Effects of exercise-induced intracellular acidosis on the phosphocreatine recovery kinetics: a 31P MRS study in three muscle groups in humans

3T 31P/1H calf muscle coil for 1H and 31P MRI/MRS integrated with NIRS data acquisition

PURPOSE

Our aim was to design and build a 3T 31P/1H calf coil that is capable of providing both good 31P and 1H transmit and receive performance, as well as being capable of accommodating a near-infrared spectroscopy (NIRS) device for simultaneous NIRS data and MRI/MRS acquisition.

METHOD

In this work, we propose a new 3T 31P/1H birdcage combination design consisting of two co-centrically positioned birdcages on the same surface to maximize transmit efficiency and sensitivity for both nuclei. The 31P birdcage is a high-pass birdcage, whereas the 1H birdcage is a low-pass one to minimize coupling. The diameter of the 31P/1H birdcage combination was designed to be large enough to accommodate a NIRS device for simultaneous NIRS data and MRI/MRS acquisition.

RESULTS

The one-layer coil structure of the birdcage combination significantly streamlines the mechanical design and coil assembly process. Full-wave simulation results show that the 31P and 1H are very well decoupled with each other, and the 1H and 31P SNR surpasses that of their standalone counterparts in the central area. Experiment results show that the inclusion of a NIRS device does not significantly affect the performance of the coil, thus enabling simultaneous NIRS and MRI readouts during exercise.

CONCLUSION

Our findings demonstrate the feasibility and effectiveness of this dual-tuned coil design for combined NIRS and MRS measurements, offering potential benefits for studying metabolic and functional changes in the skeletal muscle in vivo.

Measurements of in vivo skeletal muscle oxidative capacity are lower following sustained isometric compared with dynamic contractions

Human skeletal muscle oxidative capacity can be quantified non-invasively using 31-phosphorus magnetic resonance spectroscopy (31P-MRS) to measure the rate constant of phosphocreatine (PCr) recovery (kPCr) following contractions. In the quadricep muscles, several studies have quantified kPCr following 24-30 s of sustained maximal voluntary isometric contraction (MVIC). This approach has the advantage of simplicity but is potentially problematic because sustained MVICs inhibit perfusion, which may limit muscle oxygen availability or increase the intracellular metabolic perturbation, and thus affect kPCr. Alternatively, dynamic contractions allow reperfusion between contractions, which may avoid limitations in oxygen delivery. To determine whether dynamic contraction protocols elicit greater kPCr than sustained MVIC protocols, we used a cross-sectional design to compare quadriceps kPCr in 22 young and 11 older healthy adults following 24 s of maximal voluntary: (1) sustained MVIC and (2) dynamic (MVDC; 120°·s-1, 1 every 2 s) contractions. Muscle kPCr was ∼20% lower following the MVIC protocol compared with the MVDC protocol (p ≤ 0.001), though this was less evident in older adults (p = 0.073). Changes in skeletal muscle pH (p ≤ 0.001) and PME accumulation (p ≤ 0.001) were greater following the sustained MVIC protocol, and pH (p ≤ 0.001) and PME (p ≤ 0.001) recovery were slower. These results demonstrate that (i) a brief, sustained MVIC yields a lower value for skeletal muscle oxidative capacity than an MVDC protocol of similar duration and (ii) this difference may not be consistent across populations (e.g., young vs. old). Thus, the potential effect of contraction protocol on comparisons of kPCr in different study groups requires careful consideration in the future.

Determinants of the maximal functional reserve during repeated supramaximal exercise by humans: The roles of Nrf2/Keap1, antioxidant proteins, muscle phenotype and oxygenation

When high-intensity exercise is performed until exhaustion a "functional reserve" (FR) or capacity to produce power at the same level or higher than reached at exhaustion exists at task failure, which could be related to reactive oxygen and nitrogen species (RONS)-sensing and counteracting mechanisms. Nonetheless, the magnitude of this FR remains unknown. Repeated bouts of supramaximal exercise at 120% of VO2max interspaced with 20s recovery periods with full ischaemia were used to determine the maximal FR. Then, we determined which muscle phenotypic features could account for the variability in functional reserve in humans. Exercise performance, cardiorespiratory variables, oxygen deficit, and brain and muscle oxygenation (near-infrared spectroscopy) were measured, and resting muscle biopsies were obtained from 43 young healthy adults (30 males). Males and females had similar aerobic (VO2max per kg of lower extremities lean mass (LLM): 166.7 ± 17.1 and 166.1 ± 15.6 ml kg LLM-1.min-1, P = 0.84) and anaerobic fitness (similar performance in the Wingate test and maximal accumulated oxygen deficit when normalized to LLM). The maximal FR was similar in males and females when normalized to LLM (1.84 ± 0.50 and 2.05 ± 0.59 kJ kg LLM-1, in males and females, respectively, P = 0.218). This FR depends on an obligatory component relying on a reserve in glycolytic capacity and a putative component generated by oxidative phosphorylation. The aerobic component depends on brain oxygenation and phenotypic features of the skeletal muscles implicated in calcium handling (SERCA1 and 2 protein expression), oxygen transport and diffusion (myoglobin) and redox regulation (Keap1). The glycolytic component can be predicted by the protein expression levels of pSer40-Nrf2, the maximal accumulated oxygen deficit and the protein expression levels of SOD1. Thus, an increased capacity to modulate the expression of antioxidant proteins involved in RONS handling and calcium homeostasis may be critical for performance during high-intensity exercise in humans.

Numerous Trigger-like Interactions of Kinases/Protein Phosphatases in Human Skeletal Muscles Can Underlie Transient Processes in Activation of Signaling Pathways during Exercise

Optimizing physical training regimens to increase muscle aerobic capacity requires an understanding of the internal processes that occur during exercise that initiate subsequent adaptation. During exercise, muscle cells undergo a series of metabolic events that trigger downstream signaling pathways and induce the expression of many genes in working muscle fibers. There are a number of studies that show the dependence of changes in the activity of AMP-activated protein kinase (AMPK), one of the mediators of cellular signaling pathways, on the duration and intensity of single exercises. The activity of various AMPK isoforms can change in different directions, increasing for some isoforms and decreasing for others, depending on the intensity and duration of the load. This review summarizes research data on changes in the activity of AMPK, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), and other components of the signaling pathways in skeletal muscles during exercise. Based on these data, we hypothesize that the observed changes in AMPK activity may be largely related to metabolic and signaling transients rather than exercise intensity per se. Probably, the main events associated with these transients occur at the beginning of the exercise in a time window of about 1-10 min. We hypothesize that these transients may be partly due to putative trigger-like kinase/protein phosphatase interactions regulated by feedback loops. In addition, numerous dynamically changing factors, such as [Ca²⁺], metabolite concentration, and reactive oxygen and nitrogen species (RONS), can shift the switching thresholds and change the states of these triggers, thereby affecting the activity of kinases (in particular, AMPK and CaMKII) and phosphatases. The review considers the putative molecular mechanisms underlying trigger-like interactions. The proposed hypothesis allows for a reinterpretation of the experimental data available in the literature as well as the generation of ideas to optimize future training regimens.

Extracellular Buffering Supplements to Improve Exercise Capacity and Performance: A Comprehensive Systematic Review and Meta-analysis

BACKGROUND

Extracellular buffering supplements [sodium bicarbonate (SB), sodium citrate (SC), sodium/calcium lactate (SL/CL)] are ergogenic supplements, although questions remain about factors which may modify their effect.

OBJECTIVE

To quantify the main effect of extracellular buffering agents on exercise outcomes, and to investigate the influence of potential moderators on this effect using a systematic review and meta-analytic approach.

METHODS

This study was designed in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Three databases were searched for articles that were screened according to inclusion/exclusion criteria. Bayesian hierarchical meta-analysis and meta-regression models were used to investigate pooled effects of supplementation and moderating effects of a range of factors on exercise and biomarker responses.

RESULTS

189 articles with 2019 participants were included, 158 involving SB supplementation, 30 with SC, and seven with CL/SL; four studies provided a combination of buffering supplements together. Supplementation led to a mean estimated increase in blood bicarbonate of + 5.2 mmol L^-1 (95% credible interval (CrI) 4.7-5.7). The meta-analysis models identified a positive overall effect of supplementation on exercise capacity and performance compared to placebo [ES_0.5 = 0.17 (95% CrI 0.12-0.21)] with potential moderating effects of exercise type and duration, training status and when the exercise test was performed following prior exercise. The greatest ergogenic effects were shown for exercise durations of 0.5-10 min [ES_0.5 = 0.18 (0.13-0.24)] and > 10 min [ES_0.5 = 0.22 (0.10-0.33)]. Evidence of greater effects on exercise were obtained when blood bicarbonate increases were medium (4-6 mmol L^-1) and large (> 6 mmol L^-1) compared with small (≤ 4 mmol L^-1) [β_Small:Medium = 0.16 (95% CrI 0.02-0.32), β_Small:Large = 0.13 (95% CrI - 0.03 to 0.29)]. SB (192 outcomes) was more effective for performance compared to SC (39 outcomes) [β_SC:SB = 0.10 (95% CrI - 0.02 to 0.22)].

CONCLUSIONS

Extracellular buffering supplements generate large increases in blood bicarbonate concentration leading to positive overall effects on exercise, with sodium bicarbonate being most effective. Evidence for several group-level moderating factors were identified. These data can guide an athlete's decision as to whether supplementation with buffering agents might be beneficial for their specific aims.

Measuring Mitochondrial Function: From Organelle to Organism

Mitochondrial energy production is crucial for normal daily activities and maintenance of life. Herein, the logic and execution of two main classes of measurements are outlined to delineate mitochondrial function: ATP production and oxygen consumption. Aerobic ATP production is quantified by phosphorus magnetic resonance spectroscopy (³¹PMRS) in vivo in both human subjects and animal models using the same protocols and maintaining the same primary assumptions. Mitochondrial oxygen consumption is quantified by oxygen polarography and applied in isolated mitochondria, cultured cells, and permeabilized fibers derived from human or animal tissue biopsies. Traditionally, mitochondrial functional measures focus on maximal oxidative capacity-a flux rate that is rarely, if ever, observed outside of experimental conditions. Perhaps more physiologically relevant, both measurement classes herein focus on one principal design paradigm; submaximal mitochondrial fluxes generated by graded levels of ADP to map the function for ADP sensitivity. We propose this function defines the bioenergetic role that mitochondria fill within the myoplasm to sense and match ATP demands. Any deficit in this vital role for ATP homeostasis leads to symptoms often seen in cardiovascular and cardiopulmonary diseases, diabetes, and metabolic syndrome.

Quantifying H+ exchange from muscle cytosolic energy catabolism using metabolite flux and H+ coefficients from multiple competitive cation binding: New evidence for consideration in established theories

The purpose of this investigation was to present calculations of fractional H+ exchange (~H+e ) from the chemical reactions of non-mitochondrial energy catabolism. Data of muscle pH and metabolite accumulation were based on published research for intense exercise to contractile failure within ~3 min, from which capacities and time profiles were modeled. Data were obtained from prior research for multiple competitive cation dissociation constants of metabolites and the chemical reactions of non-mitochondrial energy catabolism, and pH dependent calculations of ~H+e from specific chemical reactions. Data revealed that the 3 min of intense exercise incurred a total ATP turnover of 142.5 mmol L-1 , with a total intramuscular ~H+ exchange (-'ve = release) of -187.9 mmol L-1 . Total ~H+ metabolic consumption was 130.6 mmol L-1 , revealing a net total ~H+e (~H+te ) of -57.3 mmol L-1 . Lactate production had a ~H+te of 44.2 mmol L-1 (for a peak accumulation = 45 mmol L-1 ). The net ~H+te for the sum of the CK, AK, and AMPD reactions was 36.33 mmol L-1 . The ~H+te from ATP turnover equaled -47.5 mmol L-1 . The total ~H+ release to lactate ratio was 4.3 (187.9/44). Muscle ~H+ release during intense exercise is up to ~4-fold larger than previously assumed based on the lactic acid construct.

In vivo muscle function and energetics in women with sickle cell anemia or trait: a 31P-magnetic resonance spectroscopy study

Sickle cell anemia (SCA) is a genetic hemoglobinopathy associated with an impaired oxygen delivery to skeletal muscle that could alter ATP production processes and increase intramuscular acidosis. These alterations have been already reported in the Townes mouse model of SCA but the corresponding changes in humans have not been documented. In the present study, we used 31-phosphorus magnetic resonance spectroscopy to investigate in vivo the metabolic changes induced by a moderate-intensity exercise in twelve SCA patients, eight sickle cell trait (SCT) carriers, and twelve controls women. The rest-exercise-recovery protocol disclosed slight differences regarding phosphocreatine (PCr) consumption and lactate accumulation between SCA patients and controls but these differences did not reach a statistical significance. On that basis, the in vivo metabolic changes associated with a moderate-intensity muscle exercise were slightly altered in SCA patients and SCT carriers but within a normal range. The present results strongly support the fact that a moderate-intensity exercise is safe and could be recommended in stable SCA patients and SCT subjects.NEW & NOTEWORTHY The main finding of the present study was that the metabolic changes associated with a moderate-intensity muscle exercise were slightly modified in stable sickle cell anemia patients and sickle cell trait carriers as compared to controls but still in the normal range. The present results strongly support the safety of a moderate-intensity exercise for stable sickle cell anemia patients and sickle cell trait carriers.

Validity and accuracy of calculating oxidative ATP synthesis in vivo during high-intensity skeletal muscle contractions

Several methods have been developed for using ³¹ P-MRS to calculate rates of oxidative ATP synthesis (ATP_OX ) during muscular contractions based on assumptions that (1) the ATP cost of force generation (ATP_COST ) remains constant or (2) Michaelis-Menten coupling between cytosolic ADP and ATP_OX does not change. However, growing evidence suggests that one, or both, of these assumptions are invalid during high-intensity fatigue protocols. Consequently, there is a need to examine the validity and accuracy of traditional ATP_OX calculation methods under these conditions. To address this gap, we measured phosphate concentrations and pH in the vastus lateralis muscle of nine young adults during four rest-contraction-recovery trials lasting 24, 60, 120, and 240 s. The initial velocity of phosphocreatine resynthesis (V_iPCr ) following each trial served as the criterion measure of ATP_OX because this method makes no assumptions of constant ATP_COST or Michaelis-Menten coupling between changes in cytosolic ADP and ATP_OX . Subsequently, we calculated ATP_OX throughout the 240 s trial using several traditional calculation methods and compared estimations of ATP_OX from each method with time-matched measurements of V_iPCr . Method 1, which assumes that ATP_COST does not change, was able to model changes in V_iPCr over time, but showed poor accuracy for predicting V_iPCr across a wide range of ATP_OX values. In contrast, Michaelis-Menten methods, which assume that the relationship between changes in cytosolic ADP and ATP_OX remains constant, were invalid because they could not model the decline in V_iPCr . However, adjusting these Michaelis-Menten methods for observed changes in maximal ATP_OX capacity (i.e., V_max ) permitted modeling of the decline in V_iPCr and markedly improved accuracy. The results of these comprehensive analyses demonstrate that valid, accurate measurements of ATP_OX can be obtained during high-intensity contractions by adjusting Michaelis-Menten ATP_OX calculations for changes in V_max observed from baseline to post-fatigue.

Multinuclear MRS at 7T Uncovers Exercise Driven Differences in Skeletal Muscle Energy Metabolism Between Young and Seniors

Purpose: Aging is associated with changes in muscle energy metabolism. Proton (¹H) and phosphorous (³¹P) magnetic resonance spectroscopy (MRS) has been successfully applied for non-invasive investigation of skeletal muscle metabolism. The aim of this study was to detect differences in adenosine triphosphate (ATP) production in the aging muscle by ³¹P-MRS and to identify potential changes associated with buffer capacity of muscle carnosine by ¹H-MRS.

Methods: Fifteen young and nineteen elderly volunteers were examined. ¹H and ³¹P-MRS spectra were acquired at high field (7T). The investigation included carnosine quantification using ¹H-MRS and resting and dynamic ³¹P-MRS, both including saturation transfer measurements of phosphocreatine (PCr), and inorganic phosphate (Pi)-to-ATP metabolic fluxes.

Results: Elderly volunteers had higher time constant of PCr recovery (τ_PCr) in comparison to the young volunteers. Exercise was connected with significant decrease in PCr-to-ATP flux in both groups. Moreover, PCr-to-ATP flux was significantly higher in young compared to elderly both at rest and during exercise. Similarly, an increment of Pi-to-ATP flux with exercise was found in both groups but the intergroup difference was only observed during exercise. Elderly had lower muscle carnosine concentration and lower postexercise pH. A strong increase in phosphomonoester (PME) concentration was observed with exercise in elderly, and a faster Pi:PCr kinetics was found in young volunteers compared to elderly during the recovery period.

Conclusion: Observations of a massive increment of PME concentration together with high Pi-to-ATP flux during exercise in seniors refer to decreased ability of the muscle to meet the metabolic requirements of exercise and thus a limited ability of seniors to effectively support the exercise load.

Benefits of Creatine Supplementation for Vegetarians Compared to Omnivorous Athletes: A Systematic Review

Background: Creatine monohydrate is a nutritional supplement often consumed by athletes in anaerobic sports. Creatine is naturally found in most meat products; therefore, vegetarians have reduced creatine stores and may benefit from supplementation. Objective: to determine the effects of creatine supplementation on vegetarians. Data sources: PubMed and SPORTDiscus. Eligibility criteria: Randomized controlled trials (parallel group, cross-over studies) or prospective studies. Participants: Vegetarians. Intervention: Creatine supplementation. Study appraisal and synthesis: A total of 64 records were identified, and eleven full-text articles (covering nine studies) were included in this systematic review. Results: Creatine supplementation in vegetarians increased total creatine, creatine, and phosphocreatine concentrations in vastus lateralis and gastrocnemius muscle, plasma, and red blood cells, often to levels greater than omnivores. Creatine supplementation had no effect on brain levels of phosphocreatine. Creatine supplementation increased lean tissue mass, type II fiber area, insulin-like growth factor-1, muscular strength, muscular endurance, Wingate mean power output, and brain function (memory and intelligence) in vegetarian participants. Studies were mixed on whether creatine supplementation improved exercise performance in vegetarians to a greater extent compared to omnivores. Limitations: Studies that were reviewed had moderate–high risk of bias. Conclusions: Overall, it appears vegetarian athletes are likely to benefit from creatine supplementation.

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

KEY POINTS

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 (ATP_COST ) or mitochondrial uncoupling; a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). To address this gap, we used 31-phosphorus magnetic resonance spectroscopy to measure changes in ATP_COST and ATP_OX in human quadriceps during repeated trials of maximal intensity knee extensions lasting up to 4 min. ATP_COST remained unchanged. In contrast, ATP_OX plateaued by ∼2 min and then declined (∼15%) over the final 2 min. The maximal capacity for ATP_OX (V_max ), as well as ADP-specific rates of ATP_OX , were also significantly diminished. Collectively, these results suggest that mitochondrial uncoupling, and not increased ATP_COST , is responsible for altering the regulation of skeletal muscle metabolism and oxygen consumption during maximal exercise.

ABSTRACT

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 (ATP_COST ) and mitochondrial uncoupling, a process that reduces the efficiency of oxidative ATP synthesis (ATP_OX ). 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 2 s) lasting 24, 60, 120, and 240 s. ATP_OX was measured at each time point from the initial velocity of PCr resynthesis, and ATP_COST was calculated as the sum of ATP synthesized by the creatine and adenylate kinase reactions, non-oxidative glycolysis, ATP_OX and net changes in [ATP]. Power output declined in a reproducible manner for all four trials. ATP_COST did not change over time (main effect P = 0.45). ATP_OX plateaued from 60 to 120 s and then decreased over the final 120 s (main effect P = 0.001). The maximal capacity for oxidative ATP synthesis (V_max ), as well as ADP-specific rates of ATP_OX , 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.

Real-time monitoring of intracellular pH in live cells with fluorescent ionic liquid

Real-time monitoring of intracellular pH is of great significance due to its essential role in physiological and pathological processes. In present work, the ionic liquid (IL) N-methyl-6-hydroxyquinolinium bis(trifluoromethylsulfonyl) imide ([6MQc][NTf₂]) is proposed as a fluorescence probe for the quantitative imaging of intracellular pH in response to external stimuli. The fluorescence of the IL [6MQc][NTf₂] exhibits a sensitive response to pH variations, as the deprotonation of [6MQc][NTf₂] generates the highly fluorescent zwitterionic product [6MQz]. pH fluctuations in the range of 6.0-7.5 can be accurately sensed by monitoring the fluorescence change at 555 nm. Moreover, this IL probe exhibits favorable biocompatibility, excellent anti-photobleaching properties, and high tolerance to ionic strength. Using the IL probe, real-time sensing of hypoxia- and drug-induced intracellular pH changes in MCF-7 cells is achieved.

31 P magnetic resonance spectroscopy in skeletal muscle: Experts' consensus recommendations

Skeletal muscle phosphorus-31 31 P MRS is the oldest MRS methodology to be applied to in vivo metabolic research. The technical requirements of 31 P MRS in skeletal muscle depend on the research question, and to assess those questions requires understanding both the relevant muscle physiology, and how 31 P MRS methods can probe it. Here we consider basic signal-acquisition parameters related to radio frequency excitation, TR, TE, spectral resolution, shim and localisation. We make specific recommendations for studies of resting and exercising muscle, including magnetisation transfer, and for data processing. We summarise the metabolic information that can be quantitatively assessed with 31 P MRS, either measured directly or derived by calculations that depend on particular metabolic models, and we give advice on potential problems of interpretation. We give expected values and tolerable ranges for some measured quantities, and minimum requirements for reporting acquisition parameters and experimental results in publications. Reliable examination depends on a reproducible setup, standardised preconditioning of the subject, and careful control of potential difficulties, and we summarise some important considerations and potential confounders. Our recommendations include the quantification and standardisation of contraction intensity, and how best to account for heterogeneous muscle recruitment. We highlight some pitfalls in the assessment of mitochondrial function by analysis of phosphocreatine (PCr) recovery kinetics. Finally, we outline how complementary techniques (near-infrared spectroscopy, arterial spin labelling, BOLD and various other MRI and 1 H MRS measurements) can help in the physiological/metabolic interpretation of 31 P MRS studies by providing information about blood flow and oxygen delivery/utilisation. Our recommendations will assist in achieving the fullest possible reliable picture of muscle physiology and pathophysiology.

Quantification of Mitochondrial Oxidative Phosphorylation in Metabolic Disease: Application to Type 2 Diabetes

Type 2 diabetes (T2D) is a growing health concern with nearly 400 million affected worldwide as of 2014. T2D presents with hyperglycemia and insulin resistance resulting in increased risk for blindness, renal failure, nerve damage, and premature death. Skeletal muscle is a major site for insulin resistance and is responsible for up to 80% of glucose uptake during euglycemic hyperglycemic clamps. Glucose uptake in skeletal muscle is driven by mitochondrial oxidative phosphorylation and for this reason mitochondrial dysfunction has been implicated in T2D. In this review we integrate mitochondrial function with physiologic function to present a broader understanding of mitochondrial functional status in T2D utilizing studies from both human and rodent models. Quantification of mitochondrial function is explained both in vitro and in vivo highlighting the use of proper controls and the complications imposed by obesity and sedentary lifestyle. This review suggests that skeletal muscle mitochondria are not necessarily dysfunctional but limited oxygen supply to working muscle creates this misperception. Finally, we propose changes in experimental design to address this question unequivocally. If mitochondrial function is not impaired it suggests that therapeutic interventions and drug development must move away from the organelle and toward the cardiovascular system.

Skeletal muscle energetics are compromised only during high-intensity contractions in the Goto-Kakizaki rat model of type 2 diabetes

Type 2 diabetes (T2D) presents with hyperglycemia and insulin resistance, affecting over 30 million people in the United States alone. Previous work has hypothesized that mitochondria are dysfunctional in T2D and results in both reduced ATP production and glucose disposal. However, a direct link between mitochondrial function and T2D has not been determined. In the current study, the Goto-Kakizaki (GK) rat model of T2D was used to quantify mitochondrial function in vitro and in vivo over a broad range of contraction-induced metabolic workloads. During high-frequency sciatic nerve stimulation, hindlimb muscle contractions at 2- and 4-Hz intensities, the GK rat failed to maintain similar bioenergetic steady states to Wistar control (WC) rats measured by phosphorus magnetic resonance spectroscopy, despite similar force production. Differences were not due to changes in mitochondrial content in red (RG) or white gastrocnemius (WG) muscles (cytochrome c oxidase, RG: 22.2 ± 1.6 vs. 23.3 ± 1.7 U/g wet wt; WG: 10.8 ± 1.1 vs. 12.1 ± 0.9 U/g wet wt; GK vs. WC, respectively). Mitochondria isolated from muscles of GK and WC rats also showed no difference in mitochondrial ATP production capacity in vitro, measured by high-resolution respirometry. At lower intensities (0.25-1 Hz) there were no detectable differences between GK and WC rats in sustained energy balance. There were similar phosphocreatine concentrations during steady-state contraction and postcontractile recovery (τ = 72 ± 6 s GK versus 71 ± 2 s WC). Taken together, these results suggest that deficiencies in skeletal muscle energetics seen at higher intensities are not due to mitochondrial dysfunction in the GK rat.

Ischaemia-induced muscle metabolic abnormalities are poorly alleviated by endurance training in a mouse model of sickle cell disease

NEW FINDINGS

What is the central question of this study? The aim of this study was to evaluate the potential beneficial effects of endurance training during an ischaemia-reperfusion protocol in a mouse model of sickle cell disease (SCD). What is the main finding and its importance? Endurance training did not reverse the metabolic defects induced by a simulated vaso-occlusive crisis in SCD mice, with regard to intramuscular acidosis, mitochondrial dysfunction or anatomical properties. Our results suggest that endurance training would reduce the number of vaso-occlusive crises rather than the complications related to vaso-occlusive crises.

ABSTRACT

The aim of this study was to investigate whether endurance training could limit the abnormalities described in a mouse model of sickle cell disease (SCD) in response to an ischaemia-reperfusion (I/R) protocol. Ten sedentary (HbSS-SED) and nine endurance-trained (HbSS-END) SCD mice were submitted to a standardized protocol of I/R of the leg, during which ATP, phosphocreatine and inorganic phosphate concentrations and intramuscular pH were measured using magnetic resonance spectroscopy. Forty-eight hours later, skeletal muscles were harvested. Oxidative stress markers were then measured. Although the time course of protons accumulation was slightly different between trained and sedentary mice (P < 0.05), the extent of acidosis was similar at the end of the ischaemic period. The initial rate of phosphocreatine resynthesis measured at blood flow restoration, illustrating mitochondrial function, was not altered in trained mice compared with sedentary mice. Although several oxidative stress markers were not different between groups (P > 0.05), the I/R-related increase of uric acid concentration observed in sedentary SCD mice (P < 0.05) was not present in the trained group. The spleen weight, generally used as a marker of the severity of the disease, was not different between groups (P > 0.05). In conclusion, endurance training did not limit the metabolic consequences of an I/R protocol in skeletal muscle of SCD mice, suggesting that the reduction in the severity of the disease previously demonstrated in the basal state would be attributable to a reduction of the occurrence of vaso-occlusive crises rather than a decrease of the deleterious effects of vaso-occlusive crises.

In Situ Imaging of Live-Cell Extracellular pH during Cell Apoptosis with Surface-Enhanced Raman Spectroscopy

Extracellular pH (pHe) is an important regulating factor that determines many cellular processes, including proliferation, differentiation, and apoptosis. In our previous work, we developed 4-MPy (4-mercaptopyridine) modified Au nanoparticles as intracellular pH sensors based on surface-enhanced Raman spectroscopy (SERS). We herein modified a Au-nanoparticle-assembled solid SERS substrate with 4-MPy molecules for in situ pHe sensing during apoptosis. We found a more acidic extracellular environment of cancer cells than that of normal cells from the pH imaging. We then in situ investigated the temporal and spatial evolution of pHe of cancer cells after addition of transforming growth factor-β (TGF-β). The pHe showed a fast decrease at the beginning, followed by a slow decrease until the complete loss of cellular functions, and the pH values in and out of the cells became similar. This work shows that our SERS substrate combined with an in situ cell culture system is well suitable for in situ pHe sensing during cell processes and will be a promising technique for understanding more pHe-related biological and pathological issues.

Exacerbated metabolic changes in skeletal muscle of sickle cell mice submitted to an acute ischemia-reperfusion paradigm

Sickle cell disease (SCD) is characterized by painful vaso-occlusive crisis. While there are several metabolic abnormalities potentially associated with muscular ischemia-reperfusion cycles that could be harmful in the context of SCD, the metabolic consequences of such events are still unknown. Ten controls (HbAA), thirteen heterozygous (HbAS), and ten homozygous (HbSS) SCD mice were submitted to a standardized protocol of rest-ischemia-reperfusion of the left leg during which adenosine triphosphate, phosphocreatine, and inorganic phosphate concentrations as well as intramuscular pH were measured using phosphorous magnetic resonance spectroscopy (MRS). Forty-eight hours later, skeletal muscles were harvested. Oxidative stress markers were then measured on the tibialis anterior. At the end of the ischemic period, HbSS mice had a lower pH value as compared with the HbAA and HbAS groups (P<0.01). During the reperfusion period, the initial rate of phosphocreatine resynthesis was lower in HbSS mice as compared with HbAA (P<0.05) and HbAS (P<0.01) animals. No significant difference among groups was observed regarding oxidative stress markers. HbSS mice displayed a higher intramuscular acidosis during the ischemic period while their mitochondrial function was impaired as compared with their HbAA and HbAS counterparts. These metabolic abnormalities could worsen the complications related to the pathology of SCD.

Dynamic multivoxel-localized 31 P MRS during plantar flexion exercise with variable knee angle

Exercise studies investigating the metabolic response of calf muscles using ³¹ P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time-resolved multivoxel ³¹ P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(∼40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end-exercise PCr depletions in gastrocnemius medialis (R² =0.8) and soleus (R² =0.53). PCr recovery times and end-exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel ³¹ P MRS during muscle exercise and recovery, and the findings should be used in future study design.

Oxidative capacity varies along the length of healthy human tibialis anterior

KEY POINTS

During exercise skeletal muscles use the energy buffer phosphocreatine. The post-exercise recovery of phosphocreatine is a measure of the oxidative capacity of muscles and is traditionally assessed by ³¹ P magnetic resonance spectroscopy of a large tissue region, assuming homogeneous energy metabolism. To test this assumption, we collected spatially resolved spectra along the length of human tibialis anterior using a home-built array of ³¹ P detection coils, and observed a striking gradient in the recovery rate of phosphocreatine, decreasing along the proximo-distal axis of the muscle. A similar gradient along this muscle was observed in signal changes recorded by ¹ H muscle functional MRI. These findings identify intra-muscular variation in the physiology of muscles in action and highlight the importance of localized sampling for any methodology investigating oxidative metabolism of this, and potentially other muscles.

ABSTRACT

The rate of phosphocreatine (PCr) recovery (k_PCr ) after exercise, characterizing muscle oxidative capacity, is traditionally assessed with unlocalized ³¹ P magnetic resonance spectroscopy (MRS) using a single surface coil. However, because of intramuscular variation in fibre type and oxygen supply, k_PCr may be non-uniform within muscles. We tested this along the length of the tibialis anterior (TA) muscle in 10 male volunteers. For this purpose, we employed a 3T MR system with a ³¹ P/¹ H volume transmit coil combined with a home-built ³¹ P phased-array receive probe, consisting of five coil elements covering the TA muscle length. Mono-exponential k_PCr was determined for all coil elements after 40 s of submaximal isometric dorsiflexion (SUBMAX) and incremental exercise to exhaustion (EXH). In addition, muscle functional MRI (¹ H mfMRI) was performed using the volume coil after another 40 s of SUBMAX. A strong gradient in k_PCr was observed along the TA (P < 0.001), being two times higher proximally vs. distally during SUBMAX and EXH. Statistical analysis showed that this gradient cannot be explained by pH variations. A similar gradient was seen in the slope of the initial post-exercise ¹ H mfMRI signal change, which was higher proximally than distally in both the TA and the extensor digitorum longus (P < 0.001) and strongly correlated with k_PCr . The pronounced differences along the TA in functional oxidative capacity identify regional variation in the physiological demand of this muscle during everyday activities and have implications for the bio-energetic assessment of interventions to modify its performance and of neuromuscular disorders involving the TA.

Endurance training reduces exercise-induced acidosis and improves muscle function in a mouse model of sickle cell disease

Sickle cell disease (SCD) mice (Townes model of SCD) presented exacerbated exercise-induced acidosis and fatigability as compared to control animals. We hypothesize that endurance training could represent a valuable approach to reverse these muscle defects. Endurance-trained HbAA (HbAA-END, n=10), HbAS (HbAS-END, n=11) and HbSS (HbSS-END, n=8) mice were compared to their sedentary counterparts (10 HbAA-SED, 10 HbAS-SED and 9 HbSS-SED mice) during two rest - exercise - recovery protocols during which muscle energetics and function were measured. In vitro analyses of some proteins involved in muscle energetics, pH regulation and oxidative stress were also performed. Exercise-induced acidosis was lower in HbSS-END mice as compared to their sedentary counterparts during both moderate (p<0.001) and intense (p<0.1) protocols. The total force production measured during both protocols was higher in trained mice compared to sedentary animals. In vitro analyses revealed that enolase/citrate synthase ratio was reduced in HbSS-END (p<0.001) and HbAS-END (p<0.01) mice compared to their sedentary counterparts. In addition, malondialdehyde concentration was reduced in trained mice (p<0.05). In conclusion, endurance training would reverse the more pronounced exercise-induced acidosis, reduce oxidative stress and ameliorate some of the muscle function parameters in SCD mice.

Competitive cation binding computations of proton balance for reactions of the phosphagen and glycolytic energy systems within skeletal muscle

Limited research and data has been published for the H⁺ coefficients for the metabolites and reactions involved in non-mitochondrial energy metabolism. The purpose of this investigation was to compute the fractional binding of H⁺, K⁺, Na⁺ and Mg²⁺ to 21 metabolites of skeletal muscle non-mitochondrial energy metabolism, resulting in 104 different metabolite-cation complexes. Fractional binding of H⁺ to these metabolite-cation complexes were applied to 17 reactions of skeletal muscle non-mitochondrial energy metabolism, and 8 conditions of the glycolytic pathway based on the source of substrate (glycogen vs. glucose), completeness of glycolytic flux, and the end-point of pyruvate vs. lactate. For pH conditions of 6.0 and 7.0, respectively, H⁺ coefficients (-‘ve values = H⁺ release) for the creatine kinase, adenylate kinase, AMP deaminase and ATPase reactions were 0.8 and 0.97, -0.13 and -0.02, 1.2 and 1.09, and -0.01 and -0.66, respectively. The glycolytic pathway is net H⁺ releasing, regardless of lactate production, which consumes 1 H⁺. For glycolysis fueled by glycogen and ending in either pyruvate or lactate, H⁺ coefficients for pH 6.0 and 7.0 were -3.97 and -2.01 (pyruvate), and -1.96 and -0.01 (lactate), respectively. When starting with glucose, the same conditions result in H⁺ coefficients of -3.98 and -2.67, and -1.97 and –0.67, respectively. The most H⁺ releasing reaction of glycolysis is the glyceraldehyde-3-phosphate dehydrogenase reaction, with H⁺ coefficients for pH 6.0 and 7.0 of -1.58 and -0.76, respectively. Incomplete flux of substrate through glycolysis would increase net H⁺ release due to the absence of the pyruvate kinase and lactate dehydrogenase reactions, which collectively result in H⁺ coefficients for pH 6.0 and 7.0 of 1.35 and 1.88, respectively. The data presented provide an extensive reference source for academics and researchers to accurately profile the balance of protons for all metabolites and reactions of non-mitochondrial energy metabolism, and reveal the greater role of glycolysis in net H⁺ release than previously assumed. The data can also be used to improve the understanding of the cause of metabolic acidosis, and reveal mechanistic connections between H⁺ release within and from muscle and the electrochemical neutrality concepts that further refine acid-base balance in biological solutions.

Combined spiroergometry and 31 P-MRS of human calf muscle during high-intensity exercise

Simultaneous measurements of pulmonary oxygen consumption (VO₂ ), carbon dioxide exhalation (VCO₂ ) and phosphorus magnetic resonance spectroscopy (³¹ P-MRS) are valuable in physiological studies to evaluate muscle metabolism during specific loads. Therefore, the aim of this study was to adapt a commercially available spirometric device to enable measurements of VO₂ and VCO₂ whilst simultaneously performing ³¹ P-MRS at 3 T. Volunteers performed intense plantar flexion of their right calf muscle inside the MR scanner against a pneumatic MR-compatible pedal ergometer. The use of a non-magnetic pneumotachograph and extension of the sampling line from 3 m to 5 m to place the spirometric device outside the MR scanner room did not affect adversely the measurements of VO₂ and VCO₂ . Response and delay times increased, on average, by at most 0.05 s and 0.79 s, respectively. Overall, we were able to demonstrate a feasible ventilation response (VO₂ = 1.05 ± 0.31 L/min; VCO₂ = 1.11 ± 0.33 L/min) during the exercise of a single calf muscle, as well as a good correlation between local energy metabolism and muscular acidification (τ_{PCr fast} and pH; R² = 0.73, p < 0.005) and global respiration (τ_{PCr fast} and VO₂ ; R²  = 0.55, p = 0.01). This provides improved insights into aerobic and anaerobic energy supply during strong muscular performances.

Exercise-induced mitochondrial dysfunction: a myth or reality?

Beneficial effects of physical activity on mitochondrial health are well substantiated in the scientific literature, with regular exercise improving mitochondrial quality and quantity in normal healthy population, and in cardiometabolic and neurodegenerative disorders and aging. However, several recent studies questioned this paradigm, suggesting that extremely heavy or exhaustive exercise fosters mitochondrial disturbances that could permanently damage its function in health and disease. Exercise-induced mitochondrial dysfunction (EIMD) might be a key proxy for negative outcomes of exhaustive exercise, being a pathophysiological substrate of heart abnormalities, chronic fatigue syndrome (CFS) or muscle degeneration. Here, we overview possible factors that mediate negative effects of exhaustive exercise on mitochondrial function and structure, and put forward alternative solutions for the management of EIMD.

Active Recovery between Interval Bouts Reduces Blood Lactate While Improving Subsequent Exercise Performance in Trained Men

This study aimed to examine the blood lactate and blood pH kinetics during high-intensity interval training. Seventeen well-trained athletes exercised on two different occasions. Exercises consisted of three 30 s bouts at a constant intensity (90% of peak power) with 4 min recovery between bouts followed by a Wingate test (WT). The recoveries were either active recovery (at 60% of the lactate threshold intensity) or passive recovery (resting at sitting position). During the exercise, blood samples were taken to determine blood gasses, blood lactate, and blood pH, and peak and average power were calculated for the WT. When performing the active recovery trials, blood pH was significantly higher (p < 0.01) and blood lactate was significantly lower (p < 0.01) compared with the passive recovery trials. WT performance was significantly higher in the active recovery trials: peak power was 671 ± 88 and 715 ± 108 watts, and average power was 510 ± 70 and 548 ± 73 watts (passive and active respectively; p < 0.01). However, no statistically significant correlations were found between the increased pH and the increased performance in the active recovery trials. These results suggest that active recovery performed during high-intensity interval exercise favors the performance in a following WT. Moreover, the blood pH variations associated with active recovery did not explain the enhanced performance.

Moderate and intense muscular exercises induce marked intramyocellular metabolic acidosis in sickle cell disease mice

Sickle cell disease (SCD) is associated with an impaired oxygen delivery to skeletal muscle that could alter ATP production processes. The present study aimed to determine the effects of sickle hemoglobin (HbS) on muscle pH homeostasis in response to exercise in homozygous (HbSS, n = 9) and heterozygous (HbAS, n = 10) SCD (Townes) mice in comparison to control (HbAA, n = 10) littermates. Magnetic resonance spectroscopy of phosphorus 31 enabled to measure intramuscular pH and phosphocreatine (PCr) concentration during rest-stimulation-recovery protocols at two different intensities. Maximal activity of some enzymes involved in muscle energetics and content of proteins involved in pH regulation were also investigated. HbSS mice presented a more pronounced exercise-induced intramuscular acidosis, whatever the intensity of exercise. Moreover, the depletion of PCr was also exacerbated in HbSS mice in response to intense exercise as compared with both HbAA and HbAS mice ( P < 0.01). While no difference was observed concerning proteins involved in muscle pH regulation, the activity of enolase (a glycolytic enzyme) was higher in both HbSS and HbAS mice as compared with controls ( P < 0.05). Interestingly, HbAS mice presented also metabolic impairments as compared with their control counterparts. This study has identified for the first time an exacerbated exercise-induced intramuscular acidosis in SCD mice.

NEW & NOTEWORTHY The main finding of the present study was that the exercise-induced intramuscular acidosis was systematically more pronounced in sickle cell disease (SCD) mice as compared with their control counterparts. This result is important since it has been demonstrated in vitro that acidosis can trigger hemoglobin polymerization. From that point of view, our results tend to support the idea that high-intensity exercise may increase the risk of hemoglobin polymerization in SCD.

Mitochondrial function assessed by 31P MRS and BOLD MRI in non-obese type 2 diabetic rats

The study aims to characterize age‐associated changes in skeletal muscle bioenergetics by evaluating the response to ischemia‐reperfusion in the skeletal muscle of the Goto‐Kakizaki (GK) rats, a rat model of non‐obese type 2 diabetes (T2D). ³¹P magnetic resonance spectroscopy (MRS) and blood oxygen level‐dependent (BOLD) MRI was performed on the hindlimb of young (12 weeks) and adult (20 weeks) GK and Wistar (control) rats. ³¹P‐MRS and BOLD‐MRI data were acquired continuously during an ischemia and reperfusion protocol to quantify changes in phosphate metabolites and muscle oxygenation. The time constant of phosphocreatine recovery, an index of mitochondrial oxidative capacity, was not statistically different between GK rats (60.8 ± 13.9 sec in young group, 83.7 ± 13.0 sec in adult group) and their age‐matched controls (62.4 ± 11.6 sec in young group, 77.5 ± 7.1 sec in adult group). During ischemia, baseline‐normalized BOLD‐MRI signal was significantly lower in GK rats than in their age‐matched controls. These results suggest that insulin resistance leads to alterations in tissue metabolism without impaired mitochondrial oxidative capacity in GK rats.

Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice

The purpose of this study was to investigate the effects of a partial suppression of monocarboxylate transporter (MCT)-1 on skeletal muscle pH, energetics, and function (MCT1^+/- mice). Twenty-four MCT1^+/- and 13 wild-type (WT) mice were subjected to a rest-exercise-recovery protocol, allowing assessment of muscle energetics (by magnetic resonance spectroscopy) and function. The study included analysis of enzyme activities and content of protein involved in pH regulation. Skeletal muscle of MCT1^+/- mice had lower MCT1 (-61%; P < 0.05) and carbonic anhydrase (CA)-II (-54%; P < 0.05) contents. Although intramuscular pH was higher in MCT1^+/- mice at rest (P < 0.001), the mice showed higher acidosis during the first minute of exercise (P < 0.01). Then, the pH time course was similar among groups until exercise completion. MCT1^+/- mice had higher specific peak (P < 0.05) and maximum tetanic (P < 0.01) forces and lower fatigability (P < 0.001) when compared to WT mice. We conclude that both MCT1 and CAII are involved in the homeostatic control of pH in skeletal muscle, both at rest and at the onset of exercise. The improved muscle function and resistance to fatigue in MCT1^+/- mice remain unexplained.-Chatel, B., Bendahan, D., Hourdé, C., Pellerin, L., Lengacher, S., Magistretti, P., Fur, Y. L., Vilmen, C., Bernard, M., Messonnier, L. A. Role of MCT1 and CAII in skeletal muscle pH homeostasis, energetics, and function: in vivo insights from MCT1 haploinsufficient mice.

Maturation-Related Differences in Neuromuscular Fatigue After a Short-Term Maximal Run

Purpose. This study investigated maturation-related differences in neuromuscular fatigue after a short-term maximal run. Methods. Eight male children, eight adolescents, and eight adults performed a maximal ca. 50-s run (300/350/400 m, respectively). Mechanisms of neuromuscular fatigue were assessed through isometric plantar flexor tests, electrical stimulation of the posterior tibial nerve, soleus electromyography, and blood tests. Results. All the groups showed a decrease in the running speed (children: -12.2 ± 6.5%; adolescents: -9.8 ± 5.1%; adults: -12.2 ± 3.1%), but only adults revealed a decline in the maximal isometric plantar flexor torque (-16.1 ± 13.0%). On the other hand, the relative pre- to post-fatigue change in the maximal isometric plantar flexor torque differed only between adults and adolescents. The peak torque in the passive twitch test decreased in adolescents (-19.2 ± 12.2%) and adults (-23.7 ± 13.7%). Moreover, post-fatigue minimum blood pH (children: 7.18 ± 0.03; adolescents: 7.14 ± 0.07; adults: 6.97 ± 0.06) differed between the groups. No changes were observed in the neural drive or mechanisms at the spinal level. Conclusions. Despite the loss of running speed, children showed no post-exercise fatigue, whereas adolescents and adults demonstrated fatigue at peripheral sites. Central fatigue could not be established for the studied groups.

Skeletal muscle ATP synthesis and cellular H(+) handling measured by localized (31)P-MRS during exercise and recovery

(31)P magnetic resonance spectroscopy (MRS) is widely used for non-invasive investigation of muscle metabolism dynamics. This study aims to extend knowledge on parameters derived from these measurements in detail and comprehensiveness: proton (H(+)) efflux, buffer capacity and the contributions of glycolytic (L) and oxidative (Q) rates to ATP synthesis were calculated from the evolutions of phosphocreatine (PCr) and pH. Data are reported for two muscles in the human calf, for each subject and over a wide range of exercise intensities. 22 subjects performed plantar flexions in a 7T MR-scanner, leading to PCr changes ranging from barely noticeable to almost complete depletion, depending on exercise protocol and muscle studied by localized MRS. Cytosolic buffer capacity was quantified for the first time non-invasively and individually, as was proton efflux evolution in early recovery. Acidification started once PCr depletion reached 60-75%. Initial and end-exercise L correlated with end-exercise levels of PCr and approximately linear with pH. Q calculated directly from PCr and pH derivatives was plausible, requiring fewer assumptions than the commonly used ADP-model. In conclusion, the evolution of parameters describing cellular energy metabolism was measured over a wide range of exercise intensities, revealing a relatively complete picture of muscle metabolism.

Accuracy and precision of quantitative 31P-MRS measurements of human skeletal muscle mitochondrial function

Although theoretically sound, the accuracy and precision of (31)P-magnetic resonance spectroscopy ((31)P-MRS) approaches to quantitatively estimate mitochondrial capacity are not well documented. Therefore, employing four differing models of respiratory control [linear, kinetic, and multipoint adenosine diphosphate (ADP) and phosphorylation potential], this study sought to determine the accuracy and precision of (31)P-MRS assessments of peak mitochondrial adenosine-triphosphate (ATP) synthesis rate utilizing directly measured peak respiration (State 3) in permeabilized skeletal muscle fibers. In 23 subjects of different fitness levels, (31)P-MRS during a 24-s maximal isometric knee extension and high-resolution respirometry in muscle fibers from the vastus lateralis was performed. Although significantly correlated with State 3 respiration (r = 0.72), both the linear (45 ± 13 mM/min) and phosphorylation potential (47 ± 16 mM/min) models grossly overestimated the calculated in vitro peak ATP synthesis rate (P < 0.05). Of the ADP models, the kinetic model was well correlated with State 3 respiration (r = 0.72, P < 0.05), but moderately overestimated ATP synthesis rate (P < 0.05), while the multipoint model, although being somewhat less well correlated with State 3 respiration (r = 0.55, P < 0.05), most accurately reflected peak ATP synthesis rate. Of note, the PCr recovery time constant (τ), a qualitative index of mitochondrial capacity, exhibited the strongest correlation with State 3 respiration (r = 0.80, P < 0.05). Therefore, this study reveals that each of the (31)P-MRS data analyses, including PCr τ, exhibit precision in terms of mitochondrial capacity. As only the multipoint ADP model did not overstimate the peak skeletal muscle mitochondrial ATP synthesis, the multipoint ADP model is the only quantitative approach to exhibit both accuracy and precision.

Localized semi-LASER dynamic (31)P magnetic resonance spectroscopy of the soleus during and following exercise at 7 T

OBJECTIVES

This study demonstrates the applicability of semi-LASER localized dynamic (31)P MRS to deeper lying areas of the exercising human soleus muscle (SOL). The effect of accurate localization and high temporal resolution on data specificity is investigated.

MATERIALS AND METHODS

To achieve high signal-to-noise ratio (SNR) at a temporal resolution of 6 s, a custom-built human calf coil array was used at 7T. The kinetics of phosphocreatine (PCr) and intracellular pH were quantified separately in SOL and gastrocnemius medialis (GM) muscle of nine volunteers, during rest, plantar flexion exercise, and recovery.

RESULTS

The average SNR of PCr at rest was [Formula: see text] in SOL ([Formula: see text] in GM). End exercise PCr depletion in SOL ([Formula: see text] %) was far lower than in GM ([Formula: see text] %). The pH in SOL increased rapidly and, in contrast to GM, remained elevated until the end of exercise.

CONCLUSION

(31)P MRS in single-shots every 6 s localized in the deeper-lying SOL enabled quantification of PCr recovery times at low depletions and of fast pH changes, like the initial rise. Both high temporal resolution and accurate spatial localization improve specificity of Pi and, thus, pH quantification by avoiding multiple, and potentially indistinguishable sources for changing the Pi peak shape.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Mitochondria, energetics, epigenetics, and cellular responses to stress

BACKGROUND

Cells respond to environmental stressors through several key pathways, including response to reactive oxygen species (ROS), nutrient and ATP sensing, DNA damage response (DDR), and epigenetic alterations. Mitochondria play a central role in these pathways not only through energetics and ATP production but also through metabolites generated in the tricarboxylic acid cycle, as well as mitochondria-nuclear signaling related to mitochondria morphology, biogenesis, fission/fusion, mitophagy, apoptosis, and epigenetic regulation.

OBJECTIVES

We investigated the concept of bidirectional interactions between mitochondria and cellular pathways in response to environmental stress with a focus on epigenetic regulation, and we examined DNA repair and DDR pathways as examples of biological processes that respond to exogenous insults through changes in homeostasis and altered mitochondrial function.

METHODS

The National Institute of Environmental Health Sciences sponsored the Workshop on Mitochondria, Energetics, Epigenetics, Environment, and DNA Damage Response on 25-26 March 2013. Here, we summarize key points and ideas emerging from this meeting.

DISCUSSION

A more comprehensive understanding of signaling mechanisms (cross-talk) between the mitochondria and nucleus is central to elucidating the integration of mitochondrial functions with other cellular response pathways in modulating the effects of environmental agents. Recent studies have highlighted the importance of mitochondrial functions in epigenetic regulation and DDR with environmental stress. Development and application of novel technologies, enhanced experimental models, and a systems-type research approach will help to discern how environmentally induced mitochondrial dysfunction affects key mechanistic pathways.

CONCLUSIONS

Understanding mitochondria-cell signaling will provide insight into individual responses to environmental hazards, improving prediction of hazard and susceptibility to environmental stressors.

Relationships between V̇O2 and blood lactate responses after all-out running exercise

To verify the effects of training status and blood lactate concentration (BLC) responses on the early excess postexercise oxygen consumption (EPOC), 8 sprinters, 7 endurance runners, and 7 untrained subjects performed an incremental test to determine maximal oxygen uptake and a 1-min all-out test to determine BLC and oxygen uptake recovery curves. BLC kinetics was evaluated to assess the quantity of lactate accumulated during exercise (QlaA), lactate removal ability (k2), and quantity of lactate removed from 0 to 10 min postexercise (QlaR). Oxygen uptake off-kinetics was evaluated to assess the decay time constants (τ1 and τ2); moreover, EPOC was measured during the first 10 min after exercise. While sprinters had 98%-100% and 94%-100% likelihood of having the highest EPOC and decay time constants, endurance runners had 98%-100% and 95%-100% likelihood of having the lowest EPOC and decay time constants. EPOC was correlated with QlaA (r = 0.74) and QlaR (r = 0.61). τ1 and τ2 were correlated with maximal oxygen uptake (r > -0.57), k2 (r > -0.48), and QlaR relative to QlaA (r > -0.60). Our findings indicate that oxygen uptake recovery is associated with fast lactate removal and aerobic training. Furthermore, the metabolites derived from anaerobic energy production seem to induce a greater EPOC after all-out exercise.

Skeletal muscle ATP turnover by 31P magnetic resonance spectroscopy during moderate and heavy bilateral knee extension

During constant-power high-intensity exercise, the expected increase in oxygen uptake (V̇O2) is supplemented by a V̇O2 slow component (V̇O2 sc ), reflecting reduced work efficiency, predominantly within the locomotor muscles. The intracellular source of inefficiency is postulated to be an increase in the ATP cost of power production (an increase in P/W). To test this hypothesis, we measured intramuscular ATP turnover with (31)P magnetic resonance spectroscopy (MRS) and whole-body V̇O2 during moderate (MOD) and heavy (HVY) bilateral knee-extension exercise in healthy participants (n = 14). Unlocalized (31)P spectra were collected from the quadriceps throughout using a dual-tuned ((1)H and (31)P) surface coil with a simple pulse-and-acquire sequence. Total ATP turnover rate (ATPtot) was estimated at exercise cessation from direct measurements of the dynamics of phosphocreatine (PCr) and proton handling. Between 3 and 8 min during MOD, there was no discernable V̇O2 sc (mean ± SD, 0.06 ± 0.12 l min(-1)) or change in [PCr] (30 ± 8 vs. 32 ± 7 mm) or ATPtot (24 ± 14 vs. 17 ± 14 mm min(-1); each P = n.s.). During HVY, the V̇O2 sc was 0.37 ± 0.16 l min(-1) (22 ± 8%), [PCr] decreased (19 ± 7 vs. 18 ± 7 mm, or 12 ± 15%; P < 0.05) and ATPtot increased (38 ± 16 vs. 44 ± 14 mm min(-1), or 26 ± 30%; P < 0.05) between 3 and 8 min. However, the increase in ATPtot (ΔATPtot) was not correlated with the V̇O2 sc during HVY (r(2) = 0.06; P = n.s.). This lack of relationship between ΔATPtot and V̇O2 sc , together with a steepening of the [PCr]-V̇O2 relationship in HVY, suggests that reduced work efficiency during heavy exercise arises from both contractile (P/W) and mitochondrial sources (the O2 cost of ATP resynthesis; P/O).

Interrelations of muscle functional MRI, diffusion-weighted MRI and (31) P-MRS in exercised lower back muscles

Exercise-induced changes of transverse proton relaxation time (T2 ), tissue perfusion and metabolic turnover were investigated in the lower back muscles of volunteers by applying muscle functional MRI (mfMRI) and diffusion-weighted imaging (DWI) before and after as well as dynamic (31) P-MRS during the exercise. Inner (M. multifidus, MF) and outer lower back muscles (M. erector spinae, ES) were examined in 14 healthy young men performing a sustained isometric trunk-extension. Significant phosphocreatine (PCr) depletions ranging from 30% (ES) to 34% (MF) and Pi accumulations between 95% (left ES) and 120%-140% (MF muscles and right ES) were observed during the exercise, which were accompanied by significantly decreased pH values in all muscles (∆pH ≈ -0.05). Baseline T2 values were similar across all investigated muscles (approximately 27 ms at 3 T), but revealed right-left asymmetric increases (T2 ,inc ) after the exercise (right ES/MF: T2 ,inc  = 11.8/9.7%; left ES/MF: T2 ,inc  = 4.6/8.9%). Analyzed muscles also showed load-induced increases in molecular diffusion D (p = .007) and perfusion fraction f (p = .002). The latter parameter was significantly higher in the MF than in the ES muscles both at rest and post exercise. Changes in PCr (p = .03), diffusion (p < .01) and perfusion (p = .03) were strongly associated with T2,inc , and linear mixed model analysis revealed that changes in PCr and perfusion both affect T2,inc (p < .001). These findings support previous assumptions that T2 changes are not only an intra-cellular phenomenon resulting from metabolic stress but are also affected by increased perfusion in loaded muscles.

High cytochrome c oxidase expression links to severe skeletal energy failure by (31)P-MRS spectroscopy in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes

AIMS

The purpose of this study was to evaluate the energy metabolism and mitochondrial function in skeletal muscle from patients with Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) or chronic progressive external ophthalmoplegia (CPEO) using phosphorus magnetic resonance spectroscopy ((31)P-MRS), to determine whether abnormally increasing cytochrome c oxidase (COX), as detected in muscle biopsy, could be a cause for MELAS.

METHODS

(31)P-MRS was performed on the quadriceps femoris muscle of 12 healthy volunteers and 11 patients diagnosed as MELAS or CPEO by muscle biopsy and genetic analysis. All subjects experienced a state of rest, 5-min exercise, and 5-min recovery protocol in a supine position.

RESULTS

Compared to CPEO, MELAS patients typically exhibited COX-positive ragged-red fibers (RRFs) as well as strongly SDH-positive blood vessels (SSVs). However, based on (31)P-MRS results, MELAS showed a higher inorganic phosphate (Pi)/phosphocreatine (PCr) ratio and lower ATP/PCr ratio during exercise and delayed Pi/PCr and ATP/PCr recovery to normal.

CONCLUSIONS

This study suggests that high COX expression contributes to severe skeletal energy failure by (31)P-MRS spectroscopy in MELAS.