1,156 research outputs found
Muscle Fatigue from the Perspective of a Single Crossbridge
The repeated intense stimulation of skeletal muscle rapidly decreases its force- and motion-generating capacity. This type of fatigue can be temporally correlated with the accumulation of metabolic by-products, including phosphate (Pi) and protons (H+). Experiments on skinned single muscle fibers demonstrate that elevated concentrations of these ions can reduce maximal isometric force, unloaded shortening velocity, and peak power, providing strong evidence for a causative role in the fatigue process. This seems to be due, in part, to their direct effect on muscleâs molecular motor, myosin, because in assays using isolated proteins, these ions directly inhibit myosinâs ability to move actin. Indeed, recent work using a single molecule laser trap assay has revealed the specific steps in the crossbridge cycle affected by these ions. In addition to their direct effects, these ions also indirectly affect myosin by decreasing the sensitivity of the myofilaments to calcium, primarily by altering the ability of the muscle regulatory proteins, troponin and tropomyosin, to govern myosin binding to actin. This effect seems to be partially due to fatigue-dependent alterations in the structure and function of specific subunits of troponin. Parallel efforts to understand the molecular basis of muscle contraction are providing new technological approaches that will allow us to gain unprecedented molecular detail of the fatigue process. This will be crucial to fully understand this ubiquitous phenomenon and develop appropriately targeted therapies to attenuate the debilitating effects of fatigue in clinical populations
Good Limestone is Pure and Fine
You can\u27t judge the purity of the limestone you buy by simply looking at a sample. But here\u27s how you can
Effects of Elevated H\u3csup\u3e+\u3c/sup\u3e And P\u3csub\u3ei\u3c/sub\u3e on The Contractile Mechanics of Skeletal Muscle Fibres From Young and Old Men: Implications for Muscle Fatigue in Humans
The present study aimed to identify the mechanisms responsible for the loss in muscle power and increased fatigability with ageing by integrating measures of wholeâmuscle function with single fibre contractile mechanics. After adjusting for the 22% smaller muscle mass in old (73â89 years, n = 6) compared to young men (20â29 years, n = 6), isometric torque and power output of the knee extensors were, respectively, 38% and 53% lower with age. Fatigability was âź2.7âfold greater with age and strongly associated with reductions in the electricallyâevoked contractile properties. To test whether crossâbridge mechanisms could explain ageârelated decrements in knee extensor function, we exposed myofibres (n = 254) from the vastus lateralis to conditions mimicking quiescent muscle and fatiguing levels of acidosis (H+) (pH 6.2) and inorganic phosphate (Pi) (30 mm). The fatigueâmimicking condition caused marked reductions in force, shortening velocity and power and inhibited the lowâ to highâforce state of the crossâbridge cycle, confirming findings from nonâhuman studies that these ions act synergistically to impair crossâbridge function. Other than severe ageârelated atrophy of fast fibres (â55%), contractile function and the depressive effects of the fatigueâmimicking condition did not differ in fibres from young and old men. The selective loss of fast myosin heavy chain II muscle was strongly associated with the ageârelated decrease in isometric torque (r = 0.785) and power (r = 0.861). These data suggest that the ageârelated loss in muscle strength and power are primarily determined by the atrophy of fast fibres, but the ageârelated increased fatigability cannot be explained by an increased sensitivity of the crossâbridge to H+ and Pi
Treatment with ActRIIB-mFc Produces Myofiber Growth and Improves Lifespan in the \u3cem\u3eActa1\u3c/em\u3e H40Y Murine Model of Nemaline Myopathy
Effect of 17 Days of Bed Rest on Peak Isometric Force and Unloaded Shortening Velocity of Human Soleus Fibers
The purpose of this study was to examine the effect of prolonged bed rest (BR) on the peak isometric force (Po) and unloaded shortening velocity (Vo) of single Ca2+-activated muscle fibers. Soleus muscle biopsies were obtained from eight adult males before and after 17 days of 6° head-down BR. Chemically permeabilized single fiber segments were mounted between a force transducer and position motor, activated with saturating levels of Ca2+, and subjected to slack length steps. Vo was determined by plotting the time for force redevelopment vs. the slack step distance. Gel electrophoresis revealed that 96% of the pre- and 87% of the post-BR fibers studied expressed only the slow type I myosin heavy chain isoform. Fibers with diameter \u3e100 Îźm made up only 14% of this post-BR type I population compared with 33% of the pre-BR type I population. Consequently, the post-BR type I fibers (n = 147) were, on average, 5% smaller in diameter than the pre-BR type I fibers (n = 218) and produced 13% less absolute Po. BR had no overall effect on Po per fiber cross-sectional area (Po/CSA), even though half of the subjects displayed a decline of 9â12% in Po/CSA after BR. Type I fiber Vo increased by an average of 34% with BR. Although the ratio of myosin light chain 3 to myosin light chain 2 also rose with BR, there was no correlation between this ratio and Vo for either the pre- or post-BR fibers. In separate fibers obtained from the original biopsies, quantitative electron microscopy revealed a 20â24% decrease in thin filament density, with no change in thick filament density. These results raise the possibility that alterations in the geometric relationships between thin and thick filaments may be at least partially responsible for the elevated Vo of the post-BR type I fibers
Mapping between dissipative and Hamiltonian systems
Theoretical studies of nonequilibrium systems are complicated by the lack of
a general framework. In this work we first show that a transformation
introduced by Ao recently (J. Phys. A {\bf 37}, L25 (2004)) is related to
previous works of Graham (Z. Physik B {\bf 26}, 397 (1977)) and Eyink {\it et
al.} (J. Stat. Phys. {\bf 83}, 385 (1996)), which can also be viewed as the
generalized application of the Helmholtz theorem in vector calculus. We then
show that systems described by ordinary stochastic differential equations with
white noise can be mapped to thermostated Hamiltonian systems. A steady-state
of a dissipative system corresponds to the equilibrium state of the
corresponding Hamiltonian system. These results provides a solid theoretical
ground for corresponding studies on nonequilibrium dynamics, especially on
nonequilibrium steady state. The mapping permits the application of established
techniques and results for Hamiltonian systems to dissipative non-Hamiltonian
systems, those for thermodynamic equilibrium states to nonequilibrium steady
states. We discuss several implications of the present work.Comment: 18 pages, no figure. final version for publication on J. Phys. A:
Math & Theo
Force-velocity-power and Force-pCa Relationships of Human Soleus Fibers After 17 Days of Bed Rest
Soleus muscle fibers from the rat display a reduction in peak power and Ca2+ sensitivity after hindlimb suspension. To examine human responses to non-weight bearing, we obtained soleus biopsies from eight adult men before and immediately after 17 days of bed rest (BR). Single chemically skinned fibers were mounted between a force transducer and a servo-controlled position motor and activated with maximal (isotonic properties) and/or submaximal (Ca2+ sensitivity) levels of free Ca2+. Gel electrophoresis indicated that all pre- and post-BR fibers expressed type I myosin heavy chain. Post-BR fibers obtained from one subject displayed increases in peak power and Ca2+ sensitivity. In contrast, post-BR fibers obtained from the seven remaining subjects showed an average 11% reduction in peak power (P \u3c 0.05), with each individual displaying a 7â27% reduction in this variable. Post-BR fibers from these subjects were smaller in diameter and produced 21% less force at the shortening velocity associated with peak power. However, the shortening velocity at peak power output was elevated 13% in the post-BR fibers, which partially compensated for their lower force. Post-BR fibers from these same seven subjects also displayed a reduced sensitivity to free Ca2+(P \u3c 0.05). These results indicate that the reduced functional capacity of human lower limb extensor muscles after BR may be in part caused by alterations in the cross-bridge mechanisms of contraction
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Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite.
Treatment and immobilization of technetium-99 (99Tc) contained in reprocessed nuclear waste and present in contaminated subsurface systems represents a major environmental challenge. One potential approach to managing this highly mobile and long-lived radionuclide is immobilization into micro- and meso-porous crystalline solids, specifically sodalite. We synthesized and characterized the structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and the structure of a mixed guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x. Perrhenate was used as a chemical analogue for pertechnetate. Bulk analyses of each solid confirm a cubic sodalite-type structure (P4Ě
3n, No. 218 space group) with rhenium and technetium in the 7+ oxidation state. High-resolution nanometer scale characterization measurements provide first-of-a-kind evidence that the ReO4- anions are distributed in a periodic array in the sample, nanoscale clustering is not observed, and the ReO4- anion occupies the center of the sodalite β-cage in Na8[AlSiO4]6(ReO4)2. We also demonstrate, for the first time, that the TcO4- anion can be incorporated into the sodalite structure. Lastly, thermochemistry measurements for the perrhenate sodalite were used to estimate the thermochemistry of pertechnetate sodalite based on a relationship between ionic potential and the enthalpy and Gibbs free energy of formation for previously measured oxyanion-bearing feldspathoid phases. The results collected in this study suggest that micro- and mesoporous crystalline solids maybe viable candidates for the treatment and immobilization of 99Tc present in reprocessed nuclear waste streams and contaminated subsurface environments
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