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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Direct stimulation of single muscle fibers from Xenopus laevis at a frequency of 1 Hz results in a decline of the peak isometric twitch tension after about 200 twitches. Fibers were chemically fixed in glutaraldehyde after a varying number of twitches and at several fatigue levels, and the ultrastructural appearance was compared with that of resting fibers treated by identical fixation methods. No gross structural abnormalities were observed but subtle changes occurred. The mitochondria of stimulated fibers contain granules of normal size and number. The inner crista width is constant but the matrix width is increased on stimulation. These changes would not compromise ATP production. The myofibrils are normal except for a slight swelling in the myosin lattice. The transverse system (T system) and sarcoplasmic reticulum are intact. The minor diameter of the transverse tubule (T tubule) is increased slightly in stimulated fibers. The gap between the T-TC membranes stays constant at about 110 A, but tiny connecting pillars are seen to cross this gap more frequently in stimulated fibers (21 +/- 5% triads) than in resting fibers (8 +/- 6%). In stimulated fibers there is a marked increase in the electron dense content of the terminal cisternae (TC). Inasmuch as the observed structural changes correlate with the number of twitches but not with the fatigue level, it is concluded that TC density and T-TC pillar formation are related to the normal mechanisms of excitation-contraction coupling.
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PMID:Structural changes in single muscle fibers after stimulation at a low frequency. 47 18

With the use of myosin adenosinetriphosphatase (ATPase) and immunofluorescence staining methods, the adaptive responses of intrafusal and extrafusal fibers to endurance swimming were studied in frozen sections of rat soleus (SOL) and extensor digitorum longus (EDL) muscles. Glycogen depletion confirmed muscle fatigue at the end of a standardized bout of exercise. No significant age-dependent changes in myosin isoforms were detected in any fibers. The 12-wk training increased type I fibers by 10.9% in the SOL and type IIa fibers in the EDL by 16.6%. In trained muscle sections, both staining methods identified a permuted chain fiber, expressed the same as the myosin isoform in the bag2 fiber. However, no exercise-induced change of myosin isoform profile was found in the bag1 and bag2 fibers. Myosin ATPase (and immunofluorescence) staining showed the percentage of permuted chain fibers increased from 0 to 6.7% (5.6%) after 6 wk of training and to 19.2% (14.1%) after 12 wk of training and that it was still at 6.1% (4.2%) 10 wks after training. A novel myosin isoform may thus be expressed in nuclear chain fibers by repetitive recruitment of muscle spindles.
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PMID:Novel myosin isoform in nuclear chain fibers of rat muscle spindles produced in response to endurance swimming. 128 26

It has long been appreciated that rates of ATP utilization and production need to be extremely closely balanced. To put it in molecular rather than molar terms, in human muscle engaged in a 15-min work protocol, approximately 3.3 x 10(20) ATP/g are used and resynthesized at approximately 100 times the resting cycling rates before fatigue, during which time only a 20-25% decrease in the ATP pool is sustained. Analysis of how such remarkable regulatory precision is achieved suggests that in resting muscle myosin behaves as a latent catalyst whose full catalytic potential 1) is realized with the arrival of an activator signal (Ca2+) and 2) is tempered with reaction products; such proactive control, initiated at ATP utilization, sets the required flux through ATP-producing pathways. For any given enzyme step in ATP-producing pathways, reaction velocity (v) becomes the independent parameter, with substrate concentration ([S], the dependent parameter) being adjusted accordingly. Because the dynamic range for muscles (change from resting to maximum ATP turnover rates) can exceed 100-fold, in many studies of working muscle the percent change in ATP turnover rate exceeds (sometimes by very large margins) the percent change in [S]. These observations are not easily explained by current metabolic regulation models but are consistent with pathway enzymes behaving as latent catalysts in resting muscle. In this view, the unmasking of such latent catalytic potential is the main explanation for how large changes in v can be achieved with modest (sometimes immeasurable) changes in [S].(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Regulating ATP turnover rates over broad dynamic work ranges in skeletal muscles. 828 52

Skeletal muscle has an inherent plasticity which allows it to undergo fibre type transformation when induced by a specific stimulus. Electrical stimulation has been used here to induce transformation of a predominantly fast type skeletal muscle towards a slow, more fatigue-resistant phenotype, which is more suitable for use in long-term cardiac assistance. Muscle samples from animals electrically stimulated for periods up to 6 months have been analysed by electrophoresis for myosin heavy chain (MHC) and myosin light chain (MLC) fast and slow isoforms. Densitometry and computer analysis have been used to determine the pattern of transformation of the different myosin subunits over this time period. MHC and MLC 2 fast to slow isoform switching preceded that of the alkali light chains (MLC1 and MLC3). After 3 months of stimulation the MHC slow isoform was found to have doubled in concentration relative to the unstimulated control muscle and by 4 months accounted for almost 50% of the total MHC content. The slow isoform accounted for 75% of the MLC2 after 4 months of stimulation. The protein products of mRNA isolated from stimulated muscle samples, translated in vitro and separated by electrophoresis, showed that transformation at the mRNA level preceded that at the protein level. By 2-4 weeks of stimulation MLC2 slow isoform mRNA represented over 60% of the total MLC2 mRNA population. An understanding of the molecular structure of muscle during transformation provides insight into its haemodynamic performance in cardiac assistance.
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PMID:Electrophoretic and computer analysis of skeletal muscle used for cardiac assistance. 171 52

Rat extensor digitorum longus (EDL) muscles were overloaded by removal of the synergist tibialis anterior (TA). The weight of the overloaded muscle was increased 15 days after the initial operation and remained higher throughout the period studied (153 days). The times to peak twitch tension and half relaxation remained unaltered, but the twitch and tetanic tensions developed by the overloaded EDL muscles increased. The overloaded EDL muscles became significantly more fatigue resistant. In a separate group of animals the overloaded EDL muscle was also chronically stimulated at 10 Hz. The additional stimulation altered the response of the EDL to overload in that the time to peak twitch tension of the muscle was slightly prolonged. There was no increase in twitch or tetanic tension in spite of the increase in muscle weight, but the electrical stimulation led to a further increase in fatigue resistance above that seen in overloaded muscles. The histochemical and immunocytochemical examination of the muscle revealed that there was a moderate increase in succinate dehydrogenase activity in the muscles overloaded only, but a considerable increase in those overloaded muscles that were also stimulated. There was no obvious change in the number of muscle fibres that reacted with an antibody to slow myosin in either overloaded only or overloaded and stimulated EDL muscles. Thus the addition of continuous activity to overload induced a slowing of contraction and prevented the increase of force usually induced by overload.
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PMID:Adaptation of rat extensor digitorum longus to overload and increased activity. 174 7

It has been postulated that the staircase response of skeletal muscle is effected by Ca2+-dependent phosphorylation of myosin light chains. Dantrolene inhibits calcium release from the sarcoplasmic reticulum, so treatment with dantrolene should prolong the time required to reach the peak of the staircase response. Furthermore, since it has been postulated that Ca2+ release is attenuated by fatigue, there should be similarities between the staircase of fatigued muscle and staircase in the presence of dantrolene. The left gastrocnemius muscle of anesthetized rats was isolated in situ and connected to an isometric transducer. In the control condition, developed tension increased for approximately 10 s with repetitive stimulation at 10 Hz. The time to reach the peak of staircase was prolonged after a 5 min period of stimulation at 10 Hz or after injection of dantrolene sodium (0.8 mg.kg-1 IV). Other similarities between the dantrolene treated group and the fatigue group include the following: contraction time was shorter in fatigue (12.8 +/- 0.8 ms) and with dantrolene (14.7 +/- 1.3 ms) than in the controls (16.4 +/- 1 ms), and half-relaxation time increased from 5 s (10.8 +/- 9 ms) to 15 s (13.8 +/- 0.8 ms) of staircase in the control but did not increase at this time in the dantrolene or fatigue groups. This study demonstrates that both fatigue and a reduction in Ca2+ release from sarcoplasmic reticulum will prolong the time required to reach the peak of staircase.
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PMID:Skeletal muscle staircase response with fatigue or dantrolene sodium. 184 39

Determinations of fatigue ratio, twitch and tetanus tension, and contraction and half-relaxation times of the isometric twitch were made in 21 single fast-twitch motor units from the rat tibialis anterior muscle. Single motor units were functionally isolated by microdissection of the ventral root, and the glycogen depletion technique was used to demonstrate the muscle fibers in the unit. Morphological and immuno- and enzyme-histochemical methods were applied to serial muscle cross sections to characterize the muscle fibers in the unit. Three of the units had muscle fibers of the IIa type according to staining both for myofibrillar adenosinetriphosphatase after acid preincubation and with the use of monoclonal antibodies specific for myosin heavy chains (MHCs), i.e., the IIa-MHC isoform. The other 18 units were of the IIb type according to enzyme-histochemistry, but immunohistochemistry showed that in six of these units the muscle fibers exhibited the novel type IIx-MHC isoform and in the other 12 units the IIb-MHC isoform. It was found that the IIx motor units have contraction and half-relaxation times similar to those of types IIa and IIb units but have morphological, physiological, and biochemical properties that distinguish them from the latter two types.
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PMID:MHC composition and enzyme-histochemical and physiological properties of a novel fast-twitch motor unit type. 185 63

Potential mechanisms of fatigue (metabolic factors) and potentiation (phosphate incorporation by myosin phosphorylatable light chains) were investigated during recovery from a 60-s maximal voluntary isometric contraction (MVC) in the quadriceps muscle of 12 subjects. On separate days before and for 2 h after the 60-s MVC, either a 1-s MVC or electrically stimulated contractions were used as indexes to test muscle performance. Torque at the end of the 60-s MVC was 57% of the initial level, whereas torques from a 1-s MVC and 50-Hz stimulation were most depressed in the immediate recovery period. At this time, muscle biopsy analyses revealed significant decreases in ATP and phosphocreatine and a 19-fold increase in muscle lactate. Conversely, isometric twitch torque and torque from a 10-Hz stimulus were the least depressed of six contractile indexes and demonstrated potentiation of 25 and 34%, respectively, by 4 min of recovery (P less than 0.05). At this time, muscle lactate concentration was still 16 times greater than at rest. An increased phosphate content of the myosin phosphorylatable light chains (P less than 0.05) was also evident both immediately and 4 min after the 60-s MVC. We conclude that the 60-s MVC produced marked force decreases likely due to metabolic displacement, while the limited decline in the twitch and 10-Hz torques and their significant potentiation suggested that myosin phosphorylation may provide a mechanism to enhance contractile force under conditions of submaximal activation during fatigue.
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PMID:Simultaneous potentiation and fatigue in quadriceps after a 60-second maximal voluntary isometric contraction. 202 65

In this paper we develop a theory for calculating the chemical energy liberation and heat production of a skeletal muscle subjected to an arbitrary history of stimulation, loading, and length variation. This theory is based on and complements the distribution-moment (DM) model of muscle [Zahalak and Ma, J. biomech. Engng 112, 52-62 (1990)]. The DM model is a mathematical approximation of the A. F. Huxley cross-bridge theory and represents a muscle in terms of five (normalized) state variables: A, the muscle length, c, the sarcoplasmic free calcium concentration, and Q0, Q1, Q2, the first three moments of the actin-myosin bond-distribution function (which, respectively, have macroscopic interpretations as the muscle stiffness, force, and elastic energy stored in the contractile tissue). From this model are derived two equations which predict the chemical energy liberation and heat production rates in terms of the five DM state variables, and which take account of the following factors: (1) phosphocreatine hydrolysis associated with cross-bridge cycling; (2) phosphocreatine hydrolysis associated with sarcoplasmic-reticulum pumping of calcium; (3) passive calcium flux across the sarcoplasmic-reticulum membrane; (4) calcium-troponin bonding; (5) cross-bridge bonding at zero strain; (6) cross-bridge strain energy; (7) tendon strain energy; and (8) external work. Using estimated parameters appropriate for a frog sartorius at 0 degree C, the energy rates are calculated for several experiments reported in the literature, and reasonable agreement is found between our model and the measurements. (The selected experiments are confined to the plateau of the isometric length-tension curve, although our theory admits arbitrary length variations.) The two most important contributions to the energy rates are phosphocreatine hydrolysis associated with cross-bridge cycling and with sarcoplasmic-reticulum calcium pumping, and these two contributions are approximately equal under tetanic, isometric, steady-state conditions. The contribution of the calcium flux across the electrochemical potential gradient at the sarcoplasmic-reticulum membrane was found to be small under all conditions examined, and can be neglected. Long-term fatigue and oxidative recovery effects are not included in this theory. Also not included is the so-called 'unexplained energy' presumably associated with reactions which have not yet been identified. Within these limitations our model defines clear quantitative interrelations between the activation, mechanics, and energetics in muscle, and permits rational estimates of the energy production to be calculated for arbitrary programs of muscular work.
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PMID:A distribution-moment model of energetics in skeletal muscle. 849 84

Indices of electrically stimulated and maximal voluntary isometric muscle torgue and the phosphate content of myosin phosphorylatable light chains (P light chains) were studied during recovery following a 60-s maximal voluntary isometric contraction (MVC) in 21 human subjects. Analysis of muscle biopsy samples revealed that immediately after the 60-s MVC there were significant decreases in ATP (-15%) and phosphocreatine (-82%), and lactate concentration increased by 17-fold. All indices of muscle torque production were reduced by the 60-s MVC, but the twitch torque and torque at 10 Hz were relatively less reduced compared with the torque at 20 and 50 Hz or a 1-s MVC. Between 3 and 6 min of recovery, twitch torque and torque at 10 Hz stimulation were significantly potentiated, reaching peak values of 125 and 134%, respectively, compared with rest. Phosphate content of the fast and two slow P light chains was significantly increased over rest levels immediately after and 4 min after the 60-s MVC. These results suggest that myosin P light-chain phosphorylation could provide a mechanism to increase human muscle torque under conditions of submaximal contractile element activation following fatigue.
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PMID:Torque potentiation and myosin light-chain phosphorylation in human muscle following a fatiguing contraction. 205 43


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