Evaluating the role of nitric oxide in myogenesis in vitro.

Skeletal muscle injury activates satellite cells to proliferate as myoblasts and migrate, differentiate and fuse with existing fibres at the site of injury. Nitric oxide (NO), a free radical produced by NO synthase, is elevated and supports healing after in vivo injury. NOS-independent elevation of NO levels in vitro is possible via donors such as molsidomine (SIN-1). We hypothesized that alterations in NO levels may directly influence myogenic processes critical for skeletal muscle wound healing. This study aimed to clarify the role of NO in myoblast proliferation, migration and differentiation. Baseline NO levels were established in vitro, whereafter NO levels were manipulated during myogenesis using l-NAME (NOS inhibitor) or SIN-1. Baseline NO levels generated by myoblasts in proliferation media did not change 1 h after stimulation. Addition of a pro-proliferative dose of HGF slightly elevated NO levels 1 h post-stimulation, whereas cell numbers assessed 24 h later increased significantly; l-NAME reduced the HGF-driven increase in NO and proliferation, reducing wound closure over 16 h. In differentiation media, NO levels increased significantly within 24 h, returning to baseline over several days. Regular addition of l-NAME to differentiating cells significantly reduced NO levels and fusion. SIN-1 increased NO levels in a dose-dependent manner, reaching maximal levels 16 h post-treatment. SIN-1, added at 0, 2 and 4 days, significantly increased myofiber area (26 ± 1.8% vs 18.6 ± 3.4% in control at 5 day, p < 0.0001), without affecting proliferation or migration. In conclusion, this study demonstrates that, during skeletal muscle regeneration, increased NO specifically stimulates myoblast differentiation.

Rheumatoid cachexia: the underappreciated role of myoblast, macrophage and fibroblast interplay in the skeletal muscle niche.

Although rheumatoid arthritis affects 1% of the global population, the role of rheumatoid cachexia, which occurs in up to a third of patients, is relatively neglected as research focus, despite its significant contribution to decreased quality of life in patients. A better understanding of the cellular and molecular processes involved in rheumatoid cachexia, as well as its potential treatment, is dependent on elucidation of the intricate interactions of the cells involved, such as myoblasts, fibroblasts and macrophages. Persistent RA-associated inflammation results in a relative depletion of the capacity for regeneration and repair in the satellite cell niche. The repair that does proceed is suboptimal due to dysregulated communication from the other cellular role players in this multi-cellular environment. This includes the incomplete switch in macrophage phenotype resulting in a lingering pro-inflammatory state within the tissues, as well as fibroblast-associated dysregulation of the dynamic control of the extracellular matrix. Additional to this endogenous dysregulation, some treatment strategies for RA may exacerbate muscle wasting and no multi-cell investigation has been done in this context. This review summarizes the most recent literature characterising clinical RA cachexia and links these features to the roles of and complex communication between multiple cellular contributors in the muscle niche, highlighting the importance of a targeted approach to therapeutic intervention.

Cellular regenerative therapy for acquired noncongenital musculoskeletal disorders.

Stem cells have an inherent capacity to facilitate regeneration; this has led to unprecedented growth in their experimental use in various clinical settings, particularly in patients with diseases with few alternative treatment options. However, their approved clinical use has to date been restricted largely to haematological diseases and epidermal transplantation to treat severe burns. After thorough searching of two databases, this review illuminates the role of stem cell therapy for treatment of musculoskeletal diseases. Research suggests that successful application of stem cells as regenerative mediators is in all likelihood dependent on the ability of endogenous tissue-resident reparative mediators to respond to paracrine signals provided by the applied stem cells. Therefore, an understanding of how the pathological environment influences this process is crucial for the ultimate success of stem cell therapies. The current review presents both the progress and limitations of stem cells as regenerative mediators in the context of musculoskeletal disorders.

Redox Status and Muscle Pathology in Rheumatoid Arthritis: Insights from Various Rat Hindlimb Muscles.

Due to atrophy, muscle weakness is a common occurrence in rheumatoid arthritis (RA). The majority of human studies are conducted on the vastus lateralis muscle-a muscle with mixed fiber type-but little comparative data between multiple muscles in either rodent or human models are available. The current study therefore assessed both muscle ultrastructure and selected redox indicators across various muscles in a model of collagen-induced rheumatoid arthritis in female Sprague-Dawley rats. Only three muscles, the gastrocnemius, extensor digitorum longus (EDL), and soleus, had lower muscle mass (38%, 27%, and 25% loss of muscle mass, respectively; all at least P < 0.01), while the vastus lateralis muscle mass was increased by 35% (P < 0.01) in RA animals when compared to non-RA controls. However, all four muscles exhibited signs of deterioration indicative of rheumatoid cachexia. Cross-sectional area was similarly reduced in gastrocnemius, EDL, and soleus (60%, 58%, and 64%, respectively; all P < 0.001), but vastus lateralis (22% smaller, P < 0.05) was less affected, while collagen deposition was significantly increased in muscles. This pathology was associated with significant increases in tissue levels of reactive oxygen species (ROS) in all muscles except the vastus lateralis, while only the gastrocnemius had significantly increased levels of lipid peroxidation (TBARS) and antioxidant activity (FRAP). Current data illustrates the differential responses of different skeletal muscles of the hindlimb to a chronic inflammatory challenge both in terms of redox changes and resistance to cachexia.

Delayed wound healing and dysregulation of IL6/STAT3 signalling in MSCs derived from pre-diabetic obese mice.

Metabolic dysfunction that occurs in obesity and Type 2 diabetes results in a low-level inflammatory state which impacts on mesenchymal stem cells (MSCs) capacity to promote wound healing. The ability of either recombinant Interleukin-6 (rIL6) or pioglitazone to modulate MSC migration, essential for wound healing, by targeting the inflammation-modulated IL6/STAT3 signalling pathway was therefore investigated in bone marrow-derived MSCs from control (C57BL/6J) and pre-diabetic obese mice (B6. Cg-Lepob/J). The population doubling time, in vitro wound closure and mRNA expression profile of 84 genes involved in the IL6/STAT3 signalling pathway were assessed. IL6/STAT3 signalling dysregulation, caused by IL6 deficiency, resulted in skewing of the immune modulatory properties of MSCs to favour a pro-inflammatory profile. This could be nullified by addition of either rIL6 or conventional diabetes treatment. Therapies to improve diabetic wound healing should therefore focus on the cellular changes induced by the pathological inflammatory micro-environment.

Neutrophil and monocyte responses to downhill running: Intracellular contents of MPO, IL-6, IL-10, pstat3, and SOCS3.

High-intensity exercise results in immune activation. This study determined whether (a) there is concordance between serum MPO and neutrophil and/or monocyte intracellular MPO content; (b) peripheral blood mononuclear cells respond to inflammatory interleukins (ILs) by increasing intracellular signaling. Healthy male (n = 12) volunteers participated in high-intensity running (12 × 5 min, 10% decline, 15 km/h). Blood sample (pre, post, 4 h) analyses included serum concentrations of IL-1ß, IL-1ra, IL-4, IL-6, IL-8, IL-10, matrix metalloprotease-9 (MMP-9) and creatine kinase (CK). Intracellular IL-6, IL-10, MPO and STAT3/SOCS3 signaling were assessed in mononuclear cells. CK (1573 ± 756 u/L), MMP-9 (101 ± 27 ng/mL), neutrophil (9.89 ± 0.76 × 10(9) cells/L) and monocyte counts (1 ± 0.08 × 10(9) cells/L) increased at 4 h. At 4 h serum (7.1 ± 1.3 ng/mL) and monocyte MPO (1.7-fold) increased, whereas neutrophil MPO decreased (0.8-fold). Intracellular monocyte IL-10 and IL-6 decreased by 15% and 20-30%, respectively, coinciding with elevations in serum IL-10 of 14.5 ± 4.7 pg/mL and IL-6 of 5.4 ± 2.9 pg/mL, suggesting immune cell cytokine release in response to exercise. Intracellular PBMC p-STAT3 to total STAT3 ratio increased from pre to 4 h. Circulating monocytes are responsive to increased serum IL-6 suggesting a negative feedback loop via STAT3 signaling.

Skeletal muscle atrophy: disease-induced mechanisms may mask disuse atrophy.

Disuse atrophy is the loss of skeletal muscle mass due to inactivity or lower than 'normal' use. It is not only a furtive component of the 'modern' sedentary lifestyle but also a part of numerous pathologies, where muscle loss is linked to disease specific and/or other toxicity factors, eventually leading to wasting (cachexia). Whether disuse-or-disease induced, muscle loss leads to weakness and metabolic comorbidities with a high societal and financial cost. This review discusses the intricate network of interacting signalling pathways including Atrogin-1/MAFbx, IGF1-Akt, myostatin, glucocorticoids, NF-kB, MAPKs and caspases that seem to regulate disuse atrophy but also share common activation patterns in other states of muscle loss such as sarcopenia or cachexia. Reactive oxygen species are also important regulators of cell signalling pathways that can accelerate proteolysis and depress protein synthesis. Exercise is an effective countermeasure and antioxidants may show some benefit. We discuss how the experimental model used can crucially affect the outcome and hence our understanding of atrophy. Timing of sampling is crucial as some signalling mechanisms reach their peak early during the atrophy process to rapidly decline thereafter, while other present high levels even weeks and months after study initiation. The importance of such differences lays in future consideration of appropriate treatment targets. Apart from attempting to correct defective genes or negate their effects, technological advances in new rational ways should aim to regulate specific gene expression at precise time points for the treatment of muscle atrophy in therapeutic protocols depending on the origin of atrophy induction.

ROCK-2 is associated with focal adhesion maturation during myoblast migration.

Satellite cell migration is critical for skeletal muscle growth and regeneration. Controlled cell migration is dependent on the formation of mature focal adhesions between the cell and the underlying extracellular matrix (ECM). These cell-ECM interactions trigger the activation of signalling events such as the Rho/ROCK pathway. We have previously identified a specific role for ROCK-2 during myoblast migration. In this study we report that ROCK inhibition with Y-27632 increases C2C12 myoblast velocity, but at the expense of directional migration. In response to Y-27632 an increased number of smaller focal adhesions were distributed across adhesion sites that in turn were clearly larger than sites in untreated cells, suggesting a reduction in focal adhesion maturation. We also confirm ROCK-2 localisation to the focal adhesion sites in migrating myoblasts and demonstrate a change in the distribution of these ROCK-2 containing adhesions in response to Y-27632. Taken together, our observations provide further proof that ROCK-2 regulates directional myoblast migration through focal adhesion formation and maturation.

Greater performance impairment of black runners than white runners when running in hypoxia.

This study aimed to compare the response of performance-matched black and white runners during maximal and sub-maximal running in normoxic and hypoxic conditions. 14 well-trained runners (8 black, 6 white) performed 2 incremental maximal exercise tests and 2 fatigue resistance tests at 21% O2 (normoxia) or 14% O2 (hypoxia). Respiratory parameters, heart rate (HR), lactate concentration ([La(-)]) as well as arterial saturation (SpO2) were measured. Enzyme activities and myosin heavy chain content (MHC) were also measured. White runners reached a significantly greater peak treadmill speed and a higher HRmax than black runners in hypoxia (p<0.05). Additionally, White runners achieved a greater time to fatigue than black runners (p<0.05), with black runners displaying a greater decline in performance in hypoxia compared to normoxia (20.3% vs. 13.4%, black vs. white, respectively). However, black runners presented lower [La(-)] and higher SpO2 than white runners in hypoxia (p<0.05). Black runners had a higher proportion of MHC IIa and higher lactate dehydrogenase activity (p<0.05). The greater performance impairment observed in black runners in hypoxia suggests a greater performance sensitivity to this condition, despite the maintenance of physiological variables such as SpO2 and [La (-) ] within a smaller range than white runners.

Acute change of titin at mid-sarcomere remains despite 8 wk of plyometric training.

The purpose of this study was to investigate skeletal muscle changes induced by an acute bout of plyometric exercise (PlyEx) both before and after PlyEx training, to understand if titin is affected differently after PlyEx training. Healthy untrained individuals (N = 11) completed the 1stPlyEx (10 × 10 squat-jumps, 1-min rest). Thereafter, six subjects completed 8 wk of PlyEx, while five controls abstained from any jumping activity. Seven days after the last training session, all subjects completed the 2ndPlyEx. Blood samples were collected before and 6 h and 1, 2, 3, and 4 days after each acute bout of PlyEx, and muscle biopsies 4 days before and 3 days after each acute bout of PlyEx. The 1stPlyEx induced an increase in circulating myoglobin concentration. Muscle sample analysis revealed Z-disk streaming, a stretch or a fragmentation of titin (immunogold), and increased calpain-3 autolysis. After training, 2ndPlyEx did not induce Z-disk streaming or calpain-3 activation. The previously observed post-1stPlyEx positional change of the titin COOH terminus was still present pre-2ndPlyEx, in all trained and all control subjects. Only two controls presented with Z-disk streaming after 2ndPlyEx, while calpain-3 activation was absent in all controls. Eccentric explosive exercise induced a stretch or fragmentation of titin, which presented as a positional change of the COOH terminus. Calpain-3 activation does not occur when titin is already stretched before explosive jumping. Enzymatic digestion results in titin fragmentation, but since an increase in calpain-3 autolysis was visible only after the 1stPlyEx acute bout, fragmentation cannot explain the prolonged positional change.

Variable inflammation and intramuscular STAT3 phosphorylation and myeloperoxidase levels after downhill running.

Individual responses in creatine kinase (CK) release after eccentric exercise are divergent. This study aimed to identify whether this could be related to selected humoral or intramuscular inflammatory factors. Twenty-three subjects were divided into non-exercising (n = 5) and downhill run (DHR; n = 18) groups (12 × 5 min, 10% decline at 15 km/h). Blood samples were analyzed for white blood cell differential count, CK, myoglobin, tumor necrosis factor-α, granulocyte colony-stimulating factor, interleukin (IL)-1ß, IL-6, and IL-10. Muscle biopsies were analyzed for signal transducer and activator of transcription-3 (STAT3), IκBα, and myeloperoxidase (MPO). DHR participants clustered as early (DHR1) recovery, biphasic response (DHR2), or classic delayed exaggerated CK response (DHR3), with a delayed CK peak (4784 ± 1496 U/L) on day 4. For DHR1 and DHR2, CK peaked on day 1 (DHR1: 1198 ± 837 U/L) or on day 1 and day 4 (DHR2: 1583 ± 448 U/L; 1878 ± 427 U/L), respectively. Immediately post-DHR, IL-6 increased in DHR2 and DHR3 whereas IL-10 increased in all DHR groups. STAT3 signaling increased for DHR1 and DHR2 at 4 h, but MPO at day 2 only in DHR2. Objective cluster analysis uncovered a group of subjects with a characteristic biphasic CK release after DHR. The second elevation was related to their early cytokine response. The results provide evidence that early responses following eccentric exercise are indicative of later variation.

In vitro myoblast motility models: investigating migration dynamics for the study of skeletal muscle repair.

Skeletal muscle repair requires the migration of myoblasts (activated satellite cells) both to the injury site and then within the wound to facilitate cellular alignment in preparation for differentiation, fusion and eventual healing. Along this journey, the cells encounter a range of soluble and extracellular matrix factors which regulate their movement and ultimately determine how successful the repair process will be. Sub-optimal migration can lead to a number of scenarios, including reduced myoblast numbers entering the wound, poor alignment and insufficient differentiation to correctly repair the damage. It is therefore critical that all aspects of myoblast migration are understood, particularly in response to the changing growth and matrix factor profile prevalent following skeletal muscle injury. Since 1962, when Boyden first introduced his chemotactic chamber, numerous in vitro migration assays have been developed to mimic the wound more closely. These have increased in complexity to account for the complex micro-environment found in vivo during muscle repair and include a range of modified cell exclusion, chemotactic and three-dimensional assays. This review describes and discusses these advances and highlights the importance they have in expanding our understanding of myoblast migration dynamics.

Current evidence that exercise can increase the number of adult stem cells.

The number of adult stem cells (ASCs) is very small, limiting the regenerative potential of tissues. One of the most studied ASCs in humans is the satellite cell (SC), which proliferates and increases pool size under exercise stress and muscle damage. This review examines the growth factor response to specific types of exercise to show the potential of exercise to stimulate not only SC self-renewal, but also other ASCs. We postulate that the same factors that stimulate a high proliferation of SCs in skeletal muscle after physical exercise should also stimulate the proliferation of ASCs in the tissue in which they reside, such as heart, bone, liver and etc. Regular exercise should be promoted, not only for disease prevention, but to maintain a high ASCs reserve and progenitor cell potential for rapid activation in response to future stressors and damage.

Cytokine and satellite cell responses to muscle damage: interpretation and possible confounding factors in human studies.

It is plausible that multiple muscle biopsies following a muscle damaging intervention can exacerbate the inflammatory and subsequent satellite cell responses. To elucidate confounding effects of muscle biopsy procedure on satellite cell number, indirect markers of damage and the inflammatory response following acute downhill running (DHR) were investigated. 10 healthy male participant were divided into a non-exercising control (n = 4) and DHR (12 × 5min bouts, 10 % decline at 85 % VO(2)max) (n = 6) group. Blood samples were taken pre, post and every 24 h for 9 days. Serum was analysed for creatine kinase (CK), myoglobin (Mb), lactate dehydrogenase (LDH), TNF-α, IL-6 and IL-10. Muscle biopsies taken on days 1 and 2 post intervention from opposing legs were analysed for Pax7(+) satellite cells. In the DHR group, Mb (536 ± 277 ng mL(-1)), IL-6 (12.6 ± 4.7 pg mL(-1)) and IL-10 (27.3 ± 11.5 pg mL(-1)) peaked immediately post DHR, while CK (2651 ± 1911 U L(-1)), LDH (202 ± 47 U L(-1)) and TNF-α (25.1 ± 8.7 pg mL(-1)) peaked on day 1. A 30 % increase in Pax7(+) satellite cells on day 1 in the DHR group was no longer apparent on day 2. H&E staining show evidence of phagocytosis in the DHR group. No significant changes over time were observed in the control group for any of the variables measured. Events observed in the DHR group were as a result of the intervention protocol and subsequent muscle damage. The relationship between SC proliferation and pro-inflammatory cytokine release appears to be complex since the IL-6/IL-10 response time differs significantly from the TNF-α response.

Satellite cell count, VO(2max) , and p38 MAPK in inactive to moderately active young men.

Satellite cells (SCs) are responsible for muscle repair following strenuous exercise or injury. SC responses to intervention have been studied, but most studies do not discuss or take into account the substantial variability in SC number among young individuals. We hypothesized that an active lifestyle reflected in higher VO(2max) may be associated with greater SC number. As training alters basal p38-mitogen-activated protein kinase (MAPK) activity, which is associated with SC proliferation, SC count may also correlate with this stress signaling kinase. Muscle biopsies from vastus lateralis of eight male participants were analyzed for fiber type, myogenin, and p38/phospho-p38 MAPK using SDS-PAGE and Western blotting. Immunofluorescence was used to detect Pax7(+) SCs. Two weeks following the biopsy, subjects underwent an incremental treadmill test to determine VO(2max) . A strong positive correlation (P = 0.0087) was found between the number of Pax7(+) nuclei and VO(2max) . Pax7(+) cell number correlated negatively with phospho-p38/p38 MAPK (P = 0.0006), but had no correlation with fiber type or myogenin. SC number is proportional to VO(2max) , and hence it can be postulated that higher levels of physical activity activate SC proliferation but not fusion, underlining the relevance of exercise in stimulating SC pool size even without injury.

Specific muscle adaptations in type II fibers after high-intensity interval training of well-trained runners.

High-intensity interval training (HIIT) forms an important component of endurance athletes' training, but little is known on intramuscular metabolic and fiber type adaptations. This study investigated physiological and skeletal muscle adaptations in endurance runners subjected to 6 weeks HIIT. Eighteen well-trained endurance athletes were subjected to 6 weeks HIIT. Maximal and submaximal exercise tests and muscle biopsies were performed before and after training. Results indicated that peak treadmill speed (PTS) increased (21.0 ± 0.8 vs 22.1 ± 1.2 km/h, P<0.001) and plasma lactate decreased at 64% and 80% PTS (P<0.05) after HIIT. Cross-sectional area of type II fibers tended to have decreased (P=0.06). No changes were observed in maximal oxygen consumption, muscle fiber type, capillary supply, citrate synthase and 3-hydroxyacetyl CoA dehydrogenase activities. Lactate dehydrogenase (LDH) activity increased in homogenate (P<0.05) and type IIa fiber pools (9.3%, P<0.05). The change in the latter correlated with an absolute interval training speed (r=0.65; P<0.05). In conclusion, HIIT in trained endurance runners causes no adaptations in muscle oxidative capacity but increased LDH activity, especially in type IIa fibers and in relation to absolute HIIT speed.

Illuminating the interrelated immune and endocrine adaptations after multiple exposures to short immobilization stress by in vivo blocking of IL-6.

Intermittent psychological stress was induced in adult rats by 2 h/day of immobilization stress for 4 days, with or without blocking the function of IL-6 by using an anti-IL-6 antibody. Basal concentrations of serum corticosterone, IL-1beta, IL-6, and TNF-alpha were assessed 24 h after the last intervention, as were levels of glucocorticoid receptors (GR) and activities of glucocorticoid-inducible enzymes (tyrosine aminotransferase and glutamine synthetase) in muscle and liver. Whole blood cultures were used to assess both spontaneous and LPS-induced reactivity of peripheral blood mononuclear cells. Stress increased corticosterone concentration in a manner partially modulated by IL-6. Serum IL-1beta concentration was downregulated during stress when IL-6 was blocked (P < 0.01). LPS-induced IL-6 secretion by peripheral blood mononuclear cells in vitro correlated positively with serum IL-1beta concentration in antibody-treated groups, independently of stress (R = 0.70 in nonstressed and R = 0.78 in stressed rats; both P < 0.05), whereas serum corticosterone concentration correlated positively with LPS-induced secretion of IL-6 only in control rats (R = 0.66; P < 0.05). Reductions in liver GR levels indicated independent effects of stress (34.5%) and anti-IL-6 antibody (16.7%) and additive effects for both (62.5%). Similar results are reported for vastus muscle. Conversely, stress increased tyrosine aminotransferase and glutamine synthetase activities in muscle and liver with a significant (P < 0.05) effect of anti-IL-6 antibody only seen in stressed livers. In conclusion, IL-6 plays a role in maintaining circulating IL-1beta concentration after multiple exposures to stress, thus promoting a continued elevation of corticosterone release; in peripheral tissues, IL-6 antagonizes the effects of glucocorticoids, especially at the level of GR concentration.

Endocrine and immune effects of dexamethasone in unilateral total knee replacement.

The effect of acute pre-surgery dexamethasone treatment on the inflammatory immune and endocrine responses to orthopaedic surgery was investigated. Whole blood samples were obtained before and 5 days after surgery for immune analysis, and serum was obtained before and 6 h, 3 days and 5 days after surgery for endocrine assessment. Dexamethasone did not affect the post-surgery granulocyte response, but inhibited the increase in monocyte count (an average increase of 38.5% was seen in the control group). Peak C-reactive protein concentration (3 days after surgery) was 51.4% lower in the dexamethasone group than in the control group. Dexamethasone had a major effect on cortisol concentrations and the cortisol:testosterone and cortisol:dehydroepiandrosterone ratios, but no effect on anabolic hormone concentrations. In conclusion, acute pre-surgery dexamethasone treatment may have beneficial effects in the post-surgery period, by limiting the extent of systemic inflammation and the cortisol response.

Prolonged suppression of the innate immune system in the horse following an 80 km endurance race.

REASONS FOR PERFORMING STUDY: An increased susceptibility to bacterial and viral infections of the respiratory tract, which results in a loss of performance, has been reported in racehorses. Much research has focused on the influence of high-intensity exercise of a short duration on immune system function in horses, but scant attention has been given to prolonged endurance exercise as an immune modulator. OBJECTIVES: The objective of this study was to evaluate the effect of an 80 km endurance race on the monocyte and neutrophil oxidative burst, serum cortisol, glutamine and plasma glucose concentrations in 8 endurance-trained horses (mean +/- s.d. age 9.4 +/- 2.2 years). METHODS: Blood samples were drawn from the horses prior to and following an 80 km ride. RESULTS: Mean time for completion of the 80 km race was 306 +/- 40 mins. Immediately post race mean serum cortisol concentration, blood monocyte and neutrophil counts were higher and blood lymphocyte counts and plasma glucose concentration were lower compared with prerace values (P < 0.05). Neutrophil and monocyte oxidative burst activity decreased following the race and had not regained prerace values after 3 days of rest (P < 0.05). CONCLUSIONS: The present study indicates that long duration exercise in horses has a negative impact on the function of the innate immune system that lasts several days post race. Precise mechanisms instigating the fall in innate immune system function are unclear and multifactorial, but may be attributed, at least in part, to a high serum cortisol response during very prolonged exercise. POTENTIAL CLINICAL RELEVANCE: A prolonged bout of exercise results in a long-term suppression of the innate immune system function in horses which may, in part, account for the observed increase of infectious episodes in horses during training.

Plasma lactate concentrations for self-selected maximal effort lasting 1 h.

PURPOSE: We determined the plasma lactate concentrations for 11 well-trained endurance cyclists or triathletes during a laboratory trial to measure maximal distance cycled in 1 h. METHODS: Subjects performed three distance trials, cycling as far as possible in 1 h. Blood samples were taken from a forearm vein every 10 min during the third trial (T). Samples were analyzed by spectrophotometer for plasma lactate concentrations ([La]). RESULTS: During T, subjects cycled 40.8+/-2.2 km at an average of 83+/-4% of their predicted maximum heart rate (HRmax). Minimum and maximum [La] for each subject was noted for minutes 10, 20, 30, 40, and 50. Minimum [La] ranged between 2.8 and 10.3 mmol x L(-1), and maximum [La] ranged between 5.8 and 13.6 mmol x L(-1). The average [La] from minute 10 to 50 was calculated for each subject and ranged from 5.0 to 12.3 mmol x L(-1). This did not correlate with performance (distance covered in 1 h). Therefore, there was a wide range of individual plasma lactate responses to the same laboratory test that simulated an actual race. The overall average [La] for all subjects was 7.6+/-2.1 mmol x L(-1). CONCLUSIONS: These data indicate first that the value of 4 mmol x L(-1), commonly referred to as OBLA, may often underestimate the upper limit of tolerance to lactate during a maximal endurance performance test lasting approximately 1 h. Second, during this type of work, intersubject differences in average plasma lactate concentration do not correlate with performance.