Design of the mammalian respiratory system. VI Distribution of mitochondria and capillaries in various muscles

Microvascular anatomy suggests varying aerobic activity levels in the adipose tissues of diving tetrapods

Adipose tissue has many important functions including metabolic energy storage, endocrine functions, thermoregulation and structural support. Given these varied functions, the microvascular characteristics within the tissue will have important roles in determining rates/limits of exchange of nutrients, waste, gases and molecular signaling molecules between adipose tissue and blood. Studies on skeletal muscle have suggested that tissues with higher aerobic capacity contain higher microvascular density (MVD) with lower diffusion distances (DD) than less aerobically active tissues. However, little is known about MVD in adipose tissue of most vertebrates; therefore, we measured microvascular characteristics (MVD, DD, diameter and branching) and cell size to explore the comparative aerobic activity in the adipose tissue across diving tetrapods, a group of animals facing additional physiological and metabolic stresses associated with diving. Adipose tissues of 33 animals were examined, including seabirds, sea turtles, pinnipeds, baleen whales and toothed whales. MVD and DD varied significantly (P < 0.001) among the groups, with seabirds generally having high MVD, low DD and small adipocytes. These characteristics suggest that microvessel arrangement in short duration divers (seabirds) reflects rapid lipid turnover, compared to longer duration divers (beaked whales) which have relatively lower MVD and greater DD, perhaps reflecting the requirement for tissue with lower metabolic activity, minimizing energetic costs during diving. Across all groups, predictable scaling patterns in MVD and DD such as those observed in skeletal muscle did not emerge, likely reflecting the fact that unlike skeletal muscle, adipose tissue performs many different functions in marine organisms, often within the same tissue compartment.

Relationship between capillaries, mitochondria and maximum power of the heart: a meta-study from shrew to elephant

This meta-study uses phylogenetic scaling models across more than 30 species, spanning five orders of magnitude in body mass, to show that cardiac capillary numerical density and mitochondrial volume density decrease with body mass raised to the -0.07 ± 0.03 and -0.04 ± 0.01 exponents, respectively. Thus, while an average 10 g mammal has a cardiac capillary density of approximately 4150 mm^-2 and a mitochondrial density of 33%, a 1 t mammal has considerably lower corresponding values of 1850 mm^-2 and 21%. These similar scaling trajectories suggest quantitative matching for the primary oxygen supply and oxygen consuming structures of the heart, supporting economic design at the cellular level of the oxygen cascade in this aerobic organ. These scaling trajectories are nonetheless somewhat shallower than the exponent of -0.11 calculated for the maximum external mechanical power of the cardiac tissue, under conditions of heavy exercise, when oxygen flow between capillaries and mitochondria is probably fully exploited. This mismatch, if substantiated, implies a declining external mechanical efficiency of the heart with increasing body mass, whereby larger individuals put more energy in but get less energy out, a scenario with implications for cardiovascular design, aerobic capacity and limits of body size.

Quantifying mitochondrial volume density in phrenic motor neurons

BACKGROUND

Previous assessments of mitochondrial volume density within motor neurons used electron microscopy (EM) to image mitochondria. However, adequate identification and sampling of motor neurons within a particular motor neuron pool is largely precluded using EM. Here, we present an alternative method for determining mitochondrial volume density in identified motor neurons within the phrenic motor neuron (PhMN) pool, with greatly increased sampling.

NEW METHOD

This novel method for assessing mitochondrial volume density in PhMNs uses a combination of intrapleural injection of Alexa 488-conjugated cholera toxin B (CTB) to retrogradely label PhMNs, followed by intrathecal application of MitoTracker Red to label mitochondria. This technique was validated by comparison to 3D EM determination of mitochondrial volume density as a "gold standard".

RESULTS

A mean mitochondrial volume density of ∼11 % was observed across PhMNs using the new MitoTracker Red method. This compared favourably with mitochondrial volume density (∼11 %) measurements using EM.

COMPARISON WITH EXISTING METHOD

The range, mean and variance of mitochondrial volume density estimates in PhMNs were not different between EM and fluorescent imaging techniques.

CONCLUSIONS

Fluorescent imaging may be used to estimate mitochondrial volume density in a large sample of motor neurons, with results similar to EM, although EM did distinguish finer mitochondrion morphology compared to MitoTracker fluorescence. Compared to EM methods, the assessment of a larger sample size and unambiguous identification of motor neurons belonging to a specific motor neuron pool represent major advantages over previous methods.

Mammals to membranes: A reductionist story

This is the story of a series of reductionist studies that started with an attempt to explain what underpins the high-level of aerobic metabolism in mammals (i.e. associated with the evolution of endothermy) and almost forty years later had led to investigations into the role of membrane lipids in determining metabolism. Initial studies showed that the increase in aerobic metabolism in mammals was driven by a combination of increases in mitochondrial volume and membrane densities, organ size and changes in the molecular activity of enzymes. The increase in the capacity to produce energy was matched by an increase in energy use, notably driven by increases in H⁺, Na⁺ and K⁺ fluxes. In the case of increased Na⁺ flux, it was found this was matched by increases in Na⁺-dependent metabolism at the tissue level and increases in enzyme activity at a cellular level but not by an increase in the number of sodium pumps. To maintain Na⁺ gradient across cell membranes, increased Na⁺ flux is not controlled by an increase in sodium pump number but rather by an increase in sodium pump molecular activity (i.e. an increase the substrate turnover rate of each sodium pump) in tissues of endotherms. This increase in molecular activity is coupled to an increase in the level of highly unsaturated polyunsaturated fatty acids (PUFA) in membranes, a mechanism similar to that used by ectotherms to ameliorate decreasing activities of metabolic processes in the cold. Determination of how changes in membrane fatty acid composition can change the activities of proteins in membranes will be the next step in this story.

Functional, Ultrastructural, and Transcriptomic Changes in Rat Diaphragms with Different Durations of Cigarette Smoke Exposure

Aims

The aim of the study was to explore the functional and structural changes of the diaphragm and underlying mechanisms in response to 12 or 24 weeks of cigarette smoke (CS) exposure in rats.

Materials and Methods

Rats were exposed to CS to develop a COPD model and the rats exposed to room air served as a control group. Rats were randomly divided into four groups: CS12W, CON12W, CS24W, and CON24W. Pulmonary function, lung histopathology, and the contractile properties and ultrastructure of diaphragm muscle were examined in these rats. The changes of transcriptomic profiling of diaphragm muscle were further compared between CS and control rats by the RNA Seq.

Results

Both CS groups showed lower FEV_0.3/FVC, elevated mean linear intercept (MLI), and reduced mean alveolar numbers (MAN) vs the control groups. The fatigue index (FI) of the diaphragm muscle from the CS12W group, but not CS24W, was significantly increased. Conversely, the force–frequency curves of the diaphragm muscle from the CS24W group, but not CS12W group, were significantly decreased. Consistently, mitochondrial number density (N_A) and volume density (Vv) were increased in the CS12W diaphragm muscle, while being decreased in the CS24W group. Furthermore, the diaphragm transcriptomic profiling results showed that genes regulating cell proliferation and energy metabolic activity were un-regulated and genes regulating protein degradation were down-regulated in the CS12W diaphragm, while CS24W diaphragm showed opposite changes.

Conclusion

These observations suggested a transition of diaphragm muscle from initial compensatory to decompensatory changes in function, structure, and gene expression during the development of COPD.

Mitochondrial respiration and H2O2 emission in saponin-permeabilized murine diaphragm fibers: optimization of fiber separation and comparison to limb muscle

Diaphragm abnormalities in aging or chronic diseases include impaired mitochondrial respiration and H₂O₂ emission, which can be measured using saponin-permeabilized muscle fibers. Mouse diaphragm presents a challenge for isolation of fibers due to relatively high abundance of connective tissue in healthy muscle that is exacerbated in disease states. We tested a new approach to process mouse diaphragm for assessment of intact mitochondria respiration and ROS emission in saponin-permeabilized fibers. We used the red gastrocnemius (RG) as "standard" limb muscle. Markers of mitochondrial content were two- to fourfold higher in diaphragm (Dia) than in RG (P < 0.05). Maximal O₂ consumption (JO₂: pmol·s^-1·mg^-1) in Dia was higher with glutamate, malate, and succinate (Dia 399 ± 127, RG 148 ± 60; P < 0.05) and palmitoyl-CoA + carnitine (Dia 15 ± 5, RG 7 ± 1; P < 0.05) than in RG, but not different between muscles when JO₂ was normalized to citrate synthase activity. Absolute JO₂ for Dia was two- to fourfold higher than reported in previous studies. Mitochondrial JH₂O₂ was higher in Dia than in RG (P < 0.05), but lower in Dia than in RG when JH₂O₂ was normalized to citrate synthase activity. Our findings are consistent with an optimized diaphragm preparation for assessment of intact mitochondria in permeabilized fiber bundles. The data also suggest that higher mitochondrial content potentially makes the diaphragm more susceptible to "mitochondrial onset" myopathy. Overall, the new approach will facilitate testing and understanding of diaphragm mitochondrial function in mouse models that are used to advance biomedical research and human health.

Evolution and Functional Differentiation of the Diaphragm Muscle of Mammals

Symmorphosis is a concept of economy of biological design, whereby structural properties are matched to functional demands. According to symmorphosis, biological structures are never over designed to exceed functional demands. Based on this concept, the evolution of the diaphragm muscle (DIAm) in mammals is a tale of two structures, a membrane that separates and partitions the primitive coelomic cavity into separate abdominal and thoracic cavities and a muscle that serves as a pump to generate intra-abdominal (Pab ) and intrathoracic (Pth ) pressures. The DIAm partition evolved in reptiles from folds of the pleural and peritoneal membranes that was driven by the biological advantage of separating organs in the larger coelomic cavity into separate thoracic and abdominal cavities, especially with the evolution of aspiration breathing. The DIAm pump evolved from the advantage afforded by more effective generation of both a negative Pth for ventilation of the lungs and a positive Pab for venous return of blood to the heart and expulsive behaviors such as airway clearance, defecation, micturition, and child birth. © 2019 American Physiological Society. Compr Physiol 9:715-766, 2019.

Scaling of morphology and ultrastructure of hearts among wild African antelope

The hearts of smaller mammals tend to operate at higher mass-specific mechanical work rates than those of larger mammals. The ultrastructural characteristics of the heart that allow for such variation in work rate still is largely unknown. We have used perfusion-fixation, transmission electron microscopy and stereology to assess the morphology and anatomical aerobic power density of the heart as a function of body mass across six species of wild African antelope differing by approximately 20-fold in body mass. The survival of wild antelope, as prey animals, depends on competent cardiovascular performance. We found that relative heart mass (g kg−1 body mass) decreases with body mass according to a power equation with an exponent of –0.12±0.07 (± 95% CI) (P=0.0027). Likewise, capillary length density (km cm−3 of cardiomyocyte), mitochondrial volume density (fraction of cardiomyocyte), and mitochondrial inner membrane surface density (m2 cm−3 of mitochondria) also decrease with body mass with exponents of –0.17±0.16 (P=0.039), –0.06±0.05 (P=0.018), and –0.07±0.05 (P=0.015), respectively, trends likely to be associated with the greater mass-specific mechanical work rates of the hearts in smaller antelope. Finally, we found proportionality between quantitative characteristics of a structure responsible for the delivery of oxygen (total capillary length) and those of a structure that ultimately uses that oxygen (total mitochondrial inner membrane surface area), which provides support for the economic principle of symmorphosis at the cellular level of the oxygen cascade in an aerobic organ.

Histopathological nature of myofascial trigger points at different stages of recovery from injury in a rat model

Objective

To investigate the histopathological nature of myofascial trigger points (MTrPs) or spots (MTrSs) at different stages of recovery from injury in a rat model.

Methods

Forty Sprague–Dawley rats were randomly divided into two groups: a control group (CG) and experimental group (EG). The CG was further randomly subdivided into CG1 and CG2 subgroups. The CG2 was used for palpating the taut band and CG1 as a blank. EG was subdivided into three groups according to recovery times: 4 weeks (4W), 8 weeks (8W) and 12 weeks (12W); these groups consisted of eight rats each. All CG rats received no intervention, whereas the intervention in EG rats was by a blunt strike to the vastus medialis and eccentric exercise for 8 weeks. The taut bands with spontaneous electrical activity were then detected in the muscle to guide a muscle biopsy. The histopathological findings were investigated under optical and electron microscopes in all groups.

Results

Under optical microscopy, the differently augmented sizes of round fibres (contracture knots) with deep staining in the transverse section and fusiform shapes in a longitudinal view were clearly seen in CG2 and EGs with a large diameter; the number of contracture knots was significantly more in EGs than in CGs. Under an electron microscope, the mitochondria in EGs significantly decreased with abnormal structures. The sarcomeres were significantly shortened in the 8W and 12W EGs.

Conclusion

An injury can cause activation of MTrSs in a muscle and an activated level of MTrPs depending on the number of contracture knots in muscle with impaired energy production.

Coupling between skeletal muscle fiber size and capillarization is maintained during healthy aging

BACKGROUND

As muscle capillarization is related to the oxidative capacity of the muscle and the size of muscle fibres, capillary rarefaction may contribute to sarcopenia and functional impairment in older adults. Therefore, it is important to assess how ageing affects muscle capillarization and the interrelationship between fibre capillary supply with the oxidative capacity and size of the fibres.

METHODS

Muscle biopsies from healthy recreationally active young (22 years; 14 men and 5 women) and older (74 years; 22 men and 6 women) people were assessed for muscle capillarization and the distribution of capillaries with the method of capillary domains. Oxidative capacity of muscle fibres was assessed with quantitative histochemistry for succinate dehydrogenase (SDH) activity.

RESULTS

There was no significant age-related reduction in muscle fibre oxidative capacity. Despite 18% type II fibre atrophy (P = 0.019) and 23% fewer capillaries per fibre (P < 0.002) in the old people, there was no significant difference in capillary distribution between young and old people, irrespective of sex. The capillary supply to a fibre was primarily determined by fibre size and only to a small extent by oxidative capacity, irrespective of age and sex. Based on SDH, the maximal oxygen consumption supported by a capillary did not differ significantly between young and old people.

CONCLUSIONS

The similar quantitative and qualitative distribution of capillaries within muscle from healthy recreationally active older people and young adults indicates that the age-related capillary rarefaction, which does occur, nevertheless maintains the coupling between skeletal muscle fibre size and capillarization during healthy ageing.

Multigenerational Effects of Rearing Atmospheric Oxygen Level on the Tracheal Dimensions and Diffusing Capacities of Pupal and Adult Drosophila melanogaster

Insects are small relative to vertebrates, and were larger in the Paleozoic when atmospheric oxygen levels were higher. The safety margin for oxygen delivery does not increase in larger insects, due to an increased mass-specific investment in the tracheal system and a greater use of convection in larger insects. Prior studies have shown that the dimensions and number of tracheal system branches varies inversely with rearing oxygen in embryonic and larval insects. Here we tested whether rearing in 10, 21, or 40 kPa atmospheric oxygen atmospheres for 5-7 generations affected the tracheal dimensions and diffusing capacities of pupal and adult Drosophila. Abdominal tracheae and pupal snorkel tracheae showed weak responses to oxygen, while leg tracheae showed strong, but imperfect compensatory changes. The diffusing capacity of leg tracheae appears closely matched to predicted oxygen transport needs by diffusion, perhaps explaining the consistent and significant responses of these tracheae to rearing oxygen. The reduced investment in tracheal structure in insects reared in higher oxygen levels may be important for conserving tissue PO2 and may provide an important mechanism for insects to invest only the space and materials necessary into respiratory structure.

Plasticity in mitochondrial cristae density allows metabolic capacity modulation in human skeletal muscle

KEY POINTS

In human skeletal muscles, the current view is that the capacity for mitochondrial energy production, and thus endurance capacity, is set by the mitochondria volume. However, increasing the mitochondrial inner membrane surface comprises an alternative mechanism for increasing the energy production capacity. In the present study, we show that mitochondrial inner membranes in leg muscles of endurance-trained athletes have an increased ratio of surface per mitochondrial volume. We show a positive correlation between this ratio and whole body oxygen uptake and muscle fibre mitochondrial content. The results obtained in the present study help us to understand modulation of mitochondrial function, as well as how mitochondria can increase their oxidative capacity with increased demand.

ABSTRACT

Mitochondrial energy production involves the movement of protons down a large electrochemical gradient via ATP synthase located on the folded inner membrane, known as cristae. In mammalian skeletal muscle, the density of cristae in mitochondria is assumed to be constant. However, recent experimental studies have shown that respiration per mitochondria varies. Modelling studies have hypothesized that this variation in respiration per mitochondria depends on plasticity in cristae density, although current evidence for such a mechanism is lacking. In the present study, we confirm this hypothesis by showing that, in human skeletal muscle, and in contrast to the current view, the mitochondrial cristae density is not constant but, instead, exhibits plasticity with long-term endurance training. Furthermore, we show that frequently recruited mitochondria-enriched fibres have significantly increased cristae density and that, at the whole-body level, muscle mitochondrial cristae density is a better predictor of maximal oxygen uptake rate than muscle mitochondrial volume. Our findings establish an elevating mitochondrial cristae density as a regulatory mechanism for increasing metabolic power in human skeletal muscle. We propose that this mechanism allows evasion of the trade-off between cell occupancy by mitochondria and other cellular constituents, as well as improved metabolic capacity and fuel catabolism during prolonged elevated energy requirements.

Chicken muscle mitochondrial content appears co-ordinately regulated and is associated with performance phenotypes

Mitochondrial content is a fundamental cellular bioenergetic phenotype. Previous work has hypothesised possible links between variation in muscle mitochondrial content and animal performance. However, no population screens have been performed in any production species. Here, we have designed a high throughput molecular approach to estimate mitochondrial content in commercial broilers. Technical validity was established using several approaches, including its performance in monoclonal DF-1 cells, cross-tissue comparisons in tissues with differing metabolic demands (white fat<breast muscle<drumstick muscle<heart muscle) and, as a negative control, a near absence of mtDNA amplification from whole blood. We screened breast muscle and thigh muscle in 80 birds individually phenotyped for 11 growth and development traits. Substantial individual variation (fivefold) was discovered in both breast and thigh muscle mitochondrial content. Interestingly, across birds we detected a very strong positive relationship between breast and thigh content (correlation coefficient 0.61; P<0.0001), consistent with coordinate regulatory control across the musculature. Further, breast muscle mitochondrial content is negatively correlated with breast muscle yield (−0.27; P=0.037), abdominal fat content (−0.31; P=0.017) and carcass yield (−0.26; P=0.045). Therefore, low breast muscle mitochondrial content is associated with more muscular birds possessing higher abdominal fat, the latter being in line with biomedical models of obesity. Finally, thigh mitochondrial content is negatively correlated with the bow out leg defect (−0.30; P=0.011). Overall, our data point to mitochondrial content as a promising consideration in predictive modelling of production traits.

Summary: We have detected variation in muscle mitochondrial content across chickens. This variation relates to phenotypes and also indicates coordinate regulation of mitochondrial biogenesis across the musculature.

A structure-function analysis of the left ventricle

This study presents a structure-function analysis of the mammalian left ventricle and examines the performance of the cardiac capillary network, mitochondria, and myofibrils at rest and during simulated heavy exercise. Left ventricular external mechanical work rate was calculated from cardiac output and systemic mean arterial blood pressure in resting sheep (Ovis aries; n = 4) and goats (Capra hircus; n = 4) under mild sedation, followed by perfusion-fixation of the left ventricle and quantification of the cardiac capillary-tissue geometry and cardiomyocyte ultrastructure. The investigation was then extended to heavy exercise by increasing cardiac work according to published hemodynamics of sheep and goats performing sustained treadmill exercise. Left ventricular work rate averaged 0.017 W/cm³ of tissue at rest and was estimated to increase to ∼0.060 W/cm³ during heavy exercise. According to an oxygen transport model we applied to the left ventricular tissue, we predicted that oxygen consumption increases from 195 nmol O₂·s^-1·cm^-3 of tissue at rest to ∼600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 90% of the oxygen demand rate and consistent with work remaining predominantly aerobic. Mitochondria represent 21-22% of cardiomyocyte volume and consume oxygen at a rate of 1,150 nmol O₂·s^-1·cm^-3 of mitochondria at rest and ∼3,600 nmol O₂·s^-1·cm^-3 during heavy exercise, which is within 80% of maximum in vitro rates and consistent with mitochondria operating near their functional limits. Myofibrils represent 65-66% of cardiomyocyte volume, and according to a Laplacian model of the left ventricular chamber, generate peak fiber tensions in the range of 50 to 70 kPa at rest and during heavy exercise, which is less than maximum tension of isolated cardiac tissue (120-140 kPa) and is explained by an apparent reserve capacity for tension development built into the left ventricle.

Mitochondrial Regulation of the Muscle Microenvironment in Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe clinical presentation of peripheral arterial disease and manifests as chronic limb pain at rest and/or tissue necrosis. Current clinical interventions are largely ineffective and therapeutic angiogenesis based trials have shown little efficacy, highlighting the dire need for new ideas and novel therapeutic approaches. Despite a decade of research related to skeletal muscle as a determinant of morbidity and mortality outcomes in CLI, very little progress has been made toward an effective therapy aimed directly at the muscle myopathies of this disease. Within the muscle cell, mitochondria are well positioned to modulate the ischemic cellular response, as they are the principal sites of cellular energy production and the major regulators of cellular redox charge and cell death. In this mini review, we update the crucial importance of skeletal muscle to CLI pathology and examine the evolving influence of muscle and endothelial cell mitochondria in the complex ischemic microenvironment. Finally, we discuss the novelty of muscle mitochondria as a therapeutic target for ischemic pathology in the context of the complex co-morbidities often associated with CLI.

Insights into the skeletal muscle characteristics of three southern African antelope species

Skeletal muscle fibre type, cross-sectional area (CSA), maximum enzyme capacities and fibre oxidative capacities were investigated in three southern African antelope species. Muscle samples from blesbok (Damaliscus pygargus phillipsi), mountain reedbuck (Redunca fulvorufula) and greater kudu (Tragelaphus strepsiceros) were collected post mortem from the Vastus lateralis and analysed for myosin heavy chain (MHC) isoform content, citrate synthase (CS), 3-hydroxyacyl Co A dehydrogenase (3-HAD), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and creatine kinase (CK) activities. Histochemistry and immunohistochemistry were performed to determine relative fibre oxidative capacity, fibre type and cross-sectional area (CSA). Type IIX fibres were the most abundant fibre type in all three species, ranging from 43 to 57%. Kudu had less type IIX fibres than mountain reedbuck and blesbok (P<0.05), values confirmed by their respective MHC isoform content. Blesbok had the smallest fibres, followed by mountain reedbuck and finally kudu (P<0.001). Overall, all three species had high oxidative and glycolytic capacities, but species differences were found. Kudu had the lowest CS activity, followed by blesbok and mountain reedbuck, but the highest PFK, LDH and CK activities. This study confirmed large variation in oxidative capacities within a single fibre type, as well as overlap between the fibre types with no distinct differences between the three species. The fibre type profile of each species is discussed and confirms some of their physical attributes and capabilities.

Aging and the Skeletal Muscle Angiogenic Response to Exercise in Women

Whether aging lowers skeletal muscle basal capillarization and angiogenesis remains controversial. To investigate the effects of aging on skeletal muscle capillarization, eight young (YW) and eight aged (AW) women completed 8 weeks of exercise training. The response and relationships of muscle capillarization, interstitial vascular endothelial growth factor (VEGF), and microvascular blood flow to aerobic exercise training were investigated. Vastus lateralis biopsies were obtained before and after exercise training for the measurement of capillarization. Muscle interstitial VEGF protein and microvascular blood flow were measured at rest and during submaximal exercise at PRE, 1-WK, and 8-WKS by microdialysis. Exercise training increased (20%-25%) capillary contacts of type I, IIA, and IIB fibers in YW and AW. Interstitial VEGF protein was higher in AW than YW at rest and was higher in YW than AW during exercise independent of training status. Differences in muscle capillarization were not explained by secreted VEGF nor were differences in VEGF explained by microvascular blood flow. These results confirm that aging (57-76 years age range) does not impair the muscle angiogenic response to exercise training, although sex differences may exist in similarly trained women and men.

Mechanisms of Myofascial Pain

Myofascial pain syndrome is an important health problem. It affects a majority of the general population, impairs mobility, causes pain, and reduces the overall sense of well-being. Underlying this syndrome is the existence of painful taut bands of muscle that contain discrete, hypersensitive foci called myofascial trigger points. In spite of the significant impact on public health, a clear mechanistic understanding of the disorder does not exist. This is likely due to the complex nature of the disorder which involves the integration of cellular signaling, excitation-contraction coupling, neuromuscular inputs, local circulation, and energy metabolism. The difficulties are further exacerbated by the lack of an animal model for myofascial pain to test mechanistic hypothesis. In this review, current theories for myofascial pain are presented and their relative strengths and weaknesses are discussed. Based on new findings linking mechanoactivation of reactive oxygen species signaling to destabilized calcium signaling, we put forth a novel mechanistic hypothesis for the initiation and maintenance of myofascial trigger points. It is hoped that this lays a new foundation for understanding myofascial pain syndrome and how current therapies work, and gives key insights that will lead to the improvement of therapies for its treatment.

Skeletal muscle mitochondrial and lipid droplet content assessed with standardized grid sizes for stereology

Skeletal muscle mitochondrial (Mito) and lipid droplet (Lipid) content are often measured in human translational studies. Stereological point counting allows computing Mito and Lipid volume density (Vd) from micrographs taken with transmission electron microscopes. Former studies are not specific as to the size of individual squares that make up the grids, making reproducibility difficult, particularly when different magnifications are used. Our objective was to determine which size grid would be best at predicting fractional volume efficiently without sacrificing reliability and to test a novel method to reduce sampling bias. Methods: ten subjects underwent vastus lateralis biopsies. Samples were fixed, embedded, and cut longitudinally in ultrathin sections of 60 nm. Twenty micrographs from the intramyofibrillar region were taken per subject at ×33,000 magnification. Different grid sizes were superimposed on each micrograph: 1,000 × 1,000 nm, 500 × 500 nm, and 250 × 250 nm. Results: mean Mito and Lipid Vd were not statistically different across grids. Variability was greater when going from 1,000 × 1,000 to 500 × 500 nm grid than from 500 × 500 to 250 × 250 nm grid. Discussion: this study is the first to attempt to standardize grid size while keeping with the conventional stereology principles. This is all in hopes of producing replicable assessments that can be obtained universally across different studies looking at human skeletal muscle mitochondrial and lipid droplet content.

High muscle mitochondrial volume and aerobic capacity in a small marsupial (Sminthopsis crassicaudata) reveals flexible links between energy-use levels in mammals

We investigated the muscle structure-function relationships that underlie the aerobic capacity of an insectivorous, small (~15 g) marsupial, Sminthopsis crassicaudata (Family: Dasyuridae), to obtain further insight into energy use patterns in marsupials relative to those in placentals, their sister clade within the Theria (advanced mammals). Disparate hopping marsupials (Suborder Macropodiformes), a kangaroo (Macropus rufus) and a rat-kangaroo (Bettongia penicillata), show aerobic capabilities as high as those of 'athletic' placentals. Equivalent muscle mitochondrial volumes and cardiovascular features support these capabilities. We examined S. crassicaudata to determine whether highly developed aerobic capabilities occur elsewhere in marsupials, rather than being restricted to the more recently evolved Macropodiformes. This was the case. Treadmill-trained S. crassicaudata attained a maximal aerobic metabolic rate ( or MMR) of 272 ml O2 min(-1) kg(-1) (N=8), similar to that reported for a small (~20 g), 'athletic' placental, Apodemus sylvaticus, 264 ml O2 min(-1) kg(-1). Hopping marsupials have comparable aerobic levels when body mass variation is considered. Sminthopsis crassicaudata has a basal metabolic rate (BMR) about 75% of placental values but it has a notably large factorial aerobic scope (fAS) of 13; elevated fAS also features in hopping marsupials. The of S. crassicaudata was supported by an elevated total muscle mitochondrial volume, which was largely achieved through high muscle mitochondrial volume densities, Vv(mt,f), the mean value being 14.0±1.33%. These data were considered in relation to energy use levels in mammals, particularly field metabolic rate (FMR). BMR is consistently lower in marsupials, but this is balanced by a high fAS, such that marsupial MMR matches that of placentals. However, FMR shows different mass relationships in the two clades, with the FMR of small (<125 g) marsupials, such as S. crassicaudata, being higher than that in comparably sized placentals, with the reverse applying for larger marsupials. The flexibility of energy output in marsupials provides explanations for this pattern. Overall, our data refute widely held notions of mechanistically closely linked relationships between body mass, BMR, FMR and MMR in mammals generally.

The high aerobic capacity of a small, marsupial rat-kangaroo (Bettongia penicillata) is matched by the mitochondrial and capillary morphology of its skeletal muscles

We examined the structure-function relationships that underlie the aerobic capacities of marsupial mammals that hop. Marsupials have relatively low basal metabolic rates (BMR) and historically were seen as 'low energy' mammals. However, the red kangaroo, Macropus rufus (family Macropodidae), has aerobic capacities equivalent to athletic placentals. It has an extreme aerobic scope (fAS) and its large locomotor muscles feature high mitochondrial and capillary volumes. M. rufus belongs to a modern group of kangaroos and its high fAS is not general for marsupials. However, other hopping marsupials may have elevated aerobic capacities. Bettongia penicillata, a rat-kangaroo (family Potoroidae), is a small (1 kg), active hopper whose fAS is somewhat elevated. We examined the oxygen delivery system in its muscles to ascertain links with hopping. An elevated fAS of 23 provided a relatively high maximal aerobic oxygen consumption ( ) in B. penicillata; associated with this is a skeletal muscle mass of 44% of body mass. Ten muscles were sampled to estimate the total mitochondrial and capillary volume of the locomotor muscles. Values in B. penicillata were similar to those in M. rufus and in athletic placentals. This small hopper had high muscle mitochondrial volume densities (7.1-11.9%) and both a large total capillary volume (6 ml kg(-1) body mass) and total capillary erythrocyte volume (3.2 ml kg(-1)). Apparently, a considerable aerobic capacity is required to achieve the benefits of the extended stride in fast hopping. Of note, the ratio of to total muscle mitochondrial volume in B. penicillata was 4.9 ml O(2) min(-1) ml(-1). Similar values occur in M. rufus and also placental mammals generally, not only athletic species. If such relationships occur in other marsupials, a fundamental structure-function relationship for oxygen delivery to muscles likely originated with or before the earliest mammals.

The influence of age and aerobic fitness: effects on mitochondrial respiration in skeletal muscle

AIM

Mitochondrial function has previously been studied in ageing, but never in humans matched for maximal oxygen uptake ((V)·O2max). Furthermore, the influence of ageing on mitochondrial substrate sensitivity is not known.

METHODS

Skeletal muscle mitochondrial respiratory capacity and mitochondrial substrate sensitivity were measured by respirometry in young (23 ± 3 years) and middle-aged (53 ± 3 years) male subjects with similar (V)·O2max. Protocols for respirometry included titration of substrates for complex I (glutamate), complex II (succinate) and both (octanoyl carnitine) for calculation of substrate sensitivity (C(50) ). Myosin heavy chain (MHC) isoforms, citrate synthase (CS) and β-hydroxy-acyl-CoA-dehydrogenase (HAD) activity, mitochondrial DNA (mtDNA) content, protein levels of complexes I-V and antioxidant defence system [manganese superoxide dismutase (MnSOD)] were measured.

RESULTS

No differences were found in maximal mitochondrial respiration or C(50) with glutamate (2.0 ± 0.3 and 1.8 ± 0.3 mm), succinate (3.7 ± 0.2 and 3.8 ± 0.4 mm) or octanoyl carnitine (47 ± 8 and 56 ± 7 μm) in young and middle-aged subjects respectively. Normalizing mitochondrial respiration to mtDNA young subjects had a higher (P < 0.05) respiratory capacity per mitochondrion compared to middle-aged subjects. HAD activity and mtDNA per mg tissue were higher in middle-aged compared to young subjects. Middle-aged had a higher MHC I isoform and a lower MHC IIX isoform content compared to young subjects.

CONCLUSION

Mitochondrial substrate sensitivity is not affected by ageing. When young and middle-aged men are carefully matched for (V)·O2max, mitochondrial respiratory capacity is also similar. However, per mitochondrion respiratory capacity was lower in middle-aged compared to young subjects. Thus, when matched for (V)·O2max, middle-aged seem to require a higher mitochondrial content than young subjects.

Black wildebeest skeletal muscle exhibits high oxidative capacity and a high proportion of type IIx fibres

The aim of the study was to investigate the skeletal muscle characteristics of black wildebeest (Connochaetes gnou) in terms of fibre type and metabolism. Samples were obtained post mortem from the vastus lateralis and longissimus lumborum muscles and analysed for myosin heavy chain (MHC) content. Citrate synthase (CS), 3-hydroxyacyl co A dehydrogenase (3HAD), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and creatine kinase (CK) activities were measured spectrophotometrically to represent the major metabolic pathways in these muscles. Both muscles had less than 20% MHC I, whereas MHC IIa and MHC IIx were expressed in excess of 50% in the vastus lateralis and longissimus lumborum muscles, respectively. Overall fibre size was 2675±1034 μm(2), which is small compared with other species. Oxidative capacity (CS and 3HAD) in both muscles was high and did not differ from one another, but the longissimus lumborum had significantly (P<0.05) higher PFK, LDH and CK activities. No relationships were observed between fibre type and the oxidative and oxygen-independent metabolic capacity as measured by specific enzyme activities. This study confirms the presence of both fast-twitch fibres and high oxidative capacity in black wildebeest, indicating an animal that can run very fast but is also fatigue resistant.

Intraspecific variation in the thermal plasticity of mitochondria in killifish

Populations of the Atlantic killifish (Fundulus heteroclitus) inhabit salt marshes and estuaries along the eastern coast of North America from Newfoundland to northern Florida, and are thus exposed to a large range of temperatures. Previous studies have shown higher whole-organism metabolic rates in the northern subspecies (F. h. macrolepidotus) compared with the southern subspecies (F. h. heteroclitus) of these fish. Here, we examine phenotypic plasticity in the response to cold temperatures between the two subspecies by acclimating fish to 5, 15 and 25°C and comparing several mitochondrial and muscle properties. The relative area of oxidative muscle versus glycolytic muscle fibers was greater in the northern subspecies at the 5 and 15°C acclimation temperatures. However, there were no differences in capillary density between the two subspecies or at different temperatures. Mitochondrial volume and surface densities increased in response to cold temperature acclimation in red and white muscle, but only in the northern killifish. Citrate synthase activities also increased in the northern killifish at 5 and 15°C. The ratio of calculated [free ADP] to [ATP] increased in the 5°C acclimated southern killifish but not in the northern killifish at 5°C when compared with the 15°C acclimation group, suggesting that there are differences in adenylate signaling for mitochondrial respiration between subspecies at low temperature. Taken together, our data indicate that the northern subspecies have a greater ability to increase mitochondrial capacity at colder temperatures compared with the southern subspecies, providing one of the few examples of intraspecific variation in phenotypic plasticity in mitochondrial amount in response to cold temperatures.

Mitochondrial treason: a driver of pH decline rate in post-mortem muscle?

The rate and extent of post-mortem pH decline is an important determinant of meat quality, influencing among other things the incidence of pale, soft, exudative meat. In this perspective I argue two things. First, the accuracy of our modelling of pH decline rates might be improved by incorporation of a muscle mitochondrial content parameter. Second, that high mitochondrial content might help explain the surprisingly rapid pH decline that has sometimes been reported in red, oxidative muscles. The proposed physiological mechanism relates to the fact that mitochondria are known to act as net consumers, rather than producers, of ATP during periods of anoxia – a process called mitochondrial treason. To the best of my knowledge, the implications of the phenomenon of mitochondrial treason have never previously been applied or discussed in a meat science context.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Effects of eccentric and concentric training on capillarization and myosin heavy chain contents in rat skeletal muscles after hindlimb suspension

We studied the effects of different protocols of post-disuse rehabilitation on angiogenesis and myosin heavy chain (MHC) content in rat hindlimb muscles after caudal suspension. Thirty female Wistar rats were divided into five groups: (1) Control I, (2) Control II, (3) Suspended, (4) Suspended trained on declined treadmill, and (5) Suspended trained on flat treadmill. Fragments of the soleus and tibialis anterior (TA) muscles were frozen and processed by electrophoresis and immunohistochemistry (CD31 antibody). Hindlimb suspension caused reduction of capillary/fiber (C/F) ratios and contents of MHC type I (MHCI) in the soleus in parallel to increased capillary density. Flat treadmill protocols increased the content of the MHCI isoform. The C/F ratio was increased by concentric training after hypokinesis, but was not modified by eccentric training, which caused a greater reduction of capillary density compared to the other protocols. In the TA muscle, hindlimb suspension caused a non-significant increase in capillary density and C/F ratio with limited changes in MHC. The present data demonstrate that the different training protocols adopted and the functional performance of the muscles analyzed caused specific changes in capillarization and in the content of the various MHC types.

Rat diaphragm mitochondria have lower intrinsic respiratory rates than mitochondria in limb muscles

The mitochondrial content of skeletal muscles is proportional to activity level, with the assumption that intrinsic mitochondrial function is the same in all muscles. This may not hold true for all muscles. For example, the diaphragm is a constantly active muscle; it is possible that its mitochondria are intrinsically different compared with other muscles. This study tested the hypothesis that mitochondrial respiration rates are greater in the diaphragm compared with triceps surae (TS, a limb muscle). We isolated mitochondria from diaphragm and TS of adult male Sprague Dawley rats. Mitochondrial respiration was measured by polarography. The contents of respiratory complexes, uncoupling proteins 1, 2, and 3 (UCP1, UCP2, and UCP3), and voltage-dependent anion channel 1 (VDAC1) were determined by immunoblotting. Complex IV activity was measured by spectrophotometry. Mitochondrial respiration states 3 (substrate and ADP driven) and 5 (uncoupled) were 27 ± 8% and 24 ± 10%, respectively, lower in diaphragm than in TS (P < 0.05 for both comparisons). However, the contents of respiratory complexes III, IV, and V, UCP1, and VDAC1 were higher in diaphragm mitochondria (23 ± 6, 30 ± 8, 25 ± 8, 36 ± 15, and 18 ± 8% respectively, P ≤ 0.04 for all comparisons). Complex IV activity was 64 ± 16% higher in diaphragm mitochondria (P ≤ 0.01). Mitochondrial UCP2 and UCP3 content and complex I activity were not different between TS and diaphragm. These data indicate that diaphragm mitochondria respire at lower rates, despite a higher content of respiratory complexes. The results invalidate our initial hypothesis and indicate that mitochondrial content is not the only determinant of aerobic capacity in the diaphragm. We propose that UCP1 and VDAC1 play a role in regulating diaphragm aerobic capacity.

Redox homeostasis, oxidative stress and disuse muscle atrophy

A pivotal role has been ascribed to oxidative stress in determining the imbalance between protein synthesis and degradation leading to muscle atrophy in many pathological conditions and in disuse. However, a large variability in disuse-induced alteration of redox homeostasis through muscles, models and species emerges from the literature. Whereas the causal role of oxidative stress appears well established in the mechanical ventilation model, findings are less compelling in the hindlimb unloaded mice and very limited in humans. The mere coexistence of muscle atrophy, indirect indexes of increased reactive oxygen species (ROS) production and impairment of antioxidant defence systems, in fact, does not unequivocally support a causal role of oxidative stress in the phenomenon. We hypothesise that in some muscles, models and species only, due to a large redox imbalance, the leading phenomena are activation of proteolysis and massive oxidation of proteins, which would become more susceptible to degradation. In other conditions, due to a lower extent and variable time course of ROS production, different ROS-dependent, but also -independent intracellular pathways might dominate determining the variable extent of atrophy and even dispensable protein oxidation. The ROS production and removal are complex and finely tuned phenomena. They are indeed important intracellular signals and redox balance maintains normal muscle homeostasis and can underlie either positive or negative adaptations to exercise. A precise approach to determine the levels of ROS in living cells in various conditions appears to be of paramount importance to define and support such hypotheses.

Mitochondrial content and distribution changes specific to mouse diaphragm after chronic normobaric hypoxia

Chronic hypoxia reduces aerobic capacity (mitochondrial content) in limb skeletal muscles, and one of the causes seems to be decreased physical activity. Diaphragm and other respiratory muscles, however, may have a different pattern of adaptation as hypoxia increases the work of breathing. Thus, we hypothesized that chronic hypoxia would not reduce mitochondrial content in mouse diaphragm. Adult male C57BL/6J mice were kept in normoxia (Fi(O(2)) = 21%, control) or normobaric hypoxia (Fi(O(2)) = 10%, hypoxia) for 1, 2, and 4 wk. Mice were then killed, and the diaphragm and gastrocnemius muscles collected for analysis. In the diaphragm, cytochrome c oxidase histochemistry showed less intense staining in the hypoxia group. The total content of subunits from the electron transport chain, pyruvate dehydrogenase kinase 1 (PDK1), and voltage-dependent anion channel 1 (VDAC1) was evaluated by Western blot. These proteins decreased by 25-30% after 4 wk of hypoxia (P < 0.05 vs. control for all comparisons), matching a comparable decrease in diaphragmatic mitochondrial volume density (control 33.6 +/- 5.5% vs. hypoxia 26.8 +/- 6.7%, P = 0.013). Mitochondrial volume density or protein content did not change in gastrocnemius after hypoxia. Hypoxia decreased the content of peroxisome proliferator-activated receptor gamma (PPARgamma) and PPARgamma cofactor 1-alpha (PGC-1alpha) in diaphragm but not in gastrocnemius. PGC-1alpha mRNA levels in diaphragm were also reduced with hypoxia. BCL2/adenovirus E1B interacting protein 3 (BNIP-3) mRNA levels were upregulated after 1 and 2 wk of hypoxia in diaphragm and gastrocnemius, respectively; BNIP-3 protein content increased only in the diaphragm after 4 wk of hypoxia. Contrary to our hypothesis, these results show that chronic hypoxia decreases mitochondrial content in mouse diaphragm, despite the increase in workload. A combination of reduced mitochondrial biogenesis and increased mitophagy seems to be responsible for the decrease in mitochondrial content in the mouse diaphragm after hypoxia.

Anatomic capillarization is elevated in the medial gastrocnemius muscle of mighty mini mice

House mice selectively bred for high voluntary wheel running display a mini-muscle (MM) phenotype wherein mass-specific mitochondrial enzyme activities are double that of normal, but muscle mass is reduced by half. In addition, mini-muscles are characterized by small muscle fibers in the superficial region of the plantaris and medial gastrocnemius muscles. To determine the structural alterations facilitating aerobic metabolism in these mini-muscles, cross-sections of the medial gastrocnemius muscle of normal (N; n = 6) and mini-muscle (MM; n = 6) mice were histo- and immunochemically labeled and analyzed for fiber size, capillarization, and fiber type. On the basis of the higher mitochondrial enzyme activities in muscles of MM mice, we hypothesized that they would have greater fiber capillarization in the medial gastrocnemius than N mice. Furthermore, we hypothesized that augmented capillarization in MM would principally be a function of the smaller fibers in the superficial aspect of this muscle. On average, MM had higher capillary-to-fiber ratio and higher capillary density. Binning fibers according to size revealed that it was primarily the normal-sized fibers of the MM that had higher capillarity. The small fibers seen in the superficial region of MM were distinct from N mice in that they had heterogeneous myofibrillar ATPase staining and patchy succinate dehydrogenase staining in the interior of the fibers. These results support the hypothesis that the MM have higher indexes of capillarity, caused primarily by greater capillary number around normally sized fibers. These alterations are consistent with the superior mass-specific aerobic function of these muscles.

Lower respiratory capacity in extraocular muscle mitochondria: evidence for intrinsic differences in mitochondrial composition and function

PURPOSE

The constant activity of the extraocular muscles is supported by abundant mitochondria. These organelles may enhance energy production by increasing the content of respiratory complexes. The authors tested the hypothesis that extraocular muscle mitochondria respire faster than do mitochondria from limb muscles because of the higher content of respiratory complexes.

METHODS

Inner mitochondrial membrane density was determined by stereological analysis of triceps surae (a limb muscle) and extraocular muscles of adult male Sprague-Dawley rats. The authors measured respiration rates of isolated mitochondria using a Clark-type electrode. The activity of respiratory complexes I, II, and IV was determined by spectrophotometry. The content of respiratory complexes was estimated by Western blot.

RESULTS

States 3, 4, and 5 respiration rates in extraocular muscle mitochondria were 40% to 60% lower than in limb muscle mitochondria. Extraocular muscle inner mitochondrial membrane density was similar to that of other skeletal muscles. Activity of complexes I and IV was lower in extraocular muscle mitochondria (approximately 50% the activity in triceps), but their content was approximately 15% to 30% higher. There was no difference in complex II content or activity or complex III content. Finally, complex V was less abundant in extraocular muscle mitochondria.

CONCLUSIONS

The results demonstrate that extraocular muscle mitochondria respire at slower rates than mitochondria from limb muscles, despite similar mitochondrial ultrastructure. Instead, differences were found in the activity (I, IV) and content (I, IV, V) of electron transport chain complexes. The discrepancy between activity and content of some complexes is suggestive of alternative subunit isoform expression in the extraocular muscles compared with limb muscles.

Abnormal transition pore kinetics and cytochrome C release in muscle mitochondria of patients with chronic obstructive pulmonary disease

Skeletal muscle dysfunction (SMD) is frequent in patients with chronic obstructive pulmonary disease (COPD). Mitochondrial abnormalities appear to play a role in the pathogenesis of SMD. The mitochondrion permeability transition pore (MPTP) facilitates the leakage of mitochondrial matrix constituents, such as cytochrome c (cyto-c), and triggers apoptosis, known to occur in skeletal muscle of patients with COPD. Our objective was to study MPTP kinetics and cyto-c release in skeletal muscle mitochondria of patients with COPD. Mitochondria were isolated from the vastus lateralis (VL), external intercostalis (EI), and latissimus dorsi (LD) in 11 patients with COPD (66 +/- 9 yr; FEV(1) 66 +/- 13%) and 15 smokers with normal lung function (64 +/- 6 yr; FEV(1) 95 +/- 11%) who required thoracic surgery for a localized lung neoplasm. MPTP kinetics were determined spectrophotometrically (time to reach V'max, V'max and mitochondrial swelling) and cyto-c release by enzyme-linked immunosorbent assay. MPTP kinetics and cyto-c release were abnormal in patients with COPD in the three muscles studied. In addition, V'max of VL mitochondria was significantly related (P < 0.01) to BMI (r = -0.75 COPD, -0.67 control) and aerobic capacity (r = -0.70 COPD, -0.60 control) for the COPD group. MPTP kinetics and cyto-c release are abnormal in skeletal and respiratory muscles of patients with moderate COPD, suggesting a systemic mechanism(s) occurring early during the course of the disease.

Morphofunctional responses to anaemia in rat skeletal muscle

Adult male Sprague-Dawley rats were randomly assigned to two groups: control and anaemic. Anaemia was induced by periodical blood withdrawal. Extensor digitorum longus and soleus muscles were excised under pentobarbital sodium total anaesthesia and processed for transmission electron microscopy, histochemical and biochemical analyses. Mitochondrial volume was determined by transmission electron microscopy in three different regions of each muscle fibre: pericapillary, sarcolemmal and sarcoplasmatic. Muscle samples sections were also stained with histochemical methods (SDH and m-ATPase) to reveal the oxidative capacity and shortening velocity of each muscle fibre. Determinations of fibre and capillary densities and fibre type composition were made from micrographs of different fixed fields selected in the equatorial region of each rat muscle. Determination of metabolites (ATP, inorganic phosphate, creatine, creatine phosphate and lactate) was done using established enzymatic methods and spectrophotometric detection. Significant differences in mitochondrial volumes were found between pericapillary, sarcolemmal and sarcoplasmic regions when data from animal groups were tested independently. Moreover, it was verified that anaemic rats had significantly lower values than control animals in all the sampled regions of both muscles. These changes were associated with a significantly higher proportion of fast fibres in anaemic rat soleus muscles (slow oxidative group = 63.8%; fast glycolytic group = 8.2%; fast oxidative glycolytic group = 27.4%) than in the controls (slow oxidative group = 79.0%; fast glycolytic group = 3.9%; fast oxidative glycolytic group = 17.1%). No significant changes were detected in the extensor digitorum longus muscle. A significant increase was found in metabolite concentration in both the extensor digitorum longus and soleus muscles of the anaemic animals as compared to the control group. In conclusion, hypoxaemic hypoxia causes a reduction in mitochondrial volumes of pericapillary, sarcolemmal, and sarcoplasmic regions. However, a common proportional pattern of the zonal distribution of mitochondria was maintained within the fibres. A significant increment was found in the concentration of some metabolites and in the proportion of fast fibres in the more oxidative soleus muscle in contrast to the predominantly anaerobic extensor digitorum longus.

Volume density and distribution of mitochondria in harbor seal (Phoca vitulina) skeletal muscle

Recent studies have shown that harbor seals (Phoca vitulina) have an increased skeletal muscle mitochondrial volume density that may be an adaptation for maintaining aerobic metabolism during diving. However, these studies were based on single samples taken from locomotory muscles. In this study, we took multiple samples from a transverse section of the epaxial (primary locomotory) muscles and single samples from the m. pectoralis (secondary locomotory) muscle of five wild harbor seals. Average mitochondrial volume density of the epaxial muscles was 5.6%, which was 36.6% higher than predicted for a terrestrial mammal of similar mass, and most (82.1%) of the mitochondria were interfibrillar, unlike athletic terrestrial mammals. In the epaxial muscles, the total mitochondrial volume density was significantly greater in samples collected from the deep (6.0%) compared with superficial (5.0%) regions. Volume density of mitochondria in the pectoralis muscle was similar (5.2%) to that of the epaxial muscles. Taken together, these adaptations reduce the intracellular distance between mitochondria and oxymyoglobin and increase the mitochondrial diffusion surface area. This, in combination with elevated myoglobin concentrations, potentially increases the rate of oxygen diffusion into mitochondria and prevents diffusion limitation so that aerobic metabolism can be maintained under low oxygen partial pressure that develops during diving.

The influence of endurance and resistance exercise on muscle capillarization in the elderly: a review

AIM

Ageing is one of many factors altering skeletal muscle. Conflicting results from previous studies guided this review to identify what has been found to date regarding microvascular adaptations in skeletal muscle as the result of ageing or inactivity, and endurance or resistance training interventions. Additionally, this review attempts to identify the variety of parameters for determining capillarization and discuss why these might be contributing to the conflicting results.

METHODS

Electronic database searches were conducted for full-length articles using relevant keywords related to ageing, muscle capillarization and exercise in all fields. Cross-sectional, longitudinal, training and review papers were included. Several studies on younger subject adaptation were also included for comparison.

RESULTS

Ageing and inactivity both result in regressive structural and functional changes to skeletal muscle capillarization. The rate and magnitude of decline is still unknown. Endurance training can positively effect structural changes to capillarity. There is conflicting evidence regarding the effects of resistance training. Training intensity may be an important factor. Biopsy sampling, histological staining and several measurement protocols may be providing inaccurate estimations of capillarity.

CONCLUSION

Part of the difficulty in determining the nature of these relationships has been the inconsistency in research conducted regarding age groups and controlling for past and present activity patterns. Further difficulties comparing across studies arise due to the variety of methods and parameters used to sample and analyse muscle tissue. Standardizing methodology will allow future research to yield more consistent results.

The Effect of Fixation and Histological Preparation on Coronary Artery Dimensions

Histometric data are necessary for quantifying the cellular and tissue structure which serves as a basis for the construction of mathematical and integrative models. However, fixation and histological processing of tissues such as dehydration, embedding, sectioning and staining can cause dimensional changes and tissue distortion. The present study was designed to analyze the effects of one widely used fixation and histological preparation protocol for muscle tissue on several morphometric parameters of the coronary arteries. A total of 110 left anterior descending (LAD) artery segments obtained from eight pig hearts were fixed in 6.25% glutaraldehyde and embedded in JB-4 solution. The dimensions of the fixed rings in the loaded and no-load states and histological sections were compared with those of the fresh tissue using Bland-Altman scatter diagrams; i.e., the percent differences between measurements in two different states of various morphometric parameters (inner and outer circumference and wall thickness and area) against their means. We found that vascular elastin cannot be fixed, as seen by the retraction of the vessel dimensions when the loading of the fixed ring was removed. This retraction is time dependent and can lead to significant differences in wall thickness. The differences in dimensions between the histological sections and the fresh tissues in the no-load state were small for the inner and outer diameter (5.6 and 5.2%, respectively) and wall thickness and area (13.4 and 13.1%, respectively). These results are important for establishing an accurate quantitative data base of histological structure that can be related to vascular function or dysfunction.

Lower capillarization, VEGF protein, and VEGF mRNA response to acute exercise in the vastus lateralis muscle of aged vs. young women

In humans, the majority of studies demonstrate an age-associated reduction in the number of capillaries surrounding skeletal muscle fibers; however, recent reports in rats suggest that muscle capillarization is well maintained with advanced age. In sedentary and trained men, aging lowers the number of capillaries surrounding type II, but not type I, skeletal muscle fibers. The fiber type-specific effect of aging on muscle capillarization is unknown in women. Vascular endothelial growth factor (VEGF) is important in the basal maintenance of skeletal muscle capillarization, and lower VEGF expression is associated with increased age in nonskeletal muscle tissue of women. Compared with young women (YW), we hypothesized that aged women (AW) would demonstrate 1) lower muscle capillarization in a fiber type-specific manner and 2) lower VEGF and VEGF receptor expression at rest and in response to acute exercise. Nine sedentary AW (70 + 8 yr) and 11 YW (22 + 3 yr) had vastus lateralis muscle biopsies obtained before and at 4 h after a submaximal exercise bout for the measurement of morphometry and VEGF and VEGF receptor expression. In AW compared with YW, muscle capillary contacts were lower overall (YW: 2.36 + 0.32 capillaries; AW: 2.08 + 0.17 capillaries), specifically in type II (YW: 2.37 + 0.39 capillaries; AW: 1.91 + 0.36 capillaries) but not type I fibers (YW: 2.36 + 0.34 capillaries; AW: 2.26 + 0.24 capillaries). Muscle VEGF protein was 35% lower at rest, and the exercise-induced increase in VEGF mRNA was 50% lower in AW compared with YW. There was no effect of age on VEGF receptor expression. These results provide evidence that, in the vastus lateralis of women, 1) capillarization surrounding type II muscle fibers is lower in AW compared with YW and 2) resting VEGF protein and the VEGF mRNA response to exercise are lower in AW compared with YW.

Biomechanical consequences of scaling

To function over a lifetime of use, materials and structures must be designed to have sufficient factors of safety to avoid failure. Vertebrates are generally built from materials having similar properties. Safety factors are most commonly calculated based on the ratio of a structure's failure stress to its peak operating stress. However, yield stress is a more likely limit, and work of fracture relative to energy absorption is likely the most relevant measure of a structure's safety factor, particularly under impact loading conditions characteristic of locomotion. Yet, it is also the most difficult to obtain. For repeated loading, fatigue damage and eventual failure may be critical to the design of biological structures and will result in lower safety factors. Although area:volume scaling predicts that stresses will increase with size, interspecific comparisons of mammals and birds show that skeletal allometry is modest, with most groups scaling(l∝d0.89) closer to geometric similarity(isometry: l ∝d1.0) than to elastic similarity (l ∝d0.67) or stress similarity(l ∝d0.5). To maintain similar peak bone and muscle stresses, terrestrial mammals change posture when running, with larger mammals becoming more erect. More erect limbs increases their limb muscle mechanical advantage (EMA) or ratio of ground impulse to muscle impulse(r/R=∫G/∫Fm). The increase in limb EMA with body weight (∝W0.25) allows larger mammals to match changes in bone and muscle area (∝W0.72-0.80) to changes in muscle force generating requirements (∝W0.75), keeping bone and muscle stresses fairly constant across a size range 0.04-300 kg. Above this size, extremely large mammals exhibit more pronounced skeletal allometry and reduced locomotor ability. Patterns of ontogenetic scaling during skeletal growth need not follow broader interspecific scaling patterns. Instead,negative allometric growth (becoming more slender) is often observed and may relate to maturation of the skeleton's properties or the need for younger animals to move at faster speeds compared with adults. In contrast to bone and muscle stress patterns, selection for uniform safety factors in tendons does not appear to occur. In addition to providing elastic energy savings, tendons transmit force for control of motion of more distal limb segments. Their role in elastic savings requires that some tendons operate at high stresses (and strains), which compromises their safety factor. Other `low stress' tendons have larger safety factors, indicating that their primary design is for stiffness to reduce the amount of stretch that their muscles must overcome when contracting to control movement.

Voluntary running induces fiber type-specific angiogenesis in mouse skeletal muscle

Adult skeletal muscle undergoes adaptation in response to endurance exercise, including fast-to-slow fiber type transformation and enhanced angiogenesis. The purpose of this study was to determine the temporal and spatial changes in fiber type composition and capillary density in a mouse model of endurance training. Long-term voluntary running (4 wk) in C57BL/6 mice resulted in an approximately twofold increase in capillary density and capillary-to-fiber ratio in plantaris muscle as measured by indirect immunofluorescence with an antibody against the endothelial cell marker CD31 (466 ± 16 capillaries/mm2and 0.95 ± 0.04 capillaries/fiber in sedentary control mice vs. 909 ± 55 capillaries/mm2and 1.70 ± 0.04 capillaries/fiber in trained mice, respectively; P < 0.001). A significant increase in capillary-to-fiber ratio was present at day 7 with increased concentration of vascular endothelial growth factor (VEGF) in the muscle, before a significant increase in percentage of type IIa myofibers, suggesting that exercise-induced angiogenesis occurs first, followed by fiber type transformation. Further analysis with simultaneous staining of endothelial cells and isoforms of myosin heavy chains (MHCs) showed that the increase in capillary contact manifested transiently in type IIb + IId/x fibers at the time ( day 7) of significant increase in total capillary density. These findings suggest that endurance training induces angiogenesis in a subpopulation of type IIb + IId/x fibers before switching to type IIa fibers.

Aerobic characteristics of red kangaroo skeletal muscles: is a high aerobic capacity matched by muscle mitochondrial and capillary morphology as in placental mammals?

Marsupials and placentals together comprise the Theria, the advanced mammals, but they have had long independent evolutionary histories, with the last common ancestor occurring more than 125 million years ago. Although in the past the marsupials were considered to be metabolically `primitive', the red kangaroo Macropus rufus has been reported to have an aerobic capacity (V̇O2max)comparable to that of the most `athletic' of placentals such as dogs. However,kangaroos travel at moderate speeds with lower relative cost than quadrupedal placentals. Given the long independent evolution of the two therian groups,and their unusual locomotor energetics, do kangaroos achieve their high aerobic capacity using the same structural and functional mechanisms used by(athletic) placentals?

Red kangaroo skeletal muscle morphometry matched closely the general aerobic characteristics of placental mammals. The relationship between total mitochondrial volume in skeletal muscle and V̇O2max during exercise was identical to that in quadrupedal placentals, and differed from that in bipedal humans. As for placentals generally, red kangaroo mitochondrial oxygen consumption at V̇O2max was 4.7 ml O2 min–1 ml–1 of mitochondria. Also, the inner mitochondrial membrane densities were 35.8±0.7 m2 ml–1 of mitochondria, which is the same as for placental mammals, and the same pattern of similarity was seen for capillary densities and volumes.

The overall data for kangaroos was equivalent to that seen in athletic placentals such as dogs and pronghorns. Total skeletal muscle mass was high,being around 50% of body mass, and was concentrated around the pelvis and lower back. The majority of the muscles sampled had relatively high mitochondrial volume densities, in the range 8.8–10.6% in the major locomotor muscles. Again, capillary densities and capillary blood volumes followed the pattern seen for mitochondria. Our results indicate that the red kangaroo, despite its locomotion and extreme body form, shows fundamental aerobic/muscular relationships that appear common to both marsupials and placentals. The evolution of such metabolic relationships apparently predates the divergence of the therian groups in the early Cretaceous, and perhaps evolved in the mammal-like reptiles during the Triassic (220 million years ago) before the actual evolution of the mammals.