A validated model of passive skeletal muscle to predict force and intramuscular pressure

The passive properties of skeletal muscle are often overlooked in muscle studies, yet they play a key role in tissue function in vivo. Studies analyzing and modeling muscle passive properties, while not uncommon, have never investigated the role of fluid content within the tissue. Additionally, intramuscular pressure (IMP) has been shown to correlate with muscle force in vivo and could be used to predict muscle force in the clinic. In this study, a novel model of skeletal muscle was developed and validated to predict both muscle stress and IMP under passive conditions for the New Zealand White Rabbit tibialis anterior. This model is the first to include fluid content within the tissue and uses whole muscle geometry. A nonlinear optimization scheme was highly effective at fitting model stress output to experimental stress data (normalized mean square error or NMSE fit value of 0.993) and validation showed very good agreement to experimental data (NMSE fit values of 0.955 and 0.860 for IMP and stress, respectively). While future work to include muscle activation would broaden the physiological application of this model, the passive implementation could be used to guide surgeries where passive muscle is stretched.     

Local temperature changes and human skeletal muscle metabolism

The aim of this review is to describe the effects induced by local temperature changes on human skeletal muscle metabolism. More specifically, we will consider the influence of temperature on the mechanical properties of muscle contraction, on aerobic metabolism, anaerobic metabolism and on the Lohmann reaction. The text has been voluntarily organized on the basis of a simple bioenergetic model describing the different energy fluxes appearing in the muscle system. This approach should better highlight some of the points that still need to be investigated. Although it was not always possible to restrict the discussion to human muscle, the references report mainly data obtained directly on humans or on isolated human fibres. A short comment on skeletal muscle temperature measurement techniques, on humans, is also included.     

Energy metabolism in relation to oxygen partial pressure in human skeletal muscle during exercise.

1. The intramuscular oxygen partial pressure (pO2) in human gastrocnemius muscle was monitored during exercise and compared with metabolite concentrations reflecting the energy and the redox state in the tissue. Ten normal subjects and ten patients with peripheral vascular occlusive disease were investigated. 2. In normal subjects the pO2 at the end of exercise was related to the intensity of the exercise, expressed as effect (J/s) per contraction. 3. In both patients and normal subject the pO2 was related to the [ATP]/[ADP] ratio, the [lactate/[pyruvate] ratio and the phosphocreatine concentration in the muscle tissue at rest and during exercise. 4. At each pO2 value, a lower [lactate/[pyruvate] ratio was found in the muscle tissue of the patients compared with that of normal subjects. This was interpreted as a beneficial effect of the higher oxidative-enzyme capacity in the muscle of the patients. 5. The results show the importance of pO2 for the regulation of the energy and the redox state of the tissue. During exercise the changes induced in pO2 and thus the energy state will stimulate the respiratory rate. This might be an important link in triggering the oxidative-enzyme capacity in response to physical training as well as in peripheral vascular occlusive disease.     

Median frequency of the myoelectric signal. Effects of muscle ischemia and cooling

A study was performed to investigate the changes that occur in the median frequency of the myoelectric signal during local ischemia or reduction of intramuscular temperature produced by surface cooling. Data was obtained from experiments which involved the first dorsal interosseous muscle of 10 female and 16 male subjects. These subjects were asked to perform isometric constant-force abduction contractions of the index finger at 20% and 80% of maximal voluntary contraction level. The initial median frequency (IMF) of the myoelectric signal during the first 0.5 s of contraction was calculated. Results showed a significant reduction of the IMF in contractions performed under ischemic conditions; upon release, the IMF recovered quickly. At 80% maximal voluntary level of contraction, a greater decrease of the IMF was recorded. Similar results were demonstrated during reduction of intramuscular temperature with gradual recovery of the IMF after cooling. These results demonstrate that the median frequency of the myoelectric signal displays behavior similar to that reported for conduction velocity and this is consistent with the notion that accumulation of metabolic byproducts in muscle tissue causes a decrease in the conduction velocity of the muscle fibers.     

Diameter changes in skeletal muscle venules during arterial pressure reduction

Previous studies in skeletal muscle have shown a substantial (>100%) increase in venous vascular resistance with arterial pressure reduction to 40 mmHg, but a microcirculatory study showed no significant venular diameter changes in the horizontal direction during this procedure. To examine the possibility of venular collapse in the vertical direction, a microscope was placed horizontally to view a vertically mounted rat spinotrapezius muscle preparation. We monitored the diameters of venules (mean diameter 73. 8 +/- 37.0 microm, range 13-185 microm) oriented horizontally and vertically with a video system during acute arterial pressure reduction by hemorrhage. Our analysis showed small but significant (P < 0.0001) diameter reductions of 1.0 +/- 2.5 microm and 1.8 +/- 3. 1 microm in horizontally and vertically oriented venules, respectively, upon reduction of arterial pressure from 115.0 +/- 26. 3 to 39.8 +/- 12.3 mmHg. The venular responses were not different after red blood cell aggregation was induced by Dextran 500 infusion. We conclude that diameter changes in venules over this range of arterial pressure reduction are isotropic and would likely increase venous resistance by <10%.     

The relationship between human skeletal muscle pyruvate dehydrogenase phosphatase activity and muscle aerobic capacity

Pyruvate dehydrogenase (PDH) is a mitochondrial enzyme responsible for regulating the conversion of pyruvate to acetyl-CoA for use in the tricarboxylic acid cycle. PDH is regulated through phosphorylation and inactivation by PDH kinase (PDK) and dephosphorylation and activation by PDH phosphatase (PDP). The effect of endurance training on PDK in humans has been investigated; however, to date no study has examined the effect of endurance training on PDP in humans. Therefore, the purpose of this study was to examine differences in PDP activity and PDP1 protein content in human skeletal muscle across a range of muscle aerobic capacities. This association is important as higher PDP activity and protein content will allow for increased activation of PDH, and carbohydrate oxidation. The main findings of this study were that 1) PDP activity (r(2) = 0.399, P = 0.001) and PDP1 protein expression (r(2) = 0.153, P = 0.039) were positively correlated with citrate synthase (CS) activity as a marker for muscle aerobic capacity; 2) E1α (r(2) = 0.310, P = 0.002) and PDK2 protein (r(2) = 0.229, P =0.012) are positively correlated with muscle CS activity; and 3) although it is the most abundant isoform, PDP1 protein content only explained ～ 18% of the variance in PDP activity (r(2) = 0.184, P = 0.033). In addition, PDP1 in combination with E1α explained ～ 38% of the variance in PDP activity (r(2) = 0.383, P = 0.005), suggesting that there may be alternative regulatory mechanisms of this enzyme other than protein content. These data suggest that with higher muscle aerobic capacity (CS activity) there is a greater capacity for carbohydrate oxidation (E1α), in concert with higher potential for PDH activation (PDP activity).     

Intramuscular fluid pressure during isometric contraction of human skeletal muscle

Intramuscular fluid pressures were recorded in the vastus medialis of seven healthy male volunteers. Pressures were measured simultaneously at three different sites in the muscle by a catheter-tip transducer with extremely low volume-displacement characteristics and by two extracorporeal transducers connected to slit catheters. All three recording systems gave qualitatively similar results provided the catheters had inner diameters exceeding 0.53 mm and allowed measurement of pressures lasting as short as 1 s. Wick catheters yielded slower responses than slit catheters. At any position intramuscular fluid pressure increased linearly with force up to maximal voluntary contraction (MVC). However, slopes of these curves varied greatly mainly because the pressure was also a linear function of the distance from the fascia. The highest recorded pressure was 570 Torr. At prolonged submaximal contractions intramuscular fluid pressure oscillated independent of contraction force. The linearity of both the pressure-force relationship and the pressure-depth relationship is compatible with a simple model based on the law of Laplace because the muscle fibers are curved during contraction in this muscle. It is hypothesized that blood flow is first compromised deep in the muscle where pressure is highest and in general at lower stress or tension in short bulging muscles with great curvature of the fibers compared with long slender ones.     

Spatial interaction between tissue pressure and skeletal muscle perfusion during contraction

The vascular waterfall theory attributes decreased muscle perfusion during contraction to increased intramuscular pressure (P(IM)) and concomitant increase in venous resistance. Although P(IM) is distributed during contractions, this theory does not account for heterogeneity. This study hypothesises that pressure heterogeneity could affect the interaction between P(IM) rise and perfusion. Regional tissue perfusion during submaximum (100kPa) tetanic contraction is studied, using a finite element model of perfused contracting skeletal muscle. Capillary flow in muscles with one proximal artery and vein (SIM(1)) and with an additional distal artery and vein (SIM(2)) is compared. Blood flow and pressures at rest and P(IM) during contraction ( approximately 25kPa maximally) are similar between simulations, but capillary flow and venous pressure differ. In SIM(2), venous pressure and capillary flow correspond to P(IM) distribution, whereas capillary flow in SIM(1) is less than 10% of flow in SIM(2), in the muscle half without draining vein. This difference is caused by a high central P(IM), followed by central venous pressure rise, in agreement with the waterfall theory. The high central pressure (SIM(1)), obstructs outflow from the distal veins. Distal venous pressure rises until central blood pressure is reached, although local P(IM) is low. Adding a distal vein (SIM(2)) restores the perfusion. It is concluded that regional effects contribute to the interaction between P(IM) and perfusion during contraction. Unlike stated by the vascular waterfall theory, venous pressure may locally exceed P(IM). Although this can be explained by the principles of this theory, the theory does not include this phenomenon as such.     

Correlation between active and passive isometric force and intramuscular pressure in the isolated rabbit tibialis anterior muscle

The purpose of this study was to quantify the relationship between intramuscular pressure (IMP) and muscle force during isometric muscle contraction of the rabbit tibialis anterior (TA) absent the effect of either bone or fascia. To quantify this relationship, length-tension experiments were performed on the isolated TA of the New Zealand White rabbit (mass=2.5+/-0.5kg, n=12). The knee was fixed in a custom jig, the distal tendon of the TA was attached to a servomotor, and a 360 microm fiber optic pressure transducer was inserted into the TA. The peroneal nerve was stimulated to define optimal length (L(0)). The length-tension curve was created using 40Hz isometric contractions with 2-min rest intervals between each contraction. Measurements began at L(0)-50%L(f) and progressed to L(0)+50%L(f), changing the length-tension in 5% L(f) increments after each contraction. Qualitatively, the length-tension curve for isometric contractions was mimicked by the length-pressure curve for both active and passive conditions. Linear regression was performed individually for each animal for the ascending and descending limb of the length-tension curve and for active and passive conditions. Pressure-force coefficients of determination ranged from 0.138-0.963 for the active ascending limb and 0.343-0.947 for the active descending limb. Passive pressure coefficients of determination ranged from 0.045-0.842 for the ascending limb and 0.672-0.982 for the descending limb. These data indicate that IMP measurement provide a fairly accurate index of relative muscle force, especially at muscle lengths longer than optimal.     

Effects of exercise on mitogen- and stress-activated kinase signal transduction in human skeletal muscle

Exercise/contraction is a powerful stimulator of mitogen-activated protein (MAP) kinase cascades in skeletal muscle. Little is known regarding the physiological activation of enzymes downstream of MAP kinase. We investigated whether acute exercise results in activation of mitogen- and stress-activated kinases (MSK) 1 and 2, p90 ribosomal S6 kinase (p90rsk), and MAP kinase-activated protein kinase 2 (MAPKAPK2). Muscle biopsies were obtained from healthy volunteers before, during, and after 60 min one-leg cycle ergometry, from exercising and resting legs. MSK1 and MSK2 activities were increased 400-500% and 200-300%, respectively, in exercised muscle (P < 0.05 vs. rest). A dramatic increase in activity of p90rsk (MAPKAPK1) (>2,500%), and to a lesser extent MAPKAP2 (300%), was noted with exercise (P < 0.05 vs. rest). MSK1, MSK2, p90rsk, and MAPKAP2 activities were sustained throughout exercise. Exercise-induced activation of these enzymes was limited to working muscle, indicating that local rather than systemic factors activate these signaling cascades. Thus physical exercise leads to activation of multiple enzymes downstream of MAP kinase.     

Genome-wide DNA methylation changes in skeletal muscle between young and middle-aged pigs

Background

Age-related physiological, biochemical and functional changes in mammalian skeletal muscle have been shown to begin at the mid-point of the lifespan. However, the underlying changes in DNA methylation that occur during this turning point of the muscle aging process have not been clarified. To explore age-related genomic methylation changes in skeletal muscle, we employed young (0.5 years old) and middle-aged (7 years old) pigs as models to survey genome-wide DNA methylation in the longissimus dorsi muscle using a methylated DNA immunoprecipitation sequencing approach.

Results

We observed a tendency toward a global loss of DNA methylation in the gene-body region of the skeletal muscle of the middle-aged pigs compared with the young group. We determined the genome-wide gene expression pattern in the longissimus dorsi muscle using microarray analysis and performed a correlation analysis using DMR (differentially methylated region)-mRNA pairs, and we found a significant negative correlation between the changes in methylation levels within gene bodies and gene expression. Furthermore, we identified numerous genes that show age-related methylation changes that are potentially involved in the aging process. The methylation status of these genes was confirmed using bisulfite sequencing PCR. The genes that exhibited a hypomethylated gene body in middle-aged pigs were over-represented in various proteolysis and protein catabolic processes, suggesting an important role for these genes in age-related muscle atrophy. In addition, genes associated with tumorigenesis exhibited aged-related differences in methylation and expression levels, suggesting an increased risk of disease associated with increased age.

Conclusions

This study provides a comprehensive analysis of genome-wide DNA methylation patterns in aging pig skeletal muscle. Our findings will serve as a valuable resource in aging studies, promoting the pig as a model organism for human aging research and accelerating the development of comparative animal models in aging research.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-653) contains supplementary material, which is available to authorized users.     

The relation between skeletal and extracellular-fluid magnesium in vitro

1. Femora from control and magnesium-deficient rats were incubated in media of different magnesium concentrations. 2. Magnesium was lost at the same rate from fresh and boiled bones to a magnesium-free medium. 3. The equilibrium established was dependent on the magnesium concentrations in both the bone and the medium. 4. Parathyroid extract increased the rate of magnesium loss from bone to a magnesium-low medium containing bovine serum (50%), but had no effect in a protein-free medium.     

Pulsatile pressure and flow in the skeletal muscle microcirculation

Although blood flow in the microcirculation of the rat skeletal muscle has negligible inertia forces with very low Reynolds number and Womersley parameter, time-dependent pressure and flow variations can be observed. Such phenomena include, for example, arterial flow overshoot following a step arterial pressure, a gradual arterial pressure reduction for a step flow, or hysteresis between pressure and flow when a pulsatile pressure is applied. Arterial and venous flows do not follow the same time course during such transients. A theoretical analysis is presented for these phenomena using a microvessel with distensible viscoelastic walls and purely viscous flow subject to time variant arterial pressures. The results indicate that the vessel distensibility plays an important role in such time-dependent microvascular flow and the effects are of central physiological importance during normal muscle perfusion. In-vivo whole organ pressure-flow data in the dilated rat gracilis muscle agree in the time course with the theoretical predictions. Hemodynamic impedances of the skeletal muscle microcirculation are investigated for small arterial and venous pressure amplitudes superimposed on an initial steady flow and pressure drop along the vessel.     

Age-related changes in the reducible cross-links on connectin from human skeletal muscle.

After NaB³H₄-reduction of connectin from human skeletal muscle, the changes in the amounts of the reducible cross-links and specific radioactivity of this elastic protein were followed throughout the whole life-span from embryo to old age. The reducible cross-links, aldimine forms of lysinonorleucine and histidino-hydroxymerodesmosine, and unidentified reducible compounds, which were assumed to be cross-linking amino acids, were found to remarkably decrease with age. A progressive decrease in the incorporation of tritium into the reducible compounds was also observed. We conclude that the conversion of the reducible cross-links derived from lysine and hydroxylysine aldehydes to non-reducible compounds is an essential step in the maturation of connectin fibrils, similar to collagen fibrils.