Contractile properties of mouse single muscle fibers, a comparison with amphibian muscle fibers

Knee extensor torque-velocity relationships following anterior cruciate ligament reconstruction

BACKGROUND

The inherent nature of the torque-velocity relationship is the inverse nature between the velocity of muscle contraction and torque production and is an indication of muscle function. The purpose of this study was to characterize the torque-velocity relationship in the quadriceps following anterior cruciate ligament reconstruction compared to healthy limbs.

METHODS

681 participants were included, 493 of which were patients at least four months following anterior cruciate ligament reconstruction (23.2 ± 10.08 yr, 6.6 ± 5.37 months post-surgery) and 188 were healthy participants (21.6 ± 3.77 yr). A subset of 175 post-surgical participants completed a repeated visit (8.1 ± 1.71 months post-surgery). Participants completed isokinetic knee extension at 90°/s and 180°/s. A one-way ANOVA was used to compare torque velocity relationships by limb type (surgical, contralateral, healthy). Paired samples t-tests were conducted to analyze the torque-velocity relationship across limbs and across time.

FINDINGS

There was a large effect for limb type on torque-velocity (F(2, 1173) = 146.08, p < 0.001, η2 = 0.20). Surgical limbs demonstrated significantly lower torque-velocity relationships compared to the contralateral limbs (ACLR: 0.26 Nm/kg, contralateral:0.55 Nm/kg, p < 0.001, d = 1.18). Healthy limbs had similar torque-velocity relationships bilaterally (dominant limb: 0.48 Nm/kg, non-dominant limb: 0.49 Nm/kg, p = 0.45). The torque velocity relationship for the involved limb significantly increased in magnitude over time (+0.11 Nm/kg, p < 0.001, d = -0.61) while the contralateral limb torque-velocity relationship remained stable over time (0.0 Nm/kg difference, p = 0.60).

INTERPRETATION

Following surgery, the knee extensors appear to have altered torque-velocity relationships compared to contralateral and healthy limbs. This may indicate a specific target for assessment and rehabilitation following surgery.

Insights into Muscle Contraction Derived from the Effects of Small-Molecular Actomyosin-Modulating Compounds

Bottom-up mechanokinetic models predict ensemble function of actin and myosin based on parameter values derived from studies using isolated proteins. To be generally useful, e.g., to analyze disease effects, such models must also be able to predict ensemble function when actomyosin interaction kinetics are modified differently from normal. Here, we test this capability for a model recently shown to predict several physiological phenomena along with the effects of the small molecular compound blebbistatin. We demonstrate that this model also qualitatively predicts effects of other well-characterized drugs as well as varied concentrations of MgATP. However, the effects of one compound, amrinone, are not well accounted for quantitatively. We therefore systematically varied key model parameters to address this issue, leading to the increased amplitude of the second sub-stroke of the power stroke from 1 nm to 2.2 nm, an unchanged first sub-stroke (5.3−5.5 nm), and an effective cross-bridge attachment rate that more than doubled. In addition to better accounting for the effects of amrinone, the modified model also accounts well for normal physiological ensemble function. Moreover, a Monte Carlo simulation-based version of the model was used to evaluate force−velocity data from small myosin ensembles. We discuss our findings in relation to key aspects of actin−myosin operation mechanisms causing a non-hyperbolic shape of the force−velocity relationship at high loads. We also discuss remaining limitations of the model, including uncertainty of whether the cross-bridge elasticity is linear or not, the capability to account for contractile properties of very small actomyosin ensembles (<20 myosin heads), and the mechanism for requirements of a higher cross-bridge attachment rate during shortening compared to during isometric contraction.

Glycine receptor subunit-ß -deficiency in a mouse model of spasticity results in attenuated physical performance, growth and muscle strength

Spasticity is the most common neurological disorder associated with increased muscle contraction causing impaired movement and gait. The aim of this study was to characterize the physical performance, skeletal muscle function, and phenotype of mice with a hereditary spastic mutation (B6.Cg-Glrb^spa/J). Motor function, gait, and physical activity of juvenile and adult spastic mice and the morphological, histological, and mechanical characteristics of their soleus and gastrocnemius medialis muscles were compared with those of their wild-type (WT) littermates. Spastic mice showed attenuated growth, impaired motor function, and low physical activity. Gait of spastic mice was characterized by a typical hopping pattern. Spastic mice showed lower muscle forces, which were related to the smaller physiological cross-sectional area of spastic muscles. The muscle-tendon complex length-force relationship of adult gastrocnemius medialis was shifted toward shorter lengths, which was explained by attenuated longitudinal tibia growth. Spastic gastrocnemius medialis was more fatigue resistant than WT gastrocnemius medialis. This was largely explained by a higher mitochondrial content in muscle fibers and relatively higher percentage of slow-type muscle fibers. Muscles of juvenile spastic mice showed similar differences compared with WT juvenile mice, but these were less pronounced than between adult mice. This study shows that in spastic mice, disturbed motor function and gait is likely to be the result of hyperactivity of skeletal muscle and impaired skeletal muscle growth, which progress with age.

Myofiber stretch induces tensile and shear deformation of muscle stem cells in their native niche

Muscle stem cells (MuSCs) are requisite for skeletal muscle regeneration and homeostasis. Proper functioning of MuSCs, including activation, proliferation, and fate decision, is determined by an orchestrated series of events and communication between MuSCs and their niche. A multitude of biochemical stimuli are known to regulate MuSC fate and function. However, in addition to biochemical factors, it is conceivable that MuSCs are subjected to mechanical forces during muscle stretch-shortening cycles because of myofascial connections between MuSCs and myofibers. MuSCs respond to mechanical forces in vitro, but it remains to be proven whether physical forces are also exerted on MuSCs in their native niche and whether they contribute to the functioning and fate of MuSCs. MuSC deformation in their native niche resulting from mechanical loading of ex vivo myofiber bundles was visualized utilizing mT/mG double-fluorescent Cre-reporter mouse and multiphoton microscopy. MuSCs were subjected to 1 h pulsating fluid shear stress (PFSS) with a peak shear stress rate of 6.5 Pa/s. After PFSS treatment, nitric oxide, messenger RNA (mRNA) expression levels of genes involved in regulation of MuSC proliferation and differentiation, ERK 1/2, p38, and AKT activation were determined. Ex vivo stretching of extensor digitorum longus and soleus myofiber bundles caused compression as well as tensile and shear deformation of MuSCs in their niche. MuSCs responded to PFSS in vitro with increased nitric oxide production and an upward trend in iNOS mRNA levels. PFSS enhanced gene expression of c-Fos, Cdk4, and IL-6, whereas expression of Wnt1, MyoD, Myog, Wnt5a, COX2, Rspo1, Vangl2, Wnt10b, and MGF remained unchanged. ERK 1/2 and p38 MAPK signaling were also upregulated after PFSS treatment. We conclude that MuSCs in their native niche are subjected to force-induced deformations due to myofiber stretch-shortening. Moreover, MuSCs are mechanoresponsive, as evidenced by PFSS-mediated expression of factors by MuSCs known to promote proliferation.

Effect of myofibril architecture on the active contraction of dystrophic muscle. A mathematical model

Duchenne muscular dystrophy (DMD) is a muscle degenerative disease caused by a mutation in the dystrophin gene. The lack of dystrophin leads to persistent inflammation, degeneration/regeneration cycles of muscle fibers, Ca^2＋ dysregulation, incompletely regenerated fibers, necrosis, fibrotic tissue replacement, and alterations in the fiber ultrastructure i.e., myofibril misalignment and branched fibers. This work aims to develop a comprehensive chemo-mechanical model of muscle-skeletal tissue accounting for dispersion in myofibrillar orientations, in addition to the disorders in sarcomere pattern and the fiber branching. The model results confirm a significant correlation between the myofibrillar dispersion and the reduction of isometric force in the dystrophic muscle and indicate that the reduction of contraction velocity in the dystrophic muscle seems to be associated with the local disorders in the sarcomere patterns of the myofibrils. Also, the implemented model can predict the force-velocity response to both concentric and eccentric loading. The resulting model represents an original approach to account for defects in the muscle ultrastructure caused by pathologies as DMD.

Comparison of linear, hyperbolic and double-hyperbolic models to assess the force-velocity relationship in multi-joint exercises

AbstractThis study assessed the validity of linear, hyperbolic and double-hyperbolic models to fit measured force-velocity (F-V) data in multi-joint exercises and the influence of muscle excitation on the F-V relationship. The force-joint angle and F-V relationships were assessed in 10 cross-training athletes and 14 recreationally resistance-trained subjects in the unilateral leg press (LP) and bilateral bench press (BP) exercises, respectively. A force plate and a linear encoder were installed to register external force and velocity, respectively. Muscle excitation was assessed by surface EMG recording of the quadriceps femoris, biceps femoris and gluteus maximus muscles during the unilateral LP. Linear, Hill's (hyperbolic) and Edman's (double-hyperbolic) equations were fitted to the measured F-V data and compared. Measured F-V data were best fitted by double-hyperbolic models in both exercises (p < 0.05). F-V data deviated from the rectangular hyperbola above a breakpoint located at 90% of measured isometric force (F₀) and from the linearity at ≤45% of F₀ (both p < 0.05). Hyperbolic equations overestimated F₀ values by 13 ± 11% and 6 ± 6% in the LP and BP, respectively (p < 0.05). No differences were found between muscle excitation levels below and above the breakpoint (p > 0.05). Large associations between variables obtained from linear and double-hyperbolic models were noted for F₀, maximum muscle power, and velocity between 25% and 100% of F₀ (r = 0.70-0.99; all p < 0.05). The F-V relationship in multi-joint exercises was double-hyperbolic, which was unrelated with lower muscle excitation levels. However, linear models may be valid to assess F₀, maximal muscle power and velocity between 25% and 100% of F₀.

Measurement of Skeletal Muscle Fiber Contractility with High-Speed Traction Microscopy

We describe a technique for simultaneous quantification of the contractile forces and cytosolic calcium dynamics of muscle fibers embedded in three-dimensional biopolymer gels under auxotonic loading conditions. We derive a scaling law for linear elastic matrices such as basement membrane extract hydrogels (Matrigel) that allows us to measure contractile force from the shape of the relaxed and contracted muscle cell and the Young's modulus of the matrix without further knowledge of the matrix deformations surrounding the cell and without performing computationally intensive inverse force reconstruction algorithms. We apply our method to isolated mouse flexor digitorum brevis (FDB) fibers that are embedded in 10 mg/mL Matrigel. Upon electrical stimulation, individual FDB fibers show twitch forces of 0.37 ± 0.15 μN and tetanic forces (100-Hz stimulation frequency) of 2.38 ± 0.71 μN, corresponding to a tension of 0.44 ± 0.25 kPa and 2.53 ± 1.17 kPa, respectively. Contractile forces of FDB fibers increase in response to caffeine and the troponin-calcium stabilizer tirasemtiv, similar to responses measured in whole muscle. From simultaneous high-speed measurements of cell length changes and cytosolic calcium concentration using confocal line scanning at a frequency of 2048 Hz, we show that twitch and tetanic force responses to electric pulses follow the low-pass filtered calcium signal. In summary, we present a technically simple high-speed method for measuring contractile forces and cytosolic calcium dynamics of single muscle fibers. We expect that our method will help to reduce preparation time, costs, and the number of sacrificed animals needed for experiments such as drug testing.

On the Shape of the Force-Velocity Relationship in Skeletal Muscles: The Linear, the Hyperbolic, and the Double-Hyperbolic

The shape of the force-velocity (F-V) relationship has important implications for different aspects of muscle physiology, such as muscle efficiency and fatigue, the understanding of the pathophysiology of several myopathies or the mechanisms of muscle contraction per se, and may be of relevance for other fields, such as the development of robotics and prosthetic applications featuring natural muscle-like properties. However, different opinions regarding the shape of the F-V relationship and the underlying mechanisms exist in the literature. In this review, we summarize relevant evidence on the shape of the F-V relationship obtained over the last century. Studies performed at multiple scales ranging from the sarcomere to the organism level have described the concentric F-V relationship as linear, hyperbolic or double-hyperbolic. While the F-V relationship has most frequently been described as a rectangular hyperbola, a large number of studies have found deviations from the hyperbolic function at both ends of the F-V relation. Indeed, current evidence suggests that the F-V relation in skeletal muscles follows a double-hyperbolic pattern, with a breakpoint located at very high forces/low velocities, which may be a direct consequence of the kinetic properties of myofilament cross-bridge formation. Deviations at low forces/high velocities, by contrast, may be related to a recently discovered, calcium-independent regulatory mechanism of muscle contraction, which may also explain the low metabolic cost of very fast muscle shortening contractions. Controversial results have also been reported regarding the eccentric F-V relationship, with studies in prepared muscle specimens suggesting that maximum eccentric force is substantially greater than isometric force, whereas in vivo studies in humans show only a modest increase, no change, or even a decrease in force in lengthening contractions. This review discusses possible reasons reported in the literature for these discrepant findings, including the testing procedures (familiarization, pre-load condition, and temperature) and a potential neural inhibition at higher lengthening velocities. Finally, some unresolved questions and recommendations for F-V testing in humans are reported at the end of this document.

Extracellular matrix remodelling induced by alternating electrical and mechanical stimulations increases the contraction of engineered skeletal muscle tissues

Engineered skeletal muscles are inferior to natural muscles in terms of contractile force, hampering their potential use in practical applications. One major limitation is that the extracellular matrix (ECM) not only impedes the contraction but also ineffectively transmits the forces generated by myotubes to the load. In the present study, ECM remodelling improves contractile force in a short time, and a coordinated, combined electrical and mechanical stimulation induces the desired ECM remodelling. Notably, the application of single and combined stimulations to the engineered muscles remodels the structure of their ECM networks, which determines the mechanical properties of the ECM. Myotubes in the tissues are connected in parallel and in series to the ECM. The stiffness of the parallel ECM must be low not to impede contraction, while the stiffness of the serial ECM must be high to transmit the forces to the load. Both the experimental results and the mechanistic model suggest that the combined stimulation through coordination reorients the ECM fibres in such a way that the parallel ECM stiffness is reduced, while the serial ECM stiffness is increased. In particular, 3 and 20 minutes of alternating electrical and mechanical stimulations increase the force by 18% and 31%, respectively.

On sarcomere length stability during isometric and post-active-stretch isometric contractions

Sarcomere length (SL) instability and SL non-uniformity have been used to explain fundamental properties of skeletal muscles, such as creep, force depression following active muscle shortening, and residual force enhancement following active stretching of muscles. Regarding residual force enhancement, it has been argued that active muscle stretching causes SL instability, thereby increasing SL non-uniformity. However, we recently showed that SL non-uniformity is not increased by active muscle stretching, but it remains unclear if SL stability is affected by active stretching. Here, we used single myofibrils of rabbit psoas and measured SL non-uniformity and SL instability during isometric contractions and for isometric contractions following active stretching at average SLs corresponding to the descending limb of the force-length relationship. We defined isometric contractions as contractions during which mean SL remained constant. SL instability was quantified by the rate of change of individual SLs over the course of steady state, isometric force; and SL non-uniformity was defined as deviations of SLs from the mean SL at an instant of time. We found that while the mean SL remained constant during isometric contraction, by definition, individual SLs did not. SLs were more stable in the force-enhanced, isometric state following active stretching compared to the isometric reference state. We also found that SL instability was not correlated with the rate of change of SL non-uniformity. Also, SL non-uniformity was not different in the isometric and the post-stretch isometric contractions. We conclude that since SL is more stable but similarly non-uniform in the force-enhanced compared to the corresponding isometric reference contraction, it appears unlikely that either SL instability or SL non-uniformity contribute to the residual force enhancement property of skeletal muscle.

Assessment of the Contractile Properties of Permeabilized Skeletal Muscle Fibers

Permeabilized individual skeletal muscle fibers offer the opportunity to evaluate contractile behavior in a system that is greatly simplified, yet physiologically relevant. Here we describe the steps required to prepare, permeabilize and preserve small samples of skeletal muscle. We then detail the procedures used to isolate individual fiber segments and attach them to an experimental apparatus for the purpose of controlling activation and measuring force generation. We also describe our technique for estimating the cross-sectional area of fiber segments. The area measurement is necessary for normalizing the absolute force to obtain specific force, a measure of the intrinsic force-generating capability of the contractile system.

Length-dependent Ca2+ activation in skeletal muscle fibers from mammalians

We tested the hypotheses that 1) a decrease in activation of skeletal muscles at short sarcomere lengths (SLs) is caused by an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR), and 2) the decrease in Ca(2+) would be caused by an inhibition of action potential conduction from the periphery to the core of the fibers. Intact, single fibers dissected from the flexor digitorum brevis from mice were activated at different SLs, and intracellular Ca(2+) was imaged with confocal microscopy. Force decreased at SLs shorter than 2.1 μm, while Ca(2+) concentration decreased at SLs below 1.9 μm. The concentration of Ca(2+) at short SL was lower at the core than at the peripheries of the fiber. When the external concentration of Na(+) was decreased in the experimental media, impairing action potential conduction, Ca(2+) gradients were observed in all SLs. When caffeine was used in the experimental media, the gradients of Ca(2+) were abolished. We concluded that there is an inhibition of Ca(2+) release from the sarcoplasmic reticulum (SR) at short SLs, which results from a decreased conduction of action potential from the periphery to the core of the fibers.

The Zebrafish Heart as a Model of Mammalian Cardiac Function

Zebrafish (Danio rerio) are widely used as vertebrate model in developmental genetics and functional genomics as well as in cardiac structure-function studies. The zebrafish heart has been increasingly used as a model of human cardiac function, in part, due to the similarities in heart rate and action potential duration and morphology with respect to humans. The teleostian zebrafish is in many ways a compelling model of human cardiac function due to the clarity afforded by its ease of genetic manipulation, the wealth of developmental biological information, and inherent suitability to a variety of experimental techniques. However, in addition to the numerous advantages of the zebrafish system are also caveats related to gene duplication (resulting in paralogs not present in human or other mammals) and fundamental differences in how zebrafish hearts function. In this review, we discuss the use of zebrafish as a cardiac function model through the use of techniques such as echocardiography, optical mapping, electrocardiography, molecular investigations of excitation-contraction coupling, and their physiological implications relative to that of the human heart. While some of these techniques (e.g., echocardiography) are particularly challenging in the zebrafish because of diminutive size of the heart (~1.5 mm in diameter) critical information can be derived from these approaches and are discussed in detail in this article.

Na,K-ATPase α2 activity in mammalian skeletal muscle T-tubules is acutely stimulated by extracellular K+

The K⁺ affinity of the Na,K-ATPase α2 isoform matches its activity to the range of extracellular K⁺ concentrations in the T-tubules at rest and during contraction, maintaining the excitability of active muscle.

The Na,K-ATPase α2 isoform is the predominant Na,K-ATPase in adult skeletal muscle and the sole Na,K-ATPase in the transverse tubules (T-tubules). In quiescent muscles, the α2 isozyme operates substantially below its maximal transport capacity. Unlike the α1 isoform, the α2 isoform is not required for maintaining resting ion gradients or the resting membrane potential, canonical roles of the Na,K-ATPase in most other cells. However, α2 activity is stimulated immediately upon the start of contraction and, in working muscles, its contribution is crucial to maintaining excitation and resisting fatigue. Here, we show that α2 activity is determined in part by the K⁺ concentration in the T-tubules, through its K⁺ substrate affinity. Apparent K⁺ affinity was determined from measurements of the K_1/2 for K⁺ activation of pump current in intact, voltage-clamped mouse flexor digitorum brevis muscle fibers. Pump current generated by the α2 Na,K-ATPase, Ip, was identified as the outward current activated by K⁺ and inhibited by micromolar ouabain. Ip was outward at all potentials studied (−90 to −30 mV) and increased with depolarization in the subthreshold range, −90 to −50 mV. The Q₁₀ was 2.1 over the range of 22–37°C. The K_1/2,K of Ip was 4.3 ± 0.3 mM at −90 mV and was relatively voltage independent. This K⁺ affinity is lower than that reported for other cell types but closely matches the dynamic range of extracellular K⁺ concentrations in the T-tubules. During muscle contraction, T-tubule luminal K⁺ increases in proportion to the frequency and duration of action potential firing. This K_1/2,K predicts a low fractional occupancy of K⁺ substrate sites at the resting extracellular K⁺ concentration, with occupancy increasing in proportion to the frequency of membrane excitation. The stimulation of preexisting pumps by greater K⁺ site occupancy thus provides a rapid mechanism for increasing α2 activity in working muscles.

Measurement of Maximum Isometric Force Generated by Permeabilized Skeletal Muscle Fibers

Analysis of the contractile properties of chemically skinned, or permeabilized, skeletal muscle fibers offers a powerful means by which to assess muscle function at the level of the single muscle cell. Single muscle fiber studies are useful in both basic science and clinical studies. For basic studies, single muscle fiber contractility measurements allow investigation of fundamental mechanisms of force production, and analysis of muscle function in the context of genetic manipulations. Clinically, single muscle fiber studies provide useful insight into the impact of injury and disease on muscle function, and may be used to guide the understanding of muscular pathologies. In this video article we outline the steps required to prepare and isolate an individual skeletal muscle fiber segment, attach it to force-measuring apparatus, activate it to produce maximum isometric force, and estimate its cross-sectional area for the purpose of normalizing the force produced.

Locomotor activity influences muscle architecture and bone growth but not muscle attachment site morphology

The ability to make behavioural inferences from skeletal remains is critical to understanding the lifestyles and activities of past human populations and extinct animals. Muscle attachment site (enthesis) morphology has long been assumed to reflect muscle strength and activity during life, but little experimental evidence exists to directly link activity patterns with muscle development and the morphology of their attachments to the skeleton. We used a mouse model to experimentally test how the level and type of activity influences forelimb muscle architecture of spinodeltoideus, acromiodeltoideus, and superficial pectoralis, bone growth rate and gross morphology of their insertion sites. Over an 11-week period, we collected data on activity levels in one control group and two experimental activity groups (running, climbing) of female wild-type mice. Our results show that both activity type and level increased bone growth rates influenced muscle architecture, including differences in potential muscular excursion (fibre length) and potential force production (physiological cross-sectional area). However, despite significant influences on muscle architecture and bone development, activity had no observable effect on enthesis morphology. These results suggest that the gross morphology of entheses is less reliable than internal bone structure for making inferences about an individual's past behaviour.

The force-velocity relationship at negative loads (assisted shortening) studied in isolated, intact muscle fibres of the frog

AIM

The study was undertaken to explore the force-velocity relationship under conditions where the myofilament system is subjected to an external force that serves as a negative load and assists the shortening movement.

METHODS

The experiments were carried out on single muscle fibres isolated from the anterior tibialis muscle of Rana temporaria. The fibres, being operated under load-clamp control, were released to shorten during tetanic stimulation at sarcomere lengths where the fibres carried different degrees of passive tension. The shortening thus occurred while the sarcomeres were subjected to a force that may be characterized as a 'negative load', that is, a force assisting the shortening movement.

RESULTS

The force-velocity relationship below zero load was found to be a smooth continuation of the force-velocity curve recorded at positive loads with the shortening velocity increasing steeply at loads below zero. A negative load amounting to merely 10% of the isometric force, thus raised the shortening velocity to a level two to three times higher than V0 , the velocity recorded at zero load.

CONCLUSIONS

The results provide evidence that, even in the presence of a longitudinal compressive force, the speed of shortening of the muscle fibre is determined by the cycling rate of the interacting cross-bridges. The force-velocity relationship at negative loads may play a relevant part during fast movements of striated muscle as pointed out in the discussion.

Intrinsic stiffness of extracellular matrix increases with age in skeletal muscles of mice

Advanced age is associated with increases in muscle passive stiffness, but the contributors to the changes remain unclear. Our purpose was to determine the relative contributions of muscle fibers and extracellular matrix (ECM) to muscle passive stiffness in both adult and old animals. Passive mechanical properties were determined for isolated individual muscle fibers and bundles of muscle fibers that included their associated ECM, obtained from tibialis anterior muscles of adult (8-12 mo old) and old (28-30 mo old) mice. Maximum tangent moduli of individual muscle fibers from adult and old muscles were not different at any sarcomere length tested. In contrast, the moduli of bundles of fibers from old mice was more than twofold greater than that of fiber bundles from adult muscles at sarcomere lengths >2.5 μm. Because ECM mechanical behavior is determined by the composition and arrangement of its molecular constituents, we also examined the effect of aging on ECM collagen characteristics. With aging, muscle ECM hydroxyproline content increased twofold and advanced glycation end-product protein adducts increased threefold, whereas collagen fibril orientation and total ECM area were not different between muscles from adult and old mice. Taken together, these findings indicate that the ECM of tibialis anterior muscles from old mice has a higher modulus than the ECM of adult muscles, likely driven by an accumulation of densely packed extensively crosslinked collagen.

Force depression decays during shortening in the medial gastrocnemius of the rat

Force depression due to shortening of activated skeletal muscles has previously been described to be long lasting during isometric contractions following the shortening. In the present study, using the medial gastrocnemius of the rat, effects of force depression have been made apparent during shortening by computationally partially compensating for the direct effect of shortening velocity due to the tension-velocity relation. Evidence was found for the decay and complete disappearance of force depression already during continuation of the shortening contraction to short muscle lengths.

The excitation-contraction coupling mechanism in skeletal muscle

First coined by Alexander Sandow in 1952, the term excitation-contraction coupling (ECC) describes the rapid communication between electrical events occurring in the plasma membrane of skeletal muscle fibres and Ca2+ release from the SR, which leads to contraction. The sequence of events in twitch skeletal muscle involves: (1) initiation and propagation of an action potential along the plasma membrane, (2) spread of the potential throughout the transverse tubule system (T-tubule system), (3) dihydropyridine receptors (DHPR)-mediated detection of changes in membrane potential, (4) allosteric interaction between DHPR and sarcoplasmic reticulum (SR) ryanodine receptors (RyR), (5) release of Ca2+ from the SR and transient increase of Ca2+ concentration in the myoplasm, (6) activation of the myoplasmic Ca2+ buffering system and the contractile apparatus, followed by (7) Ca2+ disappearance from the myoplasm mediated mainly by its reuptake by the SR through the SR Ca2+ adenosine triphosphatase (SERCA), and under several conditions movement to the mitochondria and extrusion by the Na+/Ca2+ exchanger (NCX). In this text, we review the basics of ECC in skeletal muscle and the techniques used to study it. Moreover, we highlight some recent advances and point out gaps in knowledge on particular issues related to ECC such as (1) DHPR-RyR molecular interaction, (2) differences regarding fibre types, (3) its alteration during muscle fatigue, (4) the role of mitochondria and store-operated Ca2+ entry in the general ECC sequence, (5) contractile potentiators, and (6) Ca2+ sparks.

Engineered skeletal muscle tissue for soft robotics: fabrication strategies, current applications, and future challenges

Skeletal muscle is a scalable actuator system used throughout nature from the millimeter to meter length scales and over a wide range of frequencies and force regimes. This adaptability has spurred interest in using engineered skeletal muscle to power soft robotics devices and in biotechnology and medical applications. However, the challenges to doing this are similar to those facing the tissue engineering and regenerative medicine fields; specifically, how do we translate our understanding of myogenesis in vivo to the engineering of muscle constructs in vitro to achieve functional integration with devices. To do this researchers are developing a number of ways to engineer the cellular microenvironment to guide skeletal muscle tissue formation. This includes understanding the role of substrate stiffness and the mechanical environment, engineering the spatial organization of biochemical and physical cues to guide muscle alignment, and developing bioreactors for mechanical and electrical conditioning. Examples of engineered skeletal muscle that can potentially be used in soft robotics include 2D cantilever-based skeletal muscle actuators and 3D skeletal muscle tissues engineered using scaffolds or directed self-organization. Integration into devices has led to basic muscle-powered devices such as grippers and pumps as well as more sophisticated muscle-powered soft robots that walk and swim. Looking forward, current, and future challenges include identifying the best source of muscle precursor cells to expand and differentiate into myotubes, replacing cardiomyocytes with skeletal muscle tissue as the bio-actuator of choice for soft robots, and vascularization and innervation to enable control and nourishment of larger muscle tissue constructs.

Anatomical, architectural, and biochemical diversity of the murine forelimb muscles

We characterized the architecture, fiber type, titin isoform distribution, and collagen content of 27 portions of 22 muscles in the murine forelimb. The mouse forelimb was different from the human arm in that it had the extensor digitorum lateralis muscle and no brachioradialis muscle. Architecturally, the mouse forelimb differed from humans with regard to load bearing, having a much larger contribution from extensors than flexors. In mice, the extensor : flexor PCSA ratio is 2.7, whereas in humans it is only 1.4. When the architectural difference index was calculated, similarities became especially apparent between flexors and extensors of the distal forelimb, as well as pronators. Discriminant analysis revealed that biochemical measures of collagen, titin, and myosin heavy chain were all strong between-species discriminators. In terms of composition, when compared with similar muscles in humans, mice had, on average, faster muscles with higher collagen content and larger titin isoforms. This report establishes the anatomical and biochemical properties of mouse forelimb muscles. Given the prevalence of this species in biological studies, these data will be invaluable for studying the biological basis of mouse muscle structure and function.

Approximation for Cooperative Interactions of a Spatially-Detailed Cardiac Sarcomere Model

We developed a novel ordinary differential equation (ODE) model, which produced results that correlated well with the Monte Carlo (MC) simulation when applied to a spatially-detailed model of the cardiac sarcomere. Configuration of the novel ODE model was based on the Ising model of myofilaments, with the "co-operative activation" effect introduced to incorporate nearest-neighbor interactions. First, a set of parameters was estimated using arbitrary Ca transient data to reproduce the combinational probability for the states of three consecutive regulatory units, using single unit probabilities for central and neighboring units in the MC simulation. The parameter set thus obtained enabled the calculation of the state transition of each unit using the ODE model with reference to the neighboring states. The present ODE model not only provided good agreement with the MC simulation results but was also capable of reproducing a wide range of experimental results under both steady-state and dynamic conditions including shortening twitch. The simulation results suggested that the nearest-neighbor interaction is a reasonable approximation of the cooperativity based on end-to-end interactions. Utilizing the modified ODE model resulted in a reduction in computational costs but maintained spatial integrity and co-operative effects, making it a powerful tool in cardiac modeling.

Force produced by isolated sarcomeres and half-sarcomeres after an imposed stretch

When a stretch is imposed to activated muscles, there is a residual force enhancement that persists after the stretch; the force is higher than that produced during an isometric contraction in the corresponding length. The mechanisms behind the force enhancement remain elusive, and there is disagreement if it represents a sarcomeric property, or if it is associated with length nonuniformities among sarcomeres and half-sarcomeres. The purpose of this study was to investigate the effects of stretch on single sarcomeres and myofibrils with predetermined numbers of sarcomeres (n = 2, 3. . . , 8) isolated from the rabbit psoas muscle. Sarcomeres were attached between two precalibrated microneedles for force measurements, and images of the preparations were projected onto a linear photodiode array for measurements of half-sarcomere length (SL). Fully activated sarcomeres were subjected to a stretch (5-10% of initial SL, at a speed of 0.3 μm·s(-1)·SL(-1)) after which they were maintained isometric for at least 5 s before deactivation. Single sarcomeres showed two patterns: 31 sarcomeres showed a small level of force enhancement after stretch (10.46 ± 0.78%), and 28 sarcomeres did not show force enhancement (-0.54 ± 0.17%). In these preparations, there was not a strong correlation between the force enhancement and half-sarcomere length nonuniformities. When three or more sarcomeres arranged in series were stretched, force enhancement was always observed, and it increased linearly with the degree of half-sarcomere length nonuniformities. The results show that the residual force enhancement has two mechanisms: 1) stretch-induced changes in sarcomeric structure(s); we suggest that titin is responsible for this component, and 2) stretch-induced nonuniformities of half-sarcomere lengths, which significantly increases the level of force enhancement.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Micromechanical function of myofibrils isolated from skeletal and cardiac muscles of the zebrafish

The zebrafish is a potentially important and cost-effective model for studies of development, motility, regeneration, and inherited human diseases. The object of our work was to show whether myofibrils isolated from zebrafish striated muscle represent a valid subcellular contractile model. These organelles, which determine contractile function in muscle, were used in a fast kinetic mechanical technique based on an atomic force probe and video microscopy. Mechanical variables measured included rate constants of force development (k_ACT) after Ca²⁺ activation and of force decay (τ_REL⁻¹) during relaxation upon Ca²⁺ removal, isometric force at maximal (F_max) or partial Ca²⁺ activations, and force response to an external stretch applied to the relaxed myofibril (F_pass). Myotomal myofibrils from larvae developed greater active and passive forces, and contracted and relaxed faster than skeletal myofibrils from adult zebrafish, indicating developmental changes in the contractile organelles of the myotomal muscles. Compared with murine cardiac myofibrils, measurements of adult zebrafish ventricular myofibrils show that k_ACT, F_max, Ca²⁺ sensitivity of the force, and F_pass were comparable and τ_REL⁻¹ was smaller. These results suggest that cardiac myofibrils from zebrafish, like those from mice, are suitable contractile models to study cardiac function at the sarcomeric level. The results prove the practicability and usefulness of mechanical and kinetic investigations on myofibrils isolated from larval and adult zebrafish muscles. This novel approach for investigating myotomal and myocardial function in zebrafish at the subcellular level, combined with the powerful genetic manipulations that are possible in the zebrafish, will allow the investigation of the functional primary consequences of human disease–related mutations in sarcomeric proteins in the zebrafish model.

Actomyosin-ADP states, interhead cooperativity, and the force-velocity relation of skeletal muscle

Despite intense efforts to elucidate the molecular mechanisms that determine the maximum shortening velocity and the shape of the force-velocity relationship in striated muscle, our understanding of these mechanisms remains incomplete. Here, this issue is addressed by means of a four-state cross-bridge model with significant explanatory power for both shortening and lengthening contractions. Exploration of the parameter space of the model suggests that an actomyosin-ADP state (AM( *)ADP) that is separated from the actual ADP release step by a strain-dependent isomerization is important for determining both the maximum shortening velocity and the shape of the force-velocity relationship. The model requires a velocity-dependent, cross-bridge attachment rate to account for certain experimental findings. Of interest, the velocity dependence for shortening contraction is similar to that for population of the AM( *)ADP state (with a velocity-independent attachment rate). This accords with the idea that attached myosin heads in the AM( *)ADP state position the partner heads for rapid attachment to the next site along actin, corresponding to the apparent increase in attachment rate in the model.

Mechanical properties of intact single fibres from wild-type and MLC/mIgf-1 transgenic mouse muscle

The effects of overexpression of the local form of insulin like growth factor-1 (mIgf-1) on skeletal muscle were investigated by comparing the mechanical properties of single intact fibres from the flexor digitorum brevis of wild-type (WT) and (MLC/mIgf-1) transgenic mice (TG)at 21-24 degrees C. Isolated single fibres were clean enough to measure accurately the sarcomere length. The parameters investigated were: tetanic absolute and specific force, the force-velocity relationship, and the sarcomere length-tension relationship. In addition, we investigated the properties of the "static stiffness", a non-crossbridge Ca(2+)-dependent increase of fibre stiffness previously found in frog muscle. Both average cross-sectional area and tetanic force almost doubled in TG fibres, so that specific force was the same in both preparation: 312 +/- 20 and 344 +/- 34 kN m(-2) in WT and TG fibres, respectively. None of the relative force-velocity parameters was altered by Igf-1 overexpression, however, V(max) (8-10 l(0) s(-1)) was greater than previously reported in whole muscles. The sarcomere length-tension relationship was the same in TG and WT fibres showing the classical shape with a plateau region between 2.28 and 2.52 microm and a linear descending limb. The static stiffness was present in both WT and TG fibres and showed similar characteristics to that of frog skeletal muscle. In contrast to the other parameters, static stiffness in TG fibres was about 24% smaller than in WT fibres suggesting a possible effect of Igf-1 overexpression on its mechanism.

Theoretical predictions of the effects of force transmission by desmin on intersarcomere dynamics

Desmin is an intermediate filament protein in skeletal muscle that forms a meshlike network around Z-disks. A model of a muscle fiber was developed to investigate the mechanical role of desmin. A two-dimensional mesh of viscoelastic sarcomere elements was connected laterally by elastic elements representing desmin. The equations of motion for each sarcomere boundary were evaluated at quasiequilibrium to determine sarcomere stresses and strains. Simulations of passive stretch and fixed-end contractions yielded values for sarcomere misalignment and stress in wild-type and desmin null fibers. Passive sarcomere misalignment increased nonlinearly with fiber strain in both wild-type and desmin null simulations and was significantly larger without desmin. During fixed-end contraction, desmin null simulations also demonstrated greater sarcomere misalignment and reduced stress production compared with wild-type. In simulations with only a fraction of wild-type desmin present, fixed-end stress increased as a function of desmin concentration and this relationship was influenced by the cellular location of the desmin filaments. This model suggests that desmin stabilizes Z-disks and enables greater stress production by providing a mechanical tether between adjacent myofibrils and to the extracellular matrix and that the significance of the tether is a function of its location within the cell.

Contractile performance of striated muscle

The single muscle fiber preparation provides an excellent tool for studying the mechanical behaviour of the contractile system at sarcomere level. The present article gives an overview of studies based on intact single fibers from frog and mouse skeletal muscle. The following aspects of muscle function are treated: (1) The length-tension relationship. (2) The biphasic force-velocity relationship. (3) The maximum speed of shortening, its independence of sarcomere length and degree of activation. (4) Force enhancement during stretch, its relation to sarcomere length and myofilament lattice width. (5) Residual force enhancement after stretch. (6) Force reduction after loaded shortening. (7) Deactivation by active shortening. (8) Differences in kinetic properties along individual muscle fibers.

Eccentric contractions lead to myofibrillar dysfunction in muscular dystrophy

It is commonly accepted that skeletal muscles from dystrophin-deficient mdx mice are more susceptible than those from wild-type mice to damage from eccentric contractions. However, the downstream mechanisms involved in this enhanced force drop remain controversial. We studied the reduction of contractile force induced by eccentric contractions elicited in vivo in the gastrocnemius muscle of wild-type mice and three distinct models of muscle dystrophy: mdx, alpha-sarcoglycan (Sgca)-null, and collagen 6A1 (Col6a1)-null mice. In mdx and Sgca-null mice, force decreased 35% compared with 14% in wild-type mice. Drop of force in Col6a1-null mice was comparable to that in wild-type mice. To identify the determinants of the force drop, we measured force generation in permeabilized fibers dissected from gastrocnemius muscle that had been exposed in vivo to eccentric contractions and from the contralateral unstimulated muscle. A force loss in skinned fibers after in vivo eccentric contractions was detectable in fibers from mdx and Sgca-null, but not wild-type and Col6a1-null, mice. The enhanced force reduction in mdx and Sgca-null mice was observed only when eccentric contractions were elicited in vivo, since eccentric contractions elicited in vitro had identical effects in wild-type and dystrophic skinned fibers. These results suggest that 1) the enhanced force loss is due to a myofibrillar impairment that is present in all fibers, and not to individual fiber degeneration, and 2) the mechanism causing the enhanced force reduction is active in vivo and is lost after fiber permeabilization.

Cupiennius salei: biomechanical properties of the tibia-metatarsus joint and its flexing muscles

Hunting spiders are well adapted to fast locomotion. Space saving hydraulic leg extension enables leg segments, which consist almost soley of flexor muscles. As a result, the muscle cross sectional area is high despite slender legs. Considering these morphological features in context with the spider's segmented C-shaped legs, these specifics might influence the spider's muscle properties. Moreover, these properties have to be known for modeling of spider locomotion. Cupiennius salei (n = 5) were fixed in a metal frame allowing exclusive flexion of the tibia-metatarsus joint of the second leg (counted from anterior). Its flexing muscles were stimulated supramaximally using needle electrodes. Accounting for the joint geometry, the force-length and the force-velocity relationships were determined. The spider muscles produce 0.07 N cm maximum isometric moment (corresponding to 25 N/cm(2) maximum stress) at 160 degrees tibia-metatarsus joint angle. When overextended to the dorsal limit at approximately 200 degrees , the maximum isometric moments decrease to 72%, and, when flexed to the ventral hinge stop at 85 degrees , they drop to 11%. The force-velocity relation shows the typical hyperbolic shape. The mean maximum shortening velocity is 5.7 optimum muscle lengths per second and the mean curvature (a/F (iso)) of the Hill-function is 0.34. The spider muscle's properties which were determined are similar to those of other species acting as motors during locomotion (working range, curvature of Hill hyperbola, peak power at the preferred speeds), but they are relatively slow. In conjunction with the low mechanical advantage (muscle lever/load arm), the arrangement of three considerably actuated joints in series may nonetheless enable high locomotion velocities.

The mechanical behavior of individual sarcomeres of myofibrils isolated from rabbit psoas muscle

The goal of this study was to develop a system to experiment with sarcomeres mechanically isolated from skeletal muscles. Single myofibrils from rabbit psoas were transferred into a temperature-controlled (22 degrees C or 15 degrees C) experimental chamber, and sarcomeres were isolated using precalibrated glass microneedles that were pierced externally, adjacent to the Z-lines. The force produced during activation was measured by tracking the displacement of the microneedles, and the sarcomere and half-sarcomere changes were measured by continuously tracking the Z-lines and A-bands position during the experiments. Sarcomeres produced a stress (force/cross-sectional area) of 112.75 +/- 4.96 nN/microm(2) (15 degrees C) and 128.47 +/- 5.58 nN/microm(2) (22 degrees C) at lengths between 2.0 microm and 2.4 microm. The descending limb was fitted with linear regression for length between 2.4 microm and 3.5 microm, which provided an abscissa extrapolating to 3.87 microm. The force-length relation was remarkably similar to a theoretical curve based on the degree of filament overlap. During sarcomere activation, we tracked the distance between the center of the A-band and the Z-lines. At lengths below 1.6 microm, movements of A-band were not detected. A-band movements increased with length to achieve a maximum displacement of 59.40 +/- 10.1 nm from the center at 2.0 microm-2.4 microm. A-band displacement decreased linearly in sarcomere lengths between 2.6 microm and 3.6 microm. A technique for monitoring force and length in single sarcomeres isolated from myofibrils represents a reliable technique to evaluate contractile mechanisms at the most basic, intact level of muscle organization, opening the possibility to clarify long-standing issues in the field of muscle contraction.

A multisegmental cross-bridge kinetics model of the myofibril

Striated muscle is a mechanical system that develops force and generates power in serving vital activities in the body. Striated muscle is a complex biological system; a single mammalian muscle fibre contains up to hundred or even more myofibrils in parallel connected via an inter-myofibril filament network. In one single myofibril thousands of sarcomeres are lined up as a series of linear motors. We recently demonstrated that half-sarcomeres (hS) in a single myofibril operate non-uniformly. We outline a mathematical framework based on cross-bridge kinetics for the simulation of the force response and length change of individual hS in a myofibril. The model describes the muscle myofibril in contraction experiments under various conditions. The myofibril is modeled as a multisegmental mechanical system of hS models, which have active and viscoelastic properties. In the first approach, a two-state cross-bridge formalism relates the hS force to the chemical kinetics of ATP hydrolysis, as first described by Huxley [1957. Muscle structure and theories of contraction. Prog. Biophys. Mol. Biol. 7, 255-318]. Two possible types of biological variability are introduced and modeled. Numerical simulations of a myofibril composed of four to eight hS show a non-uniform hS length distribution and complex internal dynamics upon activation. We demonstrate that the steady-state approximation holds only in restricted time zones during activation. Simulations of myofibril contraction experiments that reproduce the classic steady-state force-length and force-velocity relationships, strictly constrained or "clamped" in either end-held isometric or isotonic contraction conditions, reveal a small but conspicuous effect of hS dynamics on force.

Structure and function of skeletal muscle in zebrafish early larvae

Zebrafish muscles were examined at an early developmental stage (larvae 5-7 d). Using aluminum clips, preparations (approximately 1.5 mm length, 150 microm diameter) were mounted for force registration and small angle x-ray diffraction. Sarcomeres were oriented mainly in parallel with the preparation long axis. Electrical stimulation elicited fast and reproducible single twitch contractions. Length-force relations showed an optimal sarcomere length of 2.15 microm. X-ray diffraction revealed clear equatorial 1.1/1.0 reflections, showing that myofilaments are predominantly arranged along the preparation long axis. In contrast, reflections from older (2 mo) zebrafish showed two main filament orientations each at an approximately 25 degrees angle relative to the preparation long axis. Electrical stimulation of larvae muscles increased the 1.1/1.0 intensity ratio, reflecting mass transfer to thin filaments during contraction. The apparent lattice volume was 3.42 x 10(-3) microm(3), which is smaller than that of mammalian striated muscle and more similar to that of frog muscles. The relation between force and stimulation frequency showed fusion of responses at a comparatively high frequency (approximately 186 Hz), reflecting a fast muscle phenotype. Inhibition of fast myosin with N-benzyl-p-toluene sulphonamide (BTS) showed that the later phase of the tetanus was less affected than the initial peak. This suggests that, although the main contractile phenotype is fast, slow twitch fibers can contribute to sustained contraction. A fatigue stimulation protocol with repeated 220 ms/186 Hz tetani showed that tetanic force decreased to 50% at a train rate of 0.1 s(-1). In conclusion, zebrafish larvae muscles can be examined in vitro using mechanical and x-ray methods. The muscles and myofilaments are mainly orientated in parallel with the larvae long axis and exhibit a significant fast contractile component. Sustained contractions can also involve a small contribution from slower muscle types.

In situ detection and measurement of intracellular reactive oxygen species in single isolated mature skeletal muscle fibers by real time fluorescence microscopy

Reactive oxygen species (ROS) produced by skeletal muscle stimulate adaptive responses to activity and mediate some degenerative processes. ROS activity is usually studied by measuring indirect end-points of their reactions with various biomolecules. In order to develop a method to measure the intracellular ROS generation in real-time in mature skeletal muscle fibers, these were isolated from the flexor digitorum brevis (FDB) muscle of mice and cultured on collagen-coated plates. Fibers were loaded with 5- (and 6-) chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-DCFH DA) and measurements of 5- (and 6-) chloromethyl-2',7'-dichlorofluorescin (CM-DCF) fluorescence from individual fibers obtained by microscopy over 45 min. The sensitivity of this approach was demonstrated by addition of 1 microM H(2)O(2) to the extracellular medium. Contractions of isolated fibers induced by field electrical stimulation caused a significant increase in CM-DCF fluorescence that was abolished by pre-treatment of fibers with glutathione ethyl ester. Thus, CM-DCF fluorescence microscopy can detect physiologically relevant changes in intracellular ROS activity in single isolated mature skeletal muscle fibers in real-time, and contractions generated a net increase that was abolished when the intracellular glutathione content was enhanced. This technique has advantages over previous approaches because of the maturity of the fibers and the analysis of single cells, which prevent contributions from nonmuscle cells.

Unloaded speed of shortening in voltage-clamped intact skeletal muscle fibers from wt, mdx, and transgenic minidystrophin mice using a novel high-speed acquisition system

Skeletal muscle unloaded shortening has been indirectly determined in the past. Here, we present a novel high-speed optical tracking technique that allows recording of unloaded shortening in single intact, voltage-clamped mammalian skeletal muscle fibers with 2-ms time resolution. L-type Ca(2+) currents were simultaneously recorded. The time course of shortening was biexponential: a fast initial phase, tau(1), and a slower successive phase, tau(2,) with activation energies of 59 kJ/mol and 47 kJ/mol. Maximum unloaded shortening speed, v(u,max), was faster than that derived using other techniques, e.g., approximately 14.0 L(0) s(-1) at 30 degrees C. Our technique also allowed direct determination of shortening acceleration. We applied our technique to single fibers from C57 wild-type, dystrophic mdx, and minidystrophin-expressing mice to test whether unloaded shortening was affected in the pathophysiological mechanism of Duchenne muscular dystrophy. v(u,max) and a(u,max) values were not significantly different in the three strains, whereas tau(1) and tau(2) were increased in mdx fibers. The results were complemented by myosin heavy and light chain (MLC) determinations that showed the same myosin heavy chain IIA profiles in the interossei muscles from the different strains. In mdx muscle, MLC-1f was significantly increased and MLC-2f and MLC-3f somewhat reduced. Fast initial active shortening seems almost unaffected in mdx muscle.

Glycolytic activation at the onset of contractions in isolated Xenopus laevis single myofibres

Intracellular pH (pHi) was measured in isolated Xenopus laevis single myofibres at the onset of contractions, with and without glycolytic blockade, to investigate the time course of glycolytic activation. Single myofibres (n=8; CON) were incubated in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoyxmethyl ester (10 microM; for fluorescence measurement of pHi) and stimulated for 15 s at 0.67 Hz in anoxia in the absence (control condition; CON) and presence of a glycolytic inhibitor (1 mM iodoacetic acid; IAA). Intracellular pHi and tension were continuously recorded, and the differences in pHi between conditions were used to estimate the activation time of glycolysis. An immediate and steady increase in pHi (initial alkalosis) at the onset of contractions was similar between CON and IAA trials for the first 9 s of the contractile bout. However, from six contractions (approximately 10 s) throughout the remainder of the bout, IAA demonstrated a continued rise in pHi, in contrast to a progressive decrease in pHi in CON (P<0.05). These results demonstrate, with high temporal resolution, that glycolysis is activated within six contractions (10 s at 0.67 Hz) in single Xenopus skeletal muscle fibres.

Measuring mechanical properties, including isotonic fatigue, of fast and slow MLC/mIgf-1 transgenic skeletal muscle

Contractile properties of fast-twitch (EDL) and slow-twitch (soleus) skeletal muscles were measured in MLC/mIgf-1 transgenic and wild-type mice. MLC/mIgf-1 mice express the local factor mIgf-1 under the transcriptional control of MLC promoter, selectively activated in fast-twitch muscle fibers. Isolated muscles were studied in vitro in both isometric and isotonic conditions. We used a rapid "ad hoc" testing protocol that measured, in a single procedure, contraction time, tetanic force, Hill's (F-v) curve, power curve and isotonic muscle fatigue. Transgenic soleus muscles did not differ from wild-type with regard to any measured variable. In contrast, transgenic EDL muscles displayed a hypertrophic phenotype, with a mass increase of 29.2% compared to wild-type. Absolute tetanic force increased by 21.5% and absolute maximum power by 34.1%. However, when normalized to muscle cross-sectional area and mass, specific force and normalized power were the same in transgenic and wild-type EDL muscles, revealing that mIgf-1 expression induces a functional hypertrophy without altering fibrotic tissue accumulation. Isotonic fatigue behavior did not differ between transgenic and wild-type muscles, suggesting that the ability of mIgf-1 transgenic muscle to generate a considerable higher absolute power did not affect its resistance to fatigue.

Comparison of the tension responses to ramp shortening and lengthening in intact mammalian muscle fibres: crossbridge and non-crossbridge contributions

We examined the tension responses to ramp shortening and lengthening over a range of velocities (0.1-5 L(0)/s) and at 20 degrees C and 30 degrees C in tetanized intact fibre bundles from a rat fast (flexor hallucis brevis) muscle; fibre length (L(0)) was 2.2 mm and sarcomere length approximately 2.5 microm. The tension change during ramp releases as well as ramp stretches showed an early transition (often appearing as an inflection) at 1-4 ms; the tension change at this transition and the length change at which it occurred increased with velocity. A second transition, indicated by a more gradual reduction in slope, occurred when the length had changed by 14-28 nm per half-sarcomere; the tension at this transition increased with lengthening velocity towards a plateau and it decreased with shortening velocity towards zero tension. The velocity dependence of the time to the transitions and the length change at the transitions showed some asymmetries between shortening and lengthening. Based on analyses of the velocity dependence of the tension and modelling, we propose that the first transition reflects the tension change associated with the crossbridge power stroke in shortening, or with the reversal of the power stroke in lengthening. Modelling shows that the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached. After the second transition, the tension reaches a steady level in the model whereas the tension continues to increase during lengthening and continues to decrease during shortening in the experiments; this continuous tension change is seen at a wide range of initial sarcomere lengths and when active force is reduced by the myosin inhibitor, BTS. The continuous tension decline during shortening is not abolished by caffeine, but the rate of decline is reduced when the active force is depressed by BTS. We propose that stiffening of non-crossbridge visco-elastic elements upon activation contributes to the continuous tension rise during lengthening and the release of such tension and Ca-insensitive deactivation contribute to the tension decline during shortening in muscle fibres.

The biphasic force-velocity relationship in whole rat skeletal muscle in situ

Edman has reported that the force-velocity relationship (FVR) departs from Hill's classic hyperbola near 0.80 of measured isometric force (J Physiol 404: 301-321, 1988). The purpose of this study was to investigate the biphasic nature of the FVR in the rested state and after some recovery from fatigue in the rat medial gastrocnemius muscle in situ. Force-velocity characteristics were determined before and during recovery from fatigue induced by intermittent stimulation at 170 Hz for 100 ms each second for 6 min. Force-velocity data were obtained for isotonic contractions with 100 ms of 200-Hz stimulation, including several measurements with loads above 0.80 of measured isometric force. The force-velocity data obtained in this study were fit well by a double-hyperbolic equation. A departure from Hill's classic hyperbola was found at 0.88+/-0.01 of measured isometric force, which is higher than the approximately 0.80 reported by Edman et al. for isolated frog fibers. After 45 min of recovery, maximum shortening velocity was 86+/-2% of prefatigue, but neither curvature nor predicted isometric force was significantly different from prefatigue. The location of the departure from Hill's classic hyperbola was not different after this recovery from the fatiguing contractions. Including an isometric point in the data set will not yield the same values for maximal velocity and the degree of curvature as would be obtained using the double hyperbola approach. Data up to 0.88 of measured isometric force can be used to fit data to the Hill equation.

Dynamic behaviour of half-sarcomeres during and after stretch in activated rabbit psoas myofibrils: sarcomere asymmetry but no 'sarcomere popping'

We examined length changes of individual half-sarcomeres during and after stretch in actively contracting, single rabbit psoas myofibrils containing 10-30 sarcomeres. The myofibrils were fluorescently immunostained so that both Z-lines and M-bands of sarcomeres could be monitored by video microscopy simultaneously with the force measurement. Half-sarcomere lengths were determined by processing of video images and tracking the fluorescent Z-line and M-band signals. Upon Ca2+ activation, during the rise in force, active half-sarcomeres predominantly shorten but to different extents so that an active myofibril consists of half-sarcomeres of different lengths and thus asymmetric sarcomeres, i.e. shifted A-bands, indicating different amounts of filament overlap in the two halves. When force reached a plateau, the myofibril was stretched by 15-20% resting length (L0) at a velocity of approximately 0.2 L0 s(-1). The myofibril force response to a ramp stretch is similar to that reported from muscle fibres. Despite the approximately 2.5-fold increase in force due to the stretch, the variability in half-sarcomere length remained almost constant during the stretch and A-band shifts did not progress further, independent of whether half-sarcomeres shortened or lengthened during the initial Ca2+ activation. Moreover, albeit half-sarcomeres lengthened to different extents during a stretch, rapid elongation of individual sarcomeres beyond filament overlap ('popping') was not observed. Thus, in contrast to predictions of the 'popping sarcomere' hypothesis, a stretch rather stabilizes the uniformity of half-sarcomere lengths and sarcomere symmetry. In general, the half-sarcomere length changes (dynamics) before and after stretch were slow and the dynamics after stretch were not readily predictable on the basis of the steady-state force-sarcomere length relation.