Composition and size of type I, IIA, IID/X, and IIB fibers and citrate synthase activity of rat muscle

Enduring effects of acute prenatal ischemia in rat soleus muscle, and protective role of erythropoietin

Motor disorders are considered to originate mainly from brain lesions. Placental dysfunction or maternal exposure to a persistently hypoxic environment is a major cause of further motor disorders such as cerebral palsy. Our main goal was to determine the long-term effects of mild intrauterine acute ischemic stress on rat soleus myofibres and whether erythropoietin treatment could prevent these changes. Rat embryos were subjected to ischemic stress at embryonic day E17. They then received an intraperitoneal erythropoietin injection at postnatal days 1-5. Soleus muscles were collected at postnatal day 28. Prenatal ischemic stress durably affected muscle structure, as indicated by the greater fiber cross-sectional area (+ 18%) and the greater number of mature vessels (i.e. vessels with mature endothelial cells) per myofibres (+ 43%), and muscle biochemistry, as shown by changes in signaling pathways involved in protein synthesis/degradation balance (-81% for 4EBP1; -58% for AKT) and Hif1α expression levels (+ 95%). Erythropoietin injection in ischemic pups had a weak protective effect: it increased muscle mass (+ 25% with respect to ischemic pups) and partially prevented the increase in muscle degradation pathways and mature vascularization, whereas it exacerbated the decrease in synthesis pathways. Hence, erythropoietin treatment after acute ischemic stress contributes to muscle adaptation to ischemic conditions.

Exercise-driven cellular autophagy: A bridge to systematic wellness

BACKGROUND

Exercise enhances health by supporting homeostasis, bolstering defenses, and aiding disease recovery. It activates autophagy, a conserved cellular process essential for maintaining balance, while dysregulated autophagy contributes to disease progression. Despite extensive research on exercise and autophagy independently, their interplay remains insufficiently understood.

AIM OF REVIEW

This review explores the molecular mechanisms of exercise-induced autophagy in various tissues, focusing on key transduction pathways. It examines how different types of exercise trigger specific autophagic responses, supporting cellular balance and addressing systemic dysfunctions. The review also highlights the signaling pathways involved, their roles in protecting organ function, reducing disease risk, and promoting longevity, offering a clear understanding of the link between exercise and autophagy.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Exercise-induced autophagy is governed by highly coordinated and dynamic pathways integrating direct and indirect mechanical forces and biochemical signals, linking physical activity to cellular and systemic health across multiple organ systems. Its activation is influenced by exercise modality, intensity, duration, and individual biological characteristics, including age, sex, and muscle fiber composition. Aerobic exercises primarily engage AMPK and mTOR pathways, supporting mitochondrial quality and cellular homeostasis. Anaerobic training activates PI3K/Akt signaling, modulating molecules like FOXO3a and Beclin1 to drive muscle autophagy and repair. In pathological contexts, exercise-induced autophagy enhances mitochondrial function, proteostasis, and tissue regeneration, benefiting conditions like sarcopenia, neurodegeneration, myocardial ischemia, metabolic disorders, and cancer. However, excessive exercise may lead to autophagic overactivation, leading to muscle atrophy or pathological cardiac remodeling. This underscores the critical need for balanced exercise regimens to maximize therapeutic efficacy while minimizing risks. Future research should prioritize identifying reliable biomarkers, optimizing exercise protocols, and integrating exercise with pharmacological strategies to enhance therapeutic outcomes.

Skeletal and respiratory muscle blood flow redistribution during submaximal exercise in pulmonary hypertensive rats

Pulmonary hypertension (PH) is a chronic, progressive disease characterized by pulmonary vascular remodelling, dyspnoea and exercise intolerance. Key facets of dyspnoea and exercise intolerance include skeletal and respiratory muscle contractile and metabolic disturbances; however, muscle perfusion during exercise has not been investigated. We hypothesized that diaphragm blood flow (Q ̇ $\dot{Q}$ ) would be increased and locomotory muscleQ ̇ $\dot{Q}$ would be decreased during submaximal treadmill running in PH rats compared to healthy controls. Female Sprague-Dawley rats were injected (i.p.) with monocrotaline to induce PH (n = 16), or a vehicle control (n = 15). Disease progression was monitored via echocardiography. When moderate disease severity was confirmed, maximal oxygen uptake (V ̇ O 2 max ${{\dot{V}}_{{{{\mathrm{O}}}_{{{2}^{{\mathrm{max}}}}}}}}$ ) tests were performed. Rats were given >24 h to recover, and then fluorescent microspheres were infused during treadmill running (20 m/min, 10% grade; ∼40-50% maximal speed attained during theV ̇ O 2 max ${{\dot{V}}_{{{{\mathrm{O}}}_{{{2}^{{\mathrm{max}}}}}}}}$ test) to determine tissueQ ̇ $\dot{Q}$ . In PH rats compared with healthy controls,V ̇ O 2 max ${{\dot{V}}_{{{{\mathrm{O}}}_{{{2}^{{\mathrm{max}}}}}}}}$ was lower (84 (7) vs. 67 (11) ml/min/kg; P < 0.001), exercising diaphragmQ ̇ $\dot{Q}$ was 35% higher and soleusQ ̇ $\dot{Q}$ was 28% lower. DiaphragmQ ̇ $\dot{Q}$ was negatively correlated with soleusQ ̇ $\dot{Q}$ andV ̇ O 2 max ${{\dot{V}}_{{{{\mathrm{O}}}_{{{2}^{{\mathrm{max}}}}}}}}$ in PH rats. Furthermore, there was regionalQ ̇ $\dot{Q}$ redistribution in the diaphragm in PH compared to healthy rats, which may represent or underlie diaphragmatic weakness in PH. These findings suggest the presence of a pathological respiratory muscle blood flow steal phenomenon in PH and that this may contribute to the exercise intolerance reported in patients. KEY POINTS: Pulmonary hypertension (PH) impairs exercise tolerance, which is associated with skeletal and respiratory muscle dysfunction. Increased work of breathing in PH may augment diaphragm blood flow and lower locomotory muscle blood flow during exercise, hindering exercise tolerance. Our findings demonstrate that respiratory muscle blood flow is increased while the locomotory muscle is decreased in PH compared to healthy rats during exercise, suggesting that blood flow is preferentially redistributed to sustain ventilatory demand. Furthermore, blood flow is regionally redistributed within the diaphragm in PH, which may underlie diaphragm dysfunction. Greater respiratory muscle work at a given workload in PH commands higher respiratory muscle blood flow, impairing locomotory muscle oxygen delivery and compromising exercise tolerance, which may be improved by therapeutics which target the diaphragm vasculature.

Comparative Analysis of Muscle Fibers in Selected Muscles of Working and Companion Dog Breeds

The structural and functional characteristics of skeletal muscle fibers play a crucial role in understanding the physical capabilities of dogs, particularly in relation to their breed-specific roles. This study aimed to compare the muscle fiber composition of working and companion dog breeds by analyzing the triceps brachii and biceps femoris muscles, focusing on fiber morphology, myosin heavy chain (MYH) isoform distribution, and nuclei per fiber. A total of 12 dogs, divided equally into working and companion breed groups, were used in this study. Muscle samples were collected post-mortem and prepared for histological analysis using cryosectioning. Immunohistochemical staining was employed to identify the expression of MYH isoforms, including MYH2, MYH4, and MYH7, which correspond to type IIa, IIb, and type I fibers, respectively. The results demonstrated significant differences between the two breed groups. Working dogs exhibited larger muscle fibers, a higher proportion of type IIa (MYH2) and type I (MYH7) fibers, and a greater number of nuclei per fiber, suggesting adaptations for endurance and strength. In contrast, companion dogs showed a higher proportion of type IIb (MYH4) fibers, indicative of their capacity for short bursts of activity rather than sustained exertion. Companion breeds also displayed a higher fiber density but fewer nuclei per fiber, which may contribute to slower muscle regeneration. These findings may provide insights into the muscle adaptations of dogs based on their breed-specific functional demands and highlight the importance of considering these differences in veterinary care and rehabilitation. The study underscores the influence of selective breeding on muscle structure and function in dogs and suggests further research into breed-specific muscle recovery mechanisms.

Genetic reduction of skeletal muscle glycogen synthase 1 abundance reveals that the refeeding-induced reversal of elevated insulin-stimulated glucose uptake after exercise is not attributable to achieving a high muscle glycogen concentration

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle. Postexercise refeeding induces reversal of postexercise (PEX)-enhanced ISGU concomitant with attaining high muscle glycogen in rats. To test the relationship between high glycogen and reversal of PEX-ISGU, we injected one epitrochlearis muscle from each rat with adeno-associated virus (AAV) small hairpin RNA (shRNA) that targets glycogen synthase 1 (GS1) and injected contralateral muscles with AAV-shRNA-Scrambled (Scr). Muscles from PEX and sedentary rats were collected at 3-hour PEX (3hPEX) or 6-hour PEX (6hPEX). Rats were either not refed or refed rat-chow during the recovery period. Isolated muscles were incubated with [3H]-3-O-methylglucose, with or without insulin. The results revealed: (1) GS1 abundance was substantially lower for AAV-shRNA-GS1-treated versus AAV-shRNA-Scr-treated muscles; (2) reduced GS1 abundance in refed-rats induced much lower glycogen in AAV-shRNA-GS1-treated versus AAV-shRNA-Scr-treated muscles at 3hPEX or 6hPEX; (3) PEX-ISGU was elevated in not refed-rats at either 3hPEX or 6hPEX versus sedentary controls, regardless of GS1 abundance; (4) PEX-ISGU was not reversed by 3 h of refeeding, regardless of GS1 abundance; (5) despite substantially lower glycogen in AAV-shRNA-GS1-treated versus AAV-shRNA-Scr-treated muscles, elevated PEX-ISGU was eliminated at 6hPEX in both of the paired muscles of refed-rats; and (6) 3hPEX versus sedentary non-refed rats had greater AMP-activated protein kinase-γ3 activity in both paired muscles, but this exercise effect was eliminated in both paired muscles by 3 h of refeeding. In conclusion, the results provided compelling evidence that the reversal of exercise-enhanced ISGU by refeeding was not attributable to the accumulation of high muscle glycogen concentration.

Continuous Use During Disuse: Mechanisms and Effects of Spontaneous Activity of Unloaded Postural Muscle

In most mammals, postural soleus muscles are involved in the maintenance of the stability of the body in the gravitational field of Earth. It is well established that immediately after a laboratory rat is exposed to conditions of weightlessness (parabolic flight) or simulated microgravity (hindlimb suspension/unloading), a sharp decrease in soleus muscle electrical activity occurs. However, starting from the 3rd day of mechanical unloading, soleus muscle electrical activity begins to increase and reaches baseline levels approximately by the 14th day of hindlimb suspension. This phenomenon, observed in the course of rat hindlimb suspension, was named the "spontaneous electrical activity of postural muscle". The present review discusses spinal mechanisms underlying the development of such spontaneous activity of rat soleus muscle and the effect of this activity on intracellular signaling in rat soleus muscle during mechanical unloading.

Effect of serum 25-hydroxyvitamin D level on quadriceps strength: a systematic review and meta-analysis

BACKGROUND

Vitamin D deficiency has been linked to poor muscle function, cartilage degeneration, and the development of knee osteoarthritis. However, the impact of serum 25-hydroxyvitamin D [25(OH)D] level on quadriceps muscle strength remains inconclusive, largely due to variations in study designs, differences in study populations, and the influence of confounding factors such as co-supplementation with other vitamins. The existing literature presents mixed findings, highlighting the need for a comprehensive evaluation of the available evidence.

PURPOSE

This systematic review and meta-analysis aim to summarise.

STUDY DESIGN

Systematic review; Level of evidence, 4.

METHODS

Searches were conducted using Medline (Ovid), Embase (Ovid), CINAHL (EBSCOhost), and SPORTDiscus (EBSCOhost), which aimed to summarise recent (published after 2000 and before March 1st, 2024) studies reporting the effects of serum 25(OH)D levels on quadriceps strength. Appraisal tool for Cross-Sectional Studies (AXIS) for cross-sectional studies and Quality in Prognosis Studies (QUIPS) for longitudinal studies. Results from the AXIS and QUIPS tools were used for GRADE quality assessment. The review was carried out using PRIMSA guidelines and registered in PROSPERO (ID: CRD42022313240).

RESULTS

Four hundred studies were screened and 28 studies with 5752 participants were included. 28 published studies (24 cross-sectional and 4 longitudinal) were identified. Key results supported the significant positive correlation between serum 25(OH)D levels and isokinetic quadriceps strength at 180°/s in elderly and athletic populations with a correlation coefficient of 0.245 (95%CI: 0.078-0.398, p = 0.004). However, no significant correlation was found with isometric quadriceps strength or isokinetic strength at 60°/s (r = 0.190, p = 0.085). There was only a weak negative correlation with MVC.

CONCLUSION

This review found a statistically significant positive correlation between serum 25(OH)D levels and isokinetic quadriceps strength. This has important clinical implications, especially in the elderly cohort, with higher 25(OH)D levels being associated with a reduced incidence of falls and fragility fractures.

Glucose and glycogen affects Ca2+ transient during fatigue to a greater extent in the least than in the most fatigue resistant mouse FDB fibers

The overall objective was to determine how no extracellular glucose and/or low glycogen content affect fatigue kinetics in mouse flexor digitorum brevis (FDB) single muscle fibers. High glycogen content (Hi GLY), near normal in situ level, was obtained by incubating fibers in culture medium containing glucose and insulin while low glycogen content (Lo GLY), at about 19% of normal in situ level, was achieved by incubating fibers without glucose. Neither Lo GLY nor the absence of extracellular glucose (0GLU) affected tetanic [Ca2+]i prior to fatigue. The number of contracting unfatigued fibers versus stimulus strength relationship of Lo GLY-0GLU fibers was shifted to higher voltages compared to Hi GLY fibers exposed to 5.5 mM glucose (5GLU). The relationship for Lo GLY-0GLU fibers was shifted back toward that of Hi GLY-5GLU fibers when glucose was reintroduced, whereas the removal of glucose from Hi GLY-5GLU fibers had no effect. Fatigue was elicited with one 200 ms long tetanic contraction every s for 3 min. Both Lo GLY and 0GLU increased the rate at which intracellular tetanic concentration ([Ca2+]i) declined and unstimulated [Ca2+]i increased during fatigue in the order of the least fatigue resistant > mid fatigue resistant > the most fatigue resistant fibers.

Influence of acute dietary nitrate supplementation on oxygen delivery/consumption and critical impulse during maximal effort forearm exercise in males: a randomized crossover trial

Beetroot juice supplementation (BRJ) should increase nitric oxide bioavailability under conditions of muscle deoxygenation and acidosis that are a normal consequence of the maximal effort exercise test used to identify forearm critical impulse. We hypothesized BRJ would improve oxygen delivery:demand matching and forearm critical impulse performance. Healthy males (20.8 ± 2.4 years) participated in a randomized crossover trial between October 2017 and May 2018 (Queen's University, Kingston, ON). Participants completed 10 min of rhythmic maximal effort forearm handgrip exercise 2.5 h post placebo (PL) vs. BRJ (9 completed PL/BRJ vs. 4 completed BRJ/PL) within a 2 week period. Data are presented as mean ± SD. There was a main effect of drink (PL > BRJ) for oxygen extraction (P = 0.033, ηp2 = 0.351) and oxygen consumption/force (P = 0.017, ηp2 = 0.417). There was a drink × time interaction (PL > BRJ) for oxygen consumption/force (P = 0.035, ηp2 = 0.216) between 75 and 360 s (1.25-6 min) from exercise onset. BRJ did not influence oxygen delivery (P = 0.953, ηp2 = 0.000), oxygen consumption (P = 0.064, ηp2 = 0.278), metabolites ((lactate) (P = 0.196, ηp2 = 0.135), pH (P = 0.759, ηp2 = 0.008)) or power-duration performance parameters (critical impulse (P = 0.379, d = 0.253), W' (P = 0.733, d = 0.097)). BRJ during all-out handgrip exercise does not influence oxygen delivery or exercise performance. Oxygen cost of contraction with BRJ is reduced as contraction impulse is declining during maximal effort exercise resulting in less oxygen extraction.

Porcine skeletal muscle typing in histochemical and in-situ RT-PCR analysis

Currently, there are plenty of histochemical methods to classify pig muscle fibers, which confused the naming and classification of muscle fibers. This study aims to analyze the difference and correlation of 6 different histochemical methods and select the most suitable method for muscle fiber classification at the molecular and histomological levels by in-situ RT-PCR and enzyme histochemical methods. Muscle fiber samples, including psoas (PM), semitendinosus (SM) and trapezius muscle (TM), were collected from Large Spotted (LS), Lantang (LT) and Landrace (LR) pigs at their market-ages (LS at 150 d, LT at 210 d, and LR at 150 d). 6 kinds of histochemical methods combining actomyosin adenosine triphosphatase (AM-ATPase) with succinate dehydrogenase (SDH) enzyme were conducted to differentiate fiber types. 2 types of fibers (I and II) were differentiated by acid 2-fibre (2-AC) or alkaline 2-fibre classification(2-AL), 3 types of fibers (βR, αR and αW) by 3-AC or 3-AL, and 4 types of fibers (I, IIa, IIx and IIb) by 4-AC, or 4-AL. Results showed that AC and AL muscle-fiber classification were consistent in reflecting the characteristics of muscle fibers(P > 0.05), but the color of each muscle fiber type was just opposite. AC methods may be superior to AL methods because of their clear staining background, the sensitivity to staining condition. But there were breed differences and tissue specificity in the optimal preincubation condition. The optimal acid preincubation condition for classifying muscle fibers was pH4.30 for LT, while pH 4.35 for the LS and LR pigs. Meanwhile the optimal acid preincubation condition was pH4.35 for PM, while pH4.40 for TM or SM. For further selection from 2, 3, 4-AC, in-situ RT-PCR was applied to detect the mRNA distribution of myosin heavy chain I (MyHC-I). By combining in-situ PCR with enzyme histochemistry methods, MyHC-I gene and its product - Type I fibrocytes were directly located in cells at both molecular level and morphological level. Compared with the cross-sectional area (CSA) of different muscle fibers (i.e. I, II, βR, αR, αW, IIa, IIx and IIb) identified by enzyme histochemistry, it was found that the CSAs with stronger mRNA expression signal of MyHC-Ⅰ were closer to those of the Type I muscle fiber measured by 4-AC enzyme histochemistry (P > 0.05). Therefore, 4-AC may be considered as the most proper muscle typing method to study muscle fiber typing as well as meat quality. And the combination of in-situ RT-PCR and histochemistry may help better understand porcine muscle fiber characteristics and meat quality in pigs.

Physiological Adaptations to Progressive Endurance Exercise Training in Adult and Aged Rats: Insights from the Molecular Transducers of Physical Activity Consortium (MoTrPAC)

While regular physical activity is a cornerstone of health, wellness, and vitality, the impact of endurance exercise training on molecular signaling within and across tissues remains to be delineated. The Molecular Transducers of Physical Activity Consortium (MoTrPAC) was established to characterize molecular networks underlying the adaptive response to exercise. Here, we describe the endurance exercise training studies undertaken by the Preclinical Animal Sites Studies component of MoTrPAC, in which we sought to develop and implement a standardized endurance exercise protocol in a large cohort of rats. To this end, Adult (6-mo) and Aged (18-mo) female (n = 151) and male (n = 143) Fischer 344 rats were subjected to progressive treadmill training (5 d/wk, ∼70%-75% VO2max) for 1, 2, 4, or 8 wk; sedentary rats were studied as the control group. A total of 18 solid tissues, as well as blood, plasma, and feces, were collected to establish a publicly accessible biorepository and for extensive omics-based analyses by MoTrPAC. Treadmill training was highly effective, with robust improvements in skeletal muscle citrate synthase activity in as little as 1-2 wk and improvements in maximum run speed and maximal oxygen uptake by 4-8 wk. For body mass and composition, notable age- and sex-dependent responses were observed. This work in mature, treadmill-trained rats represents the most comprehensive and publicly accessible tissue biorepository, to date, and provides an unprecedented resource for studying temporal-, sex-, and age-specific responses to endurance exercise training in a preclinical rat model.

Relationships between endurance exercise training-induced muscle fiber-type shifting and autophagy in slow- and fast-twitch skeletal muscles of mice

PURPOSE

Endurance exercise induces muscle fiber-type shifting and autophagy; however, the potential role of autophagy in muscle fiber-type transformation remains unclear. This study examined the relationship between muscle fiber-type shifting and autophagy in the soleus (SOL) and extensor digitorum longus (EDL) muscles, which are metabolically discrete muscles.

METHODS

Male C57BL/6J mice were randomly assigned to sedentary control (CON) and exercise (EXE) groups. After 1 week of acclimation to treadmill running, the mice in the EXE group ran at 12-15 m/min, 60 min/day, 5 days/week for 6 weeks. All mice were sacrificed 90 min after the last exercise session, and the targeted tissues were rapidly dissected. The right side of the tissues was used for western blot analysis, whereas the left side was subjected to immunohistochemical analysis.

RESULTS

Endurance exercise resulted in muscle fiber-type shifting (from type IIa to type I) and autophagy (an increase in LC3-II) in the SOL muscle. However, muscle fiber-type transformation and autophagy were not correlated in the SOL and EDL muscles. Interestingly, in contrast to the canonical autophagy signaling pathways, our study showed that exercise-induced autophagy concurs with enhanced anabolic (increased p-AKTSer473/AKT and p-mTOR/mTORSer2448 ratios) and suppressed catabolic (reduced p-AMPKThr172/AMPK ratio) states.

CONCLUSION

Our findings demonstrate that chronic endurance exercise-induced muscle fiber-type transformation and autophagy occur in a muscle-specific manner (e.g., SOL). More importantly, our study suggests that endurance training-induced SOL muscle fiber-type transition may underlie metabolic modulations caused by the AMPK and AKT/mTOR signaling pathways rather than autophagy.

Influence of acute dietary nitrate supplementation on oxygen delivery/consumption and limit of tolerance during progressive forearm exercise in men: a randomized crossover trial

Beetroot juice (BRJ) supplementation increases nitric oxide bioavailability with hypoxia and acidosis, characteristics of high-intensity exercise. We investigated whether BRJ improved forearm oxygen delivery:demand matching in an intensity-dependent manner. Healthy men (21 ± 2.5 years) participated in a randomized crossover trial between October 2017 and May 2018 (Queen's University, Kingston, ON, Canada). Participants completed a forearm incremental exercise test to limit of tolerance (IET-LOT) 2.5 h post placebo (PL) versus BRJ (2 completed PL/BRJ vs. 9 completed BRJ/PL) within a 2-week period. Data are presented as mean ± standard deviation. There was a significant main effect of drink (PL < BRJ; P = 0.042, ηp2 = 0.385) and drink × intensity interaction for arteriovenous oxygen difference (PL < BRJ; P = 0.03; ηp2= 0.197; 20%-50% and 90% LOT). BRJ did not influence oxygen delivery (P = 0.893, ηp2 = 0.002), forearm blood flow (P = 0.589, ηp2 = 0.03) (forearm vascular conductance (P = 0.262, ηp2 = 0.124), mean arterial pressure (P = 0.254,ηp2 = 0.128)), oxygen consumption (P = 0.194, ηp2 = 0.179) or LOT (P = 0.432, d = 0.247). In healthy men, BRJ did not improve forearm oxygen delivery (vasodilatory or pressor response) during IET-LOT. Increased arteriovenous oxygen difference at submaximal intensities did not significantly influence oxygen consumption or performance across the entire range of forearm exercise intensities. This study adds to the growing body of evidence that BRJ does not influence small muscle mass blood flow in humans regardless of exercise intensity.

Effects of chronic alcohol intoxication on aerobic exercise-induced adaptations in female mice

Chronic alcohol intoxication decreases muscle strength/function and causes mitochondrial dysfunction. Aerobic exercise training improves mitochondrial oxidative capacity and increases muscle mass and strength. Presently, the impact of chronic alcohol on aerobic exercise-induced adaptations was investigated. Female C57BL/6Hsd mice were randomly assigned to one of four groups: control sedentary (CON SED; n = 26), alcohol sedentary (ETOH SED; n = 27), control exercise (CON EX; n = 28), and alcohol exercise (ETOH EX; n = 25). Exercise mice had running wheel access for 2 h a day, 7 days a week. All mice were fed either control or an alcohol-containing liquid diet. Grip strength testing and EchoMRI were performed before and after the interventions. After 6 wk, hindlimb muscles were collected for molecular analyses. A subset of mice performed a treadmill run to fatigue (RTF), then abstained from alcohol for 2 wk and repeated the RTF. Alcohol decreased lean mass and forelimb grip strength compared with control-fed mice. Alcohol blunted the exercise-induced increase in muscle mass (plantaris and soleus), type IIa fiber percentage in the plantaris, and run time to fatigue. Mitochondrial markers (Citrate synthase activity and Complex I-IV, COXIV and Cytochrome C protein expression) were increased with exercise regardless of ETOH in the gastrocnemius but not tibialis anterior muscle. Two weeks of alcohol abstinence improved RTF time in ETOH EX but not in ETOH SED. These data suggest that alcohol impairs some exercise-induced adaptations in skeletal muscle, but not all were negatively affected, indicating that exercise may be a beneficial behavior even while consuming alcohol.NEW & NOTEWORTHY Alcohol consumption during an aerobic exercise training period prevented training-induced increases in run to fatigue time and grip strength. Cessation of alcohol allowed for recovery of endurance performance within 2 wk. The worsened exercise performance after alcohol was unrelated to impairments in markers of mitochondrial health. Therefore, some adaptations to exercise training are impaired with alcohol use (endurance performance, muscle growth, and strength), while others remain mostly unaffected (mitochondrial health).

Dietary oleic acid intake increases the proportion of type 1 and 2X muscle fibers in mice

Skeletal muscle is one of the largest metabolic tissues in mammals and is composed of four different types of muscle fibers (types 1, 2A, 2X, and 2B); however, type 2B is absent in humans. Given that slow-twitch fibers are superior to fast-twitch fibers in terms of oxidative metabolism and are rich in mitochondria, shift of muscle fiber types in direction towards slower fiber types improves metabolic disorders and endurance capacity. We previously had reported that oleic acid supplementation increases type 1 fiber formation in C2C12 myotubes; however, its function still remains unclear. This study aimed to determine the effect of oleic acid on the muscle fiber types and endurance capacity. An in vivo mouse model was used, and mice were fed a 10% oleic acid diet for 4 weeks. Two different skeletal muscles, slow soleus muscle with the predominance of slow-twitch fibers and fast extensor digitorum longus (EDL) muscle with the predominance of fast-twitch fibers, were used. We found that dietary oleic acid intake improved running endurance and altered fiber type composition of muscles, the proportion of type 1 and 2X fibers increased in the soleus muscle and type 2X increased in the EDL muscle. The fiber type shift in the EDL muscle was accompanied by an increased muscle TAG content. In addition, blood triacylglycerol (TAG) and non-esterified fatty acid levels decreased during exercise. These changes suggested that lipid utilization as an energy substrate was enhanced by oleic acid. Increased proliferator-activated receptor γ coactivator-1β protein levels were observed in the EDL muscle, which potentially enhanced the fiber type transitions towards type 2X and muscle TAG content. In conclusion, dietary oleic acid intake improved running endurance with the changes of muscle fiber type shares in mice. This study elucidated a novel functionality of oleic acid in skeletal muscle fiber types. Further studies are required to elucidate the underlying mechanisms. Our findings have the potential to contribute to the field of health and sports science through nutritional approaches, such as the development of supplements aimed at improving muscle function.

Fiber-type traps: revisiting common misconceptions about skeletal muscle fiber types with application to motor control, biomechanics, physiology, and biology

Skeletal muscle is a highly complex tissue that is studied by scientists from a wide spectrum of disciplines, including motor control, biomechanics, exercise science, physiology, cell biology, genetics, regenerative medicine, orthopedics, and engineering. Although this diversity in perspectives has led to many important discoveries, historically, there has been limited overlap in discussions across fields. This has led to misconceptions and oversimplifications about muscle biology that can create confusion and potentially slow scientific progress across fields. The purpose of this synthesis paper is to bring together research perspectives across multiple muscle fields to identify common assumptions related to muscle fiber type that are points of concern to clarify. These assumptions include 1) classification by myosin isoform and fiber oxidative capacity is equivalent, 2) fiber cross-sectional area (CSA) is a surrogate marker for myosin isoform or oxidative capacity, and 3) muscle force-generating capacity can be inferred from myosin isoform. We address these three fiber-type traps and provide some context for how these misunderstandings can and do impact experimental design, computational modeling, and interpretations of findings, from the perspective of a range of fields. We stress the dangers of generalizing findings about "muscle fiber types" among muscles or across species or sex, and we note the importance for precise use of common terminology across the muscle fields.

Non-destructive Characterization of Skeletal Muscle Extracellular Matrix Morphology by Combining Optical Coherence Tomography (OCT) Imaging with Tissue Clearing

Histology is an essential step to visualize and analyze the microstructure of any biological tissue; however, histological processing is often irreversible, and histological samples are unable to be imaged or tested further. In this work, a novel non-destructive protocol is proposed for morphological analysis of skeletal muscles, combining Optical Coherence Tomography (OCT) imaging with Tissue Clearing. Imaging combining OCT and Propylene Glycol (PG) as a tissue-clearing agent, was performed on rat tail and extensor digitorum longus (EDL) muscle. The results show that the extracellular matrix morphology of skeletal muscles, including muscular fibers and the whole microstructure architecture were clearly identified. PG improved OCT imaging as measured by image quality metric Contrast Per Pixel CPP (increases by 3.9%), Naturalness Image Quality Evaluator NIQE (decreases by 23%), and Volume of Interest VOI size (higher for CPP and lower for NIQE values). The tendon microstructure was observed with less precision, as collagen fibers could not be clearly detected. The reversibility of the optical effects of the PG on the immersed tissue (in a Phosphate-Buffered Saline solution) was studied comparing native and rehydrated OCT image acquisition from a single EDL sample. Optical properties and microstructure visibility (CPP and NIQE) have been recovered to 99% of the native sample values. Moreover, clearing process caused shrinkage of the tissue recovered to 86% of the original width. Future work will aim to employ the proposed experimental protocol to identify the local mechanical properties of biological tissues.

Fiber type and metabolic characteristics of skeletal muscle in 16 breeds of domestic dogs

The domestic dog (Canis lupus familiaris) species comprises hundreds of breeds, each differing in physical characteristics, behavior, strength, and running capability. Very little is known about the skeletal muscle composition and metabolism between the different breeds, which may explain disease susceptibility. Muscle samples from the triceps brachii (TB) and vastus lateralis (VL) were collected post mortem from 35 adult dogs, encompassing 16 breeds of varying ages and sex. Samples were analyzed for fiber type composition, fiber size, oxidative, and glycolytic metabolic capacity (citrate synthase [CS], 3-hydroxyacetyl-coA dehydrogenase [3HAD], creatine kinase [CK], and lactate dehydrogenase [LDH] enzyme activities). There was no significant difference between the TB and VL in any of the measurements. However, there were large intra species variation, with some variables confirming the physical attributes of a specific breed. Collectively, type IIA was the predominant fiber type followed by type I and type IIX. The cross-sectional areas (CSA) of the fibers were all smaller when compared to humans and similar to other wild animals. There was no difference in the CSA between the fiber types and muscle groups. Metabolically, the muscle of the dog displayed high oxidative capacity with high activities for CS and 3HAD. Lower CK and higher LDH activities than humans indicate a lower and higher flux through the high energy phosphate and glycolytic pathways, respectively. The high variability found across the different breeds may be attributed to genetics, function or lifestyle which have largely been driven through human intervention. This data may provide a foundation for future research into the role of these parameters in disease susceptibility, such as insulin resistance and diabetes, across breeds.

Lack of Musashi-2 induces type IIa fiber-dominated muscle atrophy

Skeletal muscle is a highly plastic tissue, adapting its structure and metabolism in response to diverse conditions such as contractile activity, nutrients, and diseases. Finding a novel master regulator of muscle mass and quality will provide new therapeutic targets for the prevention and treatment of muscle weakness. Musashi is an RNA-binding protein that dynamically regulates protein expression; it was originally discovered as a cell fate determination factor in neural cells. Here, we report that Musashi-2 (Msi2) is dominantly expressed in slow-type muscle fibers, fibers characterized by high metabolism and endurance. Msi2 knockout (KO) mice exhibited a decrease in both soleus myofiber size and number compared to control mice. Biochemical and histological analyses revealed that type IIa fibers, which are of the fast type but have high metabolic capacity, were decreased in Msi2 KO mice. The contraction force of isolated soleus muscle was lower in KO mice, and the expression of the metabolic proteins, cytochrome c oxidase and myoglobin, was also decreased in KO muscle. Our data demonstrate the critical role of Msi2 in the maintenance of normal fiber-type composition and metabolism.

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

Many neuromuscular disorders can have a differential impact on a specific myofibre type, forming the central premise of this review. The many different skeletal muscles in mammals contain a spectrum of slow- to fast-twitch myofibres with varying levels of protein isoforms that determine their distinctive contractile, metabolic, and other properties. The variations in functional properties across the range of classic 'slow' to 'fast' myofibres are outlined, combined with exemplars of the predominantly slow-twitch soleus and fast-twitch extensor digitorum longus muscles, species comparisons, and techniques used to study these properties. Other intrinsic and extrinsic differences are discussed in the context of slow and fast myofibres. These include inherent susceptibility to damage, myonecrosis, and regeneration, plus extrinsic nerves, extracellular matrix, and vasculature, examined in the context of growth, ageing, metabolic syndrome, and sexual dimorphism. These many differences emphasise the importance of carefully considering the influence of myofibre-type composition on manifestation of various neuromuscular disorders across the lifespan for both sexes. Equally, understanding the different responses of slow and fast myofibres due to intrinsic and extrinsic factors can provide deep insight into the precise molecular mechanisms that initiate and exacerbate various neuromuscular disorders. This focus on the influence of different myofibre types is of fundamental importance to enhance translation for clinical management and therapies for many skeletal muscle disorders.

Exercise combined with heat treatment improves insulin resistance in diet-induced obese rats

Insulin resistance is a risk factor for various cardiovascular diseases, which seriously threaten human health. Thus, finding a safe, effective and economical strategy to treat insulin resistance is urgently needed. This study aimed to investigate the effects of exercise combined with heat treatment on the insulin sensitivity in skeletal muscle of diet-induced obese (DIO) rats. Obese rats were induced by a 10-week high-fat diet and were randomly divided into normal temperature + control (NC), normal temperature + exercise (NE), heat treatment + control (HC) and heat treatment + exercise (HE) groups for 7 weeks of incremental load endurance exercise and heat treatment (exposure to a high-temperature environment room). At the end of the 7-week intervention, we measured fasting blood glucose, serum fasting insulin, serum leptin, serum adiponectin, protein expression of HSF1/HSP27 and JAK2/STAT3 pathway in soleus (primarily composed of slow-twitch fibres) and extensor digitorum longus (primarily composed of fast-twitch fibres) muscles. The results showed that exercise combined with heat treatment can effectively improve insulin resistance by regulating HSF1/HSP27 and JAK2/STAT3 pathways in the slow-twitch muscle of DIO rats. Importantly, exercise combined with heat treatment is more effective in improving insulin resistance in DIO rats than exercise or heat treatment alone. Low-moderate intensity exercise that stimulates slow-twitch muscle, combined with heat treatment is an effective strategy to treat insulin resistance.

The pathophysiology of rhabdomyolysis in ungulates and rats: towards the development of a rodent model of capture myopathy

Capture myopathy (CM), which is associated with the capture and translocation of wildlife, is a life-threatening condition that causes noteworthy morbidity and mortality in captured animals. Such wildlife deaths have a significant impact on nature conservation efforts and the socio-economic wellbeing of communities reliant on ecotourism. Several strategies are used to minimise the adverse consequences associated with wildlife capture, especially in ungulates, but no successful preventative or curative measures have yet been developed. The primary cause of death in wild animals diagnosed with CM stems from kidney or multiple organ failure as secondary complications to capture-induced rhabdomyolysis. Ergo, the development of accurate and robust model frameworks is vital to improve our understanding of CM. Still, since CM-related complications are borne from biological and behavioural factors that may be unique to wildlife, e.g. skeletal muscle architecture or flighty nature, certain differences between the physiology and stress responses of wildlife and rodents need consideration in such endeavours. Therefore, the purpose of this review is to summarise some of the major etiological and pathological mechanisms of the condition as it is observed in wildlife and what is currently known of CM-like syndromes, i.e. rhabdomyolysis, in laboratory rats. Additionally, we will highlight some key aspects for consideration in the development and application of potential future rodent models.

Stable isotope-labeled carnitine reveals its rapid transport into muscle cells and acetylation during contraction

Carnitine plays multiple roles in skeletal muscle metabolism, including fatty acid transport and buffering of excess acetyl-CoA in the mitochondria. The skeletal muscle cannot synthesize carnitine; therefore, carnitine must be taken up from the blood into the cytoplasm. Carnitine metabolism, its uptake into cells, and the subsequent reactions of carnitine are accelerated by muscle contraction. Isotope tracing enables the marking of target molecules and monitoring of tissue distribution. In this study, stable isotope-labeled carnitine tracing was combined with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) imaging to determine carnitine distribution in mouse skeletal muscle tissues. Deuterium-labeled carnitine (d3-carnitine) was intravenously injected into the mice and diffused to the skeletal muscles for 30 and 60 min. To examine whether muscle contraction changes the distribution of carnitine and its derivatives, unilateral in situ muscle contraction was performed; 60 min muscle contraction showed increased d3-carnitine and its derivative d3-acetylcarnitine in the muscle, indicating that carnitine uptake in cells is promptly converted to acetylcarnitine, consequently, buffering accumulated acetyl-CoA. While the endogenous carnitine was localized in the slow type fibers rather than fast type, the contraction-induced distributions of d3-carnitine and acetylcarnitine were not necessarily associated with muscle fiber type. In conclusion, the combination of isotope tracing and MALDI-MS imaging can reveal carnitine flux during muscle contraction and show the significance of carnitine in skeletal muscles.

Effects of comorbid type II diabetes mellitus and heart failure on rat hindlimb and respiratory muscle blood flow during treadmill exercise

In rats with type II diabetes mellitus (T2DM) compared with nondiabetic healthy controls, muscle blood flow (Q̇m) to primarily glycolytic hindlimb muscles and the diaphragm muscle are elevated during submaximal treadmill running consequent to lower skeletal muscle mass, a finding that held even when muscle mass was normalized to body mass. In rats with heart failure with reduced ejection fraction (HF-rEF) compared with healthy controls, hindlimb Q̇m was lower, whereas diaphragm Q̇m is elevated during submaximal treadmill running. Importantly, T2DM is the most common comorbidity present in patients with HF-rEF, but the effect of concurrent T2DM and HF-rEF on limb and respiratory Q̇m during exercise is unknown. We hypothesized that during treadmill running (20 m·min-1; 10% incline), hindlimb and diaphragm Q̇m would be higher in T2DM Goto-Kakizaki rats with HF-rEF (i.e., HF-rEF + T2DM) compared with nondiabetic Wistar rats with HF-rEF. Ejection fractions were not different between groups (HF-rEF: 30 ± 5; HF-rEF + T2DM: 28 ± 8%; P = 0.617), whereas blood glucose was higher in HF-rEF + T2DM (209 ± 150 mg/dL) compared with HF-rEF rats (113 ± 28 mg/dL; P = 0.040). Hindlimb muscle mass normalized to body mass was lower in rats with HF-rEF + T2DM (36.3 ± 1.6 mg/g) than in nondiabetic HF-rEF counterparts (40.3 ± 2.7 mg/g; P < 0.001). During exercise, Q̇m was elevated in rats with HF-rEF + T2DM compared with nondiabetic counterparts to the hindlimb (HF-rEF: 100 ± 28; HF-rEF + T2DM: 139 ± 23 mL·min-1·100 g-1; P < 0.001) and diaphragm (HF-rEF: 177 ± 66; HF-rEF + T2DM: 215 ± 93 mL·min-1·100g-1; P = 0.035). These data suggest that the pathophysiological consequences of T2DM on hindlimb and diaphragm Q̇m during treadmill running in the GK rat persist even in the presence of HF-rEF.NEW & NOTEWORTHY Herein, we demonstrate that rats comorbid with heart failure with reduced ejection fraction (HF-rEF) and type II diabetes mellitus (T2DM) have a higher hindlimb and respiratory muscle blood flow during submaximal treadmill running (20 m·min-1; 10% incline) compared with nondiabetic HF-rEF counterparts. These data may carry important clinical implications for roughly half of all patients with HF-rEF who present with T2DM.

Human and African ape myosin heavy chain content and the evolution of hominin skeletal muscle

Humans are unique among terrestrial mammals in our manner of walking and running, reflecting 7 to 8 Ma of musculoskeletal evolution since diverging with the genus Pan. One component of this is a shift in our skeletal muscle biology towards a predominance of myosin heavy chain (MyHC) I isoforms (i.e. slow fibers) across our pelvis and lower limbs, which distinguishes us from chimpanzees. Here, new MyHC data from 35 pelvis and hind limb muscles of a Western gorilla (Gorilla gorilla) are presented. These data are combined with a similar chimpanzee dataset to assess the MyHC I content of humans in comparison to African apes (chimpanzees and gorillas) and other terrestrial mammals. The responsiveness of human skeletal muscle to behavioral interventions is also compared to the human-African ape differential. Humans are distinct from African apes and among a small group of terrestrial mammals whose pelvis and hind/lower limb muscle is slow fiber dominant, on average. Behavioral interventions, including immobilization, bed rest, spaceflight and exercise, can induce modest decreases and increases in human MyHC I content (i.e. -9.3% to 2.3%, n = 2033 subjects), but these shifts are much smaller than the mean human-African ape differential (i.e. 31%). Taken together, these results indicate muscle fiber content is likely an evolvable trait under selection in the hominin lineage. As such, we highlight potential targets of selection in the genome (e.g. regions that regulate MyHC content) that may play an important role in hominin skeletal muscle evolution.

Muscle fiber type and metabolic profiles of four muscles from the African black ostrich

Muscle fiber type, fiber cross-sectional area (CSA), enzyme activities (citrate synthase (CS), 3-hydroxyacetyl Co A dehydrogenase (3HAD), lactate dehydrogenase (LDH) and phosphofructokinase (PFK)) and glycogen content were analyzed in the M. iliotibialis cranialis (ITC), M. iliotibialis lateralis, M. gastrocnemius (G) and M. fibularis longus (FL) muscles from 24 ostriches. Type I and II fiber proportions were similar across the 4 muscles, but the ITC had overall the smallest fibers. CS activity was the highest in the ITC, but similar between the remainder of the muscles. 3HAD activities were very low in all muscles, ranging between 1.9 and 2.7 μmol/min/g protein, indicating poor β-oxidation. The ITC also had the lowest PFK activity. Glycogen content averaged ∼85 mmol/kg dry weight across the muscles with large intramuscular variations. The 4 ostrich muscles present with low fat oxidation capacity and low glycogen content, which could have significant implications on meat quality attributes.

Force potentiation during eccentric contractions in rat skeletal muscle

Postactivation potentiation refers to an acute enhancement of contractile properties following muscle activity. Previously, the effects of prior muscle activation on eccentric force at tetanic activation frequencies have only been sparsely reported. This paper aimed to study acute activity-induced effects on eccentric force of slow and fast-twitch muscles and characterize them in relation to postactivation potentiation. We elicited eccentric contractions in isolated rat extensor digitorum longus and soleus muscles by actively lengthening muscles at a constant velocity. We assessed contractile properties by measuring force over shortly interspaced, identical eccentric, and isometric contractions. We then analyzed stretch force, isometric peak force, rate of force development, and relaxation times. Finally, we compared the time courses for the development and cessation of changes in stretch force to known features of postactivation potentiation. In extensor digitorum longus, muscles stretch force consistently increased in a contraction-to-contraction manner by up to 49% [95% confidence interval (CI): 35-64%] whereas isometric peak force simultaneously showed minor declines (8%, 95% CI: 5-10%). The development and cessation of eccentric force potentiation coincided with the development of twitch potentiation and increases in rate of force development. In soleus muscles we found no consistent eccentric potentiation. Characterization of the increase in eccentric force revealed that force only increased in the very beginning of an active stretch. Eccentric force at tetanic activation frequencies potentiates substantially in extensor digitorum longus muscles over consecutive contractions with a time course coinciding with postactivation potentiation. Such eccentric potentiation may be important in sport performance.NEW & NOTEWORTHY Force during eccentric contractions can increase to a magnitude that may have profound consequences for our understanding of skeletal muscle locomotion. This increase in eccentric force occurs over consecutive, shortly interspaced, tetanic contractions in rat extensor digitorum longus muscles-not in rat soleus muscles-and coincides with well-known traits of postactivation potentiation. Eccentric force potentiation may significantly enhance muscle performance in activities involving stretch-shortening cycles.

Greater Phosphorylation of AMPK and Multiple AMPK Substrates in the Skeletal Muscle of 24-Month-Old Calorie Restricted Compared to Ad-Libitum Fed Male Rats

AMP-activated protein kinase (AMPK), a highly conserved, heterotrimeric serine/threonine kinase with critical sensory and regulatory functions, is proposed to induce antiaging actions of caloric restriction (CR). Although earlier studies assessed CR's effects on AMPK in rodent skeletal muscle, the scope of these studies was narrow with a limited focus on older animals. This study's purpose was to fill important knowledge gaps related to CR's influence on AMPK in skeletal muscle of older animals. Therefore, using epitrochlearis muscles from 24-month-old ad-libitum fed (AL) and CR (consuming 65% of AL intake for 8 weeks), male Fischer-344 × Brown Norway F1 rats, we determined: (a) AMPK Thr172 phosphorylation (a key regulatory site) by immunoblot; (b) AMPKα1 and AMPKα2 activity (representing the 2 catalytic α-subunits of AMPK), and AMPKγ3 activity (representing AMPK complexes that include the skeletal muscle-selective regulatory γ3 subunit) using enzymatic assays; (c) phosphorylation of multiple protein substrates that are linked to CR-related effects (acetyl-CoA carboxylase [ACC], that regulates lipid oxidation; Beclin-1 and ULK1 that are autophagy regulatory proteins; Raptor, mTORC1 complex protein that regulates autophagy; TBC1D1 and TBC1D4 that regulate glucose uptake) by immunoblot; and (d) ATP and AMP concentrations (key AMPK regulators) by mass spectrometry. The results revealed significant CR-associated increases in the phosphorylation of AMPKThr172 and 4 AMPK substrates (ACC, Beclin-1, TBC1D1, and TBC1D4), without significant diet-related differences in ATP or AMP concentration or AMPKα1-, AMPKα2-, or AMPKγ3-associated activity. The enhanced phosphorylation of multiple AMPK substrates provides novel mechanistic insights linking AMPK to functionally important consequences of CR.

Effects of Endurance Training on Metabolic Enzyme Activity and Transporter Proteins in Skeletal Muscle of Ovariectomized Mice

PURPOSE

Estrogen deficiency or insufficiency can occur under several conditions, leading to negative health outcomes. To establish an effective countermeasure against estrogen loss, we investigated the effects of endurance training on ovariectomy (OVX)-induced metabolic disturbances.

METHODS

Female Institute of Cancer Research mice underwent OVX or sham operations. On day 7 of recovery, the mice were randomized to remain either sedentary or undergo 5 wk of treadmill running (15-20 m·min -1 , 60 min, 5 d·wk -1 ). During week 5 of the training, all animals performed a treadmill running test (15 m·min -1 , 60 min).

RESULTS

After the experimental period, OVX resulted in greater gains in body mass, fat mass, and triglyceride content in the gastrocnemius muscle. OVX enhanced phosphofructokinase activity in the plantaris muscle and decreased lactate dehydrogenase activity in the plantaris and soleus muscles. OVX decreased the protein content of NDUFB8, a mitochondrial respiratory chain subunit, but did not decrease other mitochondrial proteins or enzyme activities. Endurance training significantly enhanced mitochondrial enzyme activity and protein content in the skeletal muscles. Although OVX increased the respiratory exchange ratio during the treadmill running test, and postexercise blood lactate levels, endurance training normalized these parameters.

CONCLUSIONS

The present findings suggest that endurance training is a viable strategy to counteract the negative metabolic consequences in hypoestrogenism.

Cellular and Subcellular Characteristics of Neuromuscular Junctions in Muscles with Disparate Duty Cycles and Myofiber Profiles

The neuromuscular system accounts for a large portion (~40%) of whole body mass while enabling body movement, including physical work and exercise. At the core of this system is the neuromuscular junction (NMJ) which is the vital synapse transducing electrical impulses from the motor neurons to their post-synaptic myofibers. Recent findings suggest that subcellular features (active zones) of the NMJ are distinctly sensitive to changes in activity relative to cellular features (nerve terminal branches, vesicles, receptors) of the NMJ. In the present investigation, muscles with different recruitment patterns, functions, and myofiber type profiles (soleus, plantaris, extensor digitorum longus [EDL]) were studied to quantify both cellular and subcellular NMJ characteristics along with myofiber type profiles. Results indicated that, in general, dimensions of subcellular components of NMJs mirrored cellular NMJ features when examining inter-muscle NMJ architecture. Typically, it was noted that the NMJs of the soleus, with its most pronounced recruitment pattern, were larger (p < 0.05) than NMJs of less recruited muscles. Moreover, it was revealed that myofiber size did not dictate NMJ size as soleus muscles displayed the smallest fibers (p < 0.05) while the plantaris muscles exhibited the largest fibers. In total, these data show that activity determines the size of NMJs and that generally, size dimensions of cellular and subcellular components of the NMJ are matched, and that the size of NMJs and their underlying myofibers are uncoupled.

Muscle plasticity is influenced by renal function and caloric intake through the FGF23-vitamin D axis

Skeletal muscle, the main metabolic engine in the body of vertebrates, is endowed with great plasticity. The association between skeletal muscle plasticity and two highly prevalent health problems: renal dysfunction and obesity, which share etiologic links as well as many comorbidities, is a subject of great relevance. It is important to know how these alterations impact on the structure and function of skeletal muscle because the changes in muscle phenotype have a major influence on the quality of life of the patients. This literature review aims to discuss the influence of a nontraditional axis involving kidney, bone, and muscle on skeletal muscle plasticity. In this axis, the kidneys play a role as the main site for vitamin D activation. Renal disease leads to a direct decrease in 1,25(OH)₂-vitamin D, secondary to reduction in renal functional mass, and has an indirect effect, through phosphate retention, that contributes to stimulate fibroblast growth factor 23 (FGF23) secretion by bone cells. FGF23 downregulates the renal synthesis of 1,25(OH)₂-vitamin D and upregulates its metabolism. Skeletal production of FGF23 is also regulated by caloric intake: it is increased in obesity and decreased by caloric restriction, and these changes impact on 1,25(OH)₂-vitamin D concentrations, which are decreased in obesity and increased after caloric restriction. Thus, both phosphate retention, that develops secondary to renal failure, and caloric intake influence 1,25(OH)₂-vitamin D that in turn plays a key role in muscle anabolism.

Skeletal muscle structure, physiology, and function

Contractions of skeletal muscles provide the stability and power for all body movements. Consequently, any impairment in skeletal muscle function results in some degree of instability or immobility. Factors that influence skeletal muscle structure and function are therefore of great interest scientifically and clinically. Injury, neuromuscular disease, and old age are among the factors that commonly contribute to impairments in skeletal muscle function. The goal of this chapter is to summarize the fundamentals of skeletal muscle structure and function to provide foundational knowledge for this Handbook volume. We examine the molecular interactions that provide the basis for the generation of force and movement, discuss mechanisms of the regulation of contraction at the level of myofibers, and introduce concepts of the activation and control of muscle function in vivo. Where appropriate, the chapter updates the emerging science that will increase understanding of muscle function.

TrkB signaling is correlated with muscular fatigue resistance and less vulnerability to neurodegeneration

At the neuromuscular junction (NMJ), motor neurons and myocytes maintain a bidirectional communication that guarantees adequate functionality. Thus, motor neurons' firing pattern, which is influenced by retrograde muscle-derived neurotrophic factors, modulates myocyte contractibility. Myocytes can be fast-twitch fibers and become easily fatigued or slow-twitch fibers and resistant to fatigue. Extraocular muscles (EOM) show mixed properties that guarantee fast contraction speed and resistance to fatigue and the degeneration caused by Amyotrophic lateral sclerosis (ALS) disease. The TrkB signaling is an activity-dependent pathway implicated in the NMJ well-functioning. Therefore, it could mediate the differences between fast and slow myocytes' resistance to fatigue. The present study elucidates a specific protein expression profile concerning the TrkB signaling that correlates with higher resistance to fatigue and better neuroprotective capacity through time. The results unveil that Extra-ocular muscles (EOM) express lower levels of NT-4 that extend TrkB signaling, differential PKC expression, and a higher abundance of phosphorylated synaptic proteins that correlate with continuous neurotransmission requirements. Furthermore, common molecular features between EOM and slow soleus muscles including higher neurotrophic consumption and classic and novel PKC isoforms balance correlate with better preservation of these two muscles in ALS. Altogether, higher resistance of Soleus and EOM to fatigue and ALS seems to be associated with specific protein levels concerning the TrkB neurotrophic signaling.

Intensive Care Unit Acquired Weakness Is Associated with Rapid Changes to Skeletal Muscle Proteostasis

Intensive care unit (ICU)-acquired weakness is a frequent consequence of critical illness that impacts both the limb and respiratory muscles. The cause of ICU-acquired weakness is multifactorial, but both prolonged limb muscle inactivity and mechanical ventilation are risk factors for muscle wasting, which predisposes ICU patients to both short-term complications and long-term disabilities resulting from muscle weakness. Unfortunately, the current research does not provide a detailed understanding of the cellular etiology of ICU-acquired weakness, and no standard treatment exists. Therefore, improving knowledge of the mechanisms promoting muscle atrophy in critically ill patients is essential to developing therapeutic strategies to protect against ICU-induced skeletal muscle wasting. To advance our understanding of the mechanism(s) responsible for ICU-acquired weakness, we tested the hypothesis that ICU-induced muscle inactivity promotes a rapid decrease in anabolic signaling/protein synthesis and accelerates proteolysis in both limb and respiratory muscles. To investigate ICU-induced changes in skeletal muscle proteostasis, adult Sprague Dawley rats were anesthetized and mechanically ventilated for 12 h to simulate ICU care. Measurements of anabolic signaling, protein synthesis, and proteolytic activity in the limb muscles (plantaris and soleus) and respiratory muscles (parasternal and intercostal) revealed ICU-induced reductions in both anabolic signaling (i.e., AKT/mTOR pathway) and muscle protein synthesis. Moreover, simulated ICU care resulted in increased biomarkers of accelerated proteolysis in both limb and respiratory muscles. These novel findings reveal that disturbances in limb and respiratory muscle proteostasis occur rapidly during ICU-induced muscle inactivity, irrespective of the muscle function or muscle fiber type.

Ultrasonographic assessment of skeletal muscles after experimentally induced neurogenic inflammation (facet injury) in rats

This study set out to examine ultrasonographic attributes of non-neurosegmentally (pectoral-forelimb) and neurosegmentally linked (hindlimb) myotomes in an experimental model that leads to neurogenic inflammation in segmentally linked myotomes, and to evaluate quantitative correlations among ultrasonographic attributes of the muscles, relative content of various inflammatory mediators, and nociceptive thresholds (hot and mechanical) in rats. Twelve male Wistar Kyoto rats were randomly divided into two equinumerous groups: surgery group, in which the left lumbar (L4-L6) facet joints were compressed for 3 min with modified Kelly forceps under general anesthesia, and sham-operated rats. All ultrasonograms were obtained with the Vevo 2100 Visual Sonic scanner connected to a 24-MHz transducer at four different time points: pre-surgery and 7, 14, and 21 days after surgical procedures. Digital ultrasonographic images of quadriceps femoris, hamstring, and pectoral-brachial muscle groups were analyzed using a polygonal meter region of interest placed on the largest cross-sectional area of the muscles displayed in Image ProPlus^® analytical software to compute numerical pixel values and pixel heterogeneity (standard deviation of mean pixel values). On day 21, pain behavior tests (hot plate and von Frey) were performed and then all animals were euthanized. Protein expression of inflammatory mediators in biceps brachii and rectus femoris muscles was measured by Western blot. The most prominent differences in muscle echotextural attributes between the two subsets of rats occurred 14 days post-surgery in pectoral-brachial and quadriceps femoris muscles. The expression of calcitonin-gene-related peptide was directly related to both echotextural variables only in biceps brachii (pixel intensity: r = 0.65, P = 0.02; and heterogeneity: r = 0.66, P = 0.02, respectively). Our findings have revealed the occurrence of echotextural changes in skeletal muscles of rats during myositis; however, the accumulation of inflammatory mediators and the outcomes of sensory tests did not relate to the changes in first-order echotextural characteristics of affected hindlimb muscles.

Voluntary wheel exercise training affects locomotor muscle, but not the diaphragm in the rat

Introduction: Functional tests and training regimens intensity-controlled by an individual are used in sport practice, clinical rehabilitation, and space medicine. The model of voluntary wheel running in rats can be used to explore molecular mechanisms of such training regimens in humans. Respiratory and locomotor muscles demonstrate diverse adaptations to treadmill exercise, but the effects of voluntary exercise training on these muscle types have not been compared yet. Therefore, this work aimed at the effects of voluntary ET on rat triceps brachii and diaphragm muscles with special attention to reactive oxygen species, which regulate muscle plasticity during exercise. Methods: Male Wistar rats were distributed into exercise trained (ET) and sedentary (Sed) groups. ET group had free access to running wheels, running activity was continuously recorded and analyzed using the original hardware/software complex. After 8 weeks, muscle protein contents were studied using Western blotting. Results: ET rats had increased heart ventricular weights but decreased visceral/epididymal fat weights and blood triglyceride level compared to Sed. The training did not change corticosterone, testosterone, and thyroid hormone levels, but decreased TBARS content in the blood. ET rats demonstrated higher contents of OXPHOS complexes in the triceps brachii muscle, but not in the diaphragm. The content of SOD2 increased, and the contents of NOX2 and SOD3 decreased in the triceps brachii muscle of ET rats, while there were no such changes in the diaphragm. Conclusion: Voluntary wheel running in rats is intensive enough to govern specific adaptations of muscle fibers in locomotor, but not respiratory muscle.

Application of FTIR Spectroscopy to Detect Changes in Skeletal Muscle Composition Due to Obesity with Insulin Resistance and STZ-Induced Diabetes

Age, obesity, and diabetes mellitus are pathophysiologically interconnected factors that significantly contribute to the global burden of non-communicable diseases. These metabolic conditions are associated with impaired insulin function, which disrupts the metabolism of carbohydrates, lipids, and proteins and can lead to structural and functional changes in skeletal muscle. Therefore, the alterations in the macromolecular composition of skeletal muscle may provide an indication of the underlying mechanisms of insulin-related disorders. The aim of this study was to investigate the potential of Fourier transform infrared (FTIR) spectroscopy to reveal the changes in macromolecular composition in weight-bearing and non-weight-bearing muscles of old, obese, insulin-resistant, and young streptozotocin (STZ)-induced diabetic mice. The efficiency of FTIR spectroscopy was evaluated by comparison with the results of gold-standard histochemical techniques. The differences in biomolecular phenotypes and the alterations in muscle composition in relation to their functional properties observed from FTIR spectra suggest that FTIR spectroscopy can detect most of the changes observed in muscle tissue by histochemical analyses and more. Therefore, it could be used as an effective alternative because it allows for the complete characterization of macromolecular composition in a single, relatively simple experiment, avoiding some obvious drawbacks of histochemical methods.

Exercise-Induced Improvement in Insulin-Stimulated Glucose Uptake by Rat Skeletal Muscle Is Absent in Male AS160-Knockout Rats, Partially Restored by Muscle Expression of Phosphomutated AS160, and Fully Restored by Muscle Expression of Wild-Type AS160

One exercise session can elevate insulin-stimulated glucose uptake (ISGU) in skeletal muscle, but the mechanisms remain elusive. Circumstantial evidence suggests a role for Akt substrate of 160 kDa (AS160 or TBC1D4). We used genetic approaches to rigorously test this idea. The initial experiment evaluated the role of AS160 in postexercise increase in ISGU using muscles from male wild-type (WT) and AS160-knockout (KO) rats. The next experiment used AS160-KO rats with an adeno-associated virus (AAV) approach to determine if rescuing muscle AS160 deficiency could restore the ability of exercise to improve ISGU. The third experiment tested if eliminating the muscle GLUT4 deficit in AS160-KO rats via AAV-delivered GLUT4 would enable postexercise enhancement of ISGU. The final experiment used AS160-KO rats and AAV delivery of AS160 mutated to prevent phosphorylation of Ser588, Thr642, and Ser704 to evaluate their role in postexercise ISGU. We discovered the following: 1) AS160 expression was essential for postexercise increase in ISGU; 2) rescuing muscle AS160 expression of AS160-KO rats restored postexercise enhancement of ISGU; 3) restoring GLUT4 expression in AS160-KO muscle did not rescue the postexercise increase in ISGU; and 4) although AS160 phosphorylation on three key sites was not required for postexercise elevation in ISGU, it was essential for the full exercise effect.

Capillary hemodynamics and contracting skeletal muscle oxygen pressures in male rats with heart failure: Impact of soluble guanylyl cyclase activator

In heart failure with reduced ejection fraction (HFrEF), nitric oxide-soluble guanylyl cyclase (sGC) pathway dysfunction impairs skeletal muscle arteriolar vasodilation and thus capillary hemodynamics, contributing to impaired oxygen uptake (V̇O2) kinetics. Targeting this pathway with sGC activators offers a new treatment approach to HFrEF. We tested the hypotheses that sGC activator administration would increase the O2 delivery (Q̇O2)-to-V̇O2 ratio in the skeletal muscle interstitial space (PO2is) of HFrEF rats during twitch contractions due, in part, to increases in red blood cell (RBC) flux (fRBC), velocity (VRBC), and capillary hematocrit (Hctcap). HFrEF was induced in male Sprague-Dawley rats via myocardial infarction. After 3 weeks, rats were treated with 0.3 mg/kg of the sGC activator BAY 60-2770 (HFrEF + BAY; n = 11) or solvent (HFrEF; n = 9) via gavage b.i.d for 5 days prior to phosphorescence quenching (PO2is, in contracting muscle) and intravital microscopy (resting) measurements in the spinotrapezius muscle. Intravital microscopy revealed higher fRBC (70 ± 9 vs 25 ± 8 RBC/s), VRBC (490 ± 43 vs 226 ± 35 μm/s), Hctcap (16 ± 1 vs 10 ± 1%) and a greater number of capillaries supporting flow (91 ± 3 vs 82 ± 3%) in HFrEF + BAY vs HFrEF (all P < 0.05). Additionally, PO2is was especially higher during 12-34s of contractions in HFrEF + BAY vs HFrEF (P < 0.05). Our findings suggest that sGC activators improved resting Q̇O2 via increased fRBC, VRBC, and Hctcap allowing for better Q̇O2-to-V̇O2 matching during the rest-contraction transient, supporting sGC activators as a potential therapeutic to target skeletal muscle vasomotor dysfunction in HFrEF.

Effects of pulmonary hypertension on microcirculatory hemodynamics in rat skeletal muscle

Pulmonary hypertension (PH) has previously been characterized as a disease of the pulmonary vasculature that subsequently results in myocardial dysfunction. Heart failure compromises skeletal muscle microvascular function, which contributes to exercise intolerance. Therefore, we tested the hypothesis that such changes might be present in PH. Thus, we investigated skeletal muscle oxygen (O₂) transport in the rat model of PH to determine if O₂ delivery (Q̇O₂) is impaired at the level of the microcirculation as evidenced via reduced red blood cell (RBC) flux, velocity, hematocrit, and percentage of capillaries flowing in quiescent muscle. Adult male Sprague-Dawley rats were randomized into healthy (n = 9) and PH groups (n = 9). Progressive PH was induced via a one-time intraperitoneal injection of monocrotaline (MCT; 50 mg/kg) and rats were monitored weekly via echocardiography. Intravital microscopy in the spinotrapezius muscle was performed when echocardiograms confirmed moderate PH (preceding right ventricular (RV) failure). At 25 ± 9 days post-MCT, PH rats displayed RV hypertrophy (RV/(Left ventricle + Septum): 0.28 ± 0.05 vs. 0.44 ± 0.11), pulmonary congestion, and increased right ventricular systolic pressure (21 ± 8 vs. 55 ± 14 mm Hg) compared to healthy rats (all P < 0.05). Reduced capillary RBC velocity (403 ± 140 vs. 227 ± 84 μm/s; P = 0.01), RBC flux (33 ± 12 vs. 23 ± 5 RBCs/s; P = 0.04) and % of capillaries supporting continuous RBC flux at rest (79 ± 8 vs. 56 ± 13%; P = 0.01) were evident in PH rats compared to healthy rats. When Q̇O₂ within a given field of view was quantified (RBC flux x % of capillaries supporting continuous RBC flux), PH rats demonstrated lower overall Q̇O₂ (↓ 50%; P = 0.002). These data support that microcirculatory hemodynamic impairments (↓ Q̇O₂ and therefore altered Q̇O₂-to-V̇O₂ matching) may compromise blood-myocyte O₂ transport in PH. The mechanistic bases for decreased capillary RBC flux, velocity, and percentage of capillaries supporting RBC flow remains an important topic.

Rapid Full-Cycle Technique to Control Adulteration of Meat Products: Integration of Accelerated Sample Preparation, Recombinase Polymerase Amplification, and Test-Strip Detection

Verifying the authenticity of food products is essential due to the recent increase in counterfeit meat-containing food products. The existing methods of detection have a number of disadvantages. Therefore, simple, cheap, and sensitive methods for detecting various types of meat are required. In this study, we propose a rapid full-cycle technique to control the chicken or pig adulteration of meat products, including 3 min of crude DNA extraction, 20 min of recombinase polymerase amplification (RPA) at 39 °C, and 10 min of lateral flow assay (LFA) detection. The cytochrome B gene was used in the developed RPA-based test for chicken and pig identification. The selected primers provided specific RPA without DNA nuclease and an additional oligonucleotide probe. As a result, RPA-LFA, based on designed fluorescein- and biotin-labeled primers, detected up to 0.2 pg total DNA per μL, which provided up to 0.001% w/w identification of the target meat component in the composite meat. The RPA-LFA of the chicken and pig meat identification was successfully applied to processed meat products and to meat after heating. The results were confirmed by real-time PCR. Ultimately, the developed analysis is specific and enables the detection of pork and chicken impurities with high accuracy in raw and processed meat mixtures. The proposed rapid full-cycle technique could be adopted for the authentication of other meat products.

Oxylipin profiles and levels vary by skeletal muscle type, dietary fat and sex in young rats

Polyunsaturated fatty acids (PUFA)-derived bioactive lipid mediators called oxylipins have been shown to influence muscle growth, inflammation and repair in select muscles. Since individual oxylipins have varying effects and potencies, broad profiling in differing muscle types is required to further understand their overall effects. In addition, diet and sex are key determinants of oxylipin levels. Therefore, to provide comprehensive data on oxylipin profiles in rat soleus (SO), red gastrocnemius (RG), and white gastrocnemius (WG) muscles, female and male weanling Sprague-Dawley rats were provided control or experimental diets enriched in n-3 (ω-3) or n-6 (ω-6) PUFA for 6 weeks. Free oxylipin analysis by HPLC/MS/MS revealed that SO muscle had 25% more oxylipins and 4-13 times greater oxylipin mass than WG muscle. Dietary n-3 PUFA (α-linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid) each increased n-3 oxylipins derived directly from their precursors and several that were not direct precursors, while reducing arachidonic acid derived oxylipins. Dietary linoleic acid had few effects on oxylipins. Oxylipins with a sex effect were higher in females in SO and RG. Oxylipins generally reflected the effects of diet and sex on PUFA, but there were exceptions. These fundamental oxylipin profile data provide groundwork knowledge and context for future research on muscle oxylipin functions. Novelty: Rat SO compared with RG and WG muscles have a higher number and greater mass of oxylipins. Oxylipins generally reflect diet effects on PUFA in all muscles, but there are notable exceptions. Oxylipins in SO and RG are higher in females.

Potassium-induced potentiation of subtetanic force in rat skeletal muscles: influences of β2-activation, lactic acid, and temperature

Moderate elevations of extracellular K+ concentration ([K+]o) occur during exercise and have been shown to potentiate force during contractions elicited with subtetanic frequencies. Here, we investigated whether lactic acid (reduced chloride conductance), β2-adrenoceptor activation, and increased temperature would influence the potentiating effect of potassium in slow- and fast-twitch muscles. Isometric contractions were elicited by electrical stimulation at various frequencies in isolated rat soleus and extensor digitorum longus (EDL) muscles incubated at normal (4 mM) or elevated K+, in combination with salbutamol (5 μM), lactic acid (18.1 mM), 9-anthracene-carboxylic acid (9-AC; 25 μM), or increased temperature (30-35°C). Elevating [K+]o from 4 mM to 7 mM (soleus) and 10 mM (EDL) potentiated isometric twitch and subtetanic force while slightly reducing tetanic force. In EDL, salbutamol further augmented twitch force (+27 ± 3%, P < 0.001) and subtetanic force (+22 ± 4%, P < 0.001). In contrast, salbutamol reduced subtetanic force (-28 ± 6%, P < 0.001) in soleus muscles. Lactic acid and 9-AC had no significant effects on isometric force of muscles already exposed to moderate elevations of [K+]o. The potentiating effect of elevated [K+]o was still well maintained at 35°C. Addition of salbutamol exerts a further force-potentiating effect in fast-twitch but not in slow-twitch muscles already potentiated by moderately elevated [K+]o, whereas lactic acid, 9-AC, or increased temperature does not exert any further augmentation. However, the potentiating effect of elevated [K+]o was still maintained in the presence of these, thus emphasizing the positive influence of moderately elevated [K+]o for contractile performance during exercise.

Prolonged caloric restriction ameliorates age-related atrophy in slow and fast muscle fibers of rat soleus muscle

Aging causes loss of skeletal muscle mass and function, which is called sarcopenia. While sarcopenia impairs the quality of life of older adults and is a major factor in long-term hospitalization, its detailed pathogenic mechanism and preventive measures remain to be identified. Caloric restriction (CR) suppresses age-related physiological and pathological changes in many species and prolongs the average and healthy life expectancy. It has recently been reported that CR suppresses the onset of sarcopenia; however, few studies have analyzed the effects of long-term CR on age-related skeletal muscle atrophy. Thus, we investigated the aging and CR effects on soleus (SOL) muscles of 9-, 24-, and 29-month-old ad libitum-fed rats (9AL, 24AL, and 29AL, respectively) and of 29-month-old CR (29CR) rats. The total muscle cross sectional area (mCSA) of the entire SOL muscle significantly decreased in the 29AL rats, but not in the 24AL rats, compared with the 9AL rats. SOL muscle of the 29AL rats exhibited marked muscle fiber atrophy and increases in the number of muscle fibers with a central nucleus, in fibrosis, and in adipocyte infiltration. Additionally, although the decrease in the single muscle fiber cross-sectional area (fCSA) and the muscle fibers' number occurred in both slow-type and fast-type muscle fibers, the degree of atrophy was more remarkable in the fast-type fibers. However, CR suppressed the muscle fiber atrophy observed in the 29AL rats' SOL muscle by preserving the mCSA and the number of muscle fibers that declined with aging, and by decreasing the number of muscle fibers with a central nucleus, fibrosis and denervated muscle fibers. Overall, these results revealed that advanced aging separately reduces the number and fCSA of each muscle fiber type, but long-term CR can ameliorate this age-related sarcopenic muscle atrophy.

The effects of pulmonary hypertension on skeletal muscle oxygen pressures in contracting rat spinotrapezius muscle

NEW FINDINGS

What is the central question of this study? Does impairment in the dynamics of O₂ transport in skeletal muscle during a series of contractions constitute a potential mechanism underlying reduced exercise capacity in pulmonary hypertension? What is the main finding and its importance? Pulmonary hypertension compromises the dynamic matching of skeletal muscle O₂ delivery-to-utilization following contraction onset in the rat spinotrapezius muscle. These results implicate a role for vascular dysfunction in the slow V ̇ O 2 kinetics and exercise intolerance present in pulmonary hypertension.

ABSTRACT

Pulmonary hypertension (PH) is characterized by pulmonary vascular dysfunction and exercise intolerance due, in part, to compromised pulmonary and cardiac function. We tested the hypothesis that there are peripheral (i.e., skeletal muscle) aberrations in O₂ delivery ( Q ̇ O 2 )-to-O₂ utilization ( V ̇ O 2 ) matching and vascular control that might help to explain poor exercise tolerance in PH. Furthermore, we investigated the peripheral effects of nitric oxide (NO) in attenuating these decrements. Male Sprague-Dawley rats (n = 21) were administered monocrotaline (MCT; 50 mg/kg, i.p.) to induce PH. Disease progression was monitored via echocardiography. Phosphorescence quenching determined the O₂ partial pressure in the interstitial space ( P O 2 is ) in the spinotrapezius muscle at rest and during contractions under control (SNP-) and NO-donor (sodium nitroprusside, SNP+) conditions. MCT rats displayed right ventricular (RV) hypertrophy (right ventricle/(left ventricle + septum): 0.44 (0.13) vs. 0.28 (0.05)), pulmonary congestion, increased RV systolic pressure (48 (18) vs. 20 (8) mmHg) and arterial hypoxaemia ( P a O 2 : 64 (9) vs. 82 (9) mmHg) compared to healthy controls (HC) (P < 0.05). P O 2 is was significantly lower in MCT rats during the first 30 s of SNP- contractions. SNP superfusion elevated P O 2 is in both groups; however, MCT rats demonstrated a lower P O 2 is throughout SNP+ contractions versus HC (P < 0.05). Thus, for small muscle mass exercise in MCT rats, muscle oxygenation is impaired across the rest-to-contractions transition and exogenous NO does not raise the Q ̇ O 2 -to- V ̇ O 2 ratio in contracting muscle to the same levels as HC. These data support muscle Q ̇ O 2 -to- V ̇ O 2 mismatch as a potential contributor to slow V ̇ O 2 kinetics and therefore exercise intolerance in PH and suggest peripheral vascular dysfunction or remodelling as a possible mechanism.

Alterations in renin-angiotensin receptors are not responsible for exercise preconditioning of skeletal muscle fibers

Endurance exercise training promotes a protective phenotype in skeletal muscle known as exercise preconditioning. Exercise preconditioning protects muscle fibers against a variety of threats including inactivity-induced muscle atrophy. The mechanism(s) responsible for exercise preconditioning remain unknown and are explored in these experiments. Specifically, we investigated the impact of endurance exercise training on key components of the renin-angiotensin system (RAS). The RAS was targeted because activation of the classical axis of the RAS pathway via angiotensin II type I receptors (AT1Rs) promotes muscle atrophy whereas activation of the non-classical RAS axis via Mas receptors (MasRs) inhibits the atrophic signaling of the classical RAS pathway. Guided by prior studies, we hypothesized that an exercise-induced decrease in AT1Rs and/or increases in MasRs in skeletal muscle fibers is a potential mechanism responsible for exercise preconditioning. Following endurance exercise training in rats, we examined the abundance of AT1Rs and MasRs in both locomotor and respiratory muscles. Our results indicate that endurance exercise training does not alter the protein abundance of AT1Rs or MasRs in muscle fibers from the diaphragm, plantaris, and soleus muscles compared to sedentary controls (p ​> ​0.05). Furthermore, fluorescent angiotensin II (AngII) binding analyses confirm our results that exercise preconditioning does not alter the protein abundance of AT1Rs in the diaphragm, plantaris, and soleus (p ​> ​0.05). This study confirms that exercise-induced changes in RAS receptors are not a key mechanism that contributes to the beneficial effects of exercise preconditioning in skeletal muscle fibers.

In vivo cooling-induced intracellular Ca2+ elevation and tension in rat skeletal muscle

It is an open question as to whether cooling‐induced muscle contraction occurs in the in vivo environment. In this investigation, we tested the hypotheses that a rise in intracellular Ca²⁺ concentration ([Ca²⁺]i) and concomitant muscle contraction could be evoked in vivo by reducing muscle temperature and that this phenomenon would be facilitated or inhibited by specific pharmacological interventions designed to impact Ca²⁺‐induced Ca²⁺‐release (CICR). Progressive temperature reductions were imposed on the spinotrapezius muscle of Wistar rats under isoflurane anesthesia by means of cold fluid immersion. The magnitude, location, and temporal profile of [Ca²⁺]i were estimated using fura‐2 loading. Caffeine (1.25–5.0 mM) and procaine (1.6–25.6 mM) loading were applied in separatum to evaluate response plasticity by promoting or inhibiting CICR, respectively. Lowering the temperature of the muscle surface to ~5°C produced active tension and discrete sites with elevated [Ca²⁺]i. This [Ca²⁺]i elevation differed in magnitude from fiber to fiber and also from site to site within a fiber. Caffeine at 1.25 and 5.0 mM reduced the magnitude of cooling necessary to elevate [Ca²⁺]i (i.e., from ~5°C to ~8 and ~16°C, respectively, both p < 0.05) and tension. Conversely, 25.6 mM procaine lowered the temperature at which [Ca²⁺]i elevation and tension were detected to ~2°C (p < 0.05). Herein we demonstrate the spatial and temporal relationship between cooling‐induced [Ca²⁺]i elevation and muscle contractile force in vivo and the plasticity of these responses with CICR promotion and inhibition.