and myosin heavy chain isoform I (but not II) protein and mRNA levels were reduced on chronic TNF-α stimulation. TNF-α also reduced mitochondrial respiration in a nuclear factor kappaB (NF-κB) -dependent manner. Importantly, TNF-α-induced NF-κB activation decreased promoter transactivation and transcriptional activity of regulators of mitochondrial biogenesis and muscle oxphen. In conclusion, these results demonstrate that TNF-α impairs muscle oxphen in a NF-κB-dependent manner.

Peroxisome proliferator-activated receptors: a therapeutic target in COPD?

Extrapulmonary pathology significantly impairs clinical outcome in chronic obstructive pulmonary disease (COPD). The peroxisome proliferator-activated receptors (PPARs) are implicated in the regulation of several hallmarks of systemic COPD pathology, including cachexia, decreased oxidative muscle metabolism, oxidative stress and systemic inflammation. Recently, expression of PPARs and related cofactors was shown to be reduced in peripheral skeletal muscle of patients with moderate-to-severe COPD and muscle weakness. The current authors hypothesise that impaired peroxisome proliferator-activated receptor signalling may underlie some of the muscular disturbances in chronic obstructive pulmonary disease. Proposed mechanisms will be outlined in the present article, as well as the therapeutic potential of peroxisome proliferator-activated receptor modulation in the treatment of skeletal muscle dysfunction.

Cytokine profile in quadriceps muscles of patients with severe COPD.

BACKGROUND: Systemic proinflammatory cytokines and oxidative stress have been described in association with peripheral muscle wasting and weakness of patients with severe chronic obstructive pulmonary disease (COPD), but their expression in skeletal muscle is unknown. The objectives of the present study were to determine muscle protein levels of selected cytokines in patients with COPD and to study their relationships with protein carbonylation as a marker of oxidative stress, quadriceps function and exercise capacity. METHODS: We conducted a cross sectional study in which 36 cytokines were detected using a human antibody array in quadriceps specimens obtained from 19 patients with severe COPD and seven healthy controls. Subsequently, selected cytokines (tumour necrosis factor (TNF)alpha, TNFalpha receptors I and II, interleukin (IL) 6, interferon gamma, transforming growth factor (TGF) beta and vascular endothelial growth factor (VEGF)), as well as protein carbonylation (oxidative stress index) were determined using an enzyme linked immunosorbent assay (ELISA) in all muscles. RESULTS: Compared with controls, the vastus lateralis of patients with COPD showed significantly lower protein ELISA levels of TNFalpha, which positively correlated with their quadriceps function, TNFalpha receptor II and VEGF. Protein ELISA levels of IL6, interferon gamma and TGFbeta did not differ between patients and controls. Quadriceps protein carbonylation was greater in patients and inversely correlated with quadriceps strength among them. CONCLUSIONS: These findings do not support the presence of a proinflammatory environment within the quadriceps muscles of clinically and weight stable patients with severe COPD, despite evidence for increased oxidative stress and the presence of muscle weakness.

A novel technique for nonvolitional assessment of quadriceps muscle endurance in humans.

Assessment of quadriceps endurance is of interest to investigators studying human disease. We hypothesized that repetitive magnetic stimulation (rMS) of the intramuscular branches of the femoral nerve could be used to induce and quantify quadriceps endurance. To test this hypothesis, we used a novel stimulating coil to compare the quadriceps endurance properties in eight normal humans and, to confirm that the technique could be used in clinical practice, in eight patients with advanced chronic obstructive pulmonary disease (COPD). To validate the method, we compared in vivo contractile properties of the quadriceps muscle with the fiber-type composition and oxidative enzyme capacity. We used a Magstim Rapid(2) magnetic nerve stimulator with the coil wrapped around the quadriceps. Stimuli were given at 30 Hz, a duty cycle of 0.4 (2 s on, 3 s off), and for 50 trains. Force generation and the surface electromyogram were measured throughout. Quadriceps twitch force, elicited by supramaximal magnetic stimulation of the femoral nerve, was measured before and after the protocol. Quadriceps muscle biopsies were analyzed for oxidative (citrate synthase, CS) and glycolytic (phosphofructokinase, PFK) enzyme activity and myosin heavy chain isoform protein expression. The time for force to fall to 70% of baseline (T(70)) was shorter in the COPD group than the control group: 55.6 +/- 26.0 vs. 121 +/- 38.7 s (P = 0.0014). Considering patients and controls together, positive correlations were observed between T(70) and the proportion of type I fibers (r = 0.68, P = 0.004) and CS-to-PFK ratio (CS/PFK) (r = 0.67, P = 0.005). We conclude that quadriceps endurance assessed using rMS is feasible in clinical studies.

Peroxisome proliferator-activated receptor expression is reduced in skeletal muscle in COPD.

Chronic obstructive pulmonary disease (COPD) is a multiorgan systemic disease. The systemic features are skeletal muscle weakness and cachexia, the latter being associated with systemic inflammation. The exact mechanisms underlying skeletal muscle dysfunction in COPD remain obscure. Recent evidence suggests involvement of the peroxisome proliferator-activated receptors (PPARs) and PPAR-gamma coactivator (PGC)-1alpha in regulation of skeletal muscle morphology and metabolism, and mitochondrial transcription factor A (TFAM) has been implicated in the process of mitochondrial biogenesis. The aim of the present exploratory study was, therefore, to compare these factors in the skeletal muscle of nine healthy control subjects and 14 COPD patients stratified by cachexia. PPAR-gamma, PPAR-delta and TFAM were measured at the mRNA and protein level by real-time quantitative PCR and Western blotting, respectively. PPAR-alpha and PGC-1alpha were meansured at the mRNA level. PPAR-delta and TFAM protein content, as well as PGC-1alpha mRNA levels, were decreased in the skeletal muscle of COPD patients compared with healthy controls. The cachectic COPD subgroup was further characterised by decreased PPAR-alpha mRNA expression and decreased TFAM protein and mRNA levels compared with noncachectic COPD patients. In addition, PPAR-alpha mRNA levels in skeletal muscle correlated negatively with inflammatory markers in plasma. Therefore, a disturbed expression of these regulatory factors may well underlie the disturbed skeletal muscle functioning in chronic obstructive pulmonary disease.

Skeletal muscle fibre-type shifting and metabolic profile in patients with chronic obstructive pulmonary disease.

The aim of this study was to examine the nature of fibre-type redistribution in relation to fibre metabolic profile in the vastus lateralis in chronic obstructive pulmonary disease (COPD) and COPD subtypes. Fifteen COPD patients (eight with emphysema stratified by high-resolution computed tomography) and 15 healthy control subjects were studied. A combination of myofibrillar adenosine triphosphatase staining and immunohistochemistry was used to identify pure, as well as hybrid fibre types. For oxidative capacity, fibres were stained for cytochrome c oxidase and succinate dehydrogenase activities, and glycogen phosphorylase for glycolytic capacity. The proportion of type-I fibres in COPD patients was markedly lower (16% versus 42%), especially in emphysema, and the proportion of hybrid fibres was higher (29% versus 16%) compared to controls. The proportion of fibres staining positive for oxidative enzymes was lower in COPD patients, which correlated with the proportion of type-I fibres. In COPD oxidative capacity was lower within IIA fibres. The authors conclude that fibre-type transitions are involved in the fibre-type redistribution in chronic obstructive pulmonary disease. Low oxidative capacity is closely related to the proportion of type-I fibres, but an additional reduction of oxidative enzyme activity is present within IIA fibres. Fibre-type abnormalities may be aggravated in emphysema.

Exercise-induced lactate increase in relation to muscle substrates in patients with chronic obstructive pulmonary disease.

Early lactic acidosis has been suggested as negatively influencing the exercise capacity of patients with chronic obstructive pulmonary disease (COPD). We conducted a study to investigate whether the early lactate (La) response to exercise in COPD is related to alterations in exercise-related substrate levels in resting muscle, associated with physical inactivity. Twenty-seven COPD patients and 22 controls (physically inactive [PI] subjects, n = 15; and physically active [PA] subjects, n = 7) performed an incremental cycle test. Venous blood was sampled for La analyses, and the oxygen uptake at which the La level began to rise (La threshold) was calculated. Vastus lateralis biopsy specimens were obtained at rest. In the PA group, muscle glutamate (GLU) and glycogen were higher, but muscle La, pyruvate, and glucose were not different than in the PI group. Moreover, the La threshold was higher in the PA group. The COPD group had lower values for La threshold and muscle GLU, and higher values for muscle La and pyruvate levels than did the PI group. Stratification of patients into those with and without macroscopic emphysema (EMPH+, EMPH-, respectively), with comparable physical activity levels on the basis of previous observations, revealed lower values for La threshold and GLU in EMPH+ patients. Diffusing capacity for carbon monoxide (DL(CO)) and arterial oxygen tension (Pa(O(2))) in the four study groups were positively related to GLU and La threshold. Moreover, La threshold was positively related to GLU. This study illustrates that the early lactic acidosis during exercise in patients with COPD is associated with the physical inactivity-related reduction in these patient's muscle GLU. However, factors other than physical inactivity, such as Pa(O(2)) or DL(CO), play a role in the different La responses during exercise in subjects with different subtypes of COPD.

Muscle metabolic status in patients with severe COPD with and without long-term prednisolone.

Both abnormalities in high energy phosphate metabolism and a decreased oxidative enzyme capacity have been reported in skeletal muscle of stable chronic obstructive pulmonary disease (COPD) patients. The first aim of this study was to investigate whether these findings are present in anterior tibialis muscle and whether or not they are associated. Abnormalities in mitochondrial structure and function as well as signs of myopathy have been found during corticosteroid treatment. The second aim of this study, therefore, was to investigate whether in COPD patients prolonged use of low dose prednisolone has effects on muscle energy metabolism and qualitative morphology. In a cross-sectional study 15 COPD patients (forced expiratory volume in one second (FEV1) 33+/-9 (mean+/-SD) % predicted) who were steroid-naive (CORT-) were compared with 10 healthy control subjects (HC) and with 14 COPD patients (FEV1 30+/-11 % pred), who had been using oral prednisolone for at least 1 yr (CORT+). It was found that adenosine triphosphate (ATP)/adenosine diphosphate was lower in CORT- compared to HC (5.7 versus 6.2, p=0.03). Inosine monophosphate was detected in 13 of 15 CORT- compared to 3 of 10 HC (p=0.004). However, although indications were found for an imbalance in production and utilization of ATP, comparing CORT- and HC, no differences in oxidative (citrate synthase and 3-hydroxy-acyl-coenzyme A dehydrogenase) and glycolytic (hexokinase, lactate dehydrogenase and phosphofructokinase) enzyme capacities were found. When, comparing steroid-treated and steroid-naive patient subgroups, no differences in the above mentioned parameters of muscle energy metabolism and of muscle qualitative morphology were found.