Combination of hyperoxia and a right-shifted HbO2 dissociation curve delays V̇O2 kinetics during maximal contractions in isolated muscle

Acute enhancement of peripheral O₂ diffusion may accelerate skeletal muscle O₂ uptake (V̇O₂) kinetics and lessen fatigue during transitions from rest to maximal contractions. Surgically isolated canine gastrocnemius muscles in situ (n=6) were studied during transitions from rest to 4-min of electrically stimulated isometric tetanic contractions at V̇O₂peak, in two conditions: normoxia (CTRL) and hyperoxia (FIO₂=1.00) + administration of a drug (RSR-13) which right-shifts the Hb-O₂ dissociation curve (Hyperoxia+RSR13). Prior to and during contractions muscles were pump-perfused with blood at constant elevated flow ( ) and infused with the vasodilator adenosine. Arterial (CaO₂) and muscle venous (CvO₂) O₂ concentrations were determined at rest and at 5-7 s intervals during contractions; V̇O₂ was calculated as ^.(CaO₂-CvO₂). PO₂ at 50% of Hb saturation (standard P₅₀) and mean microvascular PO₂ (PmvO₂) were calculated by the Hill equation and a numerical integration technique. P₅₀ (42±7 (x±SD) mmHg vs. 33±2, P=0.02) and PmvO₂ (218±73 mmHg vs. 49±4, P=0.003) were higher Hyperoxia+RSR13. Muscle force and fatigue were not different in the two conditions. V̇O₂ kinetics (monoexponential fitting) were unexpectedly slower in Hyperoxia+RSR13, due to a longer time delay (TD) (9.9±1.7 s vs. 4.4±2.2 [P=0.001]), whereas the time constant (t) was not different (13.7±4.3 s vs. 12.3±1.9 [P=0.37]); the mean response time (TD+t) was longer in Hyperoxia+RSR13 (23.6±3.5 s vs. 16.7±3.2 [P=0.003]). Increased O₂ availability deriving, in Hyperoxia+RSR13, from higher PmvO₂ and from presumably greater intramuscular O₂ stores, did not accelerate the primary component of the V̇O₂ kinetics, and delayed the metabolic activation of oxidative phosphorylation.

Role of parvalbumin in fatigue-induced changes in force and cytosolic calcium transients in intact single mouse myofibers

One of the most important cytosolic Ca²⁺ buffers present in mouse fast-twitch myofibers, but not in human myofibers, is parvalbumin (PV). Previous work using conventional PV knockout mice suggests that lifelong PV ablation increases fatigue resistance, possibly due to compensations in mitochondrial volume. In this work, PV gene ablation was induced only in adult mice (PV-KO), and contractile and cytosolic Ca²⁺ responses during fatigue were studied in isolated muscle and intact single myofibers. Results were compared to control littermates (PV-Ctr). We hypothesized that the reduced myofiber cytosolic Ca²⁺ buffering developed only in adult PV-KO mice leads to a larger cytosolic free Ca²⁺ concentration ([Ca²⁺]_c) during repetitive contractions, increasing myofiber fatigue resistance. Extensor digitorum longus (EDL) muscles from PV-KO mice had higher force in unfused stimulations (~50%, P<0.05) and slowed relaxation (~46% higher relaxation time, P<0.05) vs PV-Ctr, but muscle fatigue resistance or fatigue-induced changes in relaxation were not different between genotypes (P>0.05). In intact single myofibers from flexor digitorum brevis (FDB) muscles, basal and tetanic [Ca²⁺]_c during fatiguing contractions were higher in PV-KO (P<0.05), accompanied by a greater slowing in estimated sarcoplasmic reticulum (SR) Ca²⁺ pumping vs PV-Ctr myofibers (~84% reduction, P<0.05), but myofiber fatigue resistance was not different between genotypes (P>0.05). Our results demonstrate that although the estimated SR Ca²⁺ uptake was accelerated in PV-KO, the total energy demand by the major energy consumers in myofibers, the cross-bridges and SR Ca²⁺ ATPase, were not altered enough to affect the energy supply for contractions, and therefore fatigue resistance remained unaffected.

Effect of acute nitrite infusion on contractile economy and metabolism in isolated skeletal muscle in situ during hypoxia

KEY POINTS

Increased plasma nitrite concentrations may have beneficial effects on skeletal muscle function. The physiological basis explaining these observations has not been clearly defined and it may involve positive effects on muscle contraction force, microvascular O₂ delivery and skeletal muscle oxidative metabolism. In the isolated canine gastrocnemius model, we evaluated the effects of acute nitrite infusion on muscle force and skeletal muscle oxidative metabolism. Under hypoxic conditions, but in the presence of normal convective O₂ delivery, an elevated plasma nitrite concentration affects neither muscle force, nor muscle contractile economy. In accordance with previous results suggesting limited or no effects of nitrate/nitrite administrations in highly oxidative and highly perfused muscle, our data suggest that neither mitochondrial respiration, nor muscle force generation are affected by acute increased concentrations of NO precursors in hypoxia.

ABSTRACT

Contrasting findings have been reported concerning the effects of augmented nitric oxide (NO) on skeletal muscle force production and oxygen consumption ( V ̇ O 2 ). The present study examined skeletal muscle mitochondrial respiration and contractile economy in an isolated muscle preparation during hypoxia (but normal convective O₂ delivery) with nitrite infusion. Isolated canine gastrocnemius muscles in situ (n = 8) were studied during 3 min of electrically stimulated isometric tetanic contractions corresponding to ∼35% of V ̇ O 2 peak . During contractions, sodium nitrite (NITRITE) or sodium chloride (SALINE) was infused into the popliteal artery. V ̇ O 2 was calculated from the Fick principle. Experiments were carried out in hypoxia ( F I O 2  = 0.12), whereas convective O₂ delivery was maintained at normal levels under both conditions by pump-driven blood flow ( Q ̇ ). Muscle biopsies were taken and mitochondrial respiration was evaluated by respirometry. Nitrite infusion significantly increased both nitrite and nitrate concentrations in plasma. No differences in force were observed between conditions. V ̇ O 2 was not significantly different between NITRITE (6.1 ± 1.8 mL 100 g^-1  min^-1 ) and SALINE (6.2 ± 1.8 mL 100 g^-1  min^-1 ), even after being 'normalized' per unit of developed force (muscle contractile economy). No differences between conditions were found for maximal ADP-stimulated mitochondrial respiration (both for complex I and complex II), leak respiration and oxidative phosphorylation coupling. In conclusion, in the absence of changes in convective O₂ delivery, muscle force, muscle contractile economy and mitochondrial respiration were not affected by acute infusion of nitrite. The previously reported positive effects of elevated plasma nitrite concentrations are presumably mediated by the increased microvascular O₂ availability.

Effects of (-)-epicatechin on neuroinflammation and hyperphosphorylation of tau in the hippocampus of aged mice

Evidence has implicated oxidative stress (OS) and inflammation as drivers of neurodegenerative pathologies. We previously reported on the beneficial effects of (-)-epicatechin (Epi) treatment on aging-induced OS and its capacity to restore modulators of mitochondrial biogenesis in the prefrontal cortex of 26-month-old male mice. In the present study using the same mouse model of aging, we examined the capacity of Epi to mitigate hippocampus OS, inflammation, hyperphosphorylation of tau protein, soluble β-amyloid protein levels, cell survival, memory, anxiety-like behavior levels and systemic inflammation. Mice were subjected to 4 weeks of Epi treatment (1 mg kg^-1 day^-1) and samples of the hippocampus were obtained. Assessments of the OS markers, protein carbonyls, and malondialdehyde levels demonstrated their significant increase (∼3 fold) with aging that were partially suppressed by Epi. The protein levels of the glial fibrillary acidic protein, inflammatory factor 1 (Iba1), pro-inflammatory cytokines, interleukins (IL-1β, IL-3, 5, 6 and 15), cyclooxygenase 2, tumor necrosis factor α, nuclear factor-activated B cells and interferon γ increase with aging and were also significantly decreased with Epi treatment. However, anti-inflammatory cytokines, IL-1ra, IL-10 and 11 decrease with aging and were restored with Epi. Epi also reversed the aging effects on the hyperphosphorylation of tau, increased soluble β-amyloid levels (∼2 fold), cellular death (as per caspase 3 and 9 activity), and reduced nerve growth factor and triggering receptor expressed on myeloid cells 2 levels. Measures of anxiety like-behavior and memory demonstrated improvements with Epi treatment. Indicators of systemic inflammation increase with aging and Epi was capable of decreasing blood inflammatory markers. Altogether, the results show a significant capacity of Epi to mitigate hippocampus OS and inflammation leading to improved brain function.

Incubation with sodium nitrite attenuates fatigue development in intact single mouse fibres at physiological P O 2

KEY POINTS

Dietary nitrate supplementation increases plasma nitrite concentration, which provides an oxygen-independent source of nitric oxide and can delay skeletal muscle fatigue. Nitrate supplementation has been shown to increase myofibre calcium release and force production in mouse skeletal muscle during contractions at a supra-physiological oxygen tension, but it is unclear whether nitrite exposure can delay fatigue development and improve myofibre calcium handling at a near-physiological oxygen tension. Single mouse muscle fibres acutely treated with nitrite had a lower force and cytosolic calcium concentration during single non-fatiguing contractions at a near-physiological oxygen tension. Nitrite treatment delayed fatigue development during repeated fatiguing isometric contractions at near-physiological, but not at supra-physiological, oxygen tension in combination with better maintenance of myofilament calcium sensitivity and sarcoplasmic reticulum calcium pumping. These findings improve understanding of the mechanisms by which increased skeletal muscle nitrite exposure might be ergogenic and imply that this is related to improved calcium handling.

ABSTRACT

Dietary nitrate (NO₃ ^- ) supplementation, which increases plasma nitrite (NO₂ ^- ) concentration, has been reported to attenuate skeletal muscle fatigue development. Sarcoplasmic reticulum (SR) calcium (Ca²⁺ ) release is enhanced in isolated single skeletal muscle fibres following NO₃ ^- supplementation or NO₂ ^- incubation at a supra-physiological P O 2 but it is unclear whether NO₂ ^- incubation can alter Ca²⁺ handling and fatigue development at a near-physiological P O 2 . We hypothesised that NO₂ ^- treatment would improve Ca²⁺ handling and delay fatigue at a physiological P O 2 in intact single mouse skeletal muscle fibres. Each muscle fibre was perfused with Tyrode solution pre-equilibrated with either 20% ( P O 2 ∼150 Torr) or 2% O₂ ( P O 2  = 15.6 Torr) in the absence and presence of 100 µM NaNO₂ . At supra-physiological P O 2 (i.e. 20% O₂ ), time to fatigue was lowered by 34% with NaNO₂ (control: 257 ± 94 vs. NaNO₂ : 159 ± 46 s, Cohen's d = 1.63, P < 0.05), but extended by 21% with NaNO₂ at 2% O₂ (control: 308 ± 217 vs. NaNO₂ : 368 ± 242 s, d = 1.14, P < 0.01). During the fatiguing contraction protocol completed with NaNO₂ at 2% O₂ , peak cytosolic Ca²⁺ concentration ([Ca²⁺ ]_c ) was not different (P > 0.05) but [Ca²⁺ ]_c accumulation between contractions was lower, concomitant with a greater SR Ca²⁺ pumping rate (P < 0.05) compared to the control condition. These results demonstrate that increased exposure to NO₂ ^- blunts fatigue development at near-physiological, but not at supra-physiological, P O 2 through enhancing SR Ca²⁺ pumping rate in single skeletal muscle fibres. These findings extend our understanding of the mechanisms by which increased NO₂ ^- exposure can mitigate skeletal muscle fatigue development.

A mitochondrial-targeted antioxidant improves myofilament Ca2+ sensitivity during prolonged low frequency force depression at low PO2

KEY POINTS

Skeletal muscle contractile activity is associated with an enhanced reactive oxygen species (ROS) generation. At very low PO2, ROS generation by mitochondria can be elevated in intact cells. An elevated intracellular oxidant activity may affect muscle force development and recovery from fatigue. We treated intact single muscle fibres with a mitochondrial antioxidant and stimulated the fibres to contract at a low extracellular PO2 that is similar to the intracellular PO2 that is observed during moderate to intense exercise in vivo. The mitochondrial antioxidant prevented a sustained decrease in the myofibrillar Ca²⁺ sensitivity and improved muscle submaximal force development after fatigue at low extracellular PO2.

ABSTRACT

Skeletal muscle can develop a prolonged low frequency-stimulation force depression (PLFFD) following fatigue-inducing contractions. Increased levels of reactive oxygen species (ROS) have been implicated in the development of PLFFD. During exercise the skeletal muscle intracellular PO2 decreases to relatively low levels, and can be further decreased when there is an impairment in O₂ diffusion or availability, such as in certain chronic diseases and during exercise at high altitude. Since ROS generation by mitochondria is elevated at very low PO2 in cells, we tested the hypothesis that treatment of muscle fibres with a mitochondrial-targeted antioxidant at a very low, near hypoxic, PO2 can attenuate PLFFD. We treated intact single fibres from mice with the mitochondrial-specific antioxidant SS31, and measured force development and intracellular [Ca²⁺ ] 30 min after fatigue at an extracellular PO2 of ∼5 Torr. After 30 min following the end of the fatiguing contractions, fibres treated with SS31 showed significantly less impairment in force development compared to untreated fibres at submaximal frequencies of stimulation. The cytosolic peak [Ca²⁺ ] transients (peak [Ca²⁺ ]_c ) were equally decreased in both groups compared to pre-fatigue values. The combined force and peak [Ca²⁺ ]_c data demonstrated that myofibrillar Ca²⁺ sensitivity was diminished in the untreated fibres 30 min after fatigue compared to pre-fatigue values, but Ca²⁺ sensitivity was unaltered in the SS31 treated fibres. These results demonstrate that at a very low PO2, treatment of skeletal muscle fibres with a mitochondrial antioxidant prevents a decrease in the myofibrillar Ca²⁺ sensitivity, which alleviates the fatigue induced PLFFD.

No Muscle Is an Island: Integrative Perspectives on Muscle Fatigue

Muscle fatigue has been studied with a variety approaches, tools and technologies. The foci of these studies have ranged tremendously, from molecules to the entire organism. Single cell and animal models have been used to gain mechanistic insight into the fatigue process. The theme of this review is the concept that the mechanisms of muscle fatigue do not occur in isolation in vivo: muscular work is supported by many complex physiological systems, any of which could fail during exercise and thus contribute to fatigue. To advance our overall understanding of fatigue, a combination of models and approaches is necessary. In this review, we examine the roles that neuromuscular properties, intracellular glycogen, oxygen metabolism, and blood flow play in the fatigue process during exercise and pathological conditions.

Skeletal myofiber VEGF regulates contraction-induced perfusion and exercise capacity but not muscle capillarity in adult mice

A single bout of exhaustive exercise signals expression of vascular endothelial growth factor (VEGF) in the exercising muscle. Previous studies have reported that mice with life-long deletion of skeletal myofiber VEGF have fewer capillaries and a severe reduction in endurance exercise. However, in adult mice, VEGF gene deletion conditionally targeted to skeletal myofibers limits exercise capacity without evidence of capillary regression. To explain this, we hypothesized that adult skeletal myofiber VEGF acutely regulates skeletal muscle perfusion during muscle contraction. A tamoxifen-inducible skeletal myofiber-specific VEGF gene deletion mouse (skmVEGF-/-) was used to reduce skeletal muscle VEGF protein by 90% in adult mice. Three weeks after inducing deletion of the skeletal myofiber VEGF gene, skmVEGF-/- mice exhibited diminished maximum running speed (-10%, P < 0.05) and endurance capacity (-47%; P < 0.05), which did not persist after 8 wk. In skmVEGF-/- mice, gastrocnemius complex time to fatigue measured in situ was 71% lower than control mice. Contraction-induced perfusion measured by optical imaging during a period of electrically stimulated muscle contraction was 85% lower in skmVEGF-/- than control mice. No evidence of capillary rarefication was detected in the soleus, gastrocnemius, and extensor digitorum longus (EDL) up to 8 wk after tamoxifen-induced VEGF ablation, and contractility and fatigue resistance of the soleus measured ex vivo were also unchanged. The force-frequency of the EDL showed a small right shift, but fatigue resistance did not differ between EDL from control and skmVEGF-/- mice. These data suggest myofiber VEGF is required for regulating perfusion during periods of contraction and may in this manner affect endurance capacity.

Cytosolic calcium transients are a determinant of contraction-induced HSP72 transcription in single skeletal muscle fibers

The intrinsic activating factors that induce transcription of heat shock protein 72 (HSP72) in skeletal muscle following exercise remain unclear. We hypothesized that the cytosolic Ca(2+) transient that occurs with depolarization is a determinant. We utilized intact, single skeletal muscle fibers from Xenopus laevis to test the role of the cytosolic Ca(2+) transient and several other exercise-related factors (fatigue, hypoxia, AMP kinase, and cross-bridge cycling) on the activation of HSP72 transcription. HSP72 and HSP60 mRNA levels were assessed with real-time quantitative PCR; cytosolic Ca(2+) concentration ([Ca(2+)]cyt) was assessed with fura-2. Both fatiguing and nonfatiguing contractions resulted in a significant increase in HSP72 mRNA. As expected, peak [Ca(2+)]cyt remained tightly coupled with peak developed tension in contracting fibers. Pretreatment with N-benzyl-p-toluene sulfonamide (BTS) resulted in depressed peak developed tension with stimulation, while peak [Ca(2+)]cyt remained largely unchanged from control values. Despite excitation-contraction uncoupling, BTS-treated fibers displayed a significant increase in HSP72 mRNA. Treatment of fibers with hypoxia (Po2: <3 mmHg) or AMP kinase activation had no effect on HSP72 mRNA levels. These results suggest that the intermittent cytosolic Ca(2+) transient that occurs with skeletal muscle depolarization provides a sufficient activating stimulus for HSP72 transcription. Metabolic or mechanical factors associated with fatigue development and cross-bridge cycling likely play a more limited role.

Somatostatin analogues improve health-related quality of life in polycystic liver disease: a pooled analysis of two randomised, placebo-controlled trials

BACKGROUND

Polycystic liver disease is associated with impaired health-related quality of life (HRQL). Somatostatin analogues reduce hepatomegaly in polycystic liver disease.

AIM

To determine whether somatostatin analogues improve HRQL and to identify factors associated with change in HRQL in polycystic liver disease.

METHODS

We pooled data from two randomized, double-blind, placebo-controlled trials that evaluated HRQL using the Short-Form 36 (SF-36) in 96 polycystic liver disease patients treated 6-12 months with somatostatin analogues or placebo. The SF-36 contains a summarizing physical and mental component score and was administered at baseline and at the end of treatment. We used random effect models to delineate the effect of somatostatin analogues on HRQL. We determined the effect of demographics, height-adjusted liver volume, change in liver volume, somatostatin analogue-associated side effects with change in HRQL. In patients with autosomal dominant polycystic kidney disease, we estimated the effect of height-adjusted kidney volume and change in kidney volume in relation to HRQL.

RESULTS

Physical component scores improved with somatostatin analogues, but remained unchanged with placebo (3.41 ± 1.29 vs. -0.71 ± 1.54, P = 0.044). Treatment had no impact on the mental component score. Large liver volume was independently associated with larger HRQL decline during follow up (-4.04 ± 2.02 points per logarithm liver volume, P = 0.049). In autosomal dominant polycystic kidney disease, patients with large liver and kidney volumes had larger decline in HRQL (5.36 ± 2.54 points per logarithm liver volume; P = 0.040 and -4.00 ± 1.88 per logarithm kidney volume; P = 0.039).

CONCLUSION

Somatostatin analogues improve HRQL in symptomatic polycystic liver disease. Halting the progressive nature of polycystic liver disease is necessary to prevent further decline of HRQL in severe hepatomegaly.

Recovery of Indicators of Mitochondrial Biogenesis, Oxidative Stress, and Aging With (-)-Epicatechin in Senile Mice

There is evidence implicating oxidative stress (OS) as the cause of the deleterious effects of aging. In this study, we evaluated the capacity of the flavanol (-)-epicatechin (Epi) to reduce aging-induced OS and restore mitochondrial biogenesis, as well as, structural and functional endpoints in aged mice. Senile (S; 26-month-old) C57BL/6 male mice were randomly assigned to receive either water (vehicle) or 1mg/kg of Epi via oral gavage (twice daily) for 15 days. Young (Y; 6-month-old) mice were used as controls. In S brain, kidney, heart, and skeletal muscle (compared with Y animals) an increase in OS was observed as evidenced by increased protein-free carbonyls and decreased reduced glutathione levels as well as sirtuin 3, superoxide dismutase 2, catalase, thioredoxin and glutathione peroxidase protein levels. Well-recognized factors (eg, sirtuin 1) that regulate mitochondrial biogenesis and mitochondrial structure- and/or function-related endpoints (eg, mitofilin and citrate synthase) protein levels were also reduced in S organs. In contrast, the aging biomarker senescence-associated β-galactosidase was increased in S compared with Y animals, and Epi administration reduced levels towards those observed in Y animals. Altogether, these data suggest that Epi is capable of shifting the biology of S mice towards that of Y animals.

Skeletal myofiber VEGF is essential for the exercise training response in adult mice

Vascular endothelial growth factor (VEGF) is exercise responsive, pro-angiogenic, and expressed in several muscle cell types. We hypothesized that in adult mice, VEGF generated within skeletal myofibers (and not other cells within muscle) is necessary for the angiogenic response to exercise training. This was tested in adult conditional, skeletal myofiber-specific VEGF gene-deleted mice (skmVEGF-/-), with VEGF levels reduced by >80%. After 8 wk of daily treadmill training, speed and endurance were unaltered in skmVEGF-/- mice, but increased by 18% and 99% (P < 0.01), respectively, in controls trained at identical absolute speed, incline, and duration. In vitro, isolated soleus and extensor digitorum longus contractile function was not impaired in skmVEGF-/- mice. However, training-induced angiogenesis was inhibited in plantaris (wild type, 38%, skmVEGF-/- 18%, P < 0.01), and gastrocnemius (wild type, 43%, P < 0.01; skmVEGF-/-, 7%, not significant). Capillarity was maintained (different from VEGF gene deletion targeted to multiple cell types) in untrained skmVEGF-/- mice. Arteriogenesis (smooth muscle actin+, artery number, and diameter) and remodeling [vimentin+, 5'-bromodeoxycytidine (BrdU)+, and F4/80+ cells] occurred in skmVEGF-/- mice, even in the absence of training. skmVEGF-/- mice also displayed a limited oxidative enzyme [citrate synthase and β-hydroxyacyl CoA dehydrogenase (β-HAD)] training response; β-HAD activity levels were elevated in the untrained state. These data suggest that myofiber expressed VEGF is necessary for training responses in capillarity and oxidative capacity and for improved running speed and endurance.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Superoxide release from contracting skeletal muscle in pulmonary TNF-α overexpression mice

Chronic obstructive pulmonary disease (COPD) often results in increased levels of tumor necrosis factor-α (TNF-α), a proinflammatory cytokine, which circulates in the blood. However, it is not clear whether pulmonary TNF-α overexpression (a COPD mimic) induces excessive reactive oxygen species (ROS) formation in skeletal muscle and thereby may contribute to the muscle impairment often seen in COPD. We hypothesized that ROS generation in contracting skeletal muscle is elevated when there is TNF-α overproduction in the lung and that this can induce muscle dysfunction. Cytochrome c (cyt c) in the perfusate was used to assay superoxide (O2(·-)) release from isolated contracting soleus muscles from transgenic mice of pulmonary TNF-α overexpression (Tg(+)) and wild-type (WT) mice. Our results showed that Tg(+) muscle released significantly higher levels of O2(·-) than WT during a period of intense contractile activity (in nmol/mg wt; 17.5 ± 2.3 vs. 4.4 ± 1.3, respectively; n = 5; P < 0.05). In addition, the soleus muscle demonstrated a significantly reduced fatigue resistance in Tg(+) mice compared with WT mice. Perfusion of the contracting soleus muscle with superoxide dismutase, which specifically scavenges O2(·-) in the perfusate, resulted in significantly less cyt c reduction, thereby indicating that the type of ROS released from the Tg(+) muscles is O2(·-). Our results demonstrate that pulmonary TNF-α overexpression leads to a greater O2(·-) release from contracting soleus muscle in Tg(+) compared with WT and that the excessive formation of O2(·-) in the contracting muscle of Tg(+) mice leads to earlier fatigue.

Ca²⁺-pumping impairment during repetitive fatiguing contractions in single myofibers: role of cross-bridge cycling

The energy cost of contractions in skeletal muscle involves activation of both actomyosin and sarcoplasmic reticulum (SR) Ca²⁺-pump (SERCA) ATPases, which together determine the overall ATP demand. During repetitive contractions leading to fatigue, the relaxation rate and Ca²⁺ pumping become slowed, possibly because of intracellular metabolite accumulation. The role of the energy cost of cross-bridge cycling during contractile activity on Ca²⁺-pumping properties has not been investigated. Therefore, we inhibited cross-bridge cycling by incubating isolated Xenopus single fibers with N-benzyl-p-toluene sulfonamide (BTS) to study the mechanisms by which SR Ca²⁺ pumping is impaired during fatiguing contractions. Fibers were stimulated in the absence (control) and presence of BTS and cytosolic calcium ([Ca²⁺]c) transients or intracellular pH (pHi) changes were measured. BTS treatment allowed normal [Ca²⁺]c transients during stimulation without cross-bridge activation. At the time point that tension was reduced to 50% in the control condition, the fall in the peak [Ca²⁺]c and the increase in basal [Ca²⁺]c did not occur with BTS incubation. The progressively slower Ca²⁺ pumping rate and the fall in pHi during repetitive contractions were reduced during BTS conditions. However, when mitochondrial ATP supply was blocked during contractions with BTS present (BTS + cyanide), there was no further slowing in SR Ca²⁺ pumping during contractions compared with the BTS-alone condition. Furthermore, the fall in pHi was significantly less during the BTS + cyanide condition than in the control conditions. These results demonstrate that factors related to the energetic cost of cross-bridge cycling, possibly the accumulation of metabolites, inhibit the Ca²⁺ pumping rate during fatiguing contractions.

Low Po₂ conditions induce reactive oxygen species formation during contractions in single skeletal muscle fibers

Contractions in whole skeletal muscle during hypoxia are known to generate reactive oxygen species (ROS); however, identification of real-time ROS formation within isolated single skeletal muscle fibers has been challenging. Consequently, there is no convincing evidence showing increased ROS production in intact contracting fibers under low Po₂ conditions. Therefore, we hypothesized that intracellular ROS generation in single contracting skeletal myofibers increases during low Po₂ compared with a value approximating normal resting Po₂. Dihydrofluorescein was loaded into single frog (Xenopus) fibers, and fluorescence was used to monitor ROS using confocal microscopy. Myofibers were exposed to two maximal tetanic contractile periods (1 contraction/3 s for 2 min, separated by a 60-min rest period), each consisting of one of the following treatments: high Po₂ (30 Torr), low Po₂ (3-5 Torr), high Po₂ with ebselen (antioxidant), or low Po₂ with ebselen. Ebselen (10 μM) was administered before the designated contractile period. ROS formation during low Po₂ treatment was greater than during high Po₂ treatment, and ebselen decreased ROS generation in both low- and high-Po₂ conditions (P < 0.05). ROS accumulated at a faster rate in low vs. high Po₂. Force was reduced >30% for each condition except low Po₂ with ebselen, which only decreased ~15%. We concluded that single myofibers under low Po₂ conditions develop accelerated and more oxidative stress than at Po₂ = 30 Torr (normal human resting Po₂). Ebselen decreases ROS formation in both low and high Po₂, but only mitigates skeletal muscle fatigue during reduced Po₂ conditions.

Increasing temperature speeds intracellular PO2 kinetics during contractions in single Xenopus skeletal muscle fibers

Precise determination of the effect of muscle temperature (T(m)) on mitochondrial oxygen consumption kinetics has proven difficult in humans, in part due to the complexities in controlling for T(m)-related variations in blood flow, fiber recruitment, muscle metabolism, and contractile properties. To address this issue, intracellular Po(2) (P(i)(O(2))) was measured continuously by phosphorescence quenching following the onset of contractions in single Xenopus myofibers (n = 24) while controlling extracellular temperature. Fibers were subjected to two identical contraction bouts, in random order, at 15°C (cold, C) and 20°C (normal, N; n = 12), or at N and 25°C (hot, H; n = 12). Contractile properties were determined for every contraction. The time delay of the P(i)(O(2)) response was significantly greater in C (59 ± 35 s) compared with N (35 ± 26 s, P = 0.01) and H (27 ± 14 s, P = 0.01). The time constant for the decline in P(i)(O(2)) was significantly greater in C (89 ± 34 s) compared with N (52 ± 15 s; P < 0.01) and H (37 ± 10 s; P < 0.01). There was a linear relationship between the rate constant for P(i)(O(2)) kinetics and T(m) (r = 0.322, P = 0.03). Estimated ATP turnover was significantly greater in H than in C (P < 0.01), but this increased energy requirement alone with increased T(m) could not account for the differences observed in P(i)(O(2)) kinetics among conditions. These results demonstrate that P(i)(O(2)) kinetics in single contracting myofibers are dependent on T(m), likely caused by temperature-induced differences in metabolic demand and by temperature-dependent processes underlying mitochondrial activation at the start of muscle contractions.

Mitochondrial activation at the onset of contractions in isolated myofibres during successive contractile periods

At the onset of skeletal muscle repetitive contractions, there is a significant delay in the time to achieve oxidative phosphorylation steady state. The purpose of the present study was to examine the factors that limit oxidative phosphorylation at the onset of contractions. NAD(P)H was measured in real time during two contractile periods (2 min each) separated by 5 min of rest in intact single muscle fibres (n = 7) isolated from Xenopus laevis. The fibres were then loaded with the dye tetramethylrhodamine methyl ester perchlorate (TMRM) to evaluate the kinetics of the mitochondrial membrane potential (Δψ (m)) during two further successive contractile periods. At the onset of contractions in the first period, NAD(P)H exhibited a time delay (14.1 ± 1.3 s) before decreasing toward a steady state. In contrast, Δψ(m) decreased immediately after the first contraction and started to be reestablished after 10.7 ± 0.9 s, with restoration to the pre-stimulation values after approximately 32 s. In the second contractile period (5 min after the first), NAD(P)H decreased immediately (i.e. no time delay) after the first contraction and had a significantly shorter time constant compared to the first contractile bout (3.3 ± 0.3 vs. 5.0 ± 0.2 s, P < 0.05). During the second bout, Δψ(m) remained unchanged from pre-stimulation values. These results suggest: (1) that at the onset of contractions, oxidative phosphorylation is primarily limited by the activity of the electron transport chain complexes rather than by a limited level of substrates; and (2) when the muscle is 'primed' by previous contractile activity, the faster enhancement of the cellular respiratory rate is due to intrinsic factors within the myofibre.

(-)-Epicatechin enhances fatigue resistance and oxidative capacity in mouse muscle

The flavanol (-)-epicatechin, a component of cacao (cocoa), has been shown to have multiple health benefits in humans. Using 1-year-old male mice, we examined the effects of 15 days of (-)-epicatechin treatment and regular exercise on: (1) exercise performance, (2) muscle fatigue, (3) capillarity, and (4) mitochondrial biogenesis in mouse hindlimb and heart muscles. Twenty-five male mice (C57BL/6N) were randomized into four groups: (1) water, (2) water-exercise (W-Ex), (3) (-)-epicatechin ((-)-Epi), and (4) (-)-epicatechin-exercise ((-)-Epi-Ex). Animals received 1 mg kg(-1) of (-)-epicatechin or water (vehicle) via oral gavage (twice daily). Exercise groups underwent 15 days of treadmill exercise. Significant increases in treadmill performance (∼50%) and enhanced in situ muscle fatigue resistance (∼30%) were observed with (-)-epicatechin. Components of oxidative phosphorylation complexes, mitofilin, porin, nNOS, p-nNOS, and Tfam as well as mitochondrial volume and cristae abundance were significantly higher with (-)-epicatechin treatment for hindlimb and cardiac muscles than exercise alone. In addition, there were significant increases in skeletal muscle capillarity. The combination of (-)-epicatechin and exercise resulted in further increases in oxidative phosphorylation-complex proteins, mitofilin, porin and capillarity than (-)-epicatechin alone. These findings indicate that (-)-epicatechin alone or in combination with exercise induces an integrated response that includes structural and metabolic changes in skeletal and cardiac muscles resulting in greater endurance capacity. These results, therefore, warrant the further evaluation of the underlying mechanism of action of (-)-epicatechin and its potential clinical application as an exercise mimetic.

Sirtuin 1 (SIRT1) deacetylase activity is not required for mitochondrial biogenesis or peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) deacetylation following endurance exercise

The protein deacetylase, sirtuin 1 (SIRT1), is a proposed master regulator of exercise-induced mitochondrial biogenesis in skeletal muscle, primarily via its ability to deacetylate and activate peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). To investigate regulation of mitochondrial biogenesis by SIRT1 in vivo, we generated mice lacking SIRT1 deacetylase activity in skeletal muscle (mKO). We hypothesized that deacetylation of PGC-1α and mitochondrial biogenesis in sedentary mice and after endurance exercise would be impaired in mKO mice. Skeletal muscle contractile characteristics were determined in extensor digitorum longus muscle ex vivo. Mitochondrial biogenesis was assessed after 20 days of voluntary wheel running by measuring electron transport chain protein content, enzyme activity, and mitochondrial DNA expression. PGC-1α expression, nuclear localization, acetylation, and interacting protein association were determined following an acute bout of treadmill exercise (AEX) using co-immunoprecipitation and immunoblotting. Contrary to our hypothesis, skeletal muscle endurance, electron transport chain activity, and voluntary wheel running-induced mitochondrial biogenesis were not impaired in mKO versus wild-type (WT) mice. Moreover, PGC-1α expression, nuclear translocation, activity, and deacetylation after AEX were similar in mKO versus WT mice. Alternatively, we made the novel observation that deacetylation of PGC-1α after AEX occurs in parallel with reduced nuclear abundance of the acetyltransferase, general control of amino-acid synthesis 5 (GCN5), as well as reduced association between GCN5 and nuclear PGC-1α. These findings demonstrate that SIRT1 deacetylase activity is not required for exercise-induced deacetylation of PGC-1α or mitochondrial biogenesis in skeletal muscle and suggest that changes in GCN5 acetyltransferase activity may be an important regulator of PGC-1α activity after exercise.

Reactive oxygen species formation during tetanic contractions in single isolated Xenopus myofibers

Contracting skeletal muscle produces reactive oxygen species (ROS) that have been shown to affect muscle function and adaptation. However, real-time measurement of ROS in contracting myofibers has proven to be difficult. We used amphibian (Xenopus laevis) muscle to test the hypothesis that ROS are formed during contractile activity in isolated single skeletal muscle fibers and that this contraction-induced ROS formation affects fatigue development. Single myofibers were loaded with 5 μM dihydrofluorescein-DA (Hfluor-DA), a fluorescent probe that reacts with ROS and results in the formation of fluorescein (Fluor) to precisely monitor ROS generation within single myofibers in real time using confocal miscroscopy. Three identical periods of maximal tetanic contractions (1 contraction/3 s for 2 min, separated by 60 min of rest) were conducted by each myofiber (n = 6) at 20°C. Ebselen (an antioxidant) was present in the perfusate (10 μM) during the second contractile period. Force was reduced by ∼30% during each of the three contraction periods, with no significant difference in fatigue development among the three periods. The Fluor signal, indicative of ROS generation, increased significantly above baseline in both the first (42 ± 14%) and third periods (39 ± 10%), with no significant difference in the increase in fluorescence between the first and third periods. There was no increase of Fluor in the presence of ebselen during the second contractile period. These results demonstrated that, in isolated intact Xenopus myofibers, 1) ROS can be measured in real time during tetanic contractions, 2) contractile activity induced a significant increase above resting levels of ROS production, and 3) ebselen treatment reduced ROS generation to baseline levels but had no effect on myofiber contractility and fatigue development.

Effect of pulmonary TNF-α overexpression on mouse isolated skeletal muscle function

TNF-α is a proinflammatory cytokine that is involved in numerous pathological processes including chronic obstructive pulmonary disease (COPD). In the present study, we used a transgenic mouse model that overexpresses TNF-α in the lung (Tg(+)) to test the hypothesis that chronic exposure to TNF-α (as seen in COPD) reduces skeletal muscle force production and fatigue resistance, particularly under low Po(2) conditions. At 7-12 mo, body and muscle weight of both extensor digitorum longus (EDL) and soleus were significantly smaller in Tg(+) compared with littermate wild-type (WT) mice; however, the body-to-muscle weight ratio was not different between groups. EDL and soleus muscles were subjected to in vitro fatiguing contractile periods under high (∼550 Torr) and low Po(2) (∼40 Torr). Although all muscles were less fatigue-resistant during low Po(2) compared with high Po(2), only the soleus fatigued more rapidly in Tg(+) mice (∼12%) compared with WT at high Po(2). The maximal tension of EDL was equally reduced in Tg(+) mice (28-34% decrease from WT under both Po(2) conditions); but for soleus this parameter was smaller only under low Po(2) in Tg(+) mice (∼31% decrease from WT). The peak rate of relaxation and the peak rate of contraction were both significantly reduced in Tg(+) EDL muscles compared with WT EDL under low Po(2) conditions, but not in soleus. These results demonstrate that TNF-α upregulation in the lung impairs peripheral skeletal muscle function but affects fast- and slow-twitch muscles differentially at high and low Po(2).

Kinetic control of oxygen consumption during contractions in self-perfused skeletal muscle

Fast kinetics of muscle oxygen consumption (VO2) is characteristic of effective physiological systems integration. The mechanism of VO2 kinetic control in vivo is equivocal as measurements are complicated by the twin difficulties of making high-frequency direct measurements of VO2 and intramuscular metabolites, and in attaining high [ADP]; complexities that can be overcome utilising highly aerobic canine muscle for the investigation of the transition from rest to contractions at maximal VO2. Isometric tetanic contractions of the gastrocnemius complex of six anaesthetised, ventilated dogs were elicited via sciatic nerve stimulation (50 Hz; 200 ms duration; 1 contraction s(−1)). Muscle VO2 and lactate efflux were determined from direct Fick measurements. Muscle biopsies were taken at rest and every ∼10 s during the transient and analysed for [phosphates], [lactate] and pH. The temporal VO2 vs. [PCr] and [ADP] relationships were not well fitted by linear or classical hyperbolic models (respectively), due to the high sensitivity of VO2 to metabolic perturbations early in the transient. The time course of this apparent sensitisation was closely aligned to that of ATP turnover, which was lower in the first ∼25 s of contractions compared to the steady state. These findings provide the first direct measurements of skeletal muscle VO2 and [PCr] in the non-steady state, and suggest that simple phosphate feedback models (which are adequate for steady-state observations in vitro) are not sufficient to explain the dynamic control of VO2 in situ. Rather an allosteric or 'parallel activation' mechanism of energy consuming and producing processes is required to explain the kinetic control of VO2 in mammalian skeletal muscle.

Faster O₂ uptake kinetics in canine skeletal muscle in situ after acute creatine kinase inhibition

Creatine kinase (CK) plays a key role both in energy provision and in signal transduction for the increase in skeletal muscle O2 uptake () at exercise onset. The effects of acute CK inhibition by iodoacetamide (IA; 5 mm) on kinetics were studied in isolated canine gastrocnemius muscles in situ (n = 6) during transitions from rest to 3 min of electrically stimulated contractions eliciting ∼70% of muscle peak , and were compared to control (Ctrl) conditions. In both IA and Ctrl muscles were pump-perfused with constantly elevated blood flows. Arterial and venous [O2] were determined at rest and every 5-7 s during contractions. was calculated by Fick's principle. Muscle biopsies were obtained at rest and after ∼3 min of contractions. Muscle force was measured continuously. There was no fatigue in Ctrl (final force/initial force (fatigue index, FI) = 0.97 ± 0.06 (x ± s.d.)), whereas in IA force was significantly lower during the first contractions, slightly recovered at 15-20 s and then decreased (FI 0.67 ± 0.17). [Phosphocreatine] was not different in the two conditions at rest, and decreased during contractions in Ctrl, but not in IA. at 3 min was lower in IA (4.7 ± 2.9 ml 100 g-1 min-1) vs. Ctrl (16.6 ± 2.5 ml 100 g-1 min-1). The time constant (τ) of kinetics was faster in IA (8.1 ± 4.8 s) vs. Ctrl (16.6 ± 2.6 s). A second control condition (Ctrl-Mod) was produced by modelling a response that accounted for the 'non-square' force profile in IA, which by itself could have influenced kinetics. However, τ in IA was faster than in Ctrl-Mod (13.8 ± 2.8 s). The faster kinetics due to IA suggest that in mammalian skeletal muscle in situ, following contractions onset, temporal energy buffering by CK slows the kinetics of signal transduction for the activation of oxidative phosphorylation.

Phenol increases intracellular [Ca2+] during twitch contractions in intact Xenopus skeletal myofibers

Phenol is a neurolytic agent used for management of spasticity in patients with either motoneuron lesions or stroke. In addition, compounds that enhance muscle contractility (i.e., polyphenols, etc.) may affect muscle function through the phenol group. However, the effects of phenol on muscle function are unknown, and it was, therefore, the purpose of the present investigation to examine the effects of phenol on tension development and Ca(2+) release in intact skeletal muscle fibers. Dissected intact muscle fibers from Xenopus laevis were electrically stimulated, and cytosolic Ca(2+) concentration ([Ca(2+)](c)) and tension development were recorded. During single twitches and unfused tetani, phenol significantly increased [Ca(2+)](c) and tension without affecting myofilament Ca(2+) sensitivity. To investigate the phenol effects on Ca(2+) channel/ryanodine receptors, single fibers were treated with different concentrations of caffeine in the presence and absence of phenol. Low concentrations of phenol significantly increased the caffeine sensitivity (P < 0.01) and reduced the caffeine concentrations necessary to produce nonstimulated contraction (contracture). However, at high phenol concentrations, caffeine did not increase tension or Ca(2+) release. These results suggest that phenol affects the ability of caffeine to release Ca(2+) through an effect on the ryanodine receptors, or on the sarcoplasmic reticulum Ca(2+) pump. During tetanic contractions inducing fatigue, phenol application decreased the time to fatigue. In summary, phenol increases intracellular [Ca(2+)] during twitch contractions in muscle fibers without altering myofilament Ca(2+) sensitivity and may be used as a new agent to study skeletal muscle Ca(2+) handling.

The O2 cost of the tension-time integral in isolated single myocytes during fatigue

One proposed explanation for the Vo(2) slow component is that lower-threshold motor units may fatigue and develop little or no tension but continue to use O(2), thereby resulting in a dissociation of cellular respiration from force generation. The present study used intact isolated single myocytes with differing fatigue resistance profiles to investigate the relationship between fatigue, tension development, and aerobic metabolism. Single Xenopus skeletal muscle myofibers were allocated to a fast-fatiguing (FF) or a slow-fatiguing (SF) group, based on the contraction frequency required to elicit a fall in tension to 60% of peak. Phosphorescence quenching of a porphyrin compound was used to determine Delta intracellular Po(2) (Pi(O(2)); a proxy for Vo(2)), and developed isometric tension was monitored to allow calculation of the time-integrated tension (TxT). Although peak DeltaPi(O(2)) was not different between groups (P = 0.36), peak tension was lower (P < 0.05) in SF vs. FF (1.97 +/- 0. 17 V vs. 2. 73 +/- 0.30 V, respectively) and time to 60% of peak tension was significantly longer in SF vs. FF (242 +/- 10 s vs. 203 +/- 10 s, respectively). Before fatigue, both DeltaPi(O(2)) and TxT rose proportionally with contraction frequency in SF and FF, resulting in DeltaPi(O(2))/TxT being identical between groups. At fatigue, TxT fell dramatically in both groups, but DeltaPi(O(2)) decreased proportionately only in the FF group, resulting in an increase in DeltaPi(O(2))/TxT in the SF group relative to the prefatigue condition. These data show that more fatigue-resistant fibers maintain aerobic metabolism as they fatigue, resulting in an increased O(2) cost of contractions that could contribute to the Vo(2) slow component seen in whole body exercise.

Effect of physiological levels of caffeine on Ca2+ handling and fatigue development in Xenopus isolated single myofibers

The purpose of the present study was to determine whether exposure to exogenous physiological concentrations of caffeine influence contractility, Ca(2+) handling, and fatigue development in isolated single Xenopus laevis skeletal muscle fibers. After isolation, two identical contractile periods (separated by 60-min rest) were conducted in each single myofiber (n = 8) at 20 degrees C. During the first contractile period, four fibers were perfused with a noncaffeinated Ringer solution, while the other four fibers were perfused with a caffeinated (70 microM) Ringer solution. The order was reversed for the second contractile period. The single myofibers were stimulated during each contractile period at increasing frequencies (0.16, 0.20, 0.25, 0.33, 0.50, and 1.0 tetanic contractions/s), with each stimulation frequency lasting 2 min until fatigue ensued, defined in this study as a fall in tension development to 66% of maximum. Tension development and free cytosolic [Ca(2+)] (fura-2 fluorescence spectroscopy) were simultaneously measured. There was no significant difference in the peak force generation, time to fatigue, cytosolic Ca(2+) levels, or relaxation times between the noncaffeinated and caffeinated trials. These results demonstrate that physiological levels of caffeine have no significant effect on Xenopus single myofiber contractility, Ca(2+) handling, and fatigue development, and suggest that any ergogenic effects of physiological levels of caffeine on muscle performance during contractions of moderate to high intensity are likely related to factors extraneous to the muscle fiber.

Elevation in heat shock protein 72 mRNA following contractions in isolated single skeletal muscle fibers

The purpose of the present study was 1) to develop a stable model for measuring contraction-induced elevations in mRNA in single skeletal muscle fibers and 2) to utilize this model to investigate the response of heat shock protein 72 (HSP72) mRNA following an acute bout of fatiguing contractions. Living, intact skeletal muscle fibers were microdissected from lumbrical muscle of Xenopus laevis and either electrically stimulated for 15 min of tetanic contractions (EX; n=26) or not stimulated to contract (REST; n=14). The relative mean developed tension of EX fibers decreased to 29+/-7% of initial peak tension at the stimulation end point. Following treatment, individual fibers were allowed to recover for 1 (n=9), 2 (n=8), or 4 h (n=9) prior to isolation of total cellular mRNA. HSP72, HSP60, and cardiac alpha-actin mRNA content were then assessed in individual fibers using quantitative PCR detection. Relative HSP72 mRNA content was significantly (P<0.05) elevated at the 2-h postcontraction time point relative to REST fibers when normalized to either HSP60 (18.5+/-7.5-fold) or cardiac alpha-actin (14.7+/-4.3-fold), although not at the 1- or 4-h time points. These data indicate that 1) extraction of RNA followed by relative quantification of mRNA of select genes in isolated single skeletal muscle fibers can be reliably performed, 2) HSP60 and cardiac alpha-actin are suitable endogenous normalizing genes in skeletal muscle following contractions, and 3) a significantly elevated content of HSP72 mRNA is detectable in skeletal muscle 2 h after a single bout of fatiguing contractions, despite minimal temperature changes and without influence from extracellular sources.