Skeletal Muscle Oxygen Consumption Research Articles

Sourcebook update: using near-infrared spectroscopy to assess skeletal muscle oxygen uptake.

Monitoring the metabolic cost or oxygen consumption associated with rest and exercise is crucial to understanding the impact of disease or physical training on the health of individuals. Traditionally, measuring the skeletal muscle oxygen cost associated with exercise/muscle contractions can be rather expensive or invasive (i.e., muscle biopsies). More recently, specific protocols designed around the use of near-infrared spectroscopy (NIRS) have been shown to provide a quick, noninvasive easy-to-use tool to measure skeletal muscle oxygen consumption ([Formula: see text]). However, the data and results from NIRS devices are often misunderstood. Thus the primary purpose of this sourcebook update is to provide several experimental protocols students can utilize to improve their understanding of NIRS technology, learn how to analyze results from NIRS devices, and better understand how muscle contraction intensity and type (isometric, concentric, or eccentric) influence the oxygen cost of muscle contractions.NEW & NOTEWORTHY Compared to traditional methods, near-infrared spectroscopy (NIRS) provides a relatively cheap and easy-to-use noninvasive technique to measure skeletal muscle oxygen uptake following exercise. This laboratory not only enables students to learn about the basics of NIRS and muscle energetics but also addresses more complex questions regarding skeletal muscle physiology.

Effects of hibernation on two important contractile tissues in tibetan frogs, Nanorana parkeri: a perspective from transcriptomics and metabolomics approaches

BackgroundIn response to seasonal cold and food shortage, the Xizang plateau frogs, Nanorana parkeri (Anura: Dicroglossidae), enter a reversible hypometabolic state where heart rate and oxygen consumption in skeletal muscle are strongly suppressed. However, the effect of winter hibernation on gene expression and metabolic profiling in these two tissues remains unknown. In the present study, we conducted transcriptomic and metabolomic analyses of heart and skeletal muscle from summer- and winter-collected N. parkeri to explore mechanisms involved in seasonal hibernation.ResultsWe identified 2407 differentially expressed genes (DEGs) in heart and 2938 DEGs in skeletal muscle. Enrichment analysis showed that shared DEGs in both tissues were enriched mainly in translation and metabolic processes. Of these, the expression of genes functionally categorized as “response to stress”, “defense mechanisms”, or “muscle contraction” were particularly associated with hibernation. Metabolomic analysis identified 24 and 22 differentially expressed metabolites (DEMs) in myocardium and skeletal muscle, respectively. In particular, pathway analysis showed that DEMs in myocardium were involved in the pentose phosphate pathway, glycerolipid metabolism, pyruvate metabolism, citrate cycle (TCA cycle), and glycolysis/gluconeogenesis. By contrast, DEMs in skeletal muscle were mainly involved in amino acid metabolism.ConclusionsIn summary, natural adaptations of myocardium and skeletal muscle in hibernating N. parkeri involved transcriptional alterations in translation, stress response, protective mechanisms, and muscle contraction processes as well as metabolic remodeling. This study provides new insights into the transcriptional and metabolic adjustments that aid winter survival of high-altitude frogs N. parkeri.

Noninvasive estimation of skeletal muscle oxygen consumption rate and microvascular reactivity in chronic stroke survivors using near-infrared spectroscopy.

Understanding post-stroke changes in skeletal muscle oxidative metabolism and microvascular reactivity could help create therapeutic targets that optimize rehabilitative interventions. Due to disuse atrophy, we hypothesized that basal muscle oxygen consumption rate and microvascular endothelial function would be impaired in the tibialis anterior (TA) muscle of the affected leg of chronic stroke survivors compared with the nonaffected leg and versus matched controls. Fifteen chronic stroke survivors (10 females) and 15 matched controls (9 females) completed this study. A near-infrared spectroscopy oximeter measured tissue oxygen saturation (StO2) of the TA in both legs of stroke survivors and the dominant leg of controls. A cuff was placed around the thigh and inflated to 225 mmHg for 5 min while StO2 was continuously measured. The rate of change in StO2 was calculated during cuff occlusion and immediately post-cuff release. The rate of oxygen desaturation was similar between the legs of the stroke survivors (paretic -0.12 ± 0.04%·s-1 vs. nonparetic -0.16 ± 011%·s-1; P = 0.49), but the paretic leg had a reduced desaturation rate versus controls (-0.25 ± 0.18%·s-1; P = 0.007 vs. paretic leg). After cuff release, there was a greater oxygen resaturation rate in the nonparetic leg compared with the paretic leg (3.13 ± 2.08%·s-1 vs. 1.60 ± 1.11%·s-1, respectively; P = 0.01). The control leg had a similar resaturation rate versus the nonparetic leg (control = 3.41 ± 1.79%·s-1; P = 0.69) but was greater than the paretic leg (P = 0.003). The TA in the paretic leg had an impaired muscle oxygen consumption rate and reduced microvascular endothelial function compared with controls.NEW & NOTEWORTHY Secondary consequences of stroke are not well described. In this study, we show that basal muscle oxidative consumption and microvascular endothelial function are reduced in the paretic tibialis anterior muscle of chronic stroke survivors compared with matched controls using near-infrared spectroscopy and the vascular occlusion technique. There was a moderately strong correlation between microvascular endothelial function and paretic leg strength.

Analysis of electrical stimulation and voluntary muscle contraction on skeletal muscle oxygen uptake and mitochondrial recovery using near-infrared spectroscopy.

This investigation was to compare differences in skeletal muscle oxygen consumption ([Formula: see text]) and mitochondrial recovery between voluntary (VOL) and electrically stimulated (ES) plantarflexion contractions. Twelve men and women (26 ± 4.0years; 171.8 ± 5.1cm; 74.0 ± 13.7kg) were seated in a chair with their right knee fully extended and right foot secured to a force transducer. ES electrodes and a near-infrared spectroscopy device were placed on the gastrocnemius. Participants performed ES plantarflexion contractions across a range of stimulation intensities at frequencies of 1 and 2Hz and similar VOL contractions. Cuff occlusion occurred immediately following each series of contractions to measure [Formula: see text]. A standardized mitochondrial function assessment protocol was also performed to calculate K-constants between work-matched ES and VOL contractions. For mitochondrial assessments, there were no significant differences between ES and VOL rate constants (2.03 ± 0.98 vs. 1.25 ± 1.35min-1, p = 0.266). ES resulted in a significantly greater workrate-[Formula: see text] slope at 1Hz (0.007 ± 0.007 vs. 0.001 ± 0.002% [Formula: see text]/s/N, p = 0.014) and 2Hz (0.010 ± 0.010 vs. 0.001 ± 0.001% [Formula: see text]/s/N, p = 0.012), as well as a significantly greater workrate-[Formula: see text] Y-intercept at 2Hz (1.603 ± 1.513 vs. 0.556 ± 0.564% [Formula: see text]/s, p = 0.035) but not 1Hz (0.579 ± 0.448 vs. 0.442 ± 0.357% mV̇O2/s, p = 0.535) when compared to VOL. ES results in a significantly greater [Formula: see text] at similar work rates compared to VOL, however, the mitochondrial recovery rate constants were similar. The greater mVO2 with ES may partially contribute to the increased rate of fatigue during ES exercise in individuals with muscle paralysis.

Skeletal Muscle Cells Derived From Old Donors Show Altered Mitochondrial Morphology And Reduced Respiratory Capacity

With age, skeletal muscle mitochondria lose their oxidative capacity and their ability to respond to energy demand. These phenomena can lead to a reduction in skeletal muscle oxygen consumption, atrophy, and increased risk of developing age-related diseases such as sarcopenia. Whether age-derived changes in mitochondrial function correspond to structural changes in the mitochondrial reticulum remains unknown. PURPOSE: Investigate age-related changes in mitochondrial morphology and function using primary skeletal muscle cells derived from healthy young and old men. METHODS: Primary skeletal muscle progenitor cells (SkM) derived from the Rectus abdominis muscle of healthy active 18-19-year-old men (SkM Young), and 66-69-year-old men (SkM Old) were obtained from Cook MyoSite Inc. (Pittsburgh, PA). The mitochondrial network was analyzed in live cells using confocal microscopy. Oxygen Consumption Rate (OCR) was measured in intact cells using extracellular flux assays and a Seahorse analyzer (Agilent Technologies; Santa Clara, CA). RESULTS: Primary cells derived from SkM Young revealed a higher Basal and Maximal OCR compared to SkM Old (Basal: 23.20 ± 1.49 vs. 11.34 ± 2.31; Maximal: 41.50 ± 4.72 vs. 18.36 ± 1.65 pmol/min/protein). SkM Young also had higher ATP production and Spare Respiratory Capacity (ATP: 19.08 ± 1.21 vs. 9.19 ± 2.10; SRC: 18.30 ± 3.47 vs. 7.03 ± 1.29 pmol/min/protein). No differences were revealed in morphology (Individuals: 37.80 ± 23.18 vs. 46.67 ± 22.43 counts; Networks: 10.20 ± 2.48 vs. 15.33 ± 5.75 counts; Mean Branches per Network: 15.52 ± 6.41 vs. 11.27 ± 3.67 counts). CONCLUSIONS: These preliminary results show primary skeletal muscle progenitor cells derived from healthy young donors have higher respiration, produce more ATP, and have a greater capacity to adapt to energy demands when compared to SkM derived from Old donors. Further analyses can give us an insight into human skeletal muscle-derived cellular physiological capacity. Technology to observe human muscle mitochondrial dysfunction in vitro helps us understand the effects of aging on skeletal muscle mitochondria and whole-body declines of resting and exercise metabolic rates. Additional studies will be required on aging individuals to determine if predicted decrements in energy coupling efficiency occur with aging in vivo.

Mitochondria from Skeletal Muscle Cells Derived from Old Donors Show Reduced Energy Production and Respiratory Capacity

With age, skeletal muscle mitochondria lose their oxidative capacity and their ability to respond to energy demand. These phenomena can lead to a reduction in skeletal muscle oxygen consumption, atrophy, and increased risk of developing age‐related diseases such as sarcopenia. Whether age‐derived changes in mitochondrial function correspond to morphological changes in the mitochondrial reticulum remains unknown. Our objective is to investigate age‐related changes in mitochondrial morphology and function using primary skeletal muscle cells derived from healthy young and old men. Primary skeletal muscle progenitor cells (SkM) derived from the Rectus abdominis muscle of healthy active 18‐19‐year‐old men (Young), and 66‐69‐year‐old men (Old) were obtained from Cook MyoSite Inc. (Pittsburgh, PA). Mitochondrial morphology was analyzed in live cells using confocal microscopy. Oxygen Consumption Rate (OCR) was measured in intact cells using the extracellular flux assay and a Seahorse analyzer (Agilent Technologies; Santa Clara, CA). Seahorse Cell Mito Stress Test of primary cells derived from Young revealed a higher Basal and Maximal OCR compared to Old (Basal: 23.20 ± 1.49 vs. 11.34 ± 2.31; Maximal: 41.50 ± 4.72 vs. 18.36 ± 1.65 pmol/min/protein). Young also had higher ATP production and Spare Respiratory Capacity (ATP: 19.08 ± 1.21 vs. 9.19 ± 2.10; SRC: 18.30 ± 3.47 vs. 7.03 ± 1.29 pmol/min/protein). No differences were revealed in morphology (Individuals: 37.80 ± 23.18 vs. 46.67 ± 22.43 counts; Networks: 10.20 ± 2.48 vs. 15.33 ± 5.75 counts; Mean Branches per Network: 15.52 ± 6.41 vs. 11.27 ± 3.67 counts). These preliminary results show primary skeletal muscle progenitor cells derived from old donors have lower respiration, produce less ATP, and have a reduced capacity to adapt to energy demands when compared to SkM derived from young donors. Further analyses can give us an insight into human skeletal muscle‐derived cellular physiological capacity. Technology to observe human muscle mitochondrial dysfunction in vitro helps us understand the effects of aging on skeletal muscle mitochondria and whole‐body declines of resting and exercise metabolic rates. Studies on aging individuals will be required to determine if predicted decrements in energy coupling efficiency occur with aging in vivo.

A NIRS‐derived Skeletal Muscle Oxygen Consumption Method Accurately Reflects the Gold Standard Direct Fick Method during Single‐leg Knee Extension Exercise

BackgroundQuantifying the oxygen consumption (V̇O2) of skeletal muscle, which represents the largest metabolically‐active tissue mass, is an important assessment in exercise physiology. The gold standard for quantifying muscle V̇O2 in the exercising human is the direct Fick method, however, technical challenges (e.g. arterial catheter insertion) preclude the widespread use of this technique, justifying the need for alternative approaches for the quantification of muscle V̇O2. Recently, we developed a noninvasive method of quantifying muscle V̇O2 using Doppler ultrasound measurements of blood flow (Q̇) and near‐infrared spectroscopy (NIRS)‐derived deoxygenated‐[Heme] in a modified Fick method calculation, but, to date, this has not been validated against the gold standard method.Purpose and HypothesesTherefore, the purpose of this investigation was to compare NIRS‐derived and direct Fick quantification of muscle V̇O2 at multiple levels of single‐leg knee‐extensor exercise. We tested the hypotheses that: 1) the muscle V̇O2 assessed by direct Fick and the NIRS‐derived muscle V̇O2 would not be statistically different from each other across work rates, and 2) the V̇O2 assessed by the two techniques would be strongly related.MethodsSeven healthy young volunteers (3 women, 4 men; 24 ± 5 years) performed an incremental work rate test on a dynamic single‐leg knee‐extensor ergometer. Measurements of femoral artery Q̇ (Doppler ultrasound), vastus lateralis deoxygenated‐[Heme] (NIRS), and femoral artery and vein oxygen content were taken at rest, 0, 5, 10, 15, and 20 Watts (W) and used to estimate direct Fick and NIRS‐derived muscle V̇O­2.ResultsNIRS‐derived V̇O2 was not different from direct Fick V̇O2 at any point (rest: 45 ± 21 vs 25 ± 17 ml/min; 0 W: 153 ± 70 vs 133 ± 68 ml/min; 5 W: 188 ± 81 vs 160 ± 65 ml/min; 10 W: 242 ± 123 vs 214 ± 91 ml/min; 15 W: 279 ± 131 vs 247 ± 89 ml/min; 20 W: 343 ± 163 vs 319 ± 122 ml/min; all P = 0.26 – 0.44). Muscle V̇O2determined by the two techniques was strongly related (r2: 0.97 ± 0.03) with a slope of 1.1 ± 0.3 and Y‐intercept of ‐21 ± 22 ml/min.DiscussionTaken together, these findings indicate that the NIRS‐derived, noninvasive assessment of skeletal muscle V̇O2 accurately reflects the gold standard quantification of muscle V̇O2over a range of work rates (with a constant ≍20 ml/min offset). This investigation provides the first evidence to support the use of the NIRS‐derived muscle V̇O2 estimate during free‐flow conditions (i.e. not using venous or arterial occlusions like previous work), introducing a novel, noninvasive technique to quantify exercising skeletal muscle V̇O2.

Sarco/Endoplasmic Reticulum Ca2+ ATPase 2 Activator Ameliorates Endothelial Dysfunction; Insulin Resistance in Diabetic Mice.

Background: Sarco/endoplasmic reticulum Ca2+-ATPase2 (SERCA2) is impaired in various organs in animal models of diabetes. The purpose of this study was to test the effects of an allosteric SERCA2 activator (CDN1163) on glucose intolerance, hepatosteatosis, skeletal muscle function, and endothelial dysfunction in diabetic (db/db) mice. Methods: Either CDN1163 or vehicle was injected intraperitoneally into 16-week-old male control and db/db mice for 5 consecutive days. Results: SERCA2 protein expression was decreased in the aorta of db/db mice. In isometric tension measurements of aortic rings from db/db mice treated with CDN1163, acetylcholine (ACh)-induced relaxation was improved. In vivo intraperitoneal administrations of CDN 1163 also increased ACh-induced relaxation. Moreover, CDN1163 significantly decreased blood glucose in db/db mice at 60 and 120 min during a glucose tolerance test; it also decreased serum insulin levels, hepatosteatosis, and oxygen consumption in skeletal muscle during the early period of exercise in db/db mice. Conclusions: CDN1163 directly improved aortic endothelial dysfunction in db/db mice. Moreover, CDN1163 improved hepatosteatosis, skeletal muscle function, and insulin resistance in db/db mice. The activation of SERCA2 might be a strategy for the all the tissue expressed SERCA2a improvement of endothelial dysfunction and the target for the organs related to insulin resistance.

The Role of Oxidative Stress in Skeletal Muscle Myogenesis and Muscle Disease.

The contractile activity, high oxygen consumption and metabolic rate of skeletal muscle cause it to continuously produce moderate levels of oxidant species, such as reactive oxygen species (ROS) and reactive nitrogen species (RNS). Under normal physiological conditions, there is a dynamic balance between the production and elimination of ROS/RNS. However, when the oxidation products exceed the antioxidant defense capacity, the body enters a state of oxidative stress. Myogenesis is an important process to maintain muscle homeostasis and the physiological function of skeletal muscle. Accumulating evidence suggests that oxidative stress plays a key role in myogenesis and skeletal muscle physiology and pathology. In this review, we summarize the sources of reactive oxygen species in skeletal muscle and the causes of oxidative stress and analyze the key role of oxidative stress in myogenesis. Then, we discuss the relationship between oxidative stress and muscle homeostasis and physiopathology. This work systematically summarizes the role of oxidative stress in myogenesis and muscle diseases and provides targets for subsequent antioxidant therapy and repair of inflammatory damage in noninflammatory muscle diseases.

Comparison of Different Approaches Estimating Skeletal Muscle Oxygen Consumption Using Continuous-Wave Near-Infrared Spectroscopy at a Submaximal Contraction Level—A Comparative Study

Continuous-wave near-infrared spectroscopy (CW-NIRS) is a method used to non-invasively estimate skeletal muscle oxygen consumption (mVO2). Three different signals are provided by CW-NIRS devices: (1) oxygenated hemoglobin (O2Hb); (2) deoxygenated hemoglobin (HHb); and (3) tissue saturation index (TSI). Typically, the signal’s slope is interpreted with respect to high or low mVO2 during a muscle action. What signal (or combination of signals) is used for slope interpretation differs according to what approach is used, and there are several published in literature. It is unclear if resulting mVO2 estimates can be used interchangeably. Hence, this work aimed to compare five commonly used approaches on the same set of CW-NIRS data regarding their agreement in estimated mVO2. A controlled, lab-based study setting was used for this experiment. Data are based on isometric dorsiflexion contractions of 15 subjects at 30% of voluntary maximum torque, at two different ankle angles. CW-NIRS was placed on the m. tibialis anterior and blood flow was occluded. The approaches for mVO2 estimation included calculations based on (1) TSI, (2) the difference between O2Hb and HHb (Hbdiff), (3) the mean of slopes from O2Hb and HHb (Hbmean), (4) the HHb signal, and (5) the O2Hb signal. Linear regression modelling was used to calculate respective slopes (r2 > 0.99). Repeated measures ANOVA identified significant differences between the approaches (p < 0.001, ω2 = 0.258). Post-hoc tests revealed that only TSI vs. Hbmean and Hbdiff vs. HHb gave comparable results (p > 0.271). In addition, Bland–Altman plots showed good accuracy (mean bias ~2%) but low precision (±20%) between the comparisons. Thus, the different approaches to estimate mVO2 cannot be used interchangeably. The results from different studies using different approaches should be compared with caution.

Effects of the exercise training on skeletal muscle oxygen consumption in heart failure patients with reduced ejection fraction

AimsSkeletal muscle dysfunction is a systemic consequence of heart failure (HF) that correlates with functional capacity. However, the impairment within the skeletal muscle is not well established. We investigated the effect of exercise training on peripheral muscular performance and oxygenation in HF patients. Methods and resultsHF patients with ejection fraction ≤40% were randomized 2:1 to exercise training or control for 12 weeks. Muscle tissue oxygen was measured noninvasively by near-infrared spectroscopy (NIRS) during rest and a symptom-limited cardiopulmonary exercise test (CPET) before and after intervention. Measurements included skeletal muscle oxygenated hemoglobin concentration, deoxygenated hemoglobin concentration, total hemoglobin concentration, VO2 peak, VE/VCO2 slope, and heart rate. Muscle sympathetic nerve activity by microneurography, and muscle blood flow by plethysmography were also assessed at rest pre and post 12 weeks. Twenty-four participants (47.5 ± 7.4 years, 58% men, 75% no ischemic) were allocated to exercise training (ET, n = 16) or control (CG, n = 8). At baseline, no differences between groups were found. Exercise improved VO2 peak, slope VE/VCO2, and heart rate. After the intervention, significant improvements at rest were seen in the ET group in muscle sympathetic nerve activity and muscle blood flow. Concomitantly, a significant decreased in Oxy-Hb (from 29.4 ± 20.4 to 15.7 ± 9.0 μmol, p = 0.01), Deoxi-Hb (from 16.3 ± 8.2 to 12.2 ± 6.0 μmol, p = 0.003) and HbT (from 45.7 ± 27.6 to 27.7 ± 13.4 μmol, p = 0.008) was detected at peak exercise after training. No changes were observed in the control group. ConclusionExercise training improves skeletal muscle function and functional capacity in HF patients with reduced ejection fraction. This improvement was associated with increased oxygenation of the peripheral muscles, increased muscle blood flow, and decreased sympathetic nerve activity.

One Bout of Aerobic Exercise Elicits Alterations in The Expression of Mitochondrial Unfolded Protein Response (UPRmt) Markers in Skeletal Muscle

Muscle contractions during aerobic exercise stimulate molecular pathways to promote the production of new mitochondria, and improve their function. These adaptations translate into improved oxygen consumption and metabolic capacity of skeletal muscle. However, the increased drive for mitochondrial biogenesis during contractile activity causes an accumulation of misfolded proteins, disrupting mitochondrial homeostasis. The action of the UPRmt, a cytoprotective stress response pathway, is required to mitigate the associated proteotoxicity. This protein quality control mechanism favours the transcription of mitochondrial chaperones and proteases to refold and degrade the defective proteins, while inhibiting global protein translation. However, the regulation and activity of the UPRmt in helping skeletal muscle adapt to exercise stress during and after an acute bout of exercise has yet to be determined. Thus, to investigate this, mice were separated into sedentary (SED), acute exercise (AE) and acute exercise with recovery (AER) groups (n=4/group). The exercise consisted of 90-minutes of treadmill running at 15m/min. Tibialis anterior muscles were extracted from the AE groups immediately following exercise and from the AER group 3 hours post-exercise. Total RNA was isolated and mRNA levels were measured using qPCR. In line with previous data, PGC-1α mRNA increased by 50% in the AE group and increased further to 3.4-fold in the AER group. Activating transcription factor 5 (ATF5), an important regulator of UPRmt gene expression, did not change with acute exercise or recovery. ATF4, another transcription factor for stress-responsive genes, was also unchanged at the end of exercise, but was upregulated by 28% in the recovery period. mRNA levels of downstream targets of ATF5, Lon protease (LonP) and chaperones (HSP60 and GRP75) were upregulated by 18%, 24% and 77%, respectively. HSP60 mRNA returned to control levels during recovery, while GRP75 and LonP mRNAs remained elevated in the AER group. Our data indicate that acute exercise influences the expression of genes involved in both mitochondrial biogenesis and those of the UPRmt, contributing to the maintenance of organelle proteostasis in exercised muscle. Investigating the regulatory mechanisms governing this acute mitochondrial stress response is critical, as preservation of cellular function in the face of exercise-induced stress is essential for metabolic adaptations in muscle and thus, for whole-body homeostasis.

Impaired Skeletal Muscle Mitochondrial Efficiency in Smokers

Introduction Cigarette smoke can directly contribute to the development of skeletal muscle dysfunction. Mitochondrial oxidative capacity, a product of energy production efficiency and oxygen consumption, is important for skeletal muscle health. Whether mitochondrial energy production efficiency and muscle oxygen consumption play a role into skeletal muscle dysfunction in smokers has yet to be investigated. Thus, this study sought to test the hypothesis that mitochondrial oxidative capacity and mitochondrial efficiency are reduced in smokers compared to non-smokers. Methods 30 active smokers (Age: 33 ± 4 yrs, BMI: 30.8 ± 11.7 kg/m2) and 30 demographically matched healthy controls (Age: 33 ± 3 yrs, BMI: 32.5 ± 9.1 kg/m2) participated in this study. Participants self-reported the number of cigarettes smoked per day in the past 30 days. Near infrared spectroscopy (NIRS) was used to assess changes in oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin/myoglobin of the gastrocnemius following 15s of electrical stimulation and multiple rapid arterial occlusions. The phosphocreatine (PCr) recovery rate constant (kPCr), an index of mitochondrial oxidative capacity, was calculated by fitting post-stimulation measurements to a monoexponential curve. Muscle oxygen consumption (mVO2 = abs[([ΔO2Hb – Δ HHb]/2] × 60)/(10 × 1.04) x4] × 22.4/1000) was calculated for each occlusion and the slopes of the total hemoglobin (tHb = O2Hb+HHb) signal following each occlusion were converted to muscle blood flow [mBF = 1/hemoglobin (mmol/L) × ΔtHb/Δtime]. Mitochondrial efficiency (MEFF) was calculated by the dividing the estimated mVO2 needed to achieve kPcr at 100% (mVO2estimated) efficiency by the observed mVO2 [Meff = (mVO2estimated/mVO2observed) × 100]. Results The average cigarettes smoked was 5 ± 3 cigarettes per day. kPcr was significantly (p = 0.022) reduced in smokers (1.99 ± 0.19 min-1) compared to non-smokers (2.54 ± 0.19 min-1). Compared to non-smokers (157.2 ± 45.5%), MEFF was significantly (p = 0.025) attenuated in smokers (46.5 ± 10.1%) and significantly associated with mVO2 (r = -0.409, p = 0.009) and kPcr (r = 0.461, p =0.003). Average mVO2 (p = 0.231) and mBF (p = 0.993)were not significantly different in smokers compared to non-smokers. Conclusion Muscle mitochondrial oxidative capacity and mitochondrial efficiency are reduced in smokers compared to non-smokers. However, skeletal muscle blood flow and oxygen consumption were not different between groups. Taken together, mitochondrial oxidative capacity may be reduced prior to alterations in blood flow and oxygen consumption or delivery in young smokers.

Rates of oxidative ATP synthesis are not augmented beyond the pH threshold in human vastus lateralis muscles during a stepwise contraction protocol.

The oxygen cost of high-intensity exercise at power outputs above an individual's lactate threshold (LT) is greater than would be predicted by the linear oxygen consumption-power relationship observed below the LT. However, whether these augmentations are caused by an increased ATP cost of force generation (ATPCOST ) or an increased oxygen cost of ATP synthesis is unclear. We used 31 P-MRS to measure changes in cytosolic [ADP] (intramyocellular marker of oxidative metabolism), oxidative ATP synthesis (ATPOX ) and ATPCOST during a 6-stage, stepwise knee extension protocol. ATPCOST was unchanged across stages. The relationship between [ADP] and muscle power output was augmented at workloads above the pH threshold (pHT ; proxy for LT), whereas increases in ATPOX were attenuated. These results suggest the greater oxygen cost of contractions at workloads beyond the pHT is not caused by mechanisms that increase ATPCOST , but rather mechanisms that alter intrinsic mitochondrial function or capacity. Increases in skeletal muscle metabolism and oxygen consumption are linearly related to muscle power output for workloads below the lactate threshold (LT), but are augmented (i.e. greater rate of increase relative to workload) thereafter. Presently, it is unclear whether these metabolic augmentations are caused by increases in the ATP cost of force generation (ATPCOST ) or changes in the efficiency of mitochondrial oxygen consumption and oxidative ATP synthesis (ATPOX ). To partition these two hypotheses in vivo, we used 31 P-MRS to calculate slopes relating step-changes in muscle work to concurrent changes in cytosolic phosphates and ATPOX before and after the pH threshold (pHT ; used here as a proxy for LT) within the vastus lateralis muscle of eight young adults during a stepwise knee extension test. Changes in muscle phosphates and ATPOX were linearly related to workload below the pHT . However, slopes above the pHT were greater for muscle phosphates (P<0.05) and lower for ATPOX (P<0.05) than were the slopes observed below the pHT . The maximal capacity for ATPOX ( ) and ADP-specific ATPOX also declined beyond the pHT (P<0.05), whereas ATPCOST was unchanged (P=0.10). These results oppose the hypothesis that high-intensity contractions increase ATPCOST and suggest that greater oxidative metabolism at workloads beyond the pHT is caused by mechanisms that affect intrinsic mitochondrial function or capacity, such as alterations in substrate selection or electron entry into the electron transport chain, temperature-mediated changes in mitochondrial permeability to protons, or stimulation of mitochondrial uncoupling by reactive oxygen species generation.

Quantitative and simultaneous measurement of oxygen consumption rates in rat brain and skeletal muscle using 17 O MRS imaging at 16.4T.

Oxygen-17 (17 O) MRS imaging, successfully used in the brain, is extended by imaging the oxygen metabolic rate in the resting skeletal muscle and used to determine the total whole-body oxygen metabolic rate in the rat. During and after inhalations of 17 O2 gas, dynamic 17 O MRSI was performed in rats (n = 8) ventilated with N2 O or N2 at 16.4 T. Time courses of the H217 O concentration from regions of interest located in brain and muscle tissue were examined and used to fit an animal-adapted 3-phase metabolic model of oxygen consumption. CBF was determined with an independent washout method. Finally, body oxygen metabolic rate was calculated using a global steady-state approach. Cerebral metabolic rate of oxygen consumption was 1.97 ± 0.19 μmol/g/min on average. The resting metabolic rate of oxygen consumption in skeletal muscle was 0.32 ± 0.12 μmol/g/min and >6 times lower than cerebral metabolic rate of oxygen consumption. Global oxygen consumed by the body was 24.2 ± 3.6 mL O2 /kg body weight/min. CBF was estimated to be 0.28 ± 0.02 mL/g/min and 0.34 ± 0.06 mL/g/min for the N2 and N2 O ventilation condition, respectively. We have evaluated the feasibility of 17 O MRSI for imaging and quantifying the oxygen consumption rate in low metabolizing organs such as the skeletal muscle at rest. Additionally, we have shown that CBF is slightly increased in the case of ventilation with N2 O. We expect this study to be beneficial to the application of 17 O MRSI to a wider range of organs, although further validation is advised.

Skeletal Muscle Endurance And Oxygen Uptake Kinetics During Cycling In Patients With High Affinity Hemoglobin

Skeletal muscle oxygen (O2) consumption is linked to the metabolic demand of the exercising skeletal muscle. In hypoxic conditions (e.g. high altitude) O2 consumption may be the limiting factor of exercise tolerance. Patients with increased O2 content, secondary to an increased oxygen affinity (HAH), provide an experiment of nature to investigate the effects of increased flux through the O2 transport pathway on exercise tolerance. PURPOSE: To determine the effect of HAH on O2 uptake kinetics and exercise tolerance during high-intensity exercise under normoxia (NORM) and hypoxia (HYP) conditions. METHODS: Five healthy controls (CTL; 4 men, 41±8 years, P50=27±1 mmHg; Hemoglobin concentration ([Hb])= 14.2±1.3 g· dL-1; hematocrit (Hct)= 43±4%) and five patients with high-affinity hemoglobin (HAH; 3 men, 37±12 years, P50=15±2 mmHg; [Hb]: 19.8±2.3 g·dL-1, Hct= 59±7%) cycled during unloaded pedaling then at a power output that elicited 85% V̇O2max until volitional exhaustion during two different environmental conditions: 1) NORM, (FiO2=0.21), and 2) HYP, (FiO2=0.15). O2 uptake kinetics were modeled as a double-exponential rise to maximum from continuous measurements of inspired/expired gases. Two-way ANOVA with group (HAH, CTL) and inspirate (NORM, HYP) as between-subjects factors were used to compare dependent variables. RESULTS: HAH patients had marked polycythemia (higher [Hb] and Hct, P<0.05 for both). There was no effect of inspirate on any of the parameters of O2 kinetics, all P>0.175). There was no main effect of group or inspirate on V̇O2 during unloaded pedaling (A0, P>0.24), V̇O2 of the primary component (A1, P>0.13), or the V̇O2 slow component (P>0.10). HAH exhibited a trend towards slower O2 kinetics (HAH=64.3±17.7s vs. CTL=49.2±17.0s, P=0.08) and significantly lower primary component amplitude (HAH=1.14±0.66 L·min-1 vs. CTL=1.92±0.67 L·min-1, P=0.02). There was a trend towards reduced time-to-exhaustion in HYP (P=0.09), but no main effect of group (P=0.21). CONCLUSION: Patients with HAH had slower and blunted V̇O2 kinetics, which may be due to 1) blunted O2 off-loading to the contracting skeletal muscle or 2) adaptations of skeletal muscle (e.g. myosin heavy chain expression) to HAH.

Age- And Sex-related Differences In Skeletal Muscle Oxygen Consumption Rate And Microvascular Reactivity

PURPOSE: To examine the influence of age and sex on non-invasive measurements of oxygen consumption rate and microvascular reactivity. METHODS: Seven young females (YF; mean ± SD, age = 20.9±2 y), 6 older females (OF; 57.5±10 y), 14 younger males (YM; 22.2±2 y), and 8 older males (OM; 59.5±5 y) visited the laboratory on one occasion during which skeletal muscle oxygen utilization and microvascular function were assessed following a 10-h fast using the near-infrared spectroscopy with vascular occlusion test (NIRS-VOT). During the NIRS-VOT, tissue saturation (StO2) was monitored, and the rate of decrease in StO2 (Slope 1) during cuff inflation (i.e., ischemia) was quantified, as were the reperfusion magnitude (RepMAG), rate of increase in StO2 (Slope 2), and the reperfusion area under the curve (StO2AUC) following cuff deflation. Two-way (age × sex) ANOVAs were used to examine the differences in Slope 1, RepMAG, Slope 2, and StO2AUC among the YW, OW, YM, and OM. Means ± 95% CIs are reported. RESULTS: There were significant age and sex effects for Slope 1 (p = 0.01 and 0.02, respectively), which was more negative (i.e., steeper) in males versus females (-0.132±0.12 vs. -0.108±0.15 %1s-1) and in the young versus older adults (-0.133±0.13 vs. -0.107±0.14 %1s-1). There was an age effect for RepMAG (p < 0.001), which was greater in the younger than older adults (50.7±4.0 vs. 38.5±4.6%). While there was no effect of age on Slope 2, Slope 2 was steeper (p = 0.002) in males than females (1.71±0.24 vs. 1.06±0.34 %1s-1). Finally, there were age (p = 0.02) and sex (p = 0.03) main effects for StO2AUC, which was greater in males versus females (1228.5±148.8 vs. 960.1±178.9 %1s) and in the young versus older adults (1240.7±148.8 vs. 947.9±178.9 %1s). CONCLUSIONS: Overall, our results suggest that there are age and sex-related differences in skeletal muscle oxygen consumption rate and microvascular reactivity, as assessed using the NIRS-VOT technique. However, because the degree of tissue desaturation is the stimulus for subsequent reperfusion responses and the rate of desaturation was greater in the younger than older adults and in males than females, it is not clear if the differences in Slope 2 and StO2AUC reflect differences in microvascular reactivity, per se. Additional studies are needed to more fully explore this interplay.

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

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. Contrasting findings have been reported concerning the effects of augmented nitric oxide (NO) on skeletal muscle force production and oxygen consumption ( ). 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 3min of electrically stimulated isometric tetanic contractions corresponding to ∼35% of . During contractions, sodium nitrite (NITRITE) or sodium chloride (SALINE) was infused into the popliteal artery. was calculated from the Fick principle. Experiments were carried out in hypoxia ( =0.12), whereas convective O2 delivery was maintained at normal levels under both conditions by pump-driven blood flow ( ). 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. was not significantly different between NITRITE (6.1±1.8mL 100g-1 min-1 ) and SALINE (6.2±1.8mL100g-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.

Oxidative ATP synthesis in human quadriceps declines during 4minutes of maximal contractions.

During maximal exercise, skeletal muscle metabolism and oxygen consumption remain elevated despite precipitous declines in power. Presently, it is unclear whether these responses are caused by an increased ATP cost of force generation (ATPCOST ) or mitochondrial uncoupling; a process that reduces the efficiency of oxidative ATP synthesis (ATPOX ). To address this gap, we used 31-phosphorus magnetic resonance spectroscopy to measure changes in ATPCOST and ATPOX in human quadriceps during repeated trials of maximal intensity knee extensions lasting up to 4min. ATPCOST remained unchanged. In contrast, ATPOX plateaued by ∼2min and then declined (∼15%) over the final 2min. The maximal capacity for ATPOX (Vmax ), as well as ADP-specific rates of ATPOX , were also significantly diminished. Collectively, these results suggest that mitochondrial uncoupling, and not increased ATPCOST , is responsible for altering the regulation of skeletal muscle metabolism and oxygen consumption during maximal exercise. The relationship between skeletal muscle oxygen consumption and power output is augmented during exercise at workloads above the lactate threshold. Potential mechanisms for this response have been hypothesized, including increased ATP cost of force generation (ATPCOST ) and mitochondrial uncoupling, a process that reduces the efficiency of oxidative ATP synthesis (ATPOX ). To test these hypotheses, we used phosphorus magnetic resonance spectroscopy to non-invasively measure changes in phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during repeated trials of maximal, all-out dynamic knee extensions (120°s-1 , 1 every 2s) lasting 24, 60, 120, and 240s. ATPOX was measured at each time point from the initial velocity of PCr resynthesis, and ATPCOST was calculated as the sum of ATP synthesized by the creatine and adenylate kinase reactions, non-oxidative glycolysis, ATPOX and net changes in [ATP]. Power output declined in a reproducible manner for all four trials. ATPCOST did not change over time (main effect P=0.45). ATPOX plateaued from 60 to 120s and then decreased over the final 120s (main effect P=0.001). The maximal capacity for oxidative ATP synthesis (Vmax ), as well as ADP-specific rates of ATPOX , also decreased over time (main effect P=0.001, both). Collectively, these results demonstrate that prolonged maximal contraction protocols impair oxidative energetics and implicate mitochondrial uncoupling as the mechanism for this response. The causes of mitochondrial uncoupling are presently unknown but may offer a potential explanation for the dissociation between skeletal muscle power output and oxygen consumption during maximal, all-out exercise protocols.

Improving biologic predictors of cycling endurance performance with near-infrared spectroscopy derived measures of skeletal muscle respiration: E pluribus unum.

The study aim was to compare the predictive validity of the often referenced traditional model of human endurance performance (i.e. oxygen consumption, VO2, or power at maximal effort, fatigue threshold values, and indices of exercise efficiency) versus measures of skeletal muscle oxidative potential in relation to endurance cycling performance. We hypothesized that skeletal muscle oxidative potential would more completely explain endurance performance than the traditional model, which has never been collectively verified with cycling. Accordingly, we obtained nine measures of VO2 or power at maximal efforts, 20 measures reflective of various fatigue threshold values, 14 indices of cycling efficiency, and near‐infrared spectroscopy‐derived measures reflecting in vivo skeletal muscle oxidative potential. Forward regression modeling identified variable combinations that best explained 25‐km time trial time‐to‐completion (TTC) across a group of trained male participants (n = 24). The time constant for skeletal muscle oxygen consumption recovery, a validated measure of maximal skeletal muscle respiration, explained 92.7% of TTC variance by itself (Adj R 2 = .927, F = 294.2, SEE = 71.2, p < .001). Alternatively, the best complete traditional model of performance, including VO2max (L·min−1), %VO2max determined by the ventilatory equivalents method, and cycling economy at 50 W, only explained 76.2% of TTC variance (Adj R 2 = .762, F = 25.6, SEE = 128.7, p < .001). These results confirm our hypothesis by demonstrating that maximal rates of skeletal muscle respiration more completely explain cycling endurance performance than even the best combination of traditional variables long postulated to predict human endurance performance.