Sarcomere length dispersion in single skeletal muscle fibers and fiber bundles.

Light diffraction patterns produced by single skeletal muscle fibers and small fiber bundles of Rana pipiens semitendinosus have been examined at rest and during tetanic contraction. The muscle diffraction patterns were recorded with a vidicon camera interfaced to a minicomputer. Digitized video output was analyzed on-line to determine mean sarcomere length, line intensity, and the distribution of sarcomere lengths. The occurrence of first-order line intensity and peak amplitude maxima at approximately 3.0 mum is interpreted in terms of simple scattering theory. Measurements made along the length of a singel fiber reveal small variations in calculated mean sarcomere length (SD about 1.2%) and its percent dispersion (2.1% +/- 0.8%). Dispersion in small multifiber preparations increases approximately linearly with fiber number (about 0.2% per fiber) to a maximum of 8-10% in large bundles. Dispersion measurements based upon diffraction line analysis are comparable to SDs calculated from length distribution histograms obtained by light micrography of the fiber. First-order line intensity decreases by about 40% during tetanus; larger multifibered bundles exhibit substantial increases in sarcomere dispersion during contraction, but single fibers show no appreciable dispersion change. These results suggest the occurrence of asynchronous static or dynamic axial disordering of thick filaments, with a persistence in long range order of sarcomere spacing during contraction in single fibers.     

Tension relaxation after stretch in resting mammalian muscle fibers: stretch activation at physiological temperatures.

Tension responses to ramp stretches of 1-3% Lo (fiber length) in amplitude were examined in resting muscle fibers of the rat at temperatures ranging from 10 degrees C to 36 degrees C. Experiments were done using bundles of approximately 10 intact fibers isolated from the extensor digitorum longus (a fast muscle) and the soleus (a slow muscle). At low temperatures (below approximately 20 degrees C), the tension response consisted of an initial rise to a peak during the ramp followed by a complex tension decay to a plateau level; the tension decay occurred at approximately constant sarcomere length. The tension decay after a standard stretch at approximately 3-4.Lo/s contained a fast, an intermediate, and a (small amplitude) slow component, which at 10 degrees C (sarcomere length approximately 2.5 microns) were approximately 2000.s-1, approximately 150.s-1, and approximately 25.s-1 for fast fibers and approximately 2000.s-1, approximately 70.s-1 and approximately 8.s-1 for slow fibers, respectively. The fast component may represent the decay of interfilamentary viscous resistance, and the intermediate component may be due to viscoelasticity in the gap (titin, connectin) filament. The two- to threefold fast-slow muscle difference in the rate of passive tension relaxation (in the intermediate and the slow components) compares with previously reported differences in the speed of their active contractions; this suggests that "passive viscoelasticity" is appropriately matched to contraction speed in different muscle fiber types. At approximately 35 degrees C, the fast and intermediate components of tension relaxation were followed by a delayed tension rise at approximately 10.s-1 (fast fibers) and 2.5.s-1 (slow fibers); the delayed tension rise was accompanied by sarcomere shortening. BDM (5-10 mM) reduced the active twitch and tetanic tension responses and the delayed tension rise at 35 degrees C; the results indicate stretch sensitive activation in mammalian sarcomeres at physiological temperatures.     

The sarcomere length-tension relation in skeletal muscle

Tension development during isometric tetani in single fibers of frog semitendinosus muscle occurs in three phases: (a) in initial fast-rise phase; (b) a slow-rise phase; and (c) a plateau, which lasts greater than 10 s. The slow-rise phase has previously been assumed to rise out of a progressive increase of sarcomere length dispersion along the fiber (Gordon et al. 1966. J. Physiol. [Lond.]. 184:143--169;184:170-- 192). Consequently, the "true" tetanic tension has been considered to be the one existing before the onset of the slow-rise phase; this is obtained by extrapolating the slowly rising tension back to the start of the tetanus. In the study by Gordon et al. (1966. J. Physiol. [Lond.] 184:170--192), as well as in the present study, the relation between this extrapolated tension and sarcomere length gave the familiar linear descending limb of the length-tension relation. We tested the assumption that the slow rise of tension was due to a progressive increase in sarcomere length dispersion. During the fast rise, the slow rise, and the plateau of tension, the sarcomere length dispersion at any area along the muscle was less than 4% of the average sarcomere length. Therefore, a progressive increase of sarcomere length dispersion during contraction appears unable to account for the slow rise of tetanic tension. A sarcomere length-tension relation was constructed from the levels of tension and sarcomere length measured during the plateau. Tension was independent of sarcomere length between 1.9 and 2.6 microgram, and declined to 50% maximal at 3.4 microgram. This result is difficult to reconcile with the cross-bridge model of force generation.     

A quantitative model of intersarcomere dynamics during fixed-end contractions of single frog muscle fibers.

A numerical model of a muscle fiber as 400 sarcomeres, identical except for their initial lengths, was used to simulate fixed-end tetanic contractions of frog single fibers at sarcomere lengths above the optimum. The sarcomeres were represented by a lumped model, constructed from the passive and active sarcomere length-tension curves, the force-velocity curve, and the observed active elasticity of a single frog muscle fiber. An intersarcomere force was included to prevent large disparities in lengths of neighboring sarcomeres. The model duplicated the fast rise, slow creep rise, peak, and slow decline of tension seen in tetanic contractions of stretched living fibers. Decreasing the initial non-uniformity of sarcomere length reduced the rate of rise of tension during the creep phase, but did not decrease the peak tension reached. Limitations of the model, and other processes that might contribute to the shape of the fixed end tetanic tension record are discussed. Taking account of model and experimental results, it is concluded that the distinctive features of the tension records of fixed end tetanic contraction at lengths beyond optimum can be explained by internal motion within the fiber.     

Resting tension characteristics in differentiating intact rat fast- and slow-twitch muscle fibers.

The postnatal changes in resting muscle tension were investigated at 20 degrees C by using small muscle fiber bundles isolated from either the extensor digitorum longus or the soleus of both neonatal (7-21 days old) and adult rats. The results show that the tension-extension characteristics of the bundles depended on the age of the rats. For example, both the extensor digitorum longus and soleus bundles of rats older than 14 days showed characteristic differences that were absent in bundles from younger rats. Furthermore, the tension-extension relation of the adult slow muscle fiber bundles were similar to those of the two neonatal muscles and were shifted to longer sarcomere lengths relative to those of the adult fast-fiber bundles. Thus, at the extended sarcomere length of 2.9 microm, the adult fast muscle fiber bundles developed higher resting tensions (5.6 +/- 0.5 kN/m2) than either the two neonatal ( approximately 3 kN/m2) or the adult slow (3.1 +/- 0.4 kN/m2) muscle fiber bundles. At all ages examined, the resting tension responses to a ramp stretch were qualitatively similar and consisted of three components: a viscous, a viscoelastic, and an elastic tension. However, in rats older than 14 days, all three tension components showed clear fast- and slow-fiber type differences that were absent in younger rats. Bundles from 7-day-old rats also developed significantly lower resting tensions than the corresponding adult ones. Additionally, the resting tension characteristics of the adult muscles were not affected by chemical skinning. From these results, we conclude that in rats resting muscle tension, like active tension, differentiates within the first 3 wk after birth.     

Sprint training attenuates the deficits of force and dynamic stiffness in rat soleus muscle caused by eccentric contractions

We investigated whether sprint training attenuates the deficits in force and dynamic stiffness caused by eccentric contractions to the soleus muscles of Wistar rats. Two groups of male rats were analyzed: sedentary (C, n=8) and trained (T, n=8). T rats were sprint trained for 10 weeks. Subsequently, the right soleus muscles of rats were freed under anesthesia, leaving the bone insertion and blood supply intact. Eccentric contractions were induced by lengthening muscles during tetanic contractions. Force and dynamic stiffness were tested before and after 20 rounds of eccentric contractions. Tension decline was analyzed using a two-state model (first-order kinetics) in the context of Kramer's theory. Training improved the twitch tension (C, 6.44+/-0.6N/cm(2); T, 10.90+/-0.8N/cm(2)), tetanic force (C, 61.74+/-0.6N/cm(2); T, 85.62+/-0.8N/cm(2)), and increased the dynamic stiffness (C, 41.28+/-1.0N/cm(2); T, 49.56+/-3.2N/cm(2)). Twitch tension after eccentric contractions declined to 73% and 75% in C and T groups, respectively, while tetanic tension decreased to 60% and 36% in C and T groups, respectively. After eccentric contractions, dynamic stiffness decreases were smaller in T rats (from 49.56+/-3.2 to 36.09+/-2.1N/cm(2)) than in C rats (from 41.28+/-1.0 to 20.73+/-1.8N/cm(2)). Sprint training increased the dynamic stiffness and tetanic tension of the soleus muscle and protected against the attenuation induced by eccentric contractions. Finally, the two-state model provided evidence that the number of force-generating cross-bridges increases in trained muscle.     

江豚鼻道肌的解剖和构筑研究

江豚的鼻部肌共分为后外肌、前外肌、后内肌、前内肌和深肌5层,无间肌和大小内肌较退化,无对角膜肌。通过测定各肌的肌重、平均肌纤维长、平均肌小节长以及肌纤维角度,计算了各肌的生理横截面积,估计最大强直张力和肌鲜重对估计最大强直张力之比值等指标。鼻部肌各肌的相对肌纤维长度相似。各鼻部肌的肌纤维角度均为零。前部肌比后部肌具有较大的收缩速度和收缩位移优势,后部肌则具有较强的张力产生能力。着于额隆和唇部吻肌的张力产生能力很强。     

Sarcomere shortening in pressure overload hypertrophy

Sarcomere shortening during contraction was measured by using laser diffraction, in thin, rabbit right ventricular (RV) trabeculae from normal hearts (N) (n = 5) and from hearts subjected to RV pressure overload by pulmonary banding (H) (n = 5). Banding resulted in substantial RV hypertrophy after 2 wk. Hypertrophied preparations had the same resting muscle length (H = 3.15 +/- 0.29 mm) and resting sarcomere lengths (H = 2.16 +/- 0.005 micron) as the normal preparations (3.10 +/- 0.37 mm, 2.16 +/- 0.008 micron, respectively). Total tension at the peak of isometric twitches was the same as normal in the hypertrophied muscles (N = 8.06 +/- 1.20, H = 8.51 +/- 1.95 g/mm2). However, the amount of auxotonic sarcomere shortening was much less than normal in the hypertrophied preparations (N = 0.39 +/- 0.028, H = 0.19 +/- 0.034 micron; P less than 0.001). In isotonic contractions in which the ratio of muscle shortening to resting muscle length was the same in both the normal and hypertrophied muscles (ratio of 0.05 in both groups), the extent of sarcomere shortening relative to resting sarcomere length was less in the hypertrophied muscles than in the normal preparations (N = 0.14 +/- 0.01), H = 0.07 +/- 0.01; P less than 0.01). Series elasticity was the same as normal in the hypertrophied muscle P less than 0.05). Less auxotonic sarcomere shortening for a given level of isometric tension development and less isotonic sarcomere shortening per unit muscle shortening indicate that there is less than normal work per sarcomere during contraction in hypertrophied myocardium. These findings may have important implications for intracellular compensatory adaptation in pressure overload cardiac hypertrophy.     

Exercise training alters length dependence of contractile properties in rat myocardium.

Myocardial function is enhanced by endurance exercise training, but the cellular mechanisms underlying this improved function remain unclear. Exercise training increases the sensitivity of rat cardiac myocytes to activation by Ca(2+), and this Ca(2+) sensitivity has been shown to be highly dependent on sarcomere length. We tested the hypothesis that exercise training increases this length dependence in cardiac myocytes. Female Sprague-Dawley rats were divided into sedentary control (C) and exercise-trained (T) groups. The T rats underwent 11 wk of progressive treadmill exercise. Heart weight increased by 14% in T compared with C rats, and plantaris muscle citrate synthase activity showed a 39% increase with training. Steady-state tension was determined in permeabilized myocytes by using solutions of various Ca(2+) concentration (pCa), and tension-pCa curves were generated at two different sarcomere lengths for each myocyte (1.9 and 2.3 microm). We found an increased sarcomere length dependence of both maximal tension and pCa(50) (the Ca(2+) concentration giving 50% of maximal tension) in T compared with C myocytes. The DeltapCa(50) between the long and short sarcomere length was 0.084 +/- 0.023 (mean +/- SD) in myocytes from C hearts compared with 0.132 +/- 0.014 in myocytes from T hearts (n = 50 myocytes per group). The Deltamaximal tension was 5.11 +/- 1.42 kN/m(2) in C myocytes and 9.01 +/- 1.28 in T myocytes. We conclude that exercise training increases the length dependence of maximal and submaximal tension in cardiac myocytes, and this change may underlie, at least in part, training-induced enhancement of myocardial function.     

甲状腺功能亢进大鼠比目鱼肌肌浆网Ca2+-ATP酶活性增高可加速强直收缩疲劳

甲状腺功能亢进(甲亢)时甲状腺素分泌增加,不仅使具有神经支配的慢缩型肌纤维向快缩型转化,而且改变骨骼肌的强直收缩功能.因此,甲亢性肌病的肌肉乏力可能与骨骼肌强直收缩易发生疲劳有关.本实验在离体条件下,观测甲亢4周引起的大鼠慢缩肌--比目鱼肌(soleus, SOL)单收缩与间断强直收缩功能的变化.结果显示,甲亢4周大鼠体重明显低于同步对照组[(292±13)g vs (354±10)g],但SOL湿重没有明显改变[(107.3±8.6)mg vs (115.1±6.9)mg].甲亢大鼠SOL单收缩张力达到峰值的时间(time to peak tension, TPT)、从峰值降至75%舒张时间(time from peak tension to 75% relaxation, TR75)均明显缩短;强直收缩的TR75也明显缩短[(102.8±4.1)ms vs (178.8±15.8)ms];强直收缩的最适频率从对照组的100Hz增加到140Hz;间断强直收缩期间容易发生疲劳.甲亢大鼠SOL肌浆网Ca2 -ATP酶(sarcoplasmic-reticulum Ca2 -ATPase, SERCA)活性增高.采用SERCA特异性抑制剂CPA (1.0μmol/L)处理后,对照组与甲亢大鼠SOL间断强直收缩的TR75均延长,同时不易出现疲劳.5.0μmol/L CPA灌流虽可进一步抵抗甲亢大鼠SOL间断强直收缩引起的疲劳,但强直收缩期间的静息张力却明显升高.将CPA浓度增至10.0μmol/L,甲亢大鼠SOL间断强直收缩又趋向易发生疲劳.这些结果提示,与心肌相同,骨骼肌肌纤维SERCA活性亦可影响单收缩与强直收缩的舒张时间,SERCA活性升高可加速间断强直收缩发生疲劳.     

Muscle LIM protein plays both structural and functional roles in skeletal muscle

Muscle LIM protein (MLP) has been suggested to be an important mediator of mechanical stress in cardiac tissue, but the role that it plays in skeletal muscle remains unclear. Previous studies have shown that it is dramatically upregulated in fast-to-slow fiber-type transformation and also after eccentric contraction (EC)-induced muscle injury. The functional consequences of this upregulation, if any, are unclear. In the present study, we have examined the skeletal muscle phenotype of MLP-knockout (MLPKO) mice in terms of their response to EC-induced muscle injuries. The data suggest that while the MLPKO mice recover completely after EC-induced injury, their torque production lags behind that of heterozygous littermates in the early stages of the recovery process. This lag is accompanied by decreased expression of the muscle regulatory factor MyoD, suggesting that MLP may influence gene expression. In addition, there is evidence of type I fiber atrophy and a shorter resting sarcomere length in the MLPKO mice, but no significant differences in fiber type distribution. In summary, MLP appears to play a subtle role in the maintenance of normal muscle characteristics and in the early events of the recovery process of skeletal muscle to injury, serving both structural and gene-regulatory roles. eccentric contractions; passive tension     

Evidence for structurally different attached states of myosin cross-bridges on actin during contraction of fish muscle.

Using data from fast time-resolved x-ray diffraction experiments on the synchrotrons at Daresbury and (Deutsches Elektronen Synchrotron [DESY]), it is shown that during contraction of fish muscle there are at least two distinct configurations of myosin cross-bridges on actin, that they appear to have different tension producing properties and that they probably differ in the axial tilt of the cross-bridges on actin. Evidence is presented for newly observed myosin-based layer lines in patterns from active fish muscle, together with intensity changes of the actin layer lines. On the equator, the 110 reflection changes much faster (time for 50% change t1/2 = 21 +/- 4 ms after activation) than the 100 reflection (t1/2 = 35 +/- 8 ms) and tension (t1/2 = 41 +/- 3 ms) during the rising phase of tetanic contractions. These and higher order reflections have been used to show the time course of mass attachment at actin during this rising phase. Mass arrival (t1/2 = 25 ms) precedes tension by approximately 15 ms. Analysis has been carried out to evaluate the effects of changes in sarcomere length during the tetanus. It is shown that any such effects are very small. Difference "equatorial" electron density maps between active muscle at a time when mass arrival at actin is just complete, but the tension is still rising, and at a later time well into the tension plateau, show that the structural difference between the lower and higher force states corresponds to mass movement consistent with axial swinging of heads from a nonstereospecific actin attached state (low force) to a more stereospecific (high force) state.     

Muscle damage induced by eccentric contractions of 25% strain

Contractile and morphological properties were measured in the rabbit tibialis anterior muscle 1 h after isometric contraction (IC), passive stretch (PS), or eccentric contraction (EC). Maximal tetanic tension (Po) was reduced after 30 min of PS (P less than 0.001), IC (P less than 0.001), or EC (P less than 0.0001). However, the magnitude of the force deficit was a function of the treatment method. After 30 min of cyclic PS, Po decreased by 13%, whereas after IC or EC, Po decreased by 31 and 69%, respectively. The time course of tension decline in the various groups suggested that the EC-induced injury occurred during the first few minutes of treatment. Although the morphology of samples from the PS and IC groups appeared normal, eccentrically exercised muscles exhibited portions of abnormally large fibers (diam greater than or equal to 110 microns) when viewed in cross section. Examination of 231 such fibers from 6 muscles revealed that all enlarged fibers were exclusively of the fast-twitch glycolytic fiber type. Although no ultrastructural abnormalities were observed in any of the muscles from the IC or PS groups, a significant portion of the fibers in the EC group displayed various degrees of disorganization of the sarcomeric band pattern. Taken together, these studies highlight the importance of fiber oxidative capacity in EC-induced injury, which may be related to the damage mechanism.     

A non-cross-bridge stiffness in activated frog muscle fibers

Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14 degrees C at approximately 2.1 microm sarcomere length on fibers bathed in Ringer's solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed "static tension," and the ratio between the force and the accompanying sarcomere length change was termed "static stiffness." The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1-2.8 microm, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges.     

Length dependence of active force production in skeletal muscle.

The sliding filament and cross-bridge theories of muscle contraction provide discrete predictions of the tetanic force-length relationship of skeletal muscle that have been tested experimentally. The active force generated by a maximally activated single fiber (with sarcomere length control) is maximal when the filament overlap is optimized and is proportionally decreased when overlap is diminished. The force-length relationship is a static property of skeletal muscle and, therefore, it does not predict the consequences of dynamic contractions. Changes in sarcomere length during muscle contraction result in modulation of the active force that is not necessarily predicted by the cross-bridge theory. The results of in vivo studies of the force-length relationship suggest that muscles that operate on the ascending limb of the force-length relationship typically function in stretch-shortening cycle contractions, and muscles that operate on the descending limb typically function in shorten-stretch cycle contractions. The joint moments produced by a muscle depend on the moment arm and the sarcomere length of the muscle. Moment arm magnitude also affects the excursion (length change) of a muscle for a given change in joint angle, and the number of sarcomeres arranged in series within a muscle fiber determines the sarcomere length change associated with a given excursion.     

Metabolic correlates of fatigue and of recovery from fatigue in single frog muscle fibers

Fatigue and recovery from fatigue were related to metabolism in single fibers of the frog semitendinosus muscle. The fibers were held at a sarcomere length of 2.3 microm in oxygenated Ringer solution at 15 degrees C and were stimulated for up to 150 s by a schedule of 10-s, 20-Hz tetanic trains that were interrupted by 1-s rest periods, after which they were rapidly frozen for biochemical analysis. Two kinds of fatigue were produced in relation to stimulus duration. A rapidly reversed fatigue occurred with stimulation for under 40 s and was evidenced by a decline in tetanic tension that could be overcome by 1 s of rest. A prolonged fatigue was caused by stimulation for 100-150 s. It was evidenced during stimulation by a fall in tetanic tension that could not be overcome by 1 s of rest, and after stimulation by a reduction, lasting for up to 82 min, in the peak tension of a 200-ms test tetanus. Fiber phosphocreatine (PCr) fell logarithmically in relation to stimulus duration, from a mean of 121 +/- 8 nmol/mg protein (SEM, n = 12) to 10% of this value after 150 s of stimulation. PCr returned to normal levels after 90-120 min of rest. Stimulation for 150 s did not significantly affect fiber glycogen and reduced fiber ATP by at most 15%. It is suggested that the prolonged fatigue caused by 100-150 s of tetanic stimulation was caused by long-lasting failure of excitation-contraction coupling, as it was not accompanied by depletion of energy stores in the form of ATP. One possibility is that H+ accumulated in fatigued fibers so as to interfere with the action of Ca2+ in the coupling process.     

The extracellular matrix in hibernating myocardium - a significant factor causing structural defects and cardiac dysfunction

The time course of force generation and the time course of muscle stiffness were measured in rabbit soleus muscles during eccentric contraction to understand the underlying basis for the force loss in these muscles. Muscles were activated for 600 msec every 10 sec for 30 min. Soleus muscles contracting isometrically maintained constant tension throughout the treatment period, while muscles subjected to eccentric contraction rapidly dropped tension generation by 75% within the first few minutes and then an additional 10% by the end of 30 min. This indicated a dramatic loss in force-generating ability throughout the 30 min treatment period. To estimate the relative number of cross-bridges attached during the isometric force generation phase immediately preceding each eccentric contraction, stiffness was measured during a small stretch of a magnitude equal to 1.5% of the fiber length. Initially, muscle stiffness exceeded 1300 g/mm and, as eccentric treatment progressed, stiffness decreased to about 900 g/mm. Thus, while muscle stiffness decreased by only 30% over the 30 min treatment period, isometric force decreased by 85%. In isometrically activated muscles, stiffness remained constant throughout the treatment period. These data indicate that, while soleus muscles decreased their force generating capability significantly, there were a number of cross-bridges still attached that were not generating force. In summary, the loss of force generating capacity in the rabbit soleus muscle appears to be related to a fundamental change in myosin cross-bridge properties without the more dramatic morphological changes observed in other eccentric contraction models. These results are compared and contrasted with the observations made on muscles composed primarily of fast fibers.     

Optical Diffraction Studies of Muscle Fibers

A new technique to monitor light diffraction patterns electrically is applied to frog semitendinosus muscle fibers at various levels of stretch. The intensity of the diffraction lines, sarcomere length change, and the length-dispersion (line width) were calculated by fast analogue circuits and displayed in real time. A heliumneon laser (wavelength 6328 Å) was used as a light source. It was found that the intensity of the first-order diffraction line drops significantly (30-50%) at an optimal sarcomere length of 2.8 μm on isometric tetanic stimulation. Such stimulation produced contraction of half-sarcomeres by about 22 nm presumably by stretching inactive elements such as tendons. The dispersion of the sarcomere lengths is extremely small, and it is proportional to the sarcomere length (less than 4%). The dispersion increases on stimulation. These changes on isometric tetanic stimulation were dependent on sarcomere length. No vibration or oscillation in the averaged length of the sarcomeres was found during isometric tetanus within a resolution of 3 nm; however, our observation of increased length dispersion of the sarcomeres together with detection of the averaged shortening of the sarcomere lengths suggests the presence of asynchronous cyclic motions between thick and thin filaments. An alternative explanation is simply an increase of the length dispersion of sarcomeres without cyclic motions.     

Effects of excitation-contraction uncoupling by stretch and hypertonicity on metabolism and tension in single frog muscle fibers

Metabolism and tension were examined in single fibers of the semitendinosus muscle of Rana pipiens at 15 degree C after excitation- contraction uncoupling by stretch and hypertonicity. Interrupted tetanic stimulation at 20 HZ for 150 s, of control fibers in isotonic Ringer at a rest sarcomere length (SL) of 2.3 micrometers, resulted in a steadily declining tension, stimulated glycolysis, and significantly reduced fiber phosphocreatine (PCr) and ATP concentrations. Stretching resting muscle fibers to an SL of 4.7 micrometers did not alter metabolite concentrations, but glucose-6-phosphate rose and PCr fell markedly when the stretched fibers were stimulated tetanically, although tension was absent. Immersion of untetanized fibers in 2.5 X isotonic Ringer produced a transient rise in resting tension, an increase in glucose-6-phosphate, and a significant reduction in PCr. During the transient rise in resting tension, PCr consumption per unit of tension-time integral was the same as that in fibers stimulated tetanically in isotonic Ringer. Tetanization of fibers in hypertonic solution did not further alter metabolite concentrations or produce tension. The results indicate that exposure to hypertonicity induces an increase in both tension and consumption of high-energy phosphate bonds (approximately P) in resting fibers, but stretch does not. during tetanic stimulation, stretch interferes with contraction but does not prevent activation, whereas hypertonicity inhibits activation as well as contraction.     

Observation of fatigue unrelated to gross energy reserve of skeletal muscle during tetanic contraction--an application of 31P-MRS

The mechanism of muscle fatigue was studied by 31P-MRS. During tetanic contraction for 2 minutes(min), the tension measured with a strain gauge and Phosphocreatine(PCr)/Inorganic phosphate(Pi)+ Phosphomonoester(PME) ratio decreased to 31.5 +/- 4.4% of the control value and 0.6 +/- 0.1, respectively. The intracellular pH(pH) also decreased to 6.62 +/- 0.04. Toward the end of the stimulation, the tension decreased to 25.3 +/- 1.9% of the control value. However, during 20min stimulation, the PCr/(Pi+PME) ratio increased to 2.5 +/- 0.5 and the pH to 6.91 +/- 0.04. These results show that muscular fatigue is ascribable not to a decreased level of high energy metabolites required for actomyosin ATPase, but to an increase in the threshold intensity of excitation in excitation-contraction coupling.