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Carsten Lundby - One of the best experts on this subject based on the ideXlab platform.
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sexual dimorphism of substrate utilization differences in skeletal muscle Mitochondrial Volume Density and function
Experimental Physiology, 2018Co-Authors: David Montero, Klavs Madsen, Annekristine Meinildlundby, Fredrik Edin, Carsten LundbyAbstract:NEW FINDINGS: What is the central question of this study? Females rely to a greater extent than males on fat oxidation during exercise. Whether any difference in skeletal muscle Mitochondrial phenotype and oxidative capacity contributes to this sexual dimorphism remains incompletely explored. What is the main finding and its importance? Female prioritization of fat during exercise occurs in parallel to augmented Mitochondrial Volume Density and intrinsic fatty acid and lactate oxidation in skeletal muscle fibres compared with males, independently of aerobic exercise capacity. The enlarged metabolic machinery in skeletal muscle of females is associated with lower body size and leg mass. ABSTRACT: Fat oxidation during exercise is greater in females than in males. We sought to determine whether sex differences in substrate metabolism are paralleled by distinct skeletal muscle Mitochondrial Volume Density and oxidative capacity. Whole-body substrate (fat and carbohydrate) utilization during submaximal treadmill running was assessed, and skeletal muscle biopsies were taken to determine Mitochondrial Volume Density and function in healthy young females (n = 12) and males (n = 12) matched by aerobic exercise capacity and exercise performance. Females presented a lower respiratory exchange ratio (0.87 ± 0.04 versus 0.91 ± 0.04, P = 0.023) and whole-body carbohydrate oxidation (27.8 ± 8.3 versus 35.8 ± 6.5 mg kg-1 min-1 , P = 0.027), whereas fat oxidation was higher (8.7 ± 2.8 versus 5.9 ± 2.6 mg kg-1 min-1 , P = 0.034) during submaximal exercise compared with males. In skeletal muscle biopsies, females demonstrated augmented Mitochondrial Volume Density (7.51 ± 1.77 versus 5.90 ± 1.72%, P = 0.035) and oxidative capacity for fatty acid [36.6 ± 12.8 versus 24.5 ± 7.3 pmol O2 s-1 (mg wet weight)-1 , P = 0.009] and lactate [71.1 ± 24.4 versus 53.2 ± 14.6 pmol O2 s-1 (mg wet weight)-1 , P = 0.040]. No sex differences in respiratory exchange ratio, whole-body fat oxidation and skeletal muscle variables were detected when adjusted for anthropometric variables including body mass or leg mass, which were lower in females. In conclusion, female prioritization of fat over carbohydrate oxidation during exercise is underpinned by augmented body size-related Mitochondrial Volume Density, fatty acid and lactate oxidative capacity in skeletal muscle fibres.
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Altered skeletal muscle (Mitochondrial) properties in patients with Mitochondrial DNA single deletion myopathy
Orphanet journal of rare diseases, 2016Co-Authors: Saskia Maria Gehrig, Carsten Lundby, Violeta Mihaylova, Sebastian Frese, Sandro Manuel Mueller, Maria Ligon-auer, Christina M. Spengler, Jens A. Petersen, Hans H. JungAbstract:Mitochondrial myopathy severely affects skeletal muscle structure and function resulting in defective oxidative phosphorylation. However, the major pathomechanisms and therewith effective treatment approaches remain elusive. Therefore, the aim of the present study was to investigate disease-related impairments in skeletal muscle properties in patients with Mitochondrial myopathy. Accordingly, skeletal muscle biopsies were obtained from six patients with moleculargenetically diagnosed Mitochondrial myopathy (one male and five females, 53 ± 9 years) and eight age- and gender-matched healthy controls (two males and six females, 58 ± 14 years) to determine Mitochondrial respiratory capacity of complex I-V, Mitochondrial Volume Density and fiber type distribution. Mitochondrial Volume Density (4.0 ± 0.5 vs. 5.1 ± 0.8 %) as well as respiratory capacity of complex I-V were lower (P
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Adaptations of skeletal muscle mitochondria to exercise training.
Experimental physiology, 2015Co-Authors: Carsten Lundby, Robert A. JacobsAbstract:Mitochondrial Volume Density (Mito(VD)) is composed of two distinct Mitochondrial subpopulations--intermyofibrillar mitochondria (Mito(IMF)) and subsarcolemmal mitochondria (Mito(SS)). With exercise training, Mito(VD) may increase by up to 40% and is, for the most part, related to an increase in Mito(IMF). Exercise-induced adaptations in Mitochondrial function depend on the intensity of training and appear to be explained predominately by an increased expression of Mitochondrial enzymes that facilitate aerobic metabolism. Although Mitochondrial content often increases with training, it seems that Mitochondrial adaptations are not needed to facilitate maximal oxygen uptake, whereas such adaptations are of greater importance for endurance capacity.
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Twenty-eight days of exposure to 3454 m increases Mitochondrial Volume Density in human skeletal muscle
The Journal of physiology, 2015Co-Authors: Robert A. Jacobs, Anne-kristine Lundby, Simone Fenk, Saskia Maria Gehrig, Christoph Siebenmann, Daniela Flück, Niels Kirk, Matthias P. Hilty, Carsten LundbyAbstract:The role of hypoxia on skeletal muscle mitochondria is controversial. Studies superimposing exercise training on hypoxic exposure demonstrate an increase in skeletal muscle Mitochondrial Volume Density (Mito(VD)) over equivalent normoxic training. In contrast, reductions in both skeletal muscle mass and Mito(VD) have been reported following mountaineering expeditions. These observations may, however, be confounded by negative energy balance, which may obscure the results. Accordingly we sought to examine the effects of high altitude hypoxic exposure on Mitochondrial characteristics, with emphasis on Mito(VD), while minimizing changes in energy balance. For this purpose, skeletal muscle biopsies were obtained from nine lowlanders at sea level (Pre) and following 7 and 28 days of exposure to 3454 m. Maximal ergometer power output, whole body weight and composition, leg lean mass and skeletal muscle fibre area all remained unchanged following the altitude exposure. Transmission electron microscopy determined that intermyofibrillar (IMF) Mito(VD) was augmented (P = 0.028) by 11.5 ± 9.2% from Pre (5.05 ± 0.9%) to 28 Days (5.61 ± 0.04%). In contrast, there was no change in subsarcolemmal (SS) Mito(VD). As a result, total Mito(VD) (IMF + SS) was increased (P = 0.031) from 6.20 ± 1.5% at Pre to 6.62 ± 1.4% at 28 Days (7.8 ± 9.3%). At the same time no changes in mass-specific respiratory capacities, Mitochondrial protein or antioxidant content were found. This study demonstrates that skeletal muscle Mito(VD) may increase with 28 days acclimation to 3454 m.
Odile Mathieu-costello - One of the best experts on this subject based on the ideXlab platform.
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Acute and chronic exercise in patients with heart failure with reduced ejection fraction: evidence of structural and functional plasticity and intact angiogenic signalling in skeletal muscle
The Journal of physiology, 2018Co-Authors: Fabio Esposito, Odile Mathieu-costello, Peter D. Wagner, Russell S. RichardsonAbstract:KEY POINTS The vascular endothelial growth factor (VEGF) responses to acute submaximal exercise and training effects in patients with heart failure with reduced ejection fraction (HFrEF) were investigated. Six patients and six healthy matched controls performed knee-extensor exercise (KE) at 50% of maximum work rate before and after (only patients) KE training. Muscle biopsies were taken to assess skeletal muscle structure and the angiogenic response. Before training, during this submaximal KE exercise, patients with HFrEF exhibited higher leg vascular resistance and greater noradrenaline spillover. Skeletal muscle structure and VEGF response were generally not different between groups. Following training, resistance was no longer elevated and noradrenaline spillover was curtailed in the patients. Although, in the trained state, VEGF did not respond to acute exercise, capillarity was augmented. Muscle fibre cross-sectional area and percentage area of type I fibres increased and Mitochondrial Volume Density exceeded that of controls. Structural/functional plasticity and appropriate angiogenic signalling were observed in skeletal muscle of patients with HFrEF. ABSTRACT This study examined the response to acute submaximal exercise and the effect of training in patients with heart failure with reduced ejection fraction (HFrEF). The acute angiogenic response to submaximal exercise in HFrEF after small muscle mass training is debated. The direct Fick method, with vascular pressures, was performed across the leg during knee-extensor exercise (KE) at 50% of maximum work rate (WRmax ) in patients (n = 6) and controls (n = 6) and then after KE training in patients. Muscle biopsies facilitated the assessment of skeletal muscle structure and vascular endothelial growth factor (VEGF) mRNA levels. Prior to training, HFrEF exhibited significantly higher leg vascular resistance (LVR) (≈15%) and significantly greater noradrenaline spillover (≈385%). Apart from Mitochondrial Volume Density, which was significantly lower (≈22%) in HFrEF, initial skeletal muscle structure, including capillarity, was not different between groups. Resting VEGF mRNA levels, and the increase with exercise, was not different between patients and controls. Following training, LVR was no longer elevated and noradrenaline spillover was curtailed. Skeletal muscle capillarity increased with training, as assessed by capillary-to-fibre ratio (≈13%) and number of capillaries around a fibre (NCAF ) (≈19%). VEGF mRNA was now not significantly increased by acute exercise. Muscle fibre cross-sectional area and percentage area of type I fibres both increased significantly with training (≈18% and ≈21%, respectively), while the percentage area of type II fibres fell significantly (≈11%), and Mitochondrial Volume Density now exceeded that of controls. These data reveal structural and functional plasticity and appropriate angiogenic signalling in skeletal muscle of HFrEF patients.
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Resistance to fatigue of individual Xenopus single skeletal muscle fibres is correlated with Mitochondrial Volume Density.
Experimental physiology, 2004Co-Authors: Creed M. Stary, Odile Mathieu-costello, Michael C. HoganAbstract:The purpose of the present study was to compare the individual fatigue characteristics of isolated single skeletal muscle fibres with their Mitochondrial Volume Density (MVD), using direct histological morphometry. Single muscle fibres (n= 14) were microdissected from lumbrical muscle of adult female Xenopus laevis, and force was measured while fibres were stimulated (tetanic contractions of 200 ms trains with 70 Hz stimuli at 9 V) at progressively increasing frequencies (2 min each at 0.25, 0.33, 0.5 and 1 contractions s−1) until fatigue (
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Fiber capillarization and ultrastructure of pigeon pectoralis muscle after cold acclimation
The Journal of Experimental Biology, 1998Co-Authors: Odile Mathieu-costello, P.j. Agey, E. S. Quintana, K. Rousey, M. H. BernsteinAbstract:We investigated the effect of 2 months of exposure to cold conditions (0-5 C) on capillarization and on fiber size, distribution and ultrastructure in the pectoralis muscle of nine pigeons (Columbia livia; mean body mass 700 31 g) and compared the results with measurements from four control birds (mean mass 715 42 g) kept at normal ambient temperature (22-23 C) for the same period. Superficial and deep portions of the muscles, taken from the central area of the right or left pectoralis major muscle, were perfusion-fixed in situ, processed for electron microscopy and analyzed by morphometry. Aerobic fibers represented the vast majority of fibers (93 1 %, mean s.e.m.) in all samples. After cold-acclimation, fiber sectional area was reduced and capillary Density increased proportionally. There was no change in the degree of orientation (anisotropy) of capillaries, capillary-to-fiber ratio or fiber type distribution compared with controls. The Volume Density of mitochondria and lipid droplets in aerobic fibers and capillary diameter increased in response to cold, while the linear relationship between capillary length per fiber Volume and fiber Mitochondrial Volume Density remained unchanged. Capillary surface area, intrafiber lipid deposition and fiber Mitochondrial Volume Density were all correlated in cold-acclimated pigeons. The results indicate a close match between the aerobic capacity of the highly aerobic fibers of the pectoralis muscle and their vascularization to meet the increased energetic demand of shivering.
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Structural and biochemical characteristics of locomotory muscles of emperor penguins, Aptenodytes forsteri.
Respiration physiology, 1997Co-Authors: Paul J Ponganis, Odile Mathieu-costello, L N Starke, M.l. Costello, G L KooymanAbstract:Abstract Structural and biochemical characteristics of the primary muscles used for swimming (pectoralis, PEC and supracoracoideus, SC) were compared to those of leg muscles in emperor penguins ( Aptenodytes forsteri ). The mass of PEC-SC was four times that of the leg musculature, and Mitochondrial Volume Density in PEC and SC (4%) was two-thirds that in sartorius (S) and gastrocnemius. The differences in muscle mass and Mitochondrial Density yielded a 2.2-fold greater total Mitochondrial content in PEC-SC than leg muscles, which appears to account for the 1.8-fold greater whole-body highest oxygen consumption previously recorded in emperor penguins during swimming compared to walking. Calculation of maximal Mitochondrial O 2 consumption in PEC-SC and leg muscle yielded values of 5.8–6.9 ml O 2 ml −1 min −1 , which are similar to those in locomotory muscles of most mammals and birds. A distinct feature of emperor penguin muscle was its myoglobin content, with concentrations in PEC-SC (6.4 g 100 g −1 ) among the highest measured in any species. This resulted in a PEC-SC O 2 store greater than that of the entire blood. In addition, ratios of myoglobin content to Mitochondrial Volume Density and to citrate synthase activity were 4.4 and 2.5 times greater in PEC than in S, indicative of the significant role of myoglobin in the adaptation of muscle to cardiovascular adjustments during diving.
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Relationship Between Fiber Capillarization and Mitochondrial Volume Density in Control and Trained Rat Soleus and Plantaris Muscles
Microcirculation, 1996Co-Authors: David B. Poole, Odile Mathieu-costelloAbstract:Objective: The majority of investigations have demonstrated a strong relationship between muscle capillarity and oxidative capacity. There is, however, evidence that die capacities for O2 supply and utilization can be dissociated. Also, metabolite removal rather man O2 supply may represent the predominant design constraint placed on the capillary bed in some muscles (i.e., fast-twitch glycolytic). Recent evidence suggests mat the principal barrier to O2 diffusion in skeletal muscle resides between the red blood cell and the immediately subjacent sarcolemmal space. Consequendy, if the primary design constraint placed on the capillary bed is to facilitate O2 exchange, we hypothesized that capillary surface per fiber surface should be correlated with die Mitochondrial Volume subserved. Thus, the purpose of this study was to investigate whether one single relationship would be found between the capillary-to-fiber surface ratio and fiber Mitochondrial Volume in slow- (soleus) and fast-twitch (plantaris) muscle...
Michael C. Hogan - One of the best experts on this subject based on the ideXlab platform.
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alterations in skeletal muscle indicators of Mitochondrial structure and biogenesis in patients with type 2 diabetes and heart failure effects of epicatechin rich cocoa
Clinical and Translational Science, 2012Co-Authors: R Pam M D Taub, Michael C. Hogan, Guy A. Perkins, Israel Ramirezsanchez, Theodore P Ciaraldi, Anne N Murphy, Robert K Naviaux, S Alan M D Maisel, Robert R HenryAbstract:(-)-Epicatechin (Epi), a flavanol in cacao stimulates Mitochondrial Volume and cristae Density and protein markers of skeletal muscle (SkM) Mitochondrial biogenesis in mice. Type 2 diabetes mellitus (DM2) and heart failure (HF) are diseases associated with defects in SkM Mitochondrial structure/function. A study was implemented to assess perturbations and to determine the effects of Epi-rich cocoa in SkM Mitochondrial structure and mediators of biogenesis. Five patients with DM2 and stage II/III HF consumed dark chocolate and a beverage containing approximately 100 mg of Epi per day for 3 months. We assessed changes in protein and/or activity levels of oxidative phosphorylation proteins, porin, mitofilin, nNOS, nitric oxide, cGMP, SIRT1, PGC1α, Tfam, and mitochondria Volume and cristae abundance by electron microscopy from SkM. Apparent major losses in normal mitochondria structure were observed before treatment. Epi-rich cocoa increased protein and/or activity of mediators of biogenesis and cristae abundance while not changing Mitochondrial Volume Density. Epi-rich cocoa treatment improves SkM Mitochondrial structure and in an orchestrated manner, increases molecular markers of Mitochondrial biogenesis resulting in enhanced cristae Density. Future controlled studies are warranted using Epi-rich cocoa (or pure Epi) to translate improved Mitochondrial structure into enhanced cardiac and/or SkM muscle function. Clin Trans Sci 2012; Volume 5: 43–47
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resistance to fatigue of individual xenopus single skeletal muscle fibres is correlated with Mitochondrial Volume Density
Experimental Physiology, 2004Co-Authors: Creed M. Stary, Odile Mathieucostello, Michael C. HoganAbstract:The purpose of the present study was to compare the individual fatigue characteristics of isolated single skeletal muscle fibres with their Mitochondrial Volume Density (MVD), using direct histological morphometry. Single muscle fibres (n= 14) were microdissected from lumbrical muscle of adult female Xenopus laevis, and force was measured while fibres were stimulated (tetanic contractions of 200 ms trains with 70 Hz stimuli at 9 V) at progressively increasing frequencies (2 min each at 0.25, 0.33, 0.5 and 1 contractions s−1) until fatigue (<50% initial maximal force) had been established. Following the end of the fatigue protocol, MVD was determined by electron microscopy. Time to fatigue varied among the individual fibres from 3.3 to 10 min. MVD of individual fibres ranged from 3.0 to 9.2% and was positively correlated (r= 0.93) with time to fatigue of corresponding fibres. These results, using direct histological measurements of MVD: (1) support on a single cell basis the notion that oxidative capacity is a major determinant of muscle fatigue resistance; and (2) show that the fatigue profile of a single cell can be used to predict oxidative capacity.
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Resistance to fatigue of individual Xenopus single skeletal muscle fibres is correlated with Mitochondrial Volume Density.
Experimental physiology, 2004Co-Authors: Creed M. Stary, Odile Mathieu-costello, Michael C. HoganAbstract:The purpose of the present study was to compare the individual fatigue characteristics of isolated single skeletal muscle fibres with their Mitochondrial Volume Density (MVD), using direct histological morphometry. Single muscle fibres (n= 14) were microdissected from lumbrical muscle of adult female Xenopus laevis, and force was measured while fibres were stimulated (tetanic contractions of 200 ms trains with 70 Hz stimuli at 9 V) at progressively increasing frequencies (2 min each at 0.25, 0.33, 0.5 and 1 contractions s−1) until fatigue (
Robert A. Jacobs - One of the best experts on this subject based on the ideXlab platform.
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Exercise training increases skeletal muscle Mitochondrial Volume Density by enlargement of existing mitochondria and not de novo biogenesis.
Acta physiologica (Oxford England), 2017Co-Authors: Anne-kristine Lundby, Robert A. Jacobs, Saskia Maria Gehrig, Daniela Flück, Niels Kirk, J. De Leur, M. Hauser, Thomas Christian Bonne, Sune Dandanell, Andres KaechAbstract:Aims 1) determine whether exercise induced increases in muscle Mitochondrial Volume Density (MitoVD) is related to enlargement of existing mitochondria or de novo biogenesis, 2) establish if measures of Mitochondrial-specific enzymatic activities are valid biomarkers for exercise induced increases in MitoVD. Method Skeletal muscle samples were collected from twenty-one healthy males prior to and following 6 weeks of endurance training. Transmission electron microscopy was used for estimation of Mitochondrial densities and profiles. Biochemical assays, western blotting and high resolution respirometry were applied to detect changes in specific Mitochondrial functions. Result MitoVD increased with 55 ± 9% (P < 0.001), whereas the number of Mitochondrial profiles per area of skeletal muscle remained unchanged following training. Citrate synthase activity (CS) increased (44 ± 12%, P < 0.001) however, there were no functional changes in oxidative phosphorylation capacity (OXPHOS, CI+IIP) or cytochrome c oxidase (COX) activity. Correlations were found between MitoVD and CS (P=0.01; r=0.58), OXPHOS, CI+CIIP (P=0.01; R=0.58) and COX (P=0.02; R=0.52) before training, after training a correlation was found between MitoVD and CS activity only (P=0.04; R=0.49). Intrinsic respiratory capacities decreased (P < 0.05) with training when respiration was normalized to MitoVD. This was not the case when normalized to CS activity although the percentage change was comparable. Conclusions MitoVD was increased by inducing Mitochondrial enlargement rather than de novo biogenesis. CS activity may be appropriate to track training induced changes in MitoVD. This article is protected by copyright. All rights reserved.
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Adaptations of skeletal muscle mitochondria to exercise training.
Experimental physiology, 2015Co-Authors: Carsten Lundby, Robert A. JacobsAbstract:Mitochondrial Volume Density (Mito(VD)) is composed of two distinct Mitochondrial subpopulations--intermyofibrillar mitochondria (Mito(IMF)) and subsarcolemmal mitochondria (Mito(SS)). With exercise training, Mito(VD) may increase by up to 40% and is, for the most part, related to an increase in Mito(IMF). Exercise-induced adaptations in Mitochondrial function depend on the intensity of training and appear to be explained predominately by an increased expression of Mitochondrial enzymes that facilitate aerobic metabolism. Although Mitochondrial content often increases with training, it seems that Mitochondrial adaptations are not needed to facilitate maximal oxygen uptake, whereas such adaptations are of greater importance for endurance capacity.
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Twenty-eight days of exposure to 3454 m increases Mitochondrial Volume Density in human skeletal muscle
The Journal of physiology, 2015Co-Authors: Robert A. Jacobs, Anne-kristine Lundby, Simone Fenk, Saskia Maria Gehrig, Christoph Siebenmann, Daniela Flück, Niels Kirk, Matthias P. Hilty, Carsten LundbyAbstract:The role of hypoxia on skeletal muscle mitochondria is controversial. Studies superimposing exercise training on hypoxic exposure demonstrate an increase in skeletal muscle Mitochondrial Volume Density (Mito(VD)) over equivalent normoxic training. In contrast, reductions in both skeletal muscle mass and Mito(VD) have been reported following mountaineering expeditions. These observations may, however, be confounded by negative energy balance, which may obscure the results. Accordingly we sought to examine the effects of high altitude hypoxic exposure on Mitochondrial characteristics, with emphasis on Mito(VD), while minimizing changes in energy balance. For this purpose, skeletal muscle biopsies were obtained from nine lowlanders at sea level (Pre) and following 7 and 28 days of exposure to 3454 m. Maximal ergometer power output, whole body weight and composition, leg lean mass and skeletal muscle fibre area all remained unchanged following the altitude exposure. Transmission electron microscopy determined that intermyofibrillar (IMF) Mito(VD) was augmented (P = 0.028) by 11.5 ± 9.2% from Pre (5.05 ± 0.9%) to 28 Days (5.61 ± 0.04%). In contrast, there was no change in subsarcolemmal (SS) Mito(VD). As a result, total Mito(VD) (IMF + SS) was increased (P = 0.031) from 6.20 ± 1.5% at Pre to 6.62 ± 1.4% at 28 Days (7.8 ± 9.3%). At the same time no changes in mass-specific respiratory capacities, Mitochondrial protein or antioxidant content were found. This study demonstrates that skeletal muscle Mito(VD) may increase with 28 days acclimation to 3454 m.
Peter W. Hochachka - One of the best experts on this subject based on the ideXlab platform.
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Structural basis for oxygen delivery: Muscle capillaries and manifolds in tuna red muscle☆
Comparative biochemistry and physiology. Part A Physiology, 1996Co-Authors: Odile Mathieu-costello, Richard W. Brill, Peter W. HochachkaAbstract:We summarize our morphometric data on fiber vascularization and aerobic capacity in red muscle of tuna (Katsuwonus pelamis), compared to intensely aerobic flight muscles of hummingbird (Selasphorus rufus, BW 3-4 g) and bat (Eptesius fuscus, BW 15-16 g, Pipistrellus hesperus, BW 3-5 g). Three characteristic features of high flux paths for oxygen: (a) small fiber size, (b) dense capillary network and (c) high Mitochondrial Volume Density were found in tuna, but they were not as pronounced as in hummingbird and bat flight muscles. A particular arrangement of capillary manifolds, also seen in flight muscle of birds but not in bats, was found in tuna, forming dense envelopes of capillary branches around portions of muscle fibers. However, all indexes of fiber capillarization were relatively low in tuna red muscle for its Mitochondrial Volume, compared with other intensely aerobic muscles. Capillary length per unit Volume of mitochondria, and capillary surface per Mitochondrial inner (and outer) membrane surface area, were about one half of those in hummingbird or bat flight muscles. Consistent differences exist in the size of the capillary network for the size of the Mitochondrial compartment in highly aerobic red muscle of tuna compared with bird and mammal.
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Capillary–fiber geometrical relationships in tuna red muscle
Canadian Journal of Zoology, 1992Co-Authors: Odile Mathieu-costello, P.j. Agey, Richard B. Logemann, Richard W. Brill, Peter W. HochachkaAbstract:The aim of this study was to examine the size and geometry of the capillary network in tuna red muscle, one of the most aerobic muscles in fish. Deep red muscle of 1.5- to 2-kg skipjack tuna, Katsuwonus pelamis, was perfusion fixed in situ, processed for electron microscopy, and analyzed by morphometry. Fiber cross-sectional area was 560 ± 30 (SE) μm2 in the samples. Capillary length per fiber Volume was 4143 ± 242 (SE) mm−2 and Mitochondrial Volume Density 28.5 ± 1.0 (SE) %. Indexes of capillarity such as average number of capillaries around a fiber, capillary length and surface per fiber Volume, and capillary surface per fiber surface were high for a fish muscle. In fact, the size of the capillary–fiber interface (i.e., capillary to fiber surface) at a given Mitochondrial Volume per fiber was not significantly different in tuna red muscle compared with rat soleus muscle. However, calculation of Mitochondrial respiratory rates in tuna red muscle yielded a substantially lower value (approximately 1/20th) ...