Osmotic injury and cytotoxicity for hMSCs in contact with Me2SO: The effect of cell size distribution.

The paper discusses the impact of cell size on cytotoxicity and expansion lysis during the osmotic excursions resulting from the contact of hMSCs from UCB with Me2SO. It builds upon the mathematical model recently presented by the authors, which pertains to a population of cells with uniform size. The objective is to enhance the model's relevance by incorporating the more realistic scenario of cell size distribution, utilizing a Population Balance Equations approach. The study compares the capability of the multiple-sized model to the single-sized one to describe system behavior experimentally measured through cytofluorimetry and Coulter counter when, first, suspending hMSCs in hypertonic solutions of Me2SO (at varying osmolality, system temperature, and contact times), and then (at room temperature) pelleting by centrifugation before suspending the cells back to isotonic conditions. Simulations demonstrate that expansion lysis and cytotoxic effect are not affected by cell size for the specific system hMSCs/Me2SO, thus confirming what was found so far by the authors through a single-size model. On the other hand, simulations show that, when varying the adjustable parameters of the model that are expected to change from cell to cell lineages, expansion lysis is sensitive to cell size, while cytotoxicity is not, being mainly influenced by external CPA concentration and contact duration. More specifically, it is found that smaller cells suffer expansion lysis more than larger ones. The findings suggest that different cells from hMSCs may require a multiple-sized model to assess cell damage during osmotic excursions in cryopreservation.

Nanoparticles from Microalgae and Their Biomedical Applications.

Over the years, microalgae have been a source of useful compounds mainly used as food and dietary supplements. Recently, microalgae have been used as a source of metabolites that can participate in the synthesis of several nanoparticles through inexpensive and environmentally friendly routes alternative to chemical synthesis. Notably, the occurrence of global health threats focused attention on the microalgae application in the medicinal field. In this review, we report the influence of secondary metabolites from marine and freshwater microalgae and cyanobacteria on the synthesis of nanoparticles that were applied as therapeutics. In addition, the use of isolated compounds on the surface of nanoparticles to combat diseases has also been addressed. Although studies have proven the beneficial effect of high-value bioproducts on microalgae and their potential in medicine, there is still room for understanding their exact role in the human body and translating lab-based research into clinical trials.

Relating cardiorespiratory responses to work rate during incremental ramp exercise on treadmill in children and adolescents: sex and age differences.

PURPOSE: Evaluation of cardiopulmonary exercise testing (CPET) slopes such as [Formula: see text] (cardiac/skeletal muscle function) and [Formula: see text] (O2 delivery/utilization), using treadmill protocols is limited because the difficulties in measuring the total work rate ([Formula: see text]). To overcome this limitation, we proposed a new method in quantifying [Formula: see text] to determine CPET slopes. METHODS: CPET's were performed by healthy patients, (n = 674, 9-18 year) 300 female (F) and 374 male (M), using an incremental ramp protocol on a treadmill. For this protocol, a quantitative relationship based on biomechanical principles of human locomotion, was used to quantify the [Formula: see text] of the subject. CPET slopes were determined by linear regression of the data recorded until the gas exchange threshold occurred. RESULTS: The method to estimate [Formula: see text] was substantiated by verifying that: [Formula: see text] for treadmill exercise corresponded to an efficiency of muscular work similar to that of cycle ergometer; [Formula: see text] (mL min-1 W-1) was invariant with age and greater in M than F older than 12 years old (13-14 years: 9.6 ± 1.5(F) vs. 10.5 ± 1.8(M); 15-16 years: 9.7 ± 1.7(F) vs. 10.6 ± 2.2(M); 17-18 years: 9.6 ± 1.7(F) vs. 11.0 ± 2.3(M), p < 0.05); similar to cycle ergometer exercise, [Formula: see text] was inversely related to body weight (BW) (r = 0.71) or [Formula: see text] (r = 0.66) and [Formula: see text] was not related to BW (r = - 0.01), but had a weak relationship with [Formula: see text] (r = 0.28). CONCLUSION: The proposed approach can be used to estimate [Formula: see text] and quantify CPET slopes derived from incremental ramp protocols at submaximal exercise intensities using the treadmill, like the cycle ergometer, to infer cardiovascular and metabolic function in both healthy and diseased states.

Alterations of skeletal muscle bioenergetics in a mouse with F508del mutation leading to a cystic fibrosis-like condition.

High energy expenditure is reported in cystic fibrosis (CF) animal models and patients. Alterations in skeletal muscle oxidative capacity, fuel utilization, and the creatine kinase-phosphocreatine system suggest mitochondrial dysfunction. Studies were performed on congenic C57BL/6J and F508del (Cftrtm1kth) mice. Indirect calorimetry was used to measure gas exchange to evaluate aerobic capacity during treadmill exercise. The bioenergetic function of skeletal muscle subsarcolemmal (SSM) and interfibrillar mitochondria (IFM) was evaluated using an integrated approach combining measurement of the rate of oxidative phosphorylation by polarography and of electron transport chain activities by spectrophotometry. CF mice have reduced maximal aerobic capacity. In SSM of these mice, oxidative phosphorylation was impaired in the presence of complex I, II, III, and IV substrates except when glutamate was used as substrate. This impairment appeared to be caused by a defect in complex V activity, whereas the oxidative system of the electron transport chain was unchanged. In IFM, oxidative phosphorylation and electron transport chain activities were preserved, whereas complex V activity was reduced, in CF. Furthermore, creatine kinase activity was reduced in both SSM and IFM of CF skeletal muscle. The decreased complex V activity in SSM resulted in reduced oxidative phosphorylation, which could explain the reduced skeletal muscle response to exercise in CF mice. The decrease in mitochondrial creatine kinase activity also contributed to this poor exercise response.

Optimization of Brilliant Blue R photocatalytic degradation by silver nanoparticles synthesized using Chlorella vulgaris.

Synthesis of silver nanoparticles (Ag NPs) using microalgae is gaining recognition for its environmentally friendly and cost-effective nature while maintaining high activity of NPs. In the present study, Ag NPs were synthesized using a methanolic extract of Chlorella vulgaris and subjected to calcination. The X-ray diffraction (XRD) analysis showed a crystalline nature of the products with Ag2O and Ag phases with an average crystalline size of 16.07 nm before calcination and an Ag phase with 24.61 nm crystalline size after calcination. Fourier transform infrared spectroscopy (FTIR) revealed the capping functional groups on Ag NPs, while scanning electron microscopy (SEM) displayed their irregular morphology and agglomeration after calcination. The organic coating was examined by energy-dispersive X-ray spectroscopy (EDX) and thermogravimetric (TGA) analyses, confirming the involvement of the metabolites. The UV-Vis analysis showed a difference in optical properties due to calcination. Synthesized Ag NPs were applied for the photodegradation of hazardous dye Brilliant Blue R in visible light. Different values of light intensity, catalyst dose, initial dye concentration, and pH were tested to identify the optimal set of operating conditions. The highest degradation efficiency of 90.6% with an apparent rate constant of 0.04402 min-1 was achieved after 90 min of irradiation in the highest tested catalyst dosage.

Transcriptional programming of translation by BCL6 controls skeletal muscle proteostasis.

Skeletal muscle is dynamically controlled by the balance of protein synthesis and degradation. Here we discover an unexpected function for the transcriptional repressor B cell lymphoma 6 (BCL6) in muscle proteostasis and strength in mice. Skeletal muscle-specific Bcl6 ablation in utero or in adult mice results in over 30% decreased muscle mass and force production due to reduced protein synthesis and increased autophagy, while it promotes a shift to a slower myosin heavy chain fibre profile. Ribosome profiling reveals reduced overall translation efficiency in Bcl6-ablated muscles. Mechanistically, tandem chromatin immunoprecipitation, transcriptomic and translational analyses identify direct BCL6 repression of eukaryotic translation initiation factor 4E-binding protein 1 (Eif4ebp1) and activation of insulin-like growth factor 1 (Igf1) and androgen receptor (Ar). Together, these results uncover a bifunctional role for BCL6 in the transcriptional and translational control of muscle proteostasis.

Distinguishing the effects of convective and diffusive O2 delivery on VO2 on-kinetics in skeletal muscle contracting at moderate intensity.

With current techniques, experimental measurements alone cannot characterize the effects of oxygen blood-tissue diffusion on muscle oxygen uptake (Vo2) kinetics in contracting skeletal muscle. To complement experimental studies, a computational model is used to quantitatively distinguish the contributions of convective oxygen delivery, diffusion into cells, and oxygen utilization to Vo2 kinetics. The model is validated using previously published experimental Vo2 kinetics in response to slowed blood flow (Q) on-kinetics in canine muscle (τQ = 20 s, 46 s, and 64 s) [Goodwin ML, Hernández A, Lai N, Cabrera ME, Gladden LB. J Appl Physiol. 112:9-19, 2012]. Distinctive effects of permeability-surface area or diffusive conductance (PS) and Q on Vo2 kinetics are investigated. Model simulations quantify the relationship between PS and Q, as well as the effects of diffusion associated with PS and Q dynamics on the mean response time of Vo2. The model indicates that PS and Q are linearly related and that PS increases more with Q when convective delivery is limited by slower Q dynamics. Simulations predict that neither oxygen convective nor diffusive delivery are limiting Vo2 kinetics in the isolated canine gastrocnemius preparation under normal spontaneous conditions during transitions from rest to moderate (submaximal) energy demand, although both operate close to the tipping point.

Relationship between muscle venous blood oxygenation and near-infrared spectroscopy: quantitative analysis of the Hb and Mb contributions.

A linear relationship between skeletal muscle venous ([Formula: see text]) and oxygenated (ΔHbMbO2,N) or deoxygenated (ΔHHbMbN) near-infrared spectroscopy (NIRS) signals suggest a main hemoglobin (Hb) contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of myoglobin (Mb) to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48(10):2013-2020, 2016) were integrated into a computational model of muscle O2 transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of [Formula: see text] and NIRS signals for different O2 delivery conditions (blood flow, arterial O2 content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O2 delivery, Mb contribution to NIRS signals was significant (20%-30%) even in the presence of a linear [Formula: see text]-NIRS relationship; for a reduced O2 delivery the nonlinearity of the [Formula: see text]-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O2 delivery is reduced from 1.3 to 0.7 L kg-1·min-1 ([Formula: see text] < 10 mLO2 100 mL-1) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to ΔHHbMbN from 15% to 50% and the contribution to ΔHbMbO2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHHbMbN and ΔHbMbO2,N are significantly affected by Hb and Mb oxygenation changes.NEW & NOTEWORTHY Within the near-infrared spectroscopy (NIRS) signal, the contribution from hemoglobin is indistinguishable from that of myoglobin. A computation analysis indicates that a linear relationship between muscle venous oxygen content and NIRS signals does not necessarily indicate a negligible myoglobin contribution to the NIRS signal. A reduced oxygen delivery increases the myoglobin contribution to the NIRS signal. The integrative approach proposed is a powerful way to assist in interpreting the elements from which the NIRS signals are derived.

Teleosts as behaviour test models for social stress.

Stress is an important aspect of our everyday life and exposure to it is an unavoidable occurrence. In humans, this can come in the form of social stress or physical stress from an injury. Studies in animal models have helped researchers to understand the body's adaptive response to stress in human. Notably, the use of behavioural tests in animal models plays a pivotal role in understanding the neural, endocrine and behavioural changes induced by social stress. Under socially stressed conditions, behavioural parameters are often measured physiological and molecular parameters as changes in behaviour are direct responses to stress and are easily assessed by behavioural tests. Throughout the past few decades, the rodent model has been used as a well-established animal model for stress and behavioural changes. Recently, more attention has been drawn towards using fish as an animal model. Common fish models such as zebrafish, medaka, and African cichlids have the advantage of a higher rate of reproduction, easier handling techniques, sociability and most importantly, share evolutionary conserved genetic make-up, neural circuitry, neuropeptide molecular structure and function with mammalian species. In fact, some fish species exhibit a clear diurnal or seasonal rhythmicity in their stress response, similar to humans, as opposed to rodents. Various social stress models have been established in fish including but not limited to chronic social defeat stress, social stress avoidance, and social stress-related decision-making. The huge variety of behavioural patterns in teleost also aids in the study of more behavioural phenotypes than the mammalian species. In this review, we focus on the use of fish models as alternative models to study the effects of stress on different types of behaviours. Finally, fish behavioural tests against the typical mammalian model-based behavioural test are compared and discussed for their viability.

Model analysis of the relationship between intracellular PO2 and energy demand in skeletal muscle.

On the basis of experimental studies, the intracellular O(2) (iPo(2))-work rate (WR) relationship in skeletal muscle is not unique. One study found that iPo(2) reached a plateau at 60% of maximal WR, while another found that iPo(2) decreased linearly at higher WR, inferring capillary permeability-surface area (PS) and blood-tissue O(2) gradient, respectively, as alternative dominant factors for determining O(2) diffusion changes during exercise. This relationship is affected by several factors, including O(2) delivery and oxidative and glycolytic capacities of the muscle. In this study, these factors are examined using a mechanistic, mathematical model to analyze experimental data from contracting skeletal muscle and predict the effects of muscle contraction on O(2) transport, glycogenolysis, and iPo(2). The model describes convection, O(2) diffusion, and cellular metabolism, including anaerobic glycogenolysis. Consequently, the model simulates iPo(2) in response to muscle contraction under a variety of experimental conditions. The model was validated by comparison of simulations of O(2) uptake with corresponding experimental responses of electrically stimulated canine muscle under different O(2) content, blood flow, and contraction intensities. The model allows hypothetical variation of PS, glycogenolytic capacity, and blood flow and predictions of the distinctive effects of these factors on the iPo(2)-contraction intensity relationship in canine muscle. Although PS is the main factor regulating O(2) diffusion rate, model simulations indicate that PS and O(2) gradient have essential roles, depending on the specific conditions. Furthermore, the model predicts that different convection and diffusion patterns and metabolic factors may be responsible for different iPo(2)-WR relationships in humans.

Exercise intensity and oxygen uptake kinetics in African-American and Caucasian women.

The effect of exercise intensity on the on- and off-transient kinetics of oxygen uptake (VO(2)) was investigated in African American (AA) and Caucasian (C) women. African American (n = 7) and Caucasian (n = 6) women of similar age, body mass index and weight, performed an incremental test and bouts of square-wave exercise at moderate, heavy and very heavy intensities on a cycle ergometer. Gas exchange threshold (LT(GE)) was lower in AA (13.6 ± 2.3 mL kg(-1) min(-1)) than C (18.6 ± 5.6 mL kg(-1) min(-1)). The dynamic exercise and recovery VO(2) responses were characterized by mathematical models. There were no significant differences in (1) peak oxygen uptake (VO(2peak)) between AA (28.5 ± 5 mL kg(-1) min(-1)) and C (31.1 ± 6.6 mL kg(-1) min(-1)) and (2) VO(2) kinetics at any exercise intensity. At moderate exercise, the on- and off- VO(2) kinetics was described by a monoexponential function with similar time constants τ (1,on) (39.4 ± 12.5; 38.8 ± 15 s) and τ (1,off) (52.7 ± 10.1; 40.7 ± 4.4 s) for AA and C, respectively. At heavy and very heavy exercise, the VO(2) kinetics was described by a double-exponential function. The parameter values for heavy and very heavy exercise in the AA group were, respectively: τ (1,on) (47.0 ± 10.8; 44.3 ± 10 s), τ (2,on) (289 ± 63; 219 ± 90 s), τ (1,off) (45.9 ± 6.2; 50.7 ± 10 s), τ (2,off) (259 ± 120; 243 ± 93 s) while in the C group were, respectively: τ (1,on) (41 ± 12; 43.2 ± 15 s); τ (2, on) (277 ± 81; 215 ± 36 s), τ (1,off) (40.2 ± 3.4; 42.3 ± 7.2 s), τ (2,off) (215 ± 133; 228 ± 64 s). The on- and off-transients were symmetrical with respect to model order and dependent on exercise intensity regardless of race. Despite similar VO(2) kinetics, LT(GE) and gain of the VO(2) on-kinetics at moderate intensity were lower in AA than C. However, generalization to the African American and Caucasian populations is constrained by the small subject numbers.

Hemoglobin and myoglobin contributions to skeletal muscle oxygenation in response to exercise.

The quantitative contributions of hemoglobin and myoglobin oxygenation in skeletal muscle depend on physiological factors, especially muscle blood flow (Q( m )) and capillary permeability-surface area (PS). Near-infrared spectroscopy (NIRS) can be used to quantify total heme oxidation, but it is unable to distinguish between hemoglobin and myoglobin. Therefore, a mechanistic computational model has been developed to distinguish the contributions of oxygenated hemoglobin and myoglobin to the total NIRS signal. Model simulations predict how Q( m ) and PS can affect oxygenated hemoglobin and myoglobin.Although both hemoglobin and myoglobin oxygenation decrease with impaired Q( m ), simulations show that myoglobin provides a greater contribution to the overall NIRS signal. A decrease of PS primarily affects myoglobin oxygenation. Based on model simulations, the contribution of myoglobin oxygenation to the total NIRS signal can be significantly different under pathophysiological conditions, such as diabetes and peripheral arterial disorder.

Experiments and modeling of mine soil inertization through mechano-chemical processing: from bench to pilot scale using attritor and impact mills.

Mechano-chemical treatment has been recognized to be a promising technology for the immobilization of heavy metals (HMs) in contaminated soils without the use of additional reagents. Despite this, very few studies aiming to investigate the applicability of this technology at full scale have been published so far. In this study, a quantitative approach was developed to provide process design information to scale-up from laboratory- into pilot-scale mechano-chemical reactors for immobilizing heavy metals in contaminated mining soil. In fact, after preliminary experiments with laboratory-scale ball mills, experiments have been carried out by taking advantage of milling devices suited for pilot-scale applications. The experimental data of this work, along with literature ones, have been quantitatively interpreted by means of a mathematical model allowing to describe the effect of milling dynamics on the HM immobilization kinetics for applications at different scales. The results suggest that the mechanical process can trigger specific physico-chemical phenomena leading to a significant reduction of HMs leached from mining soils. Specifically, after suitably prolonged processing time, HM concentration in the leachate is lowered below the corresponding threshold limits. The observed behavior is well captured by the proposed model for different HMs and operating conditions. Therefore, the model might be exploited to infer design parameters for the implementation of this technique at the pilot and full scale. Moreover, it represents a valuable tool for designing and controlling mechano-chemical reactors at productive scale.

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki).

AIM: Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers. METHODS: Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4. RESULTS: CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10-3). CONCLUSION: With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Modeling oxygenation in venous blood and skeletal muscle in response to exercise using near-infrared spectroscopy.

Noninvasive, continuous measurements in vivo are commonly used to make inferences about mechanisms controlling internal and external respiration during exercise. In particular, the dynamic response of muscle oxygenation (Sm(O(2))) measured by near-infrared spectroscopy (NIRS) is assumed to be correlated to that of venous oxygen saturation (Sv(O(2))) measured invasively. However, there are situations where the dynamics of Sm(O(2)) and Sv(O(2)) do not follow the same pattern. A quantitative analysis of venous and muscle oxygenation dynamics during exercise is necessary to explain the links between different patterns observed experimentally. For this purpose, a mathematical model of oxygen transport and utilization that accounts for the relative contribution of hemoglobin (Hb) and myoglobin (Mb) to the NIRS signal was developed. This model includes changes in microvascular composition within skeletal muscle during exercise and integrates experimental data in a consistent and mechanistic manner. Three subjects (age 25.6 +/- 0.6 yr) performed square-wave moderate exercise on a cycle ergometer under normoxic and hypoxic conditions while muscle oxygenation (C(oxy)) and deoxygenation (C(deoxy)) were measured by NIRS. Under normoxia, the oxygenated Hb/Mb concentration (C(oxy)) drops rapidly at the onset of exercise and then increases monotonically. Under hypoxia, C(oxy) decreases exponentially to a steady state within approximately 2 min. In contrast, model simulations of venous oxygen concentration show an exponential decrease under both conditions due to the imbalance between oxygen delivery and consumption at the onset of exercise. Also, model simulations that distinguish the dynamic responses of oxy-and deoxygenated Hb (HbO(2), HHb) and Mb (MbO(2), HMb) concentrations (C(oxy) = HbO(2) + MbO(2); C(deoxy) = HHb + HMb) show that Hb and Mb contributions to the NIRS signal are comparable. Analysis of NIRS signal components during exercise with a mechanistic model of oxygen transport and metabolism indicates that changes in oxygenated Hb and Mb are responsible for different patterns of Sm(O(2)) and Sv(O(2)) dynamics observed under normoxia and hypoxia.

Non-invasive estimation of metabolic flux and blood flow in working muscle: effect of blood-tissue distribution.

Muscle oxygenation measurements by near infrared spectroscopy (NIRS) are frequently obtained in humans to make inferences about mechanisms of metabolic control of respiration in working skeletal muscle. However, these measurements have technical limitations that can mislead the evaluation of tissue processes. In particular, NIRS measurements of working muscle represent oxygenation of a mix of fibers with heterogeneous activation, perfusion and architecture. Specifically, the relative volume distribution of capillaries, small arteries, and venules may affect NIRS data. To determine the effect of spatial volume distribution of components of working muscle on oxygen utilization dynamics and blood flow changes, a mathematical model of oxygen transport and utilization was developed. The model includes blood volume distribution within skeletal muscle and accounts for convective, diffusive, and reactive processes of oxygen transport and metabolism in working muscle. Inputs to the model are arterial O2 concentration, cardiac output and ATP demand. Model simulations were compared to exercise data from human subjects during a rest-to-work transition. Relationships between muscle oxygen consumption, blood flow, and the rate coefficient of capillary-tissue transport are analyzed. Blood volume distribution in muscle has noticeable effects on the optimal estimates of metabolic flux and blood flow in response to an exercise stimulus.

Isolation of mitochondrial subpopulations from skeletal muscle: Optimizing recovery and preserving integrity.

AIM: The subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria in skeletal muscle appear to have distinct biochemical properties affecting metabolism in health and disease. The isolation of mitochondrial subpopulations has been a long-time challenge while the presence of a continuous mitochondrial reticulum challenges the view of distinctive SSM and IFM bioenergetics. Here, a comprehensive approach is developed to identify the best conditions to separate mitochondrial fractions. METHODS: The main modifications to the protocol to isolate SSM and IFM from rat skeletal muscle were: (a) decreased dispase content and homogenization speed; (b) trypsin treatment of SSM fractions; (c) recentrifugation of mitochondrial fractions at low speed to remove subcellular components. To identify the conditions preserving mitochondrial function, integrity, and maximizing their recovery, microscopy (light and electron) were used to monitor effectiveness and efficiency in separating mitochondrial subpopulations while respiratory and enzyme activities were employed to evaluate function, recovery, and integrity. RESULTS: With the modifications described, the total mitochondrial yield increased with a recovery of 80% of mitochondria contained in the original skeletal muscle sample. The difference between SSM and IFM oxidative capacity (10%) with complex-I substrate was significant only with a saturated ADP concentration. The inner and outer membrane damage for both subpopulations was <1% and 8%, respectively, while the respiratory control ratio was 16. CONCLUSION: Using a multidisciplinary approach, conditions were identified to maximize SSM and IFM recovery while preserving mitochondrial integrity, biochemistry, and morphology. High quality and recovery of mitochondrial subpopulations allow to study the relationship between these organelles and disease.

Multi-scale model of O2 transport and metabolism: response to exercise.

Regulation of pulmonary oxygen uptake (VO2p) during exercise depends on cellular energy demand, blood flow, ventilation, oxygen exchange across membranes, and oxygen utilization in the contracting skeletal muscle. In human and animal studies of metabolic processes that control cellular respiration in working skeletal muscle, pulmonary VO2 dynamics is measured at the mouth using indirect calorimetry. To provide information on the dynamic balance between oxygen delivery and oxygen consumption at the microvascular level, muscle oxygenation is measured using near-infrared spectroscopy. A multi-scale computational model that links O2 transport and cellular metabolism in the skeletal muscle was developed to relate the measurements and gain quantitative understanding of the regulation of VO2 at the cellular, tissue, and whole-body level. The model incorporates mechanisms of oxygen transport from the airway openings to the cell, as well as the phosphagenic and oxidative pathways of ATP synthesis in the muscle cells.

Muscle oxygen uptake differs from consumption dynamics during transients in exercise.

Relating external to internal respiration during exercise requires quantitative modeling analysis for reliable inferences with respect to metabolic rate. Often, oxygen transport and metabolism based on steady-state mass balances (Fick principle) and passive diffusion between blood and tissue are applied to link pulmonary to cellular respiration. Indeed, when the work rate does not change rapidly, a quasi-steady-state analysis based on the Fick principle is sufficient to estimate the rate of O2 consumption in working muscle. During exercise when the work rate changes quickly, however, non-invasive in vivo measurements to estimate muscle O2 consumption are not sufficient to characterize cellular respiration of working muscle. To interpret transient changes of venous O2 concentration, blood flow, and O2 consumption in working muscle, a mathematical model of O2 transport and consumption based on dynamic mass balances is required. In this study, a comparison is made of the differences between simulations of O2 uptake and O2 consumption within working skeletal muscle based on a dynamic model and quasi-steady-state approximations. The conditions are specified under which the quasi-steady-state approximation becomes invalid.

Models of muscle contraction and energetics.

How does skeletal muscle manage to regulate the pathways of ATP synthesis during large-scale changes in work rate while maintaining metabolic homeostasis remains unknown. The classic model of metabolic regulation during muscle contraction states that accelerating ATP utilization leads to increasing concentrations of ADP and Pi, which serve as substrates for oxidative phosphorylation and thus accelerate ATP synthesis. An alternative model states that both the ATP demand and ATP supply pathways are simultaneously activated. Here, we review experimental and computational models of muscle contraction and energetics at various organizational levels and compare them with respect to their pros and cons in facilitating understanding of the regulation of energy metabolism during exercise in the intact organism.