Oxygen dependence of metabolism and cellular adaptation in vertebrate muscles: a review

The key roles the cardiovascular system play in the complex distribution of blood, and consequently oxygen, have been extensively studied in vertebrates. Numerous studies have also revealed the complex and varied ways in which tissues cope with compromised oxygen supply. The links between these two processes are the subject of much current research. This article aims to review how blood supply influences tissue oxygenation and affects metabolism, and how this might have played a role in the evolution of the complex muscle arrangements which characterise vertebrates. Muscle tissue is the greatest proportion of body mass in most vertebrates and undergoes dramatic alterations in metabolism and associated oxygen flux. Special attention is given to the myotome of fishes, in which the partitioning of the fibre types contrasts with the mosaic arrangement of tetrapods. This gives us the opportunity to study pure whole vascularised muscle blocks, rather than single fibres, and further explore the interrelationship between oxygen supply and tissue energetics.

Oxygen-dependence of metabolic rate in the muscles of craniates

We present evidence that oxygen consumption (V_O2 ) is oxygen partial pressure (P_O2) dependent in striated muscles and P_O2 -independent in the vasculature in representatives of three craniate taxa: two teleost fish, a hagfish and a rat. Blood vessel V_O2 displayed varying degrees of independence in a P_O2 range of 15–95 mmHg, while V_O2 by striated muscle tissue slices from all species related linearly to P_O2 between 0 and 125 mmHg, despite V_O2 rates varying greatly between species and muscle type. In salmon red muscle, lactate concentrations fell in slices incubated at a P_O2 of either 30 or 100 mmHg, suggesting aerobic rather than anaerobic metabolism. Consistent with this finding, potential energy, a proxy of ATP turnover, was P_O2 -dependent. Our data suggest that the reduction in V_O2 with falling P_O2 results in a decrease in ATP demand, suggesting that the hypoxic signal is sensed and cellular changes effected. Viability and diffusion limitation of the preparations were investigated using salmon cardiac and skeletal muscles. Following the initial P_O2 depletion, reoxygenation of the Ringer bathing salmon cardiac muscle resulted in V_O2^s that was unchanged from the first run. V_O2 increased in all muscles uncoupled with p-trifluoromethoxylphenyl-hydrazone (FCCP) and 2,4-dinitrophenol (DNP). Mitochondrial succinate dehydrogenase activity, quantified by reduction of 3-(4,5-dimethylthiazol)-2,5-diphenyl-2H-tetrazolium bromide (MTT) to formazan, was constant over the course of the experiment. These three findings indicate that the tissues remained viable over time and ruled out diffusion-limitation as a constraint on V_O2.

Mid-femoral and mid-tibial muscle cross-sectional area as predictors of tibial bone strength in middle-aged and older men

While it is widely acknowledged that bones adapt to the site-specific prevalent loading environment, reasonable ways to estimate skeletal loads are not necessarily available. For long bone shafts, muscles acting to bend the bone may provide a more appropriate surrogate of the loading than muscles expected to cause compressive loads. Thus, the aim of this study was to investigate whether mid-thigh muscle cross-sectional area (CSA) was a better predictor of tibial mid-shaft bone strength than mid-tibia muscle CSA in middle aged and older men. 181 Caucasian men aged 50–79 years (mean±SD; 61±7 years) participated in this study. Mid-femoral and mid-tibial bone traits cortical area , density weighted polar moment of area and muscle CSA [cm²] were assessed with computed tomography. Tibial bone traits were positively associated with both the mid-femur (r=0.44 to 0.46, P<0.001) and the mid-tibia muscle CSA (r=0.35 to 0.37, P<0.001). Multivariate regression analysis, adjusting for age, weight, physical activity and femoral length, indicated that mid-femur muscle CSA predicted tibial mid-shaft bone strength indices better thn mid-tibia muscle CSA. In conclusion, the association between a given skeletal site and functionally adjacent muscles may provide a meaningful probe of the site-specific effect of loading on bone.

Artificial Muscles Based on Polypyrrole/Carbon Nanotube Laminates

A novel method for the periodical assembly of laminates of forest-drawn carbon nanotube (CNT) sheets and polypyrrole (PPy) is described. The method produces composite films in which the volume fraction and orientation of CNTs can be controlled. Actuator stroke and strength is increased and work capacity per cycle doubled when nanotube orientation is perpendicular to the actuation direction. Most importantly, these PPy/CNT laminates have dramatically decreased creep during actuation, which has been a major barrier for the application of PPy actuators.

Alterations in Notch signalling in skeletal muscles from mdx and dko dystrophic mice and patients with Duchenne muscular dystrophy.

New Findings What is the central question of this study? The Notch signalling pathway plays an important role in muscle regeneration, and activation of the pathway has been shown to enhance muscle regeneration in aged mice. It is unknown whether Notch activation will have a similarly beneficial effect on muscle regeneration in the context of Duchenne muscular dystrophy (DMD). What is the main finding and its importance? Although expression of Notch signalling components is altered in both mouse models of DMD and in human DMD patients, activation of the Notch signalling pathway does not confer any functional benefit on muscles from dystrophic mice, suggesting that other signalling pathways may be more fruitful targets for manipulation in treating DMD. Abstract In Duchenne muscular dystrophy (DMD), muscle damage and impaired regeneration lead to progressive muscle wasting, weakness and premature death. The Notch signalling pathway represents a central regulator of gene expression and is critical for cellular proliferation, differentiation and apoptotic signalling during all stages of embryonic muscle development. Notch activation improves muscle regeneration in aged mice, but its potential to restore regeneration and function in muscular dystrophy is unknown. We performed a comprehensive examination of several genes involved in Notch signalling in muscles from dystrophin-deficient mdx and dko (utrophin- and dystrophin-null) mice and DMD patients. A reduction of Notch1 and Hes1 mRNA in tibialis anterior muscles of dko mice and quadriceps muscles of DMD patients and a reduction of Hes1 mRNA in the diaphragm of the mdx mice were observed, with other targets being inconsistent across species. Activation and inhibition of Notch signalling, followed by measures of muscle regeneration and function, were performed in the mouse models of DMD. Notch activation had no effect on functional regeneration in C57BL/10, mdx or dko mice. Notch inhibition significantly depressed the frequency-force relationship in regenerating muscles of C57BL/10 and mdx mice after injury, indicating reduced force at each stimulation frequency, but enhanced the frequency-force relationship in muscles from dko mice. We conclude that while Notch inhibition produces slight functional defects in dystrophic muscle, Notch activation does not significantly improve muscle regeneration in murine models of muscular dystrophy. Furthermore, the inconsistent expression of Notch targets between murine models and DMD patients suggests caution when making interspecies comparisons.

Sociocultural influences on strategies to lose weight, gain weight, and increase muscles among ten cultural groups

This study determined how sociocultural messages to change one's body are perceived by adolescents from different cultural groups. In total, 4904 adolescents, including Australian, Chilean, Chinese, Indo-Fijian, Indigenous Fijian, Greek, Malaysian, Chinese Malaysian, Tongans in New Zealand, and Tongans in Tonga, were surveyed about messages from family, peers, and the media to lose weight, gain weight, and increase muscles. Groups were best differentiated by family pressure to gain weight. Girls were more likely to receive the messages from multiple sociocultural sources whereas boys were more likely to receive the messages from the family. Some participants in a cultural group indicated higher, and others lower, levels of these sociocultural messages. These findings highlight the differences in sociocultural messages across cultural groups, but also that adolescents receive contrasting messages within a cultural group. These results demonstrate the difficulty in representing a particular message as being characteristic of each cultural group.

Muscle atrophy, pain, and damage in bed rest reduced by resistive (vibration) exercise

The purpose of this study was to investigate the effectiveness of a short-duration (5-6 min, 3 d·wk) resistive exercise program with (RVE) or without (RE) whole-body vibration in reducing muscle atrophy in the lower limb during prolonged inactivity when compared with that in an inactive control group. METHODS: As part of the second Berlin BedRest Study, 24 male subjects underwent 60 d of head-down tilt bed rest. Using magnetic resonance imaging, muscle volumes of the individual muscles of the lower limb were calculated before and at various intervals during and after bed rest. Pain levels and markers of muscle damage were also evaluated during and after bed rest. Adjustment of P values to guard against false positives was performed via the false discovery rate method. RESULTS: On the "intent-to-treat" analysis, RE reduced atrophy of the medial and lateral gastrocnemius, soleus, vasti, tibialis posterior, flexor hallucis longus, and flexor digitorum longus (P ≤ 0.045 vs control group) and RVE reduced atrophy of the medial and lateral gastrocnemius and tibialis posterior (P ≤ 0.044). Pain intensity reports after bed rest were lower in RE at the foot (P ≤ 0.033) and whole lower limb (P = 0.01) and in RVE at the thigh (P ≤ 0.041), lower leg (P ≤ 0.01), and whole lower limb (P ≤ 0.036). Increases in sarcomere-specific creatine kinase after bed rest were less in RE (P = 0.020) and RVE (P = 0.020). No differences between RE and RVE were observed. CONCLUSIONS: In conclusion, a short-duration RVE or RE can be effective in reducing the effect of prolonged bed rest on lower extremity muscle volume loss during bed rest and muscle damage and pain after bed rest. Copyright © 2014 by the American College of Sports Medicine.

Development of a novel soft parallel robot equipped with polymeric artificial muscles

This paper presents the design, analysis and fabrication of a novel low-cost soft parallel robot for biomedical applications, including bio-micromanipulation devices. The robot consists of two active flexible polymer actuator-based links, which are connected to two rigid links by means of flexible joints. A mathematical model is established between the input voltage to the polymer actuators and the robot's end effector position. The robot has two degrees-of-freedom, making it suitable for handling planar micromanipulation tasks. Moreover, a number of robots can be configured to operate in a cooperative manner for increasing micromanipulation dexterity. Finally, the experimental results demonstrate two main motion modes of the robot.

Altered response of the anterolateral abdominal muscles to simulated weight-bearing in subjects with low back pain

An important aspect of neuromuscular control at the lumbo-pelvic region is stabilization. Subjects with low back pain (LBP) have been shown to exhibit impairments in motor control of key muscles which contribute to stabilization of the lumbo-pelvic region. However, a test of automatic recruitment that relates to function has been lacking. A previous study used ultrasound imaging to show that healthy subjects automatically recruited the transversus abdominis (TrA) and internal oblique (IO) muscles in response to a simulated weight-bearing task. This task has not been investigated in subjects with LBP. The aim of this study was to compare the automatic recruitment of the abdominal muscles among subjects with and without LBP in response to the simulated weight-bearing task. Twenty subjects with and without LBP were tested. Real-time ultrasound imaging was used to assess changes in thickness of the TrA and internal oblique IO muscles as well as lateral movement ("slide") of the anterior fascial insertion of the TrA muscle. Results showed that subjects with LBP showed significantly less shortening of the TrA muscle (P < 0.0001) and greater increases in thickness of the IO muscle (P = 0.002) with the simulated weight-bearing task. There was no significant difference between groups for changes in TrA muscle thickness (P = 0.055). This study provides evidence of changes in motor control of the abdominal muscles in subjects with LBP. This test may provide a functionally relevant and non-invasive method to investigate the automatic recruitment of the abdominal muscles in people with and without LBP.

Effect of prolonged bed rest on the anterior hip muscles

Prolonged bed rest and inactivity is known to cause muscular atrophy with previous research indicating that muscles involved in joint stabilisation are more susceptible. The anterior hip muscles are important for hip joint function and stability but little is known about the effects of prolonged inactivity on their function. This study investigated the effect of prolonged bed rest on the size of the anterior hip muscles and their pattern of recovery. The effect of resistive vibration exercise (RVE) as a countermeasure to muscle atrophy was also investigated. 12 male participants, randomly assigned to either a control or an exercise group, underwent 8 weeks of bed rest with 6 months follow-up. Changes in muscle cross-sectional area (CSA) of the iliacus, psoas, iliopsoas, sartorius and rectus femoris muscles were measured by magnetic resonance imaging at regular intervals during bed rest and recovery phases. CSAs of iliopsoas and sartorius decreased at the hip joint (p<0.05) during bed rest but iliacus, psoas, and rectus femoris CSAs were unchanged (p>0.05). No significant difference was found between the two groups for all muscles (all p>0.1), suggesting inefficacy of the countermeasure in this sample. These findings suggest that prolonged bed rest can result in the atrophy of specific muscles across the hip joint which may affect its stability and function.

The effects of rehabilitation on the muscles of the trunk following prolonged bed rest

Microgravity and inactivity due to prolonged bed rest have been shown to result in atrophy of spinal extensor muscles such as the multifidus, and either no atrophy or hypertrophy of flexor muscles such as the abdominal group and psoas muscle. These effects are long-lasting after bed rest and the potential effects of rehabilitation are unknown. This two-group intervention study aimed to investigate the effects of two rehabilitation programs on the recovery of lumbo-pelvic musculature following prolonged bed rest. 24 subjects underwent 60 days of head down tilt bed rest as part of the 2nd Berlin BedRest Study (BBR2-2). After bed rest, they underwent one of two exercise programs, trunk flexor and general strength (TFS) training or specific motor control (SMC) training. Magnetic resonance imaging of the lumbo-pelvic region was conducted at the start and end of bed rest and during the recovery period (14 and 90 days after re-ambulation). Cross-sectional areas (CSAs) of the multifidus, psoas, lumbar erector spinae and quadratus lumborum muscles were measured from L1 to L5. Morphological changes including disc volume, spinal length, lordosis angle and disc height were also measured. Both exercise programs restored the multifidus muscle to pre-bed-rest size, but further increases in psoas muscle size were seen in the TFS group up to 14 days after bed rest. There was no significant difference in the number of low back pain reports for the two rehabilitation groups (p=.59). The TFS program resulted in greater decreases in disc volume and anterior disc height. The SMC training program may be preferable to TFS training after bed rest as it restored the CSA of the multifidus muscle without generating potentially harmful compressive forces through the spine.