The elementary force generation process probed by temperature and length perturbations in muscle fibres from the rabbit

Oscillatory work and the step that generates force in single myofibrils from rabbit psoas

The elementary molecular step that generates force by cross-bridges (CBs) in active muscles has been under intense investigation in the field of muscle biophysics. It is known that an increase in the phosphate (Pi) concentration diminishes isometric force in active fibers, indicating a tight coupling between the force generation step and the Pi release step. The question asked here is whether the force generation occurs before Pi release or after release. We investigated the effect of Pi on oscillatory work production in single myofibrils and found that Pi-attached state(s) to CBs is essential for its production. Oscillatory work is the mechanism that allows an insect to fly by beating its wings, and it also has been observed in skeletal and cardiac muscle fibers, implying that it is an essential feature of all striated muscle types. With our studies, oscillatory work disappears in the absence of Pi in experiments using myofibrils. This suggests that force is generated during a transition between steps of oscillatory work production, and that the states involved in force production must have Pi attached. With sinusoidal analysis, we obtained the kinetic constants around the Pi release steps, established a CB scheme, and evaluated force generated (and supported) by each CB state. Our results demonstrate that force is generated before Pi is released, and the same force is maintained after Pi is released. Stretch activation and/or delayed tension can also be explained with this CB scheme and forms the basis of force generation and oscillatory work production.

Can animals tune tissue mechanics in response to changing environments caused by anthropogenic impacts?

Anthropogenic climate change and pollution are impacting environments across the globe. This Review summarises the potential impact of such anthropogenic effects on animal tissue mechanics, given the consequences for animal locomotor performance and behaviour. More specifically, in light of current literature, this Review focuses on evaluating the acute and chronic effects of temperature on the mechanical function of muscle tissues. For ectotherms, maximal muscle performance typically occurs at temperatures approximating the natural environment of the species. However, species vary in their ability to acclimate to chronic changes in temperature, which is likely to have longer-term effects on species range. Some species undergo periods of dormancy to avoid extreme temperature or drought. Whilst the skeletal muscle of such species generally appears to be adapted to minimise muscle atrophy and maintain performance for emergence from dormancy, the increased occurrence of extreme climatic conditions may reduce the survival of individuals in such environments. This Review also considers the likely impact of anthropogenic pollutants, such as hormones and heavy metals, on animal tissue mechanics, noting the relative paucity of literature directly investigating this key area. Future work needs to determine the direct effects of anthropogenic environmental changes on animal tissues and related changes in locomotor performance and behaviour, including accounting for currently unknown interactions between environmental factors, e.g. temperature and pollutants.

Dependence of thick filament structure in relaxed mammalian skeletal muscle on temperature and interfilament spacing

Contraction of skeletal muscle is regulated by structural changes in both actin-containing thin filaments and myosin-containing thick filaments, but myosin-based regulation is unlikely to be preserved after thick filament isolation, and its structural basis remains poorly characterized. Here, we describe the periodic features of the thick filament structure in situ by high-resolution small-angle x-ray diffraction and interference. We used both relaxed demembranated fibers and resting intact muscle preparations to assess whether thick filament regulation is preserved in demembranated fibers, which have been widely used for previous studies. We show that the thick filaments in both preparations exhibit two closely spaced axial periodicities, 43.1 nm and 45.5 nm, at near-physiological temperature. The shorter periodicity matches that of the myosin helix, and x-ray interference between the two arrays of myosin in the bipolar filament shows that all zones of the filament follow this periodicity. The 45.5-nm repeat has no helical component and originates from myosin layers closer to the filament midpoint associated with the titin super-repeat in that region. Cooling relaxed or resting muscle, which partially mimics the effects of calcium activation on thick filament structure, disrupts the helical order of the myosin motors, and they move out from the filament backbone. Compression of the filament lattice of demembranated fibers by 5% Dextran, which restores interfilament spacing to that in intact muscle, stabilizes the higher-temperature structure. The axial periodicity of the filament backbone increases on cooling, but in lattice-compressed fibers the periodicity of the myosin heads does not follow the extension of the backbone. Thick filament structure in lattice-compressed demembranated fibers at near-physiological temperature is similar to that in intact resting muscle, suggesting that the native structure of the thick filament is largely preserved after demembranation in these conditions, although not in the conditions used for most previous studies with this preparation.

Thermal robustness of biomechanical processes

Temperature influences many physiological processes that govern life as a result of the thermal sensitivity of chemical reactions. The repeated evolution of endothermy and widespread behavioral thermoregulation in animals highlight the importance of elevating tissue temperature to increase the rate of chemical processes. Yet, movement performance that is robust to changes in body temperature has been observed in numerous species. This thermally robust performance appears exceptional in light of the well-documented effects of temperature on muscle contractile properties, including shortening velocity, force, power and work. Here, we propose that the thermal robustness of movements in which mechanical processes replace or augment chemical processes is a general feature of any organismal system, spanning kingdoms. The use of recoiling elastic structures to power movement in place of direct muscle shortening is one of the most thoroughly studied mechanical processes; using these studies as a basis, we outline an analytical framework for detecting thermal robustness, relying on the comparison of temperature coefficients (Q ₁₀ values) between chemical and mechanical processes. We then highlight other biomechanical systems in which thermally robust performance that arises from mechanical processes may be identified using this framework. Studying diverse movements in the context of temperature will both reveal mechanisms underlying performance and allow the prediction of changes in performance in response to a changing thermal environment, thus deepening our understanding of the thermal ecology of many organisms.

The likely effects of thermal climate change on vertebrate skeletal muscle mechanics with possible consequences for animal movement and behaviour

Climate change can involve alteration in the local temperature that an animal is exposed to, which in turn may affect skeletal muscle temperature. The underlying effects of temperature on the mechanical performance of skeletal muscle can affect organismal performance in key activities, such as locomotion and fitness-related behaviours, including prey capture and predator avoidance. The contractile performance of skeletal muscle is optimized within a specific thermal range. An increased muscle temperature can initially cause substantial improvements in force production, faster rates of force generation, relaxation, shortening, and production of power output. However, if muscle temperature becomes too high, then maximal force production and power output can decrease. Any deleterious effects of temperature change on muscle mechanics could be exacerbated by other climatic changes, such as drought, altered water, or airflow regimes that affect the environment the animal needs to move through. Many species will change their location on a daily, or even seasonal basis, to modulate the temperature that they are exposed to, thereby improving the mechanical performance of their muscle. Some species undergo seasonal acclimation to optimize muscle mechanics to longer-term changes in temperature or undergo dormancy to avoid extreme climatic conditions. As local climate alters, species either cope with the change, adapt, avoid extreme climate, move, or undergo localized extinction events. Given that such outcomes will be determined by organismal performance within the thermal environment, the effects of climate change on muscle mechanics could have a major impact on the ability of a population to survive in a particular location.

Temperature Effects on Force and Actin⁻Myosin Interaction in Muscle: A Look Back on Some Experimental Findings

Observations made in temperature studies on mammalian muscle during force development, shortening, and lengthening, are re-examined. The isometric force in active muscle goes up substantially on warming from less than 10 °C to temperatures closer to physiological (>30 °C), and the sigmoidal temperature dependence of this force has a half-maximum at ~10 °C. During steady shortening, when force is decreased to a steady level, the sigmoidal curve is more pronounced and shifted to higher temperatures, whereas, in lengthening muscle, the curve is shifted to lower temperatures, and there is a less marked increase with temperature. Even with a small rapid temperature-jump (T-jump), force in active muscle rises in a definitive way. The rate of tension rise is slower with adenosine diphosphate (ADP) and faster with increased phosphate. Analysis showed that a T-jump enhances an early, pre-phosphate release step in the acto-myosin (crossbridge) ATPase cycle, thus inducing a force-rise. The sigmoidal dependence of steady force on temperature is due to this endothermic nature of crossbridge force generation. During shortening, the force-generating step and the ATPase cycle are accelerated, whereas during lengthening, they are inhibited. The endothermic force generation is seen in different muscle types (fast, slow, and cardiac). The underlying mechanism may involve a structural change in attached myosin heads and/or their attachments on heat absorption.

The force-generation process in active muscle is strain sensitive and endothermic: a temperature-perturbation study

In experiments on active muscle, we examined the tension decline and its temperature sensitivity at the onset of ramp shortening and at a range of velocities. A segment (∼1.5 mm long) of a skinned muscle fibre isolated from rabbit psoas muscle was held isometrically (sarcomere length ∼2.5 µm) at 8-9°C, maximally Ca²⁺-activated and a ramp shortening applied. The tension decline with a ramp shortening showed an early decrease of slope (the P₁ transition) followed by a slower decrease in slope (the P₂ transition) to the steady (isotonic) force. The tension level at the initial P₁ transition and the time to that transition decreased as the velocity was increased; the length change to this transition increased with shortening velocity to a steady value of ∼8 nm half-sarcomere^-1 A small, rapid, temperature jump (T-jump) (3-4°C, <0.2 ms) applied coincident with the onset of ramp shortening showed force enhancement by T-jump and changed the tension decline markedly. Analyses showed that the rate of T-jump-induced force rise increased linearly with increase of shortening velocity. These results provide crucial evidence that the strain-sensitive cross-bridge force generation, or a step closely coupled to it, is endothermic.

Kinetic coupling of phosphate release, force generation and rate-limiting steps in the cross-bridge cycle

A basic goal in muscle research is to understand how the cyclic ATPase activity of cross-bridges is converted into mechanical force. A direct approach to study the chemo-mechanical coupling between P_i release and the force-generating step is provided by the kinetics of force response induced by a rapid change in [P_i]. Classical studies on fibres using caged-P_i discovered that rapid increases in [P_i] induce fast force decays dependent on final [P_i] whose kinetics were interpreted to probe a fast force-generating step prior to P_i release. However, this hypothesis was called into question by studies on skeletal and cardiac myofibrils subjected to P_i jumps in both directions (increases and decreases in [P_i]) which revealed that rapid decreases in [P_i] trigger force rises with slow kinetics, similar to those of calcium-induced force development and mechanically-induced force redevelopment at the same [P_i]. A possible explanation for this discrepancy came from imaging of individual sarcomeres in cardiac myofibrils, showing that the fast force decay upon increase in [P_i] results from so-called sarcomere 'give'. The slow force rise upon decrease in [P_i] was found to better reflect overall sarcomeres cross-bridge kinetics and its [P_i] dependence, suggesting that the force generation coupled to P_i release cannot be separated from the rate-limiting transition. The reasons for the different conclusions achieved in fibre and myofibril studies are re-examined as the recent findings on cardiac myofibrils have fundamental consequences for the coupling between P_i release, rate-limiting steps and force generation. The implications from P_i-induced force kinetics of myofibrils are discussed in combination with historical and recent models of the cross-bridge cycle.

Force Responses and Sarcomere Dynamics of Cardiac Myofibrils Induced by Rapid Changes in [Pi]

The second phase of the biphasic force decay upon release of phosphate from caged phosphate was previously interpreted as a signature of kinetics of the force-generating step in the cross-bridge cycle. To test this hypothesis without using caged compounds, force responses and individual sarcomere dynamics upon rapid increases or decreases in concentration of inorganic phosphate [P_i] were investigated in calcium-activated cardiac myofibrils. Rapid increases in [P_i] induced a biphasic force decay with an initial slow decline (phase 1) and a subsequent 3-5-fold faster major decay (phase 2). Phase 2 started with the distinct elongation of a single sarcomere, the so-called sarcomere "give". "Give" then propagated from sarcomere to sarcomere along the myofibril. Propagation speed and rate constant of phase 2 (k_+Pi(2)) had a similar [P_i]-dependence, indicating that the kinetics of the major force decay (phase 2) upon rapid increase in [P_i] is determined by sarcomere dynamics. In contrast, no "give" was observed during phase 1 after rapid [P_i]-increase (rate constant k_+Pi(1)) and during the single-exponential force rise (rate constant k_-Pi) after rapid [P_i]-decrease. The values of k_+Pi(1) and k_-Pi were similar to the rate constant of mechanically induced force redevelopment (k_TR) and Ca²⁺-induced force development (k_ACT) measured at same [P_i]. These results indicate that the major phase 2 of force decay upon a P_i-jump does not reflect kinetics of the force-generating step but results from sarcomere "give". The other phases of P_i-induced force kinetics that occur in the absence of "give" yield the same information as mechanically and Ca²⁺-induced force kinetics (k_+Pi(1) ∼ k_-Pi ∼ k_TR ∼ k_ACT). Model simulations indicate that P_i-induced force kinetics neither enable the separation of P_i-release from the rate-limiting transition f into force states nor differentiate whether the "force-generating step" occurs before, along, or after the P_i-release.

Possible Cause of Nonlinear Tension Rise in Activated Muscle Fiber during Stretching

Tension in contracting muscle fiber under conditions of ramp stretching rapidly increases, but after reaching a critical stretch P_c sharply decreases. To find out the cause of these changes in muscle fiber tension, we stopped stretching before and after reaching P_c and left the fiber stretched for 50 msec. After rapid tension drop, the transient tension rise not accompanied by fiber stiffness increase was observed only in fibers heated to 25°C and stretched to P_c. Under other experimental conditions, this growth was absent. We suppose that stretch of the fiber to P_c induces transition of stereo-specifically attached myosin heads to pre-power stroke state and when the stretching is stopped, they make their step on actin and generate force. When the tension reaches P_c, all stereospecifically attached myosin heads turn out to be non-stereospecifically, or weakly attached to actin, and are unable to make the force-generating step.

The endothermic ATP hydrolysis and crossbridge attachment steps drive the increase of force with temperature in isometric and shortening muscle

The isometric tetanic tension of skeletal muscle increases with temperature because attached crossbridge states bearing a relatively low force convert to those bearing a higher force. It was previously proposed that the tension-generating step(s) in the crossbridge cycle was highly endothermic and was therefore itself directly targeted by changes in temperature. However, this did not explain why a rapid rise in temperature (a temperature jump) caused a much slower rate of rise of tension than a rapid length step. This led to suggestions that the step targeted by a temperature rise is not the tension-generating step but is an extra step in the attached pathway of the crossbridge cycle, perhaps located on a parallel pathway. This enigma has been a major obstacle to a full understanding of the operation of the crossbridge cycle. We have now used a previously developed mechano-kinetic model of the crossbridge cycle in frog muscle to simulate the temperature dependence of isometric tension and shortening velocity. We allowed all five steps in the cycle to be temperature-sensitive. Models with different starting combinations of enthalpy changes and activation enthalpies for the five steps were refined by downhill simplex runs and scored by their ability to fit experimental data on the temperature dependence of isometric tension and the relationship between force and shortening velocity in frog muscle. We conclude that the first tension-generating step may be weakly endothermic and that the rise of tension with temperature is largely driven by the preceding two strongly endothermic steps of ATP hydrolysis and attachment of M.ADP.Pi to actin. The refined model gave a reasonable fit to the available experimental data and after a temperature jump the overall rate of tension rise was much slower than after a length step as observed experimentally. The findings aid our understanding of the crossbridge cycle by showing that it may not be necessary to include an additional temperature-sensitive step.

Why muscle is an efficient shock absorber

Skeletal muscles power body movement by converting free energy of ATP hydrolysis into mechanical work. During the landing phase of running or jumping some activated skeletal muscles are subjected to stretch. Upon stretch they absorb body energy quickly and effectively thus protecting joints and bones from impact damage. This is achieved because during lengthening, skeletal muscle bears higher force and has higher instantaneous stiffness than during isometric contraction, and yet consumes very little ATP. We wish to understand how the actomyosin molecules change their structure and interaction to implement these physiologically useful mechanical and thermodynamical properties. We monitored changes in the low angle x-ray diffraction pattern of rabbit skeletal muscle fibers during ramp stretch compared to those during isometric contraction at physiological temperature using synchrotron radiation. The intensities of the off-meridional layer lines and fine interference structure of the meridional M3 myosin x-ray reflection were resolved. Mechanical and structural data show that upon stretch the fraction of actin-bound myosin heads is higher than during isometric contraction. On the other hand, the intensities of the actin layer lines are lower than during isometric contraction. Taken together, these results suggest that during stretch, a significant fraction of actin-bound heads is bound non-stereo-specifically, i.e. they are disordered azimuthally although stiff axially. As the strong or stereo-specific myosin binding to actin is necessary for actin activation of the myosin ATPase, this finding explains the low metabolic cost of energy absorption by muscle during the landing phase of locomotion.

The effects of Ca2+ and MgADP on force development during and after muscle length changes

The goal of this study was to compare the effects of Ca²⁺ and MgADP activation on force development in skeletal muscles during and after imposed length changes. Single fibres dissected from the rabbit psoas were (i) activated in pCa²⁺4.5 and pCa²⁺6.0, or (ii) activated in pCa²⁺4.5 before and after administration of 10 mM MgADP. Fibres were activated in sarcomere lengths (SL) of 2.65 µm and 2.95 µm, and subsequently stretched or shortened (5%SL at 1.0 SL.s⁻¹) to reach a final SL of 2.80 µm. The kinetics of force during stretch were not altered by pCa²⁺ or MgADP, but the fast change in the slope of force development (P₁) observed during shortening and the corresponding SL extension required to reach the change (L₁) were higher in pCa²⁺6.0 (P₁ = 0.22±0.02 P_o; L₁ = 5.26±0.24 nm.HS^.1) than in pCa²⁺4.5 (P₁ = 0.15±0.01 P_o; L₁ = 4.48±0.25 nm.HS^.1). L₁ was also increased by MgADP activation during shortening. Force enhancement after stretch was lower in pCa²⁺4.5 (14.9±5.4%) than in pCa²⁺6.0 (38.8±7.5%), while force depression after shortening was similar in both Ca²⁺ concentrations. The stiffness accompanied the force behavior after length changes in all situations. MgADP did not affect the force behavior after length changes, and stiffness did not accompany the changes in force development after stretch. Altogether, these results suggest that the mechanisms of force generation during and after stretch are different from those obtained during and after shortening.

A review of the thermal sensitivity of the mechanics of vertebrate skeletal muscle

Environmental temperature varies spatially and temporally, affecting many aspects of an organism's biology. In ectotherms, variation in environmental temperature can cause parallel changes in skeletal muscle temperature, potentially leading to significant alterations in muscle performance. Endotherms can also undergo meaningful changes in skeletal muscle temperature that can affect muscle performance. Alterations in skeletal muscle temperature can affect contractile performance in both endotherms and ectotherms, changing the rates of force generation and relaxation, shortening velocity, and consequently mechanical power. Such alterations in the mechanical performance of skeletal muscle can in turn affect locomotory performance and behaviour. For instance, as temperature increases, a consequent improvement in limb muscle performance causes some lizard species to be more likely to flee from a potential predator. However, at lower temperatures, they are much more likely to stand their ground, show threatening displays and even bite. There is no consistent pattern in reported effects of temperature on skeletal muscle fatigue resistance. This review focuses on the effects of temperature variation on skeletal muscle performance in vertebrates, and investigates the thermal sensitivity of different mechanical measures of skeletal muscle performance. The plasticity of thermal sensitivity in skeletal muscle performance has been reviewed to investigate the extent to which individuals can acclimate to chronic changes in their thermal environment. The effects of thermal sensitivity of muscle performance are placed in a wider context by relating thermal sensitivity of skeletal muscle performance to aspects of vertebrate species distribution.

Very low force-generating ability and unusually high temperature-dependency in hummingbird flight muscle fibers

Hummingbird flight muscle is estimated to have among the highest mass-specific power output among vertebrates, based on aerodynamic models. However, little is known about fundamental contractile properties of their remarkable flight muscles. We hypothesized that hummingbird pectoralis fibers generate relatively low force when activated in a tradeoff for high shortening speeds associated with the characteristic high wing beat frequencies that are required for sustained hovering. Our objective was to measure maximal force-generating ability (maximal force/cross-sectional area, Po/CSA) in single, skinned fibers from the pectoralis and supracoracoideus muscles, which power the wing downstroke and upstroke, respectively, in hummingbirds (Calypte anna) and in another similarly-sized species, zebra finch (Taeniopygia guttata), which also has a very high wingbeat frequency during flight but does not perform a sustained hover. Mean Po/CSA in hummingbird pectoralis fibers was very low - 1.6, 6.1 and 12.2 kN/m2, at 10, 15 and 20oC, respectively. Po/CSA in finch pectoralis fibers was also very low (for both species, ~5% of the reported Po/CSA of chicken pectoralis fast fibers at 15oC). Force generated at 20oC/force generated at 10oC ('Q10-force' value) was very high for hummingbird and finch pectoralis fibers (mean = 15.3 and 11.5, respectively), compared to rat slow and fast fibers (1.8 and 1.9, respectively). Po/CSA in hummingbird leg fibers was much higher than in pectoralis fibers, at each temperature, and the mean Q10-force was much lower. Thus, hummingbird and finch pectoralis fibers have an extremely low force-generating ability, compared to other bird and mammalian limb fibers, and an extremely high temperature-dependence of force generation. The extrapolated maximum force-generating ability of hummingbird pectoralis fibers in vivo (~48 kN/m2) is, however, substantially higher than the estimated requirements for hovering flight of C. anna. The unusually low Po/CSA of hummingbird and zebra finch pectoralis fibers may reflect a constraint imposed by a need for extremely high contraction frequencies, especially during hummingbird hovering.

Molecular mechanism of actin-myosin motor in muscle

The interaction of actin and myosin powers striated and smooth muscles and some other types of cell motility. Due to its highly ordered structure, skeletal muscle is a very convenient object for studying the general mechanism of the actin-myosin molecular motor. The history of investigation of the actin-myosin motor is briefly described. Modern concepts and data obtained with different techniques including protein crystallography, electron microscopy, biochemistry, and protein engineering are reviewed. Particular attention is given to X-ray diffraction studies of intact muscles and single muscle fibers with permeabilized membrane as they give insight into structural changes that underlie force generation and work production by the motor. Time-resolved low-angle X-ray diffraction on contracting muscle fibers using modern synchrotron radiation sources is used to follow movement of myosin heads with unique time and spatial resolution under near physiological conditions.

ATP binding and cross-bridge detachment steps during full Ca²⁺ activation: comparison of myofibril and muscle fibre mechanics by sinusoidal analysis

Single myofibrils 50–60 μm length and 2–3 μm diameter were isolated from rabbit psoas muscle fibres, and cross-bridge kinetics were studied by small perturbations of the length (∼0.2%) over a range of 15 frequencies (1–250 Hz). The experiments were performed at 15◦C in the presence of 0.05–10 mM MgATP, 8mM phosphate (Pi), 200 mM ionic strength with KAc (acetate), pCa 4.35–4.65, and pH 7.0. Two exponential processes, B and C, were resolved in tension transients. Their apparent rate constants (2πb and 2πc) increased as the [MgATP] was raised from 0.05 mM to 1mM, and then reached saturation at [MgATP] ≥ 1. Given that these rate constants were similar (c/b ∼1.7) at [Pi] ≥ 4 mM, they were combined to achieve an accurate estimate of the kinetic constants: their sum and product were analysed as functions of [MgATP]. These analyses yielded K1 =2.91 ± 0.31 mM −1, k2 =288 ± 36 s−1, and k−2 =10 ± 21 s−1 (±95% confidence limit, n =13 preparations), based on the cross-bridge model: AM+ATP ↔ (step 1) AM.ATP ↔ (step 2) A+M.ATP, where K1 is the ATP association constant (step 1), k2 is the rate constant of the cross-bridge detachment (step 2), and k−2 is the rate constant of its reversal step. These kinetic constants are respectively comparable to those observed in single fibres from rabbit psoas (K1 =2.35 ± 0.31 mM −1, k2 =243 ± 22 s−1, and k−2 =6 ± 14 s−1; n =8 preparations) when analysed by the same methods and under the same experimental conditions. These values are respectively not significantly different from those obtained in myofibrils, indicating that the same kinetic constants can be deduced from myofibril and muscle fibre studies, in terms of ATP binding and cross-bridge detachments steps. The fact that K1 in myofibrils is 1.2 times that in fibres (P≈0.05) may be explained by a small concentration gradient of ATP, ADP and/or Pi in single fibres.

Along the road not taken: how many myosin heads act on a single actin filament at any instant in working muscle?

We reconsider the use of stiffness measurements to estimate N, the number of myosin heads acting (working at any instant to produce tension) on a single actin filament in vertebrate striated muscle, and give reasons for our rejection of numbers produced from such measurements. We go on to present a different approach to the problem, citing and extending a model bearing on the value of N which is derived from other physiological and biochemical data and which offers insight into the fundamental actin-myosin contractile event as an impulsive force. New experimental data accumulating over the past decade support this model, in which the myosin heads act sequentially along the actin filament (this is an example of Conformational Spread). In this model only a single myosin head acts on a single actin filament to produce an impulse at any given instant in normally-contracting muscle, either in the isometric or the isotonic mode, so N = 1. However, extra impulses occur within the same time frame after quick release of length or tension. The predictions of this sequential model are in striking agreement with a large body of recent detailed biophysical and biochemical evidence. We suggest that this warrants further in-depth experimental work, specifically to explore and test the sequential model and its implications.

The fraction of myosin motors that participate in isometric contraction of rabbit muscle fibers at near-physiological temperature

The duty ratio, or the part of the working cycle in which a myosin molecule is strongly attached to actin, determines motor processivity and is required to evaluate the force generated by each molecule. In muscle, it is equal to the fraction of myosin heads that are strongly, or stereospecifically, bound to the thin filaments. Estimates of this fraction during isometric contraction based on stiffness measurements or the intensities of the equatorial or meridional x-ray reflections vary significantly. Here, we determined this value using the intensity of the first actin layer line, A1, in the low-angle x-ray diffraction patterns of permeable fibers from rabbit skeletal muscle. We calibrated the A1 intensity by considering that the intensity in the relaxed and rigor states corresponds to 0% and 100% of myosin heads bound to actin, respectively. The fibers maximally activated with Ca(2+) at 4°C were heated to 31-34°C with a Joule temperature jump (T-jump). Rigor and relaxed-state measurements were obtained on the same fibers. The intensity of the inner part of A1 during isometric contraction compared with that in rigor corresponds to 41-43% stereospecifically bound myosin heads at near-physiological temperature, or an average force produced by a head of ~6.3 pN.

Myosin heads contribute to the maintenance of filament order in relaxed rabbit muscle

Raising the temperature of rabbit skeletal muscle from ∼0°C to ∼20°C has been shown to enhance the helical organization of the myosin heads and to change the intensities of the 10 and 11 equatorial reflections. We show here by time-resolved x-ray diffraction combined with temperature jump that the movement of the heads to enhance the organized myosin helix occurs at the same fast rate as the change in the intensities of the equatorial reflections. However, model calculations indicate that the change in the equatorials cannot be explained simply in terms of the movement of myosin heads. Analysis of electron micrographs of transverse sections of relaxed muscle fibers cryofixed at ∼5°C and ∼35°C shows that in addition to the reorganization of the heads the thin and thick filaments are less constrained to their positions in the hexagonal filament lattice in the warm muscle than in the cold. Incorporating the changes in filament order in model calculations reconciles these with the observed changes in equatorial reflections. We suggest the thin filaments in the cold muscle are boxed into their positions by the thermal movement of the disordered myosin heads. In the warmer muscle, the packed-down heads leave the thin filaments more room to diffuse laterally.

Crossbridge mechanism(s) examined by temperature perturbation studies on muscle

An overall view of the contractile process that has emerged from -temperature-studies on active muscle is outlined. In isometric muscle, a small rapid temperature-jump (T-jump) enhances an early, pre-phosphate release, step in the acto-myosin (crossbridge) ATPase cycle and induces a characteristic rise in force indicating that crossbridge force generation is endothermic (force rises when heat is absorbed). Sigmoidal temperature dependence of steady force is largely due to the endothermic nature of force generation. During shortening, when muscle force is decreased, the T-jump force generation is enhanced; conversely, when a muscle is lengthening and its force increased, the T-jump force generation is inhibited. Taking T-jump force generation as a signature of the crossbridge - ATPase cycle, the results suggest that during lengthening the ATPase cycle is truncated before endothermic force generation, whereas during shortening this step and the ATPase cycle, are accelerated; this readily provides a molecular basis for the Fenn effect.

Force and power generating mechanism(s) in active muscle as revealed from temperature perturbation studies

The basic characteristics of the process of force and power generation in active muscle that have emerged from temperature studies are examined. This is done by reviewing complementary findings from temperature-dependence studies and rapid temperature-jump (T-jump) experiments and from intact and skinned fast mammalian muscle fibres. In isometric muscle, a small T-jump leads to a characteristic rise in force showing that crossbridge force generation is endothermic (heat absorbed) and associated with increased entropy (disorder). The sensitivity of the T-jump force generation to added inorganic phosphate (Pi) indicates that a T-jump enhances an early step in the actomyosin (crossbridge) ATPase cycle before Pi-release. During muscle lengthening when steady force is increased, the T-jump force generation is inhibited. Conversely, during shortening when steady force is decreased, the T-jump force generation is enhanced in a velocity-dependent manner, showing that T-jump force generation is strain sensitive. Within the temperature range of ∼5–35◦C, the temperature dependence of steady active force is sigmoidal both in isometric and in shortening muscle. However, in shortening muscle, the endothermic character of force generation becomes more pronounced with increased velocity and this can, at least partly, account for the marked increase with warming of the mechanical power output of active muscle.

Temperature jump induced force generation in rabbit muscle fibres gets faster with shortening and shows a biphasic dependence on velocity

We examined the tension responses to ramp shortening and rapid temperature jump (<0.2 ms, 3-4 degrees C T-jump) in maximally Ca(2+)-activated rabbit psoas muscle fibres at 8-9 degrees C (the fibre length (L(0)) was approximately 1.5 mm and sarcomere length 2.5 microm). The aim was to investigate the strain sensitivity of crossbridge force generation in muscle. The T-jump induced tension rise was examined during steady shortening over a wide range of velocities (V) approaching the V(max) (V range approximately 0.01 to approximately 1.5 L(0) s(1)). In the isometric state, a T-jump induced a biphasic tension rise consisting of a fast (approximately 50 s(1), phase 2b) and a slow (approximately 10 s(1), phase 3) component, but if treated as monophasic the rate was approximately 20 s(1). During steady shortening the T-jump tension rise was monophasic; the rate of tension rise increased linearly with shortening velocity, and near V(max) it was approximately 200 s(1), approximately 10x faster than in the isometric state. Relative to the tension reached after the T-jump, the amplitude increased with shortening velocity, and near V(max) it was 4x larger than in the isometric state. Thus, the temperature sensitivity of muscle force is markedly increased with velocity during steady shortening, as found in steady state experiments. The rate of tension decline during ramp shortening also increased markedly with increase of velocity. The absolute amplitude of T-jump tension rise was larger than that in the isometric state at the low velocities (<0.5 L(0) s(1)) but decreased to below that of the isometric state at the higher velocities. Such a biphasic velocity dependence of the absolute amplitude of T-jump tension rise implies interplay between, at least, two processes that have opposing effects on the tension output as the shortening velocity is increased, probably enhancement of crossbridge force generation and faster (post-stroke) crossbridge detachment by negative strain. Overall, our results show that T-jump force generation is strain sensitive and becomes considerably faster when exposed to negative strain. Thus the crossbridge force generation step in muscle is both temperature sensitive (endothermic) and strain sensitive.

Effect of phosphate and temperature on force exerted by white muscle fibres from dogfish

Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca²⁺ pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20°C were sensitive to Pi, whereas fibres activated from rigor at 12°C were insensitive to Pi in the range 5–25 mmol l⁻¹. Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12°C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12°C, normal body temperature, the force was near to its maximum value.

Tropomyosin period 3 is essential for enhancement of isometric tension in thin filament-reconstituted bovine myocardium

Tropomyosin (Tm) consists of 7 quasiequivalent repeats known as "periods," and its specific function may be associated with these periods. To test the hypothesis that either period 2 or 3 promotes force generation by inducing a positive allosteric effect on actin, we reconstituted the thin filament with mutant Tm in which either period 2 (Delta2Tm) or period 3 (Delta3Tm) was deleted. We then studied: isometric tension, stiffness, 6 kinetic constants, and the pCa-tension relationship. N-terminal acetylation of Tm did not cause any differences. The isometric tension in Delta2Tm remained unchanged, and was reduced to approximately 60% in Delta3Tm. Although the kinetic constants underwent small changes, the occupancy of strongly attached cross-bridges was not much different. The Hill factor (cooperativity) did not differ significantly between Delta2Tm (1.79 +/- 0.19) and the control (1.73 +/- 0.21), or Delta3Tm (1.35 +/- 0.22) and the control. In contrast, pCa(50) decreased slightly in Delta2Tm (5.11 +/- 0.07), and increased significantly in Delta3Tm (5.57 +/- 0.09) compared to the control (5.28 +/- 0.04). These results demonstrate that, when ions are present at physiological concentrations in the muscle fiber system, period 3 (but not period 2) is essential for the positive allosteric effect that enhances the interaction between actin and myosin, and increases isometric force of each cross-bridge.

Temperature change as a probe of muscle crossbridge kinetics: a review and discussion

Following the ideas introduced by Huxley (Huxley 1957, Prog. Biophys. Biophys. Chem. 7, 255-318), it is generally supposed that muscle contraction is produced by temporary links, called crossbridges, between myosin and actin filaments, which form and break in a cyclic process driven by ATP splitting. Here we consider the interaction of the energy in the crossbridge, in its various states, and the force exerted. We discuss experiments in which the mechanical state of the crossbridge is changed by imposed movement and the energetic consequence observed as heat output and the converse experiments in which the energy content is changed by altering temperature and the mechanical consequences are observed. The thermodynamic relationship between the experiments is explained and, at the first sight, the relationship between the results of these two types of experiment appears paradoxical. However, we describe here how both of them can be explained by a model in which mechanical and energetic changes in the crossbridges occur in separate steps in a branching cycle.

Direct modeling of X-ray diffraction pattern from contracting skeletal muscle

A direct modeling approach was used to quantitatively interpret the two-dimensional x-ray diffraction patterns obtained from contracting mammalian skeletal muscle. The dependence of the calculated layer line intensities on the number of myosin heads bound to the thin filaments, on the conformation of these heads and on their mode of attachment to actin, was studied systematically. Results of modeling are compared to experimental data collected from permeabilized fibers from rabbit skeletal muscle contracting at 5 degrees C and 30 degrees C and developing low and high isometric tension, respectively. The results of the modeling show that: i), the intensity of the first actin layer line is independent of the tilt of the light chain domains of myosin heads and can be used as a measure of the fraction of myosin heads stereospecifically attached to actin; ii), during isometric contraction at near physiological temperature, the fraction of these heads is approximately 40% and the light chain domains of the majority of them are more perpendicular to the filament axis than in rigor; and iii), at low temperature, when isometric tension is low, a majority of the attached myosin heads are bound to actin nonstereospecifically whereas at high temperature and tension they are bound stereospecifically.

Force generation examined by laser temperature-jumps in shortening and lengthening mammalian (rabbit psoas) muscle fibres

We examined the tension change induced by a rapid temperature jump (T-jump) in shortening and lengthening active muscle fibres. Experiments were done on segments of permeabilized single fibres (length (L0) approximately 2 mm, sarcomere length 2.5 microm) from rabbit psoas muscle; [MgATP] was 4.6 mm, pH 7.1, ionic strength 200 mm and temperature approximately 9 degrees C. A fibre was maximally Ca2+-activated in the isometric state and a approximately 3 degrees C, rapid (< 0.2 ms), laser T-jump applied when the tension was approximately steady in the isometric state, or during ramp shortening or ramp lengthening at a limited range of velocities (0-0.2 L0 s(-1)). The tension increased to 2- to 3 x P0 (isometric force) during ramp lengthening at velocities > 0.05 L0 s(-1), whereas the tension decreased to about < 0.5 x P0 during shortening at 0.1-0.2 L0 s(-1); the unloaded shortening velocity was approximately 1 L0 s(-1) and the curvature of the force-shortening velocity relation was high (a/P0 ratio from Hill's equation of approximately 0.05). In isometric state, a T-jump induced a tension rise of 15-20% to a new steady state; by curve fitting, the tension rise could be resolved into a fast (phase 2b, 40-50 s(-1)) and a slow (phase 3, 5-10 s(-1)) exponential component (as previously reported). During steady lengthening, a T-jump induced a small instantaneous drop in tension, followed by recovery, so that the final tension recorded with and without a T-jump was not significantly different; thus, a T-jump did not lead to a net increase of tension. During steady shortening, the T-jump induced a pronounced tension rise and both its amplitude and the rate (from a single exponential fit) increased with shortening velocity; at 0.1-0.2 L0 s(-1), the extent of fibre shortening during the T-jump tension rise was estimated to be approximately 1.2% L(0) and it was shorter at lower velocities. At a given shortening velocity and over the temperature range of 8-30 degrees C, the rate of T-jump tension rise increased with warming (Q10 approximately 2.7), similar to phase 2b (endothermic force generation) in isometric muscle. Results are discussed in relation to the previous findings in isometric muscle fibres which showed that a T-jump promotes an early step in the crossbridge-ATPase cycle that generates force. In general, the finding that the T-jump effect on active muscle tension is pronounced during shortening, but is depressed/inhibited during lengthening, is consistent with the expectations from the Fenn effect that energy liberation (and acto-myosin ATPase rate) in muscle are increased during shortening and depressed/inhibited during lengthening.

Stiffness and fraction of Myosin motors responsible for active force in permeabilized muscle fibers from rabbit psoas

The stiffness of the single myosin motor (epsilon) is determined in skinned fibers from rabbit psoas muscle by both mechanical and thermodynamic approaches. Changes in the elastic strain of the half-sarcomere (hs) are measured by fast mechanics both in rigor, when all myosin heads are attached, and during active contraction, with the isometric force (T0) modulated by changing either [Ca2+] or temperature. The hs compliance is 43.0+/-0.8 nm MPa-1 in isometric contraction at saturating [Ca2+], whereas in rigor it is 28.2+/-1.1 nm MPa-1. The equivalent compliance of myofilaments is 21.0+/-3.3 nm MPa-1. Accordingly, the stiffness of the ensemble of myosin heads attached in the hs is 45.5+/-1.7 kPa nm-1 in isometric contraction at saturating [Ca2+] (e0), and in rigor (er) it rises to 138.9+/-21.2 kPa nm-1. Epsilon, calculated from er and the lattice molecular dimensions, is 1.21+/-0.18 pN nm-1. epsilon estimated, using a thermodynamic approach, from the relation of T0 at saturating [Ca2+] versus the reciprocal of absolute temperature is 1.25+/-0.14 pN nm-1, similar to that estimated for fibers in rigor. Consequently, the ratio e0/er (0.33+/-0.05) can be used to estimate the fraction of attached heads during isometric contraction at saturating [Ca2+]. If the osmotic agent dextran T-500 (4 g/100 ml) is used to reduce the lateral filament spacing of the relaxed fiber to the value before skinning, both e0 and er increase by approximately 40%. Epsilon becomes approximately 1.7 pN nm-1 and the fraction and the force of myosin heads attached in the isometric contraction remain the same as before dextran application. The finding that the fraction of myosin heads attached to actin in an isometric contraction is 0.33 rules out the hypothesis of multiple mechanical cycles per ATP hydrolyzed.

Temperature-dependence of isometric tension and cross-bridge kinetics of cardiac muscle fibers reconstituted with a tropomyosin internal deletion mutant

The effect of temperature on isometric tension and cross-bridge kinetics was studied with a tropomyosin (Tm) internal deletion mutant AS-Delta23Tm (Ala-Ser-Tm Delta(47-123)) in bovine cardiac muscle fibers by using the thin filament extraction and reconstitution technique. The results are compared with those from actin reconstituted alone, cardiac muscle-derived control acetyl-Tm, and recombinant control AS-Tm. In all four reconstituted muscle groups, isometric tension and stiffness increased linearly with temperature in the range 5-40 degrees C for fibers activated in the presence of saturating ATP and Ca(2+). The slopes of the temperature-tension plots of the two controls were very similar, whereas the slope derived from fibers with actin alone had approximately 40% the control value, and the slope from mutant Tm had approximately 36% the control value. Sinusoidal analysis was performed to study the temperature dependence of cross-bridge kinetics. All three exponential processes A, B, and C were identified in the high temperature range (30-40 degrees C); only processes B and C were identified in the mid-temperature range (15-25 degrees C), and only process C was identified in the low temperature range (5-10 degrees C). At a given temperature, similar apparent rate constants (2pia, 2pib, 2pic) were observed in all four muscle groups, whereas their magnitudes were markedly less in the order of AS-Delta23Tm < Actin < AS-Tm approximately Acetyl-Tm groups. Our observations are consistent with the hypothesis that Tm enhances hydrophobic and stereospecific interactions (positive allosteric effect) between actin and myosin, but Delta23Tm decreases these interactions (negative allosteric effect). Our observations further indicate that tension/cross-bridge is increased by Tm, but is diminished by Delta23Tm. We conclude that Tm affects the conformation of actin so as to increase the area of hydrophobic interaction between actin and myosin molecules.

Temperature change does not affect force between regulated actin filaments and heavy meromyosin in single-molecule experiments

The temperature dependence of sliding velocity, force and the number of cross-bridges was studied on regulated actin filaments (reconstituted thin filaments) when they were placed on heavy meromyosin (HMM) attached to a glass surface. The regulated actin filaments were used because our previous study on muscle fibres demonstrated that the temperature effect was much reduced in the absence of regulatory proteins. A fluorescently labelled thin filament was attached to the gelsolin-coated surface of a polystyrene bead. The bead was trapped by optical tweezers, and HMM-thin filament interaction was performed at 20-35 degrees C to study the temperature dependence of force at the single-molecule level. Our experiments showed that there was a small increase in force with temperature (Q10 = 1.43) and sliding velocity (Q10 = 1.46). The small increase in force was correlated with the small increase in the number of cross-bridges (Q10 = 1.49), and when force was divided by the number of cross-bridges, the result did not depend on the temperature (Q(10) = 1.03). These results demonstrate that the force each cross-bridge generates is fixed and independent of temperature. Our additional experiments demonstrate that tropomyosin (Tm) in the presence of troponin (Tn) and Ca2+ enhances both force and velocity, and a truncated mutant, Delta23Tm, diminishes force and velocity. These results are consistent with the hypothesis that Tm in the presence of Tn and Ca2+ exerts a positive allosteric effect on actin to make actomyosin linkage more secure so that larger forces can be generated.

Effect of strain on actomyosin kinetics in isometric muscle fibers

Investigations were conducted into the biochemical and mechanical states of cross-bridges during isometric muscle contraction. Rapid length steps (3 or 6 nm hs(-1)) were applied to rabbit psoas fibers, permeabilized and isometric, at either 12 degrees C or 20 degrees C. Fibers were activated by photolysis of P(3)-1-(2-nitrophenyl)-ethyl ester of ATP infused into rigor fibers at saturating Ca(2+). Sarcomere length, tension, and phosphate release were recorded-the latter using the MDCC-PBP fluorescent probe. A reduction in strain, induced by a rapid release step, produced a short-lived acceleration of phosphate release. Rates of the phosphate transient and that of phases 3 and 4 of tension recovery were unaffected by step size but were elevated at higher temperatures. In contrast the amplitude of the phosphate transient was smaller at 20 degrees C than 12 degrees C. The presence of 0.5 or 1.0 mM added ADP during a release step reduced both the rate of tension recovery and the poststep isometric tension. A kinetic scheme is presented to simulate the observed data and to precisely determine the rate constants for the elementary steps of the ATPase cycle.

Kinetics of force recovery following length changes in active skinned single fibres from rabbit psoas muscle: analysis and modelling of the late recovery phase

Redevelopment of isometric force following shortening of skeletal muscle is thought to result from a redistribution of cross-bridge states. We varied the initial force and cross-bridge distribution by applying various length-change protocols to active skinned single fibres from rabbit psoas muscle, and observed the effect on the slowest phase of recovery ('late recovery') that follows transient changes. In response to step releases that reduced force to near zero ( approximately 8 nm (half sarcomere)(-1)) or prolonged shortening at high velocity, late recovery was well described by two exponentials of approximately equal amplitude and rate constants of approximately 2 s(-1) and approximately 9 s(-1) at 5 degrees C. When a large restretch was applied at the end of rapid shortening, recovery was accelerated by (1) the introduction of a slow falling component that truncated the rise in force, and (2) a relative increase in the contribution of the fast exponential component. The rate of the slow fall was similar to that observed after a small isometric step stretch, with a rate of 0.4-0.8 s(-1), and its effects could be reversed by reducing force to near zero immediately after the stretch. Force at the start of late recovery was varied in a series of shortening steps or ramps in order to probe the effect of cross-bridge strain on force redevelopment. The rate constants of the two components fell by 40-50% as initial force was raised to 75-80% of steady isometric force. As initial force increased, the relative contribution of the fast component decreased, and this was associated with a length constant of about 2 nm. The results are consistent with a two-state strain-dependent cross-bridge model. In the model there is a continuous distribution of recovery rate constants, but two-exponential fits show that the fast component results from cross-bridges initially at moderate positive strain and the slow component from cross-bridges at high positive strain.

Endothermic force generation, temperature-jump experiments and effects of increased [MgADP] in rabbit psoas muscle fibres

We studied, by experiment and by kinetic modelling, the characteristics of the force increase on heating (endothermic force) in muscle. Experiments were done on maximally Ca2+-activated, permeabilized, single fibres (length approximately 2 mm; sarcomere length, 2.5 microm) from rabbit psoas muscle; [MgATP] was 4.6 mM, pH 7.1 and ionic strength was 200 mM. A small-amplitude (approximately 3 degrees C) rapid laser temperature-jump (0.2 ms T-jump) at 8-9 degrees C induced a tension rise to a new steady state and it consisted of two (fast and slow) exponential components. The T-jump-induced tension rise became slower as [MgADP] was increased, with half-maximal effect at 0.5 mM [MgADP]; the pre- and post-T-jump tension increased approximately 20% with 4 mM added [MgADP]. As determined by the tension change to small, rapid length steps (<1.4%L0 complete in <0.5 ms), the increase of force by [MgADP] was not associated with a concomitant increase of stiffness; the quick tension recovery after length steps (Huxley-Simmons phase 2) was slower with added MgADP. In steady-state experiments, the tension was larger at higher temperatures and the plot of tension versus reciprocal absolute temperature was sigmoidal, with a half-maximal tension at 10-12 degrees C; the relation with added 4 mM MgADP was shifted upwards on the tension axis and towards lower temperatures. The potentiation of tension with 4 mM added MgADP was 20-25% at low temperatures (approximately 5-10 degrees C), but approximately 10% at the physiological temperatures (approximately 30 degrees C). The shortening velocity was decreased with increased [MgADP] at low and high temperatures. The sigmoidal relation between tension and reciprocal temperature, and the basic effects of increased [MgADP] on endothermic force, can be qualitatively simulated using a five-step kinetic scheme for the crossbridge/A-MATPase cycle where the force generating conformational change occurs in a reversible step before the release of inorganic phosphate (P(i)), it is temperature sensitive (Q10 of approximately 4) and the release of MgADP occurs by a subsequent, slower, two-step mechanism. Modelling shows that the sigmoidal relation between force and reciprocal temperature arises from conversion of preforce-generating (A-M.ADP.P(i)) states to force-bearing (A-M.ADP) states as the temperature is raised. A tension response to a simulated T-jump consists of three (one fast and two slow) components, but, by combining the two slow components, they could be reduced to two; their relative amplitudes vary with temperature. The model can qualitatively simulate features of the tension responses induced by large-T-jumps from low starting temperatures, and those induced by small-T-jumps from different starting temperatures and, also, the interactive effects of P(i) and temperature on force in muscle fibres.

The structural basis of the increase in isometric force production with temperature in frog skeletal muscle

X-ray diffraction patterns were recorded from isolated single fibres of frog skeletal muscle during isometric contraction at temperatures between 0 and 17 degrees C. Isometric force was 43 +/- 2% (mean +/- S.E.M., n = 10) higher at 17 degrees C than 0 degrees C. The intensity of the first actin layer line increased by 57 +/- 18% (n = 5), and the ratio of the intensities of the equatorial 1,1 and 1,0 reflections by 20 +/- 7% (n = 10), signalling radial or azimuthal motions of the myosin head domains. The M3 X-ray reflection from the axial repeat of the heads along the filaments was 27 +/- 4% more intense at 17 degrees C, suggesting that the heads became more perpendicular to the filaments. The ratio of the intensities of the higher and lower angle peaks of the M3 reflection (R(M3)) was 0.93 +/- 0.02 (n = 5) at 0 degrees C and 0.77 +/- 0.02 at 17 degrees C. These peaks are due to interference between the two halves of each myosin filament, and the R(M3) decrease shows that heads move towards the midpoint of the myosin filament at the higher temperature. Calculations based on a crystallographic model of the heads indicated that the observed R(M3) change corresponds to tilting of their light-chain domains by 9 deg, producing an axial displacement of 1.4 nm, which is equal to that required to strain the actin and myosin filaments under the increased force. We conclude that the higher force generated by skeletal muscle at higher temperature can be accounted for by axial tilting of the myosin heads.

The “Roll and Lock” Mechanism of Force Generation in Muscle

Muscle force results from the interaction of the globular heads of myosin-II with actin filaments. We studied the structure-function relationship in the myosin motor in contracting muscle fibers by using temperature jumps or length steps combined with time-resolved, low-angle X-ray diffraction. Both perturbations induced simultaneous changes in the active muscle force and in the extent of labeling of the actin helix by stereo-specifically bound myosin heads at a constant total number of attached heads. The generally accepted hypothesis assumes that muscle force is generated solely by tilting of the lever arm, or the light chain domain of the myosin head, about its catalytic domain firmly bound to actin. Data obtained suggest an additional force-generating step: the "roll and lock" transition of catalytic domains of non-stereo-specifically attached heads to a stereo-specifically bound state. A model based on this scheme is described to quantitatively explain the data.

Strong binding of myosin heads stretches and twists the actin helix

Calculation of the size of the power stroke of the myosin motor in contracting muscle requires knowledge of the compliance of the myofilaments. Current estimates of actin compliance vary significantly introducing uncertainty in the mechanical parameters of the motor. Using x-ray diffraction on small bundles of permeabilized fibers from rabbit muscle we show that strong binding of myosin heads changes directly the actin helix. The spacing of the 2.73-nm meridional x-ray reflection increased by 0.22% when relaxed fibers were put into low-tension rigor (<10 kN/m(2)) demonstrating that strongly bound myosin heads elongate the actin filaments even in the absence of external tension. The pitch of the 5.9-nm actin layer line increased by approximately 0.62% and that of the 5.1-nm layer line decreased by approximately 0.26%, suggesting that the elongation is accompanied by a decrease in its helical angle (approximately 166 degrees) by approximately 0.8 degrees. This effect explains the difference between actin compliance revealed from mechanical experiments with single fibers and from x-ray diffraction on whole muscles. Our measurement of actin compliance obtained by applying tension to fibers in rigor is consistent with the results of mechanical measurements.

What do we learn by studying the temperature effect on isometric tension and tension transients in mammalian striated muscle fibres?

The significance of transient analysis of isometric tension and its temperature dependence on the molecular mechanisms of contraction is reviewed. The kinetic analysis of tension transient is essential to establish the elementary steps of the cross-bridge cycle. The temperature study is essential to deduce thermodynamic parameters of the force generation step, from which surface area changes associated with hydrophobic interaction and ionic interaction can be calculated. Experimental evidence suggests that a large scale hydrophobic and stereospecific interaction takes place at the time of force generation. This interaction is promoted by regulatory proteins troponin and tropomyosin, which is the basis for endothermic force generation. The six state cross-bridge model with two apparent rate constants is capable of explaining the temperature dependence of isometric tension and tension transients induced by temperature jump experiments. This model was previously proposed to account for the tension transients induced by sinusoidal length changes [Kawai and Halvorson (1991) Biophys J 59: 329-342]. The series compliance model is suitable for explaining the temperature effect on the stiffness data as the function of temperature, leading to the conclusion that the series compliance accounts for 40 +/- 5% of the total compliance in activated psoas fibres at 20 degrees C. These results are consistent with the hypothesis that tension per cross-bridge remains the same at different temperatures, and that it is the population shift that gives rise to the characteristic temperature effect on isometric tension.

Temperature dependence of the force-generating process in single fibres from frog skeletal muscle

Generation of force and shortening in striated muscle is due to the cyclic interactions of the globular portion (the head) of the myosin molecule, extending from the thick filament, with the actin filament. The work produced in each interaction is due to a conformational change (the working stroke) driven by the hydrolysis of ATP on the catalytic site of the myosin head. However, the precise mechanism and the size of the force and length step generated in one interaction are still under question. Here we reinvestigate the endothermic nature of the force-generating process by precisely determining, in tetanized intact frog muscle fibres under sarcomere length control, the effect of temperature on both isometric force and force response to length changes. We show that raising the temperature: (1) increases the force and the strain of the myosin heads attached in the isometric contraction by the same amount (approximately 70 %, from 2 to 17 degrees C); (2) increases the rate of quick force recovery following small length steps (range between -3 and 2 nm (half-sarcomere)-1) with a Q10 (between 2 and 12 degrees C) of 1.9 (releases) and 2.3 (stretches); (3) does not affect the maximum extent of filament sliding accounted for by the working stroke in the attached heads (10 nm (half-sarcomere)-1). These results indicate that in isometric conditions the structural change leading to force generation in the attached myosin heads can be modulated by temperature at the expense of the structural change responsible for the working stroke that drives filament sliding. The energy stored in the elasticity of the attached myosin heads at the plateau of the isometric tetanus increases with temperature, but even at high temperature this energy is only a fraction of the mechanical energy released by attached heads during filament sliding.

Force generation induced by rapid temperature jumps in intact mammalian (rat) skeletal muscle fibres

We examined the tension (force) responses induced by rapid temperature jumps (T-jumps) in electrically stimulated, intact fibre bundles (5-10 fibres, fibre length approximately 2 mm) isolated from a foot muscle (flexor hallucis brevis) of the rat; the muscle contains approximately 90 % type 2 fast fibres. In steady state experiments, the temperature dependence of the twitch tension was basically similar to that previously described from other fast muscles; the tetanic tension increased 3- to 4-fold in raising the temperature from approximately 2 to 35 degrees C and the relation between the tetanic tension and the reciprocal absolute temperature was sigmoidal with half-maximal tension at 9.5 degrees C. A rapid T-jump of 3-5 degrees C was induced during a contraction by applying an infrared laser pulse (lambda = 1.32 micro, 0.2 ms) to the 50 microl trough containing the fibre bundle immersed in physiological saline. At approximately 10 degrees C, a T-jump induced a large transient tension rise when applied during the rising phase of a twitch contraction, the amplitude of which decreased when the T-jump was delayed with respect to the stimulus; a T-jump probably perturbs an early step in excitation-contraction coupling. No transient increase was seen when a T-jump was applied during twitch relaxation. When applied during the plateau of a tetanic contraction a T-jump induced a tension rise to a higher steady tension level; the tension rise after a T-jump was 2-3 times faster than the corresponding phase of the initial tension rise in a tetanus. The approach to a new steady tension level after a T-jump was biphasic with a fast (phase 2b, approximately 35 s-1 at 10 degrees C) and a slow component (phase 3, < 10 s-1). The rates of both components increased (Q10 approximately 3) but their amplitudes decreased with increase of the steady temperature. These results from tetanized intact fibres are consistent with the thesis previously proposed from studies on Ca2+-activated skinned fibres, that the elementary force generation step in muscle is enhanced by increased temperature; the findings indicate that an endothermic molecular step underlies muscle force generation.