The effect of high-intensity exercise on the respiratory capacity of skeletal muscle

Local and systemic responses to repeated gluteal muscle microbiopsies in mature sedentary horses

We aimed to test the hypothesis that repeated muscle collections would impact mitochondrial function, antioxidant status, and markers of inflammation and muscle damage. Twenty-six horses (8 geldings, 18 mares; mean ± SD 9.5 ± 3.5 y) had gluteus medius muscle biopsy samples collected at: 0 and 24h (n=7); 0 and 6h (n = 6); 0, 6, and 12h (n=7); or 0, 6, 12, and 24h (n=6). Blood was collected from all horses every 6h for 72h, starting 24h prior to the 0h muscle collection. Data were analyzed using mixed linear models. Muscle integrative (per mg tissue) electron transfer capacity of complex II decreased (P=0.004) and intrinsic (relative to citrate synthase (CS) activity) LEAK increased (P<0.03) from 0 to 6h but both returned to 0h levels by 12h. Activity of CS was greater at 0 than 12 and 24h (P≤0.02). Serum creatine kinase (CK) activity was similar from -24 through 0h but increased in all horses at 6h and remained elevated through 48h (P<0.05) though not above reference ranges. Whole blood superoxide dismutase activity fluctuated throughout the 72-h collection period (P=0.03) and serum cortisol concentration displayed a circadian pattern (P<0.0001) but neither were altered by muscle collections. No other variable, including muscle mitochondrial capacities and function, blood and muscle antioxidant status and concentrations of select cytokines, and serum amyloid A, differed by time or muscle collection. Repeated gluteal collections had limited short-term or no effect on physiological markers in unstressed, mature horses except serum CK activity, which should be interpreted with caution during repeated tissue collections.

Combined changes in temperature and pH mimicking exercise result in decreased efficiency in muscle mitochondria

It is well known that exercise efficiency declines at intensities above the lactate threshold, yet the underlying mechanisms are poorly understood. Some have suggested it is due to a decline in mitochondrial efficiency, but this is difficult to examine in vivo. Therefore, the aim of the current study was to examine how changes in temperature and pH, mimicking those that occur during exercise, affect mitochondrial efficiency in skeletal muscle mitochondria. This study was performed on quadriceps muscle of 20 wild-type mice. Muscle tissue was dissected and either permeabilized (n = 10) or homogenized for isolation of mitochondria (n = 10), and oxidative phosphorylation capacity and P/O ratio were assessed using high-resolution respirometry. Samples from each muscle were analyzed in both normal physiological conditions (37°C, pH 7.4), decreased pH (6.8), increased temperature (40°C), and a combination of both. The combination of increased temperature and decreased pH resulted in a significantly lower P/O ratio, mirrored by an increase in leak respiration and a decrease in respiratory control ratio (RCR), in isolated mitochondria. In permeabilized fibers, RCR and leak were relatively unaffected, though a main effect of temperature was observed. Oxidative phosphorylation capacity was unaffected by changes in pH and temperature in both isolated mitochondria and permeabilized fibers. These results indicate that exercise-like changes in temperature and pH lead to impaired mitochondrial efficiency. These findings offer some degree of support to the concept of decreased mitochondrial efficiency during exercise, and may have implications for the assessment of mitochondrial function related to exercise.NEW & NOTEWORTHY To the best of our knowledge, this is the first study to examine the effects of combined changes in temperature and pH, mimicking intramuscular alterations during exercise. Our findings suggest that mitochondrial efficiency is impaired during exercise of moderate to high intensity, which could be a possible mechanism contributing to the decline in exercise efficiency at intensities above the lactate threshold.

Clock genes regulate skeletal muscle energy metabolism through NAMPT/NAD+/SIRT1 following heavy-load exercise

The biological clock is an invisible "clock" in the organism, which can regulate behavior, physiology, and biochemical reactions. However, the relationship between clock genes and energy metabolism in postexercise skeletal muscle is not well known. The purpose of this study was to determine the mechanisms through which peripheral clock genes regulate energy metabolism in skeletal muscle. We analyzed the rhythm of mRNA expression of the clock genes Bmal1 and Clock in skeletal muscle following heavy-load exercise and measured related indicators of mitochondrial structure and function. We obtained the following experimental results. First, heavy-load exercise induced loss of circadian rhythm of Bmal1 between ZT0 and ZT24, and the circadian rhythm of Clock was not restored between ZT0 and ZT72. Second, analysis of mitochondrial morphology in group E showed abnormal swelling and ridge structure damage at ZT0, which recovered somewhat at ZT24 and ZT48, and the damage had essentially disappeared by ZT72. Third, the expression of NAMPT/NAD+/SIRT1 signaling axis proteins in group E was abnormal at ZT0, the content of NAMPT and the activity of SIRT1 significantly increased, and the content of NAD+ significantly decreased. Fourth, the expression of BMAL1 and PGC-1α in group E significantly increased, whereas the ATP and ADP content, as well as the activities of COXII and COXIV, were significantly changed. Finally, the colocalization of BMAL1 and SIRT1 in group E was significantly upregulated at ZT0. These results suggest that the skeletal muscle clock gene Bmal1 may regulate the energy metabolism level of skeletal muscle after exercise through the NAMPT/NAD+/SIRT1 signaling pathway.

The effect of pre-exercise alkalosis on lactate/pH regulation and mitochondrial respiration following sprint-interval exercise in humans

Purpose: The purpose of this study was to evaluate the effect of pre-exercise alkalosis, induced via ingestion of sodium bicarbonate, on changes to lactate/pH regulatory proteins and mitochondrial function induced by a sprint-interval exercise session in humans. Methods: On two occasions separated by 1 week, eight active men performed a 3 × 30-s all-out cycling test, interspersed with 20 min of recovery, following either placebo (PLA) or sodium bicarbonate (BIC) ingestion. Results: Blood bicarbonate and pH were elevated at all time points after ingestion in BIC vs PLA (p < 0.05). The protein content of monocarboxylate transporter 1 (MCT1) and basigin (CD147), at 6 h and 24 h post-exercise, and sodium/hydrogen exchanger 1 (NHE1) 24 h post-exercise, were significantly greater in BIC compared to PLA (p < 0.05), whereas monocarboxylate transporter 4 (MCT4), sodium/bicarbonate cotransporter (NBC), and carbonic anhydrase isoform II (CAII) content was unchanged. These increases in protein content in BIC vs. PLA after acute sprint-interval exercise may be associated with altered physiological responses to exercise, such as the higher blood pH and bicarbonate concentration values, and lower exercise-induced oxidative stress observed during recovery (p < 0.05). Additionally, mitochondrial respiration decreased after 24 h of recovery in the BIC condition only, with no changes in oxidative protein content in either condition. Conclusion: These data demonstrate that metabolic alkalosis induces post-exercise increases in several lactate/pH regulatory proteins, and reveal an unexpected role for acidosis in mitigating the loss of mitochondrial respiration caused by exercise in the short term.

Acute high-intensity exercise and skeletal muscle mitochondrial respiratory function: role of metabolic perturbation

Recently it was documented that fatiguing, high-intensity exercise resulted in a significant attenuation in maximal skeletal muscle mitochondrial respiratory capacity, potentially due to the intramuscular metabolic perturbation elicited by such intense exercise. With the utilization of intrathecal fentanyl to attenuate afferent feedback from group III/IV muscle afferents, permitting increased muscle activation and greater intramuscular metabolic disturbance, this study aimed to better elucidate the role of metabolic perturbation on mitochondrial respiratory function. Eight young, healthy males performed high-intensity cycle exercise in control (CTRL) and fentanyl-treated (FENT) conditions. Liquid chromatography-mass spectrometry and high-resolution respirometry were used to assess metabolites and mitochondrial respiratory function, respectively, pre- and postexercise in muscle biopsies from the vastus lateralis. Compared with CTRL, FENT yielded a significantly greater exercise-induced metabolic perturbation (PCr: -67% vs. -82%, Pi: 353% vs. 534%, pH: -0.22 vs. -0.31, lactate: 820% vs. 1,160%). Somewhat surprisingly, despite this greater metabolic perturbation in FENT compared with CTRL, with the only exception of respiratory control ratio (RCR) (-3% and -36%) for which the impact of FENT was significantly greater, the degree of attenuated mitochondrial respiratory capacity postexercise was not different between CTRL and FENT, respectively, as assessed by maximal respiratory flux through complex I (-15% and -33%), complex II (-36% and -23%), complex I + II (-31% and -20%), and state 3_CI+CII control ratio (-24% and -39%). Although a basement effect cannot be ruled out, this failure of an augmented metabolic perturbation to extensively further attenuate mitochondrial function questions the direct role of high-intensity exercise-induced metabolite accumulation in this postexercise response.

Venous Blood Acid-Base Status in Show Jumper Horses Subjected to Different Physical Exercises

The aim of this study was to assess whether acid-base profile exhibits changes in regularly trained show jumping horses undergoing increasing exercise workloads. Seven female Italian saddle horses were subjected to three different physical exercise trials of increasing workload identified as three exercise phases (EPs). During EP_I horses were subjected to a standardized exercise test consisting of 15 minutes of treadmill, during EP_II horses were subjected to a show jumping test (height, 0.9-1.1 m; course length, 300 m), during EP_III horses underwent two jumping sessions carried out over two consecutive days. Blood samples were collected at rest (T_PRE), after exercise (T_POST), and 30 minutes after the end of exercise (T_POST30). The values of pH, partial pressure of carbon dioxide (Pco₂), partial pressure of oxygen (Po₂), bicarbonate level (HCO₃^-), hemoglobin (Hb), and hematocrit (Hct) were measured. A significant effect of exercise workload and time (P < .001) on Po₂, Pco₂, HCO₃^-, Hb, and Hct values was found. The variation in the studied parameters resulted mostly reversible within T_POST30 in horses when subjected to EP_I and EP_II, whereas Po₂, Hb, and Hct remained higher at T_POST30 than T_PRE in horses when subjected to the second day of jumping section (EP_III) indicating a failure to recover. The results suggest that jumping sessions carried out over two consecutive days represent extra workload for horses, and this should be taken into account by veterinarian to prevent acid-base imbalance and for the maintenance of health and performance in equine athletes.

Sport science relevance and integration in horseracing: perceptions of UK racehorse trainers

Whilst equestrian sport science research has expanded over recent years, and technologies to positively impact training and performance have been developed, long-standing traditions and experiential learning in the racing industry still appear to impede the integration of sport science knowledge. This study used semi-structured interviews to investigate the perceptions of eleven national hunt and flat-based racehorse trainers to determine the current status of sport science integration within the racing industry, the perceived barriers to its uptake, and areas where trainers sought further knowledge. Three key higher order themes emerged from the interviews: the current training and monitoring principles for health and fitness of racehorses, trainers’ attitudes toward sport science research, and areas for potential future research and integration of sports science in training. Subjective methods grounded in personal experience were found to form the basis of racehorse training principles, with the application of sport science minimal, namely due to poor integration strategies. Negative connotations arising from a general lack of understanding of the application of knowledge and a scepticism toward adapting already successful principles, as well as pressure from industry stakeholders, appear to create barriers to sport science uptake. Trainers felt a stronger evidence base emphasising performance benefits is needed to overcome these. Where trainers identified areas of research potential, many studies had already been undertaken, highlighting the necessity for effective dissemination strategies to demonstrate how research could apply to industry practice. Increased educational initiatives to showcase technology and improve trainer understanding and application of currently available sport science knowledge is also warranted.

Acute High-Intensity Exercise Impairs Skeletal Muscle Respiratory Capacity

PURPOSE

The effect of an acute bout of exercise, especially high-intensity exercise, on the function of mitochondrial respiratory complexes is not well understood, with potential implications for both the healthy population and patients undergoing exercise-based rehabilitation. Therefore, this study sought to comprehensively examine respiratory flux through the different complexes of the electron transport chain in skeletal muscle mitochondria before and immediately after high-intensity aerobic exercise.

METHODS

Muscle biopsies of the vastus lateralis were obtained at baseline and immediately after a 5-km time trial performed on a cycle ergometer. Mitochondrial respiratory flux through the complexes of the electron transport chain was measured in permeabilized skeletal muscle fibers by high-resolution respirometry.

RESULTS

Complex I + II state 3 (state 3CI + CII) respiration, a measure of oxidative phosphorylation capacity, was diminished immediately after the exercise (pre, 27 ± 3 ρm·mg·s; post, 17 ± 2 ρm·mg·s; P < 0.05). This decreased oxidative phosphorylation capacity was predominantly the consequence of attenuated complex II-driven state 3 (state 3CII) respiration (pre, 17 ± 1 ρm·mg·s; post, 9 ± 2 ρm·mg·s; P < 0.05). Although complex I-driven state 3 (3CI) respiration was also lower (pre, 20 ± 2 ρm·mg·s; post, 14 ± 4 ρm·mg·s), this did not reach statistical significance (P = 0.27). In contrast, citrate synthase activity, proton leak (state 2 respiration), and complex IV capacity were not significantly altered immediately after the exercise.

CONCLUSIONS

These findings reveal that acute high-intensity aerobic exercise significantly inhibits skeletal muscle state 3CII and oxidative phosphorylation capacity. This, likely transient, mitochondrial defect might amplify the exercise-induced development of fatigue and play an important role in initiating exercise-induced mitochondrial adaptations.

Assessment of two methods to determine the relative contributions of the aerobic and anaerobic energy systems in racehorses

A prospective, randomized, controlled study was designed to determine relative aerobic and anaerobic (lactic and alactic) contributions at supramaximal exercise intensities using two different methods. Thoroughbred racehorses (n = 5) performed a maximal rate of oxygen consumption (V̇o2max) test and three supramaximal treadmill runs (105, 115, and 125% V̇o2max). Blood lactate concentration (BL) was measured at rest, every 15 s during runs, and 2, 5, 10, 20, 30, 40, 50, and 60 min postexercise. In method 1, oxygen demand was calculated for each supramaximal intensity based on the V̇o2max test, and relative aerobic and anaerobic contributions were calculated from measured V̇o2 and the accumulated oxygen deficit. In method 2, aerobic contribution was calculated using the trapezoidal method to determine V̇o2 during exercise. A monoexponential model was fitted to the postexercise V̇o2 curve. Alactic contribution was calculated using the coefficients of this model. Lactate anaerobic contribution was calculated by multiplying the peak to resting change in BL by 3. Linear mixed-effects models were used to examine the effects of exercise intensity and method (as fixed effects) on measured outcomes (P ≤ 0.05). Relative aerobic and anaerobic contributions were not different between methods (P = 0.20). Horses' mean contributions were 81.4, 77.6, and 72.5% (aerobic), and 18.5, 22.3, and 27.4% (anaerobic) at 105, 115, and 125% V̇o2max, respectively. Individual alactic anaerobic energy was not different between supramaximal exercise intensities (P = 0.43) and was negligible, contributing a mean of 0.11% of the total energy. Relative energy contributions can be calculated using measured V̇o2 and BL in situations where the exercise intensity is unknown. Understanding relative metabolic demands could help develop tailored training programs. NEW & NOTEWORTHY Relative energy contributions of horses can be calculated using measured V̇o2 and BL in situations where the exercise intensity is unknown. Horses' mean contributions were 81.4, 77.6, and 72.5% (aerobic), and 18.5, 22.3, and 27.4% (anaerobic) at 105, 115, and 125% of V̇o2max, respectively. Individual alactic capacity was unaltered between supramaximal exercise intensities and accounted for a mean contribution of 0.11% of energy use.

Near infrared spectroscopy-guided exercise training for claudication in peripheral arterial disease

Rationale

Supervised treadmill exercise for claudication in peripheral arterial disease is effective but poorly tolerated because of ischemic leg pain. Near infrared spectroscopy allows non-invasive detection of muscle ischemia during exercise, allowing for characterization of tissue perfusion and oxygen utilization during training.

Objective

We evaluated walking time, muscle blood flow, and muscle mitochondrial capacity in patients with peripheral artery disease after a traditional pain-based walking program and after a muscle oxygen-guided walking program.

Method and results

Patients with peripheral artery disease trained thrice weekly in 40-minute-long sessions for 12 weeks, randomized to oxygen-guided training ( n = 8, age 72 ± 9.7 years, 25% female) versus traditional pain-based training ( n = 10, age 71.6 ± 8.8 years, 20% female). Oxygen-guided training intensity was determined by maintaining a 15% reduction in skeletal muscle oxygenation by near infrared spectroscopy rather than relying on symptoms of pain to determine exercise effort. Pain free and maximal walking times were measured with a 12-minute Gardner treadmill test. Gastrocnemius mitochondrial capacity and blood flow were measured using near infrared spectroscopy. Baseline pain-free walking time was similar on a Gardner treadmill test (2.5 ± 0.9 vs. 3.6 ± 1.0 min, p = 0.5). After training, oxygen-guided cohorts improved similar to pain-guided cohorts (pain-free walking time 6.7 ± 0.9 vs. 6.9 ± 1.1 min, p < 0.01 for change from baseline and p = 0.97 between cohorts). Mitochondrial capacity improved in both groups but more so in the pain-guided cohort than in the oxygen-guided cohort (38.8 ± 8.3 vs. 14.0 ± 9.3, p = 0.018). Resting muscle blood flow did not improve significantly in either group with training.

Conclusions

Oxygen-guided exercise training improves claudication comparable to pain-based training regimens. Adaptations in mitochondrial function rather than increases in limb perfusion may account for functional improvement. Increases in mitochondrial oxidative capacity may be proportional to the degree of tissue hypoxia during exercise.

Acute exercise alters skeletal muscle mitochondrial respiration and H2O2 emission in response to hyperinsulinemic-euglycemic clamp in middle-aged obese men

Obesity, sedentary lifestyle and aging are associated with mitochondrial dysfunction and impaired insulin sensitivity. Acute exercise increases insulin sensitivity in skeletal muscle; however, whether mitochondria are involved in these processes remains unclear. The aim of this study was to investigate the effects of insulin stimulation at rest and after acute exercise on skeletal muscle mitochondrial respiratory function (JO₂) and hydrogen peroxide emission (JH₂O₂), and the associations with insulin sensitivity in obese, sedentary men. Nine men (means ± SD: 57 ± 6 years; BMI 33 ± 5 kg.m²) underwent hyperinsulinemic-euglycemic clamps in two separate trials 1–3 weeks apart: one under resting conditions, and another 1 hour after high-intensity exercise (4x4 min cycling at 95% HR_peak). Muscle biopsies were obtained at baseline, and pre/post clamp to measure JO₂ with high-resolution respirometry and JH₂O₂ via Amplex UltraRed from permeabilized fibers. Post-exercise, both JO₂ and JH₂O₂ during ADP stimulated state-3/OXPHOS respiration were lower compared to baseline (P<0.05), but not after subsequent insulin stimulation. JH₂O₂ was lower post-exercise and after subsequent insulin stimulation compared to insulin stimulation in the rest trial during succinate supported state-4/leak respiration (P<0.05). In contrast, JH₂O₂ increased during complex-I supported leak respiration with insulin after exercise compared with resting conditions (P<0.05). Resting insulin sensitivity and JH₂O₂ during complex-I leak respiration were positively correlated (r = 0.77, P<0.05). We conclude that in obese, older and sedentary men, acute exercise modifies skeletal muscle mitochondrial respiration and H₂O₂ emission responses to hyperinsulinemia in a respiratory state-specific manner, which may have implications for metabolic diseases involving insulin resistance.

Ex vivo measures of muscle mitochondrial capacity reveal quantitative limits of oxygen delivery by the circulation during exercise

Muscle mitochondrial respiratory capacity measured ex vivo provides a physiological reference to assess cellular oxidative capacity as a component in the oxygen cascade in vivo. In this article, the magnitude of muscle blood flow and oxygen uptake during exercise involving a small-to-large fraction of the body mass will be discussed in relation to mitochondrial capacity measured ex vivo. These analyses reveal that as the mass of muscle engaged in exercise increases from one-leg knee extension, to 2-arm cranking, to 2-leg cycling and x-country skiing, the magnitude of blood flow and oxygen delivery decrease. Accordingly, a 2-fold higher oxygen delivery and oxygen uptake per unit muscle mass are seen in vivo during 1-leg exercise compared to 2-leg cycling indicating a significant limitation of the circulation during exercise with a large muscle mass. This analysis also reveals that mitochondrial capacity measured ex vivo underestimates the maximal in vivo oxygen uptake of muscle by up to ∼2-fold. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

Physical fitness and mitochondrial respiratory capacity in horse skeletal muscle

BACKGROUND

Within the animal kingdom, horses are among the most powerful aerobic athletic mammals. Determination of muscle respiratory capacity and control improves our knowledge of mitochondrial physiology in horses and high aerobic performance in general.

METHODOLOGY/PRINCIPAL FINDINGS

We applied high-resolution respirometry and multiple substrate-uncoupler-inhibitor titration protocols to study mitochondrial physiology in small (1.0-2.5 mg) permeabilized muscle fibres sampled from triceps brachii of healthy horses. Oxidative phosphorylation (OXPHOS) capacity (pmol O(2) • s(-1) • mg(-1) wet weight) with combined Complex I and II (CI+II) substrate supply (malate+glutamate+succinate) increased from 77 ± 18 in overweight horses to 103 ± 18, 122 ± 15, and 129 ± 12 in untrained, trained and competitive horses (N = 3, 8, 16, and 5, respectively). Similar to human muscle mitochondria, equine OXPHOS capacity was limited by the phosphorylation system to 0.85 ± 0.10 (N = 32) of electron transfer capacity, independent of fitness level. In 15 trained horses, OXPHOS capacity increased from 119 ± 12 to 134 ± 37 when pyruvate was included in the CI+II substrate cocktail. Relative to this maximum OXPHOS capacity, Complex I (CI)-linked OXPHOS capacities were only 50% with glutamate+malate, 64% with pyruvate+malate, and 68% with pyruvate+malate+glutamate, and ~78% with CII-linked succinate+rotenone. OXPHOS capacity with glutamate+malate increased with fitness relative to CI+II-supported ETS capacity from a flux control ratio of 0.38 to 0.40, 0.41 and 0.46 in overweight to competitive horses, whereas the CII/CI+II substrate control ratio remained constant at 0.70. Therefore, the apparent deficit of the CI- over CII-linked pathway capacity was reduced with physical fitness.

CONCLUSIONS/SIGNIFICANCE

The scope of mitochondrial density-dependent OXPHOS capacity and the density-independent (qualitative) increase of CI-linked respiratory capacity with increased fitness open up new perspectives of integrative and comparative mitochondrial respiratory physiology.

Differential modeling of anaerobic and aerobic metabolism in the 800-m and 1,500-m run

This study examined the hypothesis that running speed over 800- and 1,500-m races is regulated by the prevailing anaerobic (oxygen independent) store (ANS) at each instant of the race up until the all-out phase of the race over the last several meters. Therefore, we hypothesized that the anaerobic power that allows running above the speed at maximal oxygen uptake (V̇o2max) is regulated by ANS, and as a consequence the time limit at the anaerobic power (tlim PAN = ANS/PAN) is constant until the final sprint. Eight 800-m and seven 1,500-m male runners performed an incremental test to measure V̇o2max and the minimal velocity associated with the attainment of V̇o2max ( vV̇o2max), referred to as maximal aerobic power, and ran the 800-m or 1,500-m race with the intent of achieving the lowest time possible. Anaerobic power (PAN) was measured as the difference between total power and aerobic power, and instantaneous ANS as the difference between end-race and instantaneous accumulated oxygen deficits. In 800 m and 1,500 m, tlim PAN was constant during the first 70% of race time in both races. Furthermore, the 1,500-m performance was significantly correlated with tlim PAN during this period ( r = −0.92, P < 0.01), but the 800-m performance was not ( r = −0.05, P = 0.89), although it was correlated with the end-race oxygen deficit ( r = −0.70, P = 0.05). In conclusion, this study shows that in middle-distance races over both 800 m and 1,500 m, the speed variations during the first 70% of the race time serve to maintain constant the time to exhaustion at the instantaneous anaerobic power. This observation is consistent with the hypothesis that at any instant running speed is controlled by the ANS remaining.

Training rather than walking: the test in -train out program for home-based rehabilitation in peripheral arteriopathy

BACKGROUND

Exercise training reduces walking disability in peripheral arterial disease (PAD). This non-randomized study evaluates the effects on walking ability and hemodynamic parameters of a novel approach to home-based rehabilitation, the test in -train out program (Ti-To), compared with the traditional home-based free walking exercise (Tr-E).

METHODS AND RESULTS

A total of 143 patients with claudication (117 men, average age 68+/-10 years), were included in a Ti-To (n=83) or Tr-E program (n=60). Evaluations, which were carried out upon entry and at 1, 2, 3, 4 and 6 months, included: self-reported claudication, walking ability (ie, absolute claudication distance, pain threshold speed), resting/exercise heart rates (HR), systolic/diastolic brachial pressure (SBP/DBP), ankle pressure (AP), ankle-brachial index (ABI). Ti-To involved 2 daily 10-min home walking sessions at maximal asymptomatic speed and the patient attending monthly check-ups at hospital. Tr-E involved 20-30 min of daily walking at self-selected speeds up to pain tolerance. A total of 126 patients (Ti-To, n=74; Tr-E, n=52) completed the program. Ti-To induced better relief from claudication (p=0.001). Functional parameters improved significantly for both groups (p<0.0001) with significant intergroup difference for Ti-To (p<0.0001). SBP and exercise HR decreased significantly in both groups, with Ti-To improving resting HR (p=0.0002), DBP (p=0.003), lowest AP worse limb (p=0.004) and ABI worse limb (p=0.0002).

CONCLUSIONS

In patients with PAD, a Ti-To program had more positive effects on perceived claudication, and functional and hemodynamic parameters than did a Tr-E program.

Overreaching-induced oxidative stress, enhanced HSP72 expression, antioxidant and oxidative enzymes downregulation

Overreaching (OVR) is defined as the initial phase of overtraining syndrome and is known as a metabolic imbalance leading to short-term fatigue. Exercise increases reactive oxygen species production, which can oxidize intracellular structures impairing cell function and thus leads to OVR process. The aim of this work is to study the behavior of oxidative stress markers in subjects submitted to an OVR protocol. Thirty rats were divided in exercise and control group, and submitted to an 8-week-endurance training (ET) and a 3-week-OVR protocol. Thiobarbituric acid reactive substances (TBARs), reactive carbonylated derivatives (RCD), glutathione reductase (GR), catalase (CAT) and citrate synthase (CS) activities and stress protein HSP72 were measured in soleus (SO), extensor digital longus (EDL) and semitendinuous (ST) muscles. ET induced significant enhancement (P<0.05) in CS, GR, CAT, TBARs, RCD and HSP72 in SO, EDL and ST. OVR induced higher levels (P<0.05) of TBARs, RCD and HSP72 compared with ET only in SO, while in EDL and ST all measured parameters ranged at same levels reached during ET. We concluded that stress-induced OVR protocol is fiber type dependent, the SO muscle fiber type I being the most affected by this treatment.

Effects of intermittent hypoxic training on amino and fatty acid oxidative combustion in human permeabilized muscle fibers

The effects of concurrent hypoxic/endurance training on mitochondrial respiration in permeabilized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1-W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired Po(2) (Pi(O(2)))] or hypoxic conditions ( approximately 100 Torr Pi(O(2)), approximately 3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) increased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O(2) uptake (Vo(2 max)) remained unchanged: 58.1 +/- 0.8 vs. 61.0 +/- 1.2 ml.kg(-1).min(-1) and 58.5 +/- 0.7 vs. 58.3 +/- 0.6 ml.kg(-1).min(-1) for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 +/- 0.37 vs. 8.51 +/- 0.33 mumol O(2).min(-1).g dry weight(-1)) and significantly decreased for palmitate + malate (3.88 +/- 0.23 vs. 2.77 +/- 0.08 mumol O(2).min(-1).g dry weight(-1)) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in Vo(2 max), and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.

Repeated static contractions increase mitochondrial vulnerability toward oxidative stress in human skeletal muscle

Repeated static contractions (RSC) induce large fluctuations in tissue oxygen tension and increase the generation of reactive oxygen species (ROS). This study investigated the effect of RSC on muscle contractility, mitochondrial respiratory function, and in vitro sarcoplasmic reticulum (SR) Ca(2+) kinetics in human muscle. Ten male subjects performed five bouts of static knee extension with 10-min rest in between. Each bout of RSC (target torque 66% of maximal voluntary contraction torque) was maintained to fatigue. Muscle biopsies were taken preexercise and 0.3 and 24 h postexercise from vastus lateralis. Mitochondria were isolated and respiratory function measured after incubation with H(2)O(2) (HPX) or control medium (Con). Mitochondrial function was not affected by RSC during Con. However, RSC exacerbated mitochondrial dysfunction during HPX, resulting in decreased respiratory control index, decreased mitochondrial efficiency (phosphorylated ADP-to-oxygen consumed ratio), and increased noncoupled respiration (HPX/Con post- vs. preexercise). SR Ca(2+) uptake rate was lower 0.3 vs. 24 h postexercise, whereas SR Ca(2+) release rate was unchanged. RSC resulted in long-lasting changes in muscle contractility, including reduced maximal torque, low-frequency fatigue, and faster torque relaxation. It is concluded that RSC increases mitochondrial vulnerability toward ROS, reduces SR Ca(2+) uptake rate, and causes low-frequency fatigue. Although conclusive evidence is lacking, we suggest that these changes are related to increased formation of ROS during RSC.

Flight activity, mortality rates, and lipoxidative damage in Drosophila

In this study, the effect of flight activity on mortality rates and lipoxidative damage in Drosophila was determined to identify mechanisms through which oxidative damage affects life span. The results showed that flies allowed flying throughout life had higher mortality rates and decreased median and maximum life spans compared to controls. The mortality rate of the flight activity group could be lowered, but not completely reversed by switching to control conditions; and the accrued oxidative damage could not be eliminated. The levels of reactive oxygen species produced by mitochondria isolated from high activity and control flies did not differ significantly. However, the high activity flies had altered membrane fatty acid compositions, which made them prone to increased lipid peroxidation. The effect of flight activity on insect life span differs considerably from the beneficial effects of exercise in mammals; these differences may be caused by physiological differences between the two taxa.

Carbofuran-induced oxidative stress in slow and fast skeletal muscles: prevention by memantine and atropine

Acute toxic effects of acetylcholinesterase (AChE) inhibitors on skeletal muscles are thought to involve oxidative stress with increased generation of free radicals such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Muscle hyperactivity with its increased oxygen and energy consumption appear to be the primary cause of oxidative stress. The present investigation was therefore undertaken to establish the normal levels of F(2)-isoprostanes (F(2)-IsoPs, specific markers of ROS/oxidative stress), citrulline (determinant of NO/NOS and marker of RNS), and high-energy phosphates (HEP: adenosine triphosphate, ATP and phosphocreatine, PCr) in slow (soleus) and fast (extensor digitorum longus, EDL) muscles of rats. In addition, we aimed to determine if memantine HCl (MEM), in combination with atropine sulfate (ATS), prevents carbofuran-induced changes in markers of oxidative stress. Control values were not significantly different for F(2)-IsoPs (1.142 +/- 0.027 and 1.177 +/- 0.092 ng/g) and citrulline (469.7 +/- 31.8 and 417.8 +/- 18.5 nmol/g) in soleus and EDL muscles, while the values were different for HEP (ATP, 3.66 +/- 0.11 and 5.85 +/- 0.14 micromol/g; PCr, 7.91 +/- 0.26 and 13.14 +/- 0.31 micromol/g). Rats acutely intoxicated with carbofuran (1.5 mg/kg, s.c.) showed the signs of maximal toxicity including muscle hyperactivity within 60 min of exposure. At this time, F(2)-IsoPs (177 and 153%) and citrulline (267 and 304%) levels were significantly increased, while ATP (46 and 43%) and PCr (44 and 46%) levels were decreased in soleus and EDL, respectively. Rats pretreated with MEM (18 mg/kg, s.c.) and ATS (16 mg/kg, s.c.), 60 and 15 min prior to carbofuran, respectively, showed no signs of toxicity. MEM in combination with ATS protected muscles from carbofuran-induced hyperactivity and attenuated increases in F(2)-IsoPs and citrulline, and depletion of HEP. Carbofuran-induced changes and protection by MEM and ATS were of similar magnitude in both muscles. These findings indicate that carbofuran-induced muscle hyperactivity produces oxidative stress as measured by increased ROS and RNS generation, and HEP depletion. MEM and ATS prevent the carbofuran-induced chain of events involved in oxidative stress.

Relationships between maximal muscle oxidative capacity and blood lactate removal after supramaximal exercise and fatigue indexes in humans

The present study investigated whether blood lactate removal after supramaximal exercise and fatigue indexes measured during continuous and intermittent supramaximal exercises are related to the maximal muscle oxidative capacity in humans with different training status. Lactate recovery curves were obtained after a 1-min all-out exercise. A biexponential time function was then used to determine the velocity constant of the slow phase (γ2), which denoted the blood lactate removal ability. Fatigue indexes were calculated during all-out (FIAO) and repeated 10-s cycling sprints (FISprint). Biopsies were taken from the vastus lateralis muscle, and maximal ADP-stimulated mitochondrial respiration ( Vmax) was evaluated in an oxygraph cell on saponin-permeabilized muscle fibers with pyruvate + malate and glutamate + malate as substrates. Significant relationships were found between γ2 and pyruvate + malate Vmax ( r = 0.60, P < 0.05), γ2 and glutamate + malate Vmax ( r = 0.66, P < 0.01), and γ2 and citrate synthase activity ( r = 0.76, P < 0.01). In addition, γ2, glutamate + malate Vmax, and pyruvate + malate Vmax were related to FIAO (γ2 − FIAO: r = 0.85; P < 0.01; glutamate + malate Vmax − FIAO: r = 0.70, P < 0.01; and pyruvate + malate Vmax − FIAO: r = 0.63, P < 0.01) and FISprint (γ2 − FISprint: r = 0.74, P < 0.01; glutamate + malate Vmax − FISprint: r = 0.64, P < 0.01; and pyruvate + malate Vmax − FISprint: r = 0.46, P < 0.01). In conclusion, these results suggested that the maximal muscle oxidative capacity was related to blood lactate removal ability after a 1-min all-out test. Moreover, maximal muscle oxidative capacity and blood lactate removal ability were associated with the delay in the fatigue observed during continuous and intermittent supramaximal exercises in well-trained subjects.

Effects of prolonged training, overtraining and detraining on skeletal muscle metabolites and enzymes

Thirteen Standardbred horses trained intensively for 34 weeks and detrained for 12 weeks to investigate the effects of training, overtraining and detraining on muscle metabolites, buffering capacity and enzyme activities (CS, HAD and LDH). After a standardised exercise test to fatigue at 10 m/s (approximately 100% VO2max), there was significant depletion of [ATP], [PCr] and muscle [glycogen] and accumulation of muscle and plasma [lactate], [NH3] and elevated muscle temperature. After training, associated with increased run time to fatigue (148%), there was reduced depletion of muscle [glycogen] and increased [NH3] and muscle temperature at fatigue. Training resulted in increased muscle buffering capacity (19%) and activities of CS (29%) and HAD (32%) and reduced glycogen utilisation (1.32 mmol/s in week 1 to 0.58 mmol/s in week 32). Plasma [lactate] at fatigue increased with training as opposed to muscle [lactate] implying enhanced ability to remove lactate from muscle. Overtraining resulted in reduced run time and associated effects in overtrained horses. While muscle [glycogen] prior to exercise was lower in overtrained horses, glycogen utilisation/s was not reduced and it may not, therefore, have caused the reduced run time. Prolonged high intensity training caused primarily aerobic adaptations and poor performance associated with overtraining may not be due to metabolic disturbances.

Depletion of energy metabolites following acetylcholinesterase inhibitor-induced status epilepticus: protection by antioxidants

Status epilepticus (SE)-induced neuronal injury may involve excitotoxicity, energy impairment and increased generation of reactive oxygen species (ROS). Potential treatment therefore should consider agents that protect mitochondrial function and ROS scavengers. In the present study, we examined whether the spin trapping agent N-tertbutyl-alpha-phenylnitrone (PBN) and the antioxidant vitamin E (DL-alpha-tocopherol) protect levels of high-energy phosphates during SE. In rats, SE was induced by either of two inhibitors of acetylcholinesterase (AChE), the organophosphate diisopropylphosphorofluoridate (DFP, 1.25 mg/kg, sc)- or the carbamate carbofuran (1.25 mg/kg, sc). Rats were sacrificed 1 h or 3 days after onset of seizures by head-focused microwave (power, 10 kW; duration 1.7 s) and levels of the energy-rich phosphates adenosine triphosphate (ATP) and phosphocreatine (PCr) and their metabolites adenosine diphosphate (ADP) and adenosine monophosphate (AMP), and creatine (Cr), respectively, were determined in the cortex, amygdala and hippocampus. Within 1 h of seizure activity, marked declines were seen in ATP (34-60%) and PCr (25-52%). Total adenine nucleotides (TAN = ATP + ADP + AMP) and total creatine compounds (TCC = PCr + Cr) were also reduced (TAN 38-60% and TCC 25-47%). No changes in ATP/AMP ratio were seen. Three days after the onset of seizures, recovery of ATP and PCr was significant in the amygdala and hippocampus, but not in the cortex. Pretreatment of rats with PBN (200 mg/kg, ip, in a single dose), 30 min before DFP or carbofuran administration, prevented induced seizures and partially prevented depletion of high-energy phosphates. Pretreatment with the natural antioxidant vitamin E (100 mg/kg, ip/day for 3 days), partially prevented loss of high energy phosphates without affecting seizures. In controls, citrulline, a product of nitric oxide synthesis, was found to be highest in the amygdala, followed by hippocampus, and lowest in the cortex. DFP- or carbofuran-induced seizures caused elevation of citrulline levels seven- to eight-fold in the cortex and three- to four-fold in the amygdala and hippocampus. These results suggest a close relationship between SE, excitotoxicity and energy metabolism. The involvement of oxidative stress is supported by the findings that DFP and carbofuran trigger an excessive nitric oxide (NO) production in the seizure relevant regions of the brain.

Aerobic metabolism of human quadriceps muscle: in vivo data parallel measurements on isolated mitochondria

The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake (VO2max) of human skeletal muscle. VO2max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10-18 mmol O2 x min(-1) x kg(-1) at work rates of 22-32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained between VO2max and a number of mitochondrial parameters (mitochondrial protein, cytochrome aa3, citrate synthase, and respiratory activities). The activities of citrate synthase, succinate dehydrogenase, and pyruvate dehydrogenase, measured in isolated mitochondria, corresponded to, respectively, 15, 3, and 1.1 times the rates calculated from VO2max. The respiratory chain activity also appeared sufficient. Fully coupled in vitro respiration, which is limited by the rate of ATP synthesis, could account for, at most, 60% of the VO2max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.

Antioxidants and mitochondrial respiration in lung, diaphragm, and locomotor muscles: effect of exercise

Previous studies have shown that exhaustive exercise may increase reactive oxygen species (ROS) generation in oxidative muscles that may in turn impair mitochondrial respiration. Locomotor muscles have been extensively examined, but there is few report about diaphragm or lung. The later is a privileged site for oxygen transit. To compare the antioxidant defense system and mitochondrial function in lung, diaphragm and locomotor muscles after exercise, 24 young adult male rats were randomly assigned to a control (C) or exercise (E) group. E group rats performed an exhaustive running test on a motorized treadmill at 80-85% VO2max Mean exercise duration was 66+/-2.7 min. Lung, costal diaphragm, mixed gastrocnemius, and oxidative muscles (red gastrocnemius and soleus: RG/SOL homogenate) were sampled. Mitochondrial respiration was assessed in tissue homogenates by respiratory control index (RCI: rate of uncoupled respiration/rate of basal respiration) measurement. Lipid peroxidation was evaluated by malondialdehyde concentration (MDA) and we determined the activity of two antioxidant enzymes: superoxide dismutase (SOD) and glutathione peroxidase (GPX). We found elevated basal (C group data) SOD and GPX activities in both lung and diaphragm compared to locomotor muscles (p<.001). Exercise led to a rise in GPX activity in red locomotor muscles homogenate (GR/SOL; C = 10.3+/-0.29 and E = 14.4+/-1.51 micromol x min(-1) x gww(-1); p<.05), whereas there was no significant change in lung and diaphragm. MDA concentration and mitochondrial RCI values were not significantly changed after exercise. We conclude that lung and diaphragm had higher antioxidant protection than locomotor muscles. The exercise test did not lead to significant oxidative stress or alteration in mitochondrial respiration, suggesting that antioxidant function was adequate in both lung and diaphragm in the experimental condition.

Lipid peroxidation and changes in cytochrome c oxidase and xanthine oxidase activity in organophosphorus anticholinesterase induced myopathy

A possible role of radical oxygen species (ROS) initiated lipid peroxidation in diisopropylphosphorofluoridate (DFP)-induced muscle necrosis was investigated by quantifying muscle changes in F2-isoprostanes, novel and extremely accurate markers of lipid peroxidation in vivo. A significant increase in F2-isoprostanes of 56% was found in the diaphragm of rats 60 min after DFP-induced fasciculations. As possible source of ROS initiating lipid peroxidation, the cytocrome-c oxidase (Cyt-ox) and xanthine dehydrogenase-xanthine oxidase (XD-XO) systems were investigated. Within 30 min of onset of fasciculations Cyt-ox activity was reduced by 50% from 0.526 to 0.263 mumol/mg prot/min and XO activity increased from 0.242 to 0.541 mumol/mg prot/min. Total XD-XO activity was unchanged, indicating a conversion from XD into XO. In rats pretreatment with the neuromuscular blocking agent d-tubocurarine, prevented DFP-induced fasciculations, increases in F2-isoprostanes and changes in Cyt-ox or XD-XO. The decrease in Cyt-ox and increase in XO suggest that ROS are produced during DFP induced muscle fasciculations initiating lipid peroxidation and subsequent myopathy.

Exercise intolerance in endurance horses

Endurance competition requires synchronism and development of metabolic and musculoskeletal systems. An understanding of the existence of performance-limiting factors may permit the detection of exercise intolerance that could lead to performance failure, fatigue, and exhaustion. New concepts for assessment of fitness have increased the understanding of individual capacities and deficiencies and the interaction of the different systems involved in exercise.

Effect of sprint cycle training on activities of antioxidant enzymes in human skeletal muscle

The effect of intermittent sprint cycle training on the level of muscle antioxidant enzyme protection was investigated. Resting muscle biopsies, obtained before and after 6 wk of training and 3, 24, and 72 h after the final session of an additional 1 wk of more frequent training, were analyzed for activities of the antioxidant enzymes glutathione peroxidase (GPX), glutathione reductase (GR), and superoxide dismutase (SOD). Activities of several muscle metabolic enzymes were determined to assess the effectiveness of the training. After the first 6-wk training period, no change in GPX, GR, or SOD was observed, but after the 7th week of training there was an increase in GPX from 120 +/- 12 (SE) to 164 +/- 24 mumol.min-1.g dry wt-1 (P < 0.05) and in GR from 10.8 +/- 0.8 to 16.8 +/- 2.4 mumol.min-1.g dry wt-1 (P < 0.05). There was no significant change in SOD. Sprint cycle training induced a significant (P < 0.05) elevation in the activity of phosphofructokinase and creatine kinase, implying an enhanced anaerobic capacity in the trained muscle. The present study demonstrates that intermittent sprint cycle training that induces an enhanced capacity for anaerobic energy generation also improves the level of antioxidant protection in the muscle.

Oxidative stress associated with exercise, psychological stress and life-style factors

Oxidative stress is a cellular or physiological condition of elevated concentrations of reactive oxygen species that cause molecular damage to vital structures and functions. Several factors influence the susceptibility to oxidative stress by affecting the antioxidant status or free oxygen radical generation. Here, we review the effect of alcohol, air pollution, cigarette smoke, diet, exercise, non-ionizing radiation (UV and microwaves) and psychological stress on the development of oxidative stress. Regular exercise and carbohydrate-rich diets seem to increase the resistance against oxidative stress. Air pollution, alcohol, cigarette smoke, non-ionizing radiation and psychological stress seem to increase oxidative stress. Alcohol in lower doses may act as an antioxidant on low density lipoproteins and thereby have an anti-atherosclerotic property.

Mitochondria changes in human muscle after prolonged exercise, endurance training and selenium supplementation

The functional and structural responses to acute exercise (E) and training, (T) with or without selenium supplementation (Sel), were investigated in a double-blind study on 24 young male subjects. The Sel or the placebo were given over 10 weeks of an endurance training programme. Prior to the programme and on its conclusion muscle biopsies were taken from the vastus lateralis muscle before and after an exhausting treadmill test of maximal endurance capacity (Capmax). The muscle samples were examined by electron microscopy to make a quantitative analysis of the mitochondria population in the muscle fibres. The number of mitochondria per area (QA) and the relative surface occupied by the total mitochondria profile area (AA) were estimated. The mean area per mitochondrion (â) was obtained by the quotient AA/QA. The effects of the isolated or combined independent variables T, E and Sel were analysed by nonparametric tests. Training induced significant increases in both QA (30%, P < 0.001) and AA (52%, P < 0.001), without changing â; T + Sel produced a slight rise of AA (27%, P < 0.001), which resulted in larger (24%, P < 0.001) â. The E produced an enlargement of â resembling swelling. This phenomenon was also found for the combinations E + T and E + T + Sel, but it was then far more pronounced in E + T. The training effects observed are in agreement with previous descriptions. In contrast, the changes observed after acute exercise seem to indicate a remarkable short-term plasticity of muscle mitochondria. The results in Sel would seem to suggest a dampening effect of the selenium on the mitochondria changes, both in chronic and acute exercise. The mechanism of this action on mitochondrial turnover is uncertain, but might be related to a higher efficiency of the selenium-dependent enzyme glutathione peroxidase.

Effect of exercise on hexokinase distribution and mitochondrial respiration in skeletal muscle

Horses were subjected to treadmill running at 65% (submaximal) or 100% (maximal) VO2,max to examine the effects of exercise on subcellular distribution of hexokinase (HK) and on mitochondrial respiration. It is hypothesized that the fraction of HK bound to mitochondria will be reduced due to an elevation of glucose-6-phosphate (G-6-P) concentration in the exercising muscle and that such release of HK from mitochondria will depress oxidative phosphorylation. Changes in muscle G-6-P concentration, pH, subcellular HK distribution, mitochondrial respiration and other metabolites were determined in biopsy samples pre-exercise, immediately post-exercise and during the recovery phase. The fraction of HK associated with mitochondria decreased from 38% to 7% at the end of maximal exercise; exercise at VO2,max also reduced respiratory capacity of muscle homogenates by 20% and was associated with a fivefold increase in muscle [G-6-P], a potent agent known to dissociate HK from mitochondria. The HK distribution returned to normal within 60 min after exercise and the reassociation of the HK with mitochondria parallelled the removal of muscle G-6-P. No changes in muscle HK distribution and respiration were found following the submaximal exercise despite the fact that G-6-P was slightly elevated. Muscle concentrations of adenosine triphosphate, creatine phosphate and glycogen and pH dropped after exercise while lactate concentration increased. The amount of mitochondria-bound HK was also altered in vitro in a preparation of mitochondria isolated from rat skeletal muscle to examine the effect of the bound HK on mitochondrial respiration.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic and antioxidant enzyme activities in the diaphragm: effects of acute exercise

Disruption of cellular constituents including inhibition or "downregulation" of metabolic enzyme activity has been associated with free radical stress in locomotor muscle with acute, strenuous exercise. However, the effects of acute, strenuous exercise on important metabolic and antioxidant enzyme activity levels in the diaphragm are unknown. Twenty 4-month-old and twenty 24-month-old female Fischer-344 rats were divided at random into young exercised (YE; n = 10)/old exercised (OE; n = 10); young control (YC; n = 10)/old control (OC; n = 10) groups. Animals in both young and old exercise groups ran on a treadmill (10% uphill grade) for 40 min at approximately 75% of age group VO2 max. Immediately following the treadmill run, both exercise and control groups were euthanized with sodium pentobarbital. Costal (COD) and crural diaphragm (CRD) were quickly removed and frozen in liquid nitrogen. Lipid peroxidation was significantly increased (P < 0.05) in COD of YE vs. YC rats. Activity of the antioxidant enzyme glutathione peroxidase (GPX) was unaltered in the diaphragm by acute exercise (P > 0.05) in both age groups. There was a significant increase in superoxide dismutase (SOD) activity with exercise (P < 0.05). Post-hocs revealed SOD activity was approximately 20% greater (P = 0.066) in YE CRD only. Activities of the metabolic enzymes phosphofructokinase (PFK), succinate dehydrogenase (SDH), and citrate synthase (CS) were not affected by acute exercise in YE or OE. Strenuous exercise resulted in a small trend towards a decrease in 3-hydroxyacyl-CoA dehydrogenase (HADH) activity in YE COD (P = 0.115) and YE CRD (P = 0.082). We conclude that the employed bout of exercise induces some free radical stress, while metabolic enzymes are protected, in the diaphragm.

Influence of temperature on the response time of mitochondrial oxygen consumption in isolated rabbit heart

1. In this study we determined the temperature dependence of the mean response time of cardiac mitochondrial oxygen consumption following steps in metabolic demand. Metabolic demand was altered by stepwise changes in heart rate or in left ventricular volume at 20 and 28 degrees C. 2. Ten isolated rabbit hearts were perfused with Tyrode solution at constant oxygen tension and constant arterial flow. A balloon was inserted in the left ventricle and developed pressure was measured. Coronary venous oxygen tension was measured continuously with a Clark-type oxygen electrode. 3. The mean response time of mitochondrial oxygen consumption is defined as the first statistical moment of the impulse response function. This mean response time of mitochondrial oxygen consumption, following the change in metabolic demand, is calculated from the measured mean response time for the change in coronary venous oxygen tension by subtracting the transport time resulting from diffusion and convective transport in the blood vessels. The transport time is obtained from a model for oxygen transport developed previously. Experimental data, necessary for the model calculation, were obtained from measurement of the coronary venous oxygen tension transients following stepwise changes either in arterial oxygen tension or perfusion flow. 4. The calculated mean response times of mitochondrial oxygen consumption were 26.9 +/- 3.0 s (mean +/- S.E.M.) at 20 degrees C and 14.9 +/- 1.0 s at 28 degrees C. The mean response times of mitochondrial oxygen consumption did not differ significantly for steps in heart rate and in left ventricular volume and between upward and downward steps. 5. We suggest that intracellular calcium concentration is not the sole regulator of mitochondrial oxygen consumption in the isolated rabbit heart, since steps in heart rate and in left ventricular volume showed the same time course of oxygen uptake. 6. The mean response time of mitochondrial oxygen consumption obtained in the isolated rabbit heart at 20 degrees C did not differ significantly from the mean response time of mitochondrial oxygen consumption of isolated rabbit papillary muscle. After combining our data with previously published data on empty beating hearts at 37 degrees C, a Q10, which is the factor by which the mean response time of mitochondrial oxygen consumption increases per 10 degrees C decrease in temperature, of 2.1 was calculated.

Ca2+ activation properties of myofibrillar ATPase from fatigued rat plantaris

1. Muscle fatigue following long-duration rhythmic activity is often characterized by reduced force following a single impulse and at low-frequencies of stimulation. 2. Although this response is generally attributed to an alteration in excitation-contraction coupling, the possibility that the responsiveness of myofibrillar proteins to a given Ca2+ signal is altered has never been ruled out. 3. In this study, rat plantaris muscles were subjected to an in situ regimen of contractions (100 Hz, lasting 100 msec, once every 750 msec, for 1 hr), and allowed to recover for 15 min. 4. Twitch, 100 Hz, and 200 Hz forces were reduced by 79%, 49% and 17% respectively, at this time. 5. In myofibrils isolated from these muscles, maximum activity of Ca2+ activated myofibrillar ATPase, Ca2+ sensitivity (pCa 50), and co-operatively (Hill n), were not different from non-fatigued muscles. 6. It appears, therefore, that the Ca2+ activation properties of myofibrillar ATPase do not contribute to this pattern of fatigue.