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

The force of the myosin motor sets cooperativity in thin filament activation of skeletal muscles

Contraction of striated muscle is regulated by a dual mechanism involving both thin, actin-containing filament and thick, myosin-containing filament. Thin filament is activated by Ca2+ binding to troponin, leading to tropomyosin displacement that exposes actin sites for interaction with myosin motors, extending from the neighbouring stress-activated thick filaments. Motor attachment to actin contributes to spreading activation along the thin filament, through a cooperative mechanism, still unclear, that determines the slope of the sigmoidal relation between isometric force and pCa (−log[Ca2+]), estimated by Hill coefficient nH. We use sarcomere-level mechanics in demembranated fibres of rabbit skeletal muscle activated by Ca2+ at different temperatures (12–35 °C) to show that nH depends on the motor force at constant number of attached motors. The definition of the role of motor force provides fundamental constraints for modelling the dynamics of thin filament activation and defining the action of small molecules as possible therapeutic tools.

Analysis methods and quality criteria for investigating muscle physiology using x-ray diffraction

X-ray diffraction studies of muscle have been tremendously powerful in providing fundamental insights into the structures of, for example, the myosin and actin filaments in a variety of muscles and the physiology of the cross-bridge mechanism during the contractile cycle. However, interpretation of x-ray diffraction patterns is far from trivial, and if modeling of the observed diffraction intensities is required it needs to be performed carefully with full knowledge of the possible pitfalls. Here, we discuss (1) how x-ray diffraction can be used as a tool to monitor various specific muscle properties and (2) how to get the most out of the rest of the observed muscle x-ray diffraction patterns by modeling where the reliability of the modeling conclusions can be objectively tested. In other x-ray diffraction methods, such as protein crystallography, the reliability of every step of the process is estimated and quoted in published papers. In this way, the quality of the structure determination can be properly assessed. To be honest with ourselves in the muscle field, we need to do as near to the same as we can, within the limitations of the techniques that we are using. We discuss how this can be done. We also use test cases to reveal the dos and don’ts of using x-ray diffraction to study muscle physiology.

Cold temperatures put a freeze on myosin activation

JGP study reveals that low temperatures reduce force production in mammalian skeletal muscle by trapping myosin motors in a refractory state unable to bind actin.

JGP study reveals that low temperatures reduce force production in mammalian skeletal muscle by trapping myosin motors in a refractory state unable to bind actin.

Low temperature traps myosin motors of mammalian muscle in a refractory state that prevents activation

The active force of mammalian skeletal muscle is reduced at low temperatures. Caremani et al. reveal that this is due to the rise of a population of myosin motors captured in a refractory state insensitive to muscle activation.

Myosin motors in the thick filament of resting striated (skeletal and cardiac) muscle are trapped in an OFF state, in which the motors are packed in helical tracks on the filament surface, inhibiting their interactions with actin and utilization of ATP. To investigate the structural changes induced in the thick filament of mammalian skeletal muscle by changes in temperature, we collected x-ray diffraction patterns from the fast skeletal muscle extensor digitorum longus of the mouse in the temperature range from near physiological (35°C) to 10°C, in which the maximal isometric force (T₀) shows a threefold decrease. In resting muscle, x-ray reflections signaling the OFF state of the thick filament indicate that cooling produces a progressive disruption of the OFF state with motors moving away from the ordered helical tracks on the surface of the thick filament. We find that the number of myosin motors in the OFF state at 10°C is half of that at 35°C. At T₀, changes in the x-ray signals that report the fraction and conformation of actin-attached motors can be explained if the threefold decrease in force associated with lowering temperature is due not only to a decrease in the force-generating transition in the actin-attached motors but also to a twofold decrease in the number of such motors. Thus, lowering the temperature reduces to the same extent the fraction of motors in the OFF state at rest and the fraction of motors attached to actin at T₀, suggesting that motors that leave the OFF state accumulate in a disordered refractory state that makes them unavailable for interaction with actin upon stimulation. This regulatory effect of temperature on the thick filament of mammalian skeletal muscle could represent an energetically convenient mechanism for hibernating animals.

Temperature effect on the chemomechanical regulation of substeps within the power stroke of a single Myosin II

Myosin IIs in the skeletal muscle are highly efficient nanoscale machines evolved in nature. Understanding how they function can not only bring insights into various biological processes but also provide guidelines to engineer synthetic nanoscale motors working in the vicinity of thermal noise. Though it was clearly demonstrated that the behavior of a skeletal muscle fiber, or that of a single myosin was strongly affected by the temperature, how exactly the temperature affects the kinetics of a single myosin is not fully understood. By adapting the newly developed transitional state model, which successfully explained the intriguing motor force regulation during skeletal muscle contraction, here we systematically explain how exactly the power stroke of a single myosin proceeds, with the consideration of the chemomechanical regulation of sub-steps within the stroke. The adapted theory is then utilized to investigate the temperature effect on various aspects of the power stroke. Our analysis suggests that, though swing rates, the isometric force, and the maximal stroke size all strongly vary with the temperature, the temperature can have a very small effect on the releasable elastic energy within the power stroke.

Self-Regulation of Motor Force Through Chemomechanical Coupling in Skeletal Muscle Contraction

It is intriguing how the mechanics of molecular motors is regulated to perform the mechanical work in living systems. In sharp contrast to the conventional wisdom, recent experiments indicated that motor force maintains ∼6 pN upon a wide range of filament loads during skeletal muscle contraction at the steady state. Here we find that this rather precise regulation which takes place in an essentially chaotic system, can be due to that a “working” motor is arrested in a transitional state when the motor force is ∼6 pN. Our analysis suggests that the motor force can be self-regulated through chemomechanical coupling, and motor force homeostasis is a built-in feature at the level of a single motor, which provides insights to understanding the coordinated function of multiple molecular motors existing in various physiological processes. With a coupled stochastic-elastic numerical framework, the kinetic model for a Actin-myosin-ATP cycle constructed in this work might pave the way to decently investigate the transient behaviors of the skeletal muscle or other actomyosin complex structures.

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.

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.

Motor force homeostasis in skeletal muscle contraction

In active biological contractile processes such as skeletal muscle contraction, cellular mitosis, and neuronal growth, an interesting common observation is that multiple motors can perform coordinated and synchronous actions, whereas individual myosin motors appear to randomly attach to and detach from actin filaments. Recent experiment has demonstrated that, during skeletal muscle shortening at a wide range of velocities, individual myosin motors maintain a force of ~6 pN during a working stroke. To understand how such force-homeostasis can be so precisely regulated in an apparently chaotic system, here we develop a molecular model within a coupled stochastic-elastic theoretical framework. The model reveals that the unique force-stretch relation of myosin motor and the stochastic behavior of actin-myosin binding cause the average number of working motors to increase in linear proportion to the filament load, so that the force on each working motor is regulated at ~6 pN, in excellent agreement with experiment. This study suggests that it might be a general principle to use catch bonds together with a force-stretch relation similar to that of myosin motors to regulate force homeostasis in many biological processes.

Geometrical conditions indispensable for muscle contraction

Computer simulation has uncovered the geometrical conditions under which the vertebrate striated muscle sarcomere can contract. First, all thick filaments should have identical structure, namely: three myosin cross-bridges, building a crown, should be aligned at angles of 0°, 120°, 180°, and the successive crowns and the two filament halves should be turned around 120°. Second, all thick filaments should act simultaneously. Third, coordination in action of the myosin cross-bridges should exist, namely: the three cross-bridges of a crown should act simultaneously and the cross-bridge crowns axially 43 and 14.333 nm apart should act, respectively, simultaneously and with a phase shift. Fifth, six thin filaments surrounding the thick filament should be turned around 180° to each other in each sarcomere half. Sixth, thin filaments should be oppositely oriented in relation to the sarcomere middle. Finally, the structure of each of the thin filaments should change in consequence of strong interaction with myosin heads, namely: the axial distance and the angular alignment between neighboring actin monomers should be, respectively, 2.867 nm and 168° instead of 2.75 nm and 166.15°. These conditions ensure the stereo-specific interaction between actin and myosin and good agreement with the data gathered by electron microscopy and X-ray diffraction methods. The results suggest that the force is generated not only by the myosin cross-bridges but also by the thin filaments; the former acts by cyclical unwrapping and wrapping the thick filament backbone, and the latter byelongation.

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.

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.

The mechanism of the resistance to stretch of isometrically contracting single muscle fibres

Rapid attachment to actin of the detached motor domain of myosin dimers with one motor domain already attached has been hypothesized to explain the stretch-induced changes in X-ray interference and stiffness of active muscle. Here, using half-sarcomere mechanics in single frog muscle fibres (2.15 microm sarcomere length and 4 degrees C), we show that: (1) an increase in stiffness of the half-sarcomere under stretch is specific to isometric contraction and does not occur in rigor, indicating that the mechanism of stiffness increase is an increase in the number of attached motors; (2) 2 ms after 100 micros stretches (amplitude 2-8 nm per half-sarcomere) imposed during an isometric tetanus, the stiffness of the array of myosin motors in each half-sarcomere (e(m)) increases above the isometric value (e(m0)); (3) e(m) has a sigmoidal dependence on the distortion of the motor domains (Delta z) attached in isometric contraction, with a maximum approximately 2 e(m0) for a distortion of approximately 6 nm; e(m) is influenced by detachment of motors at z > 6 nm; (4) at the end of the 100 micros stretch the relation between e(m)/e(m0) and Delta z lies slightly but not significantly above that at 2 ms. These results support the idea that stretch-induced sliding of the actin filament distorts the actin-attached motor domain of the myosin dimers away from the centre of the sarcomere, providing the steric conditions for rapid attachment of the second motor domain. The rate of new motor attachment must be as high as 7.5 x 10(4) s(1) and explains the rapid and efficient increase of the resistance of active muscle to stretch.

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.

The temperature dependence of cell mechanics measured by atomic force microscopy

The cytoskeleton is a complex polymer network that regulates the structural stability of living cells. Although the cytoskeleton plays a key role in many important cell functions, the mechanisms that regulate its mechanical behaviour are poorly understood. Potential mechanisms include the entropic elasticity of cytoskeletal filaments, glassy-like inelastic rearrangements of cross-linking proteins and the activity of contractile molecular motors that sets the tensional stress (prestress) borne by the cytoskeleton filaments. The contribution of these mechanisms can be assessed by studying how cell mechanics depends on temperature. The aim of this work was to elucidate the effect of temperature on cell mechanics using atomic force microscopy. We measured the complex shear modulus (G*) of human alveolar epithelial cells over a wide frequency range (0.1-25.6 Hz) at different temperatures (13-37 degrees C). In addition, we probed cell prestress by mapping the contractile forces that cells exert on the substrate by means of traction microscopy. To assess the role of actomyosin contraction in the temperature-induced changes in G* and cell prestress, we inhibited the Rho kinase pathway of the myosin light chain phosphorylation with Y-27632. Our results show that with increasing temperature, cells become stiffer and more solid-like. Cell prestress also increases with temperature. Inhibiting actomyosin contraction attenuated the temperature dependence of G* and prestress. We conclude that the dependence of cell mechanics with temperature is dominated by the contractile activity of molecular motors.

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.

Residual force enhancement in myofibrils and sarcomeres

Residual force enhancement has been observed following active stretch of skeletal muscles and single fibres. However, there has been intense debate whether force enhancement is a sarcomeric property, or is associated with sarcomere length instability and the associated development of non-uniformities. Here, we studied force enhancement for the first time in isolated myofibrils (n=18) that, owing to the strict in series arrangement, allowed for evaluation of this property in individual sarcomeres (n=79). We found consistent force enhancement following stretch in all myofibrils and each sarcomere, and forces in the enhanced state typically exceeded the isometric forces on the plateau of the force-length relationship. Measurements were made on the plateau and the descending limb of the force-length relationship and revealed gross sarcomere length non-uniformities prior to and following active myofibril stretching, but in contrast to previous accounts, revealed that sarcomere lengths were perfectly stable under these experimental conditions. We conclude that force enhancement is a sarcomeric property that does not depend on sarcomere length instability, that force enhancement varies greatly for different sarcomeres within the same myofibril and that sarcomeres with vastly different amounts of actin-myosin overlap produce the same isometric steady-state forces. This last finding was not explained by differences in the amount of contractile proteins within sarcomeres, vastly different passive properties of individual sarcomeres or (half-) sarcomere length instabilities, suggesting that the basic mechanical properties of muscles, such as force enhancement, force depression and creep, which have traditionally been associated with sarcomere instabilities and the corresponding dynamic redistribution of sarcomere lengths, are not caused by such instabilities, but rather seem to be inherent properties of the mechanisms of contraction.

Effect of temperature on cross-bridge properties in intact frog muscle fibers

It is well known that the force developed by skeletal muscles increases with temperature. Despite the work done on this subject, the mechanism of force potentiation is still debated. Most of the published papers suggest that force enhancement is due to the increase of the individual cross-bridge force. However, reports on skinned fibers and single-molecule experiments suggest that cross-bridge force is temperature independent. The effects of temperature on cross-bridge properties in intact frog fibers were investigated in this study by applying fast stretches at various tension levels (P) on the tetanus rise at 5 degrees C and 14 degrees C to induce cross-bridge detachment. Cross-bridge number was measured from the force (critical force, P(c)) needed to detach the cross-bridge ensemble, and the average cross-bridge strain was calculated from the sarcomere elongation needed to reach P(c) (critical length, L(c)). Our results show that P(c) increased linearly with the force developed at both temperatures, but the P(c)/P ratio was considerably smaller at 14 degrees C. This means that the average force per cross bridge is greater at high temperature. This mechanism accounts for all the tetanic force enhancement. The critical length L(c) was independent of the tension developed at both temperatures but was significantly lower at high temperature suggesting that cross bridges at 14 degrees C are more strained. The increased cross-bridge strain accounts for the greater average force developed.

Skeletal muscle resists stretch by rapid binding of the second motor domain of myosin to actin

A shortening muscle is a machine that converts metabolic energy into mechanical work, but, when a muscle is stretched, it acts as a brake, generating a high resistive force at low metabolic cost. The braking action of muscle can be activated with remarkable speed, as when the leg extensor muscles rapidly decelerate the body at the end of a jump. Here we used time-resolved x-ray and mechanical measurements on isolated muscle cells to elucidate the molecular basis of muscle braking and its rapid control. We show that a stretch of only 5 nm between each overlapping set of myosin and actin filaments in a muscle sarcomere is sufficient to double the number of myosin motors attached to actin within a few milliseconds. Each myosin molecule has two motor domains, only one of which is attached to actin during shortening or activation at constant length. A stretch strains the attached motor domain, and we propose that combined steric and mechanical coupling between the two domains promotes attachment of the second motor domain. This mechanism allows skeletal muscle to resist external stretch without increasing the force per motor and provides an answer to the longstanding question of the functional role of the dimeric structure of muscle myosin.

Sarcomere length dependence of rat skinned cardiac myocyte mechanical properties: dependence on myosin heavy chain

The effects of sarcomere length (SL) on sarcomeric loaded shortening velocity, power output and rates of force development were examined in rat skinned cardiac myocytes that contained either alpha-myosin heavy chain (alpha-MyHC) or beta-MyHC at 12 +/- 1 degrees C. When SL was decreased from 2.3 microm to 2.0 microm submaximal isometric force decreased approximately 40% in both alpha-MyHC and beta-MyHC myocytes while peak absolute power output decreased 55% in alpha-MyHC myocytes and 70% in beta-MyHC myocytes. After normalization for the fall in force, peak power output decreased about twice as much in beta-MyHC as in alpha-MyHC myocytes (41% versus 20%). To determine whether the fall in normalized power was due to the lower force levels, [Ca(2+)] was increased at short SL to match force at long SL. Surprisingly, this led to a 32% greater peak normalized power output at short SL compared to long SL in alpha-MyHC myocytes, whereas in beta-MyHC myocytes peak normalized power output remained depressed at short SL. The role that interfilament spacing plays in determining SL dependence of power was tested by myocyte compression at short SL. Addition of 2% dextran at short SL decreased myocyte width and increased force to levels obtained at long SL, and increased peak normalized power output to values greater than at long SL in both alpha-MyHC and beta-MyHC myocytes. The rate constant of force development (k(tr)) was also measured and was not different between long and short SL at the same [Ca(2+)] in alpha-MyHC myocytes but was greater at short SL in beta-MyHC myocytes. At short SL with matched force by either dextran or [Ca(2+)], k(tr) was greater than at long SL in both alpha-MyHC and beta-MyHC myocytes. Overall, these results are consistent with the idea that an intrinsic length component increases loaded crossbridge cycling rates at short SL and beta-MyHC myocytes exhibit a greater sarcomere length dependence of power output.

An accelerated state of myosin-based actin motility

We use an in vitro motility assay to determine the biochemical basis for a hypermotile state of myosin-based actin sliding. It is widely assumed that the sole biochemical determinant of actin-sliding velocities, V, is actin-myosin detachment kinetics (1/tauon), yet we recently reported that, above a critical ATP concentration of approximately 100 microM, V exceeds the detachment limit by more than 2-fold. To determine the biochemical basis for this hypermotile state, we measure the effects of ATP and inorganic phosphate, Pi, on V and observe that at low [ATP] V decreases as ln [Pi], whereas above 100 microM ATP the hypermotile V is independent of Pi. The ln [Pi] dependence of V at low [ATP] is consistent with a macroscopic model of muscle shortening, similar to Hill's contractile component, which predicts that V varies linearly with an internal force (Hill's active state) that drives actin movement against the viscous drag of myosin heads strongly bound to actin (Hill's dashpot). At high [ATP], we suggest that the hypermotile V is caused by shear thinning of the resistive population of strongly bound myosin heads. Our data and analysis indicate that, in addition to contributions from tauon and myosin's step size, d, V is influenced by the biochemistry of myosin's working step as well as resistive properties of actin and myosin.

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.

Structural changes in the myosin filament and cross-bridges during active force development in single intact frog muscle fibres: stiffness and X-ray diffraction measurements

Structural and mechanical changes occurring in the myosin filament and myosin head domains during the development of the isometric tetanus have been investigated in intact frog muscle fibres at 4 degrees C and 2.15 microm sarcomere length, using sarcomere level mechanics and X-ray diffraction at beamline ID2 of the European Synchrotron Radiation Facility (Grenoble, France). The time courses of changes in both the M3 and M6 myosin-based reflections were recorded with 5 ms frames using the gas-filled RAPID detector (MicroGap Technology). Following the end of the latent period (11 ms after the start of stimulation), force increases to the tetanus plateau value (T(0)) with a half-time of 40 ms, and the spacings of the M3 and M6 reflections (S(M3) and S(M6)) increase by 1.5% from their resting values, with time courses that lead that of force by approximately 10 and approximately 20 ms, respectively. These temporal relations are maintained when the increase of force is delayed by approximately 10 ms by imposing, from 5 ms after the first stimulus, 50 nm (half-sarcomere)(-1) shortening at the velocity (V(0)) that maintains zero force. Shortening at V(0) transiently reduces S(M3) following the latent period and delays the subsequent increase in S(M3), but only delays the S(M6) increase without a transient decrease. Shortening at V(0) imposed at the tetanus plateau causes an abrupt reduction of the intensity of the M3 reflection (I(M3)), whereas the intensity of the M6 reflection (I(M6)) is only slightly reduced. The changes in half-sarcomere stiffness indicate that the isometric force at each time point is proportional to the number of myosin heads bound to actin. The different sensitivities of the intensity and spacing of the M3 and M6 reflections to the mechanical responses support the view that the M3 reflection in active muscle originates mainly from the myosin heads attached to the actin filament and the M6 reflection originates mainly from a fixed structure in the myosin filament signalling myosin filament length changes during the tetanus rise.

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 temperature on the working stroke of muscle myosin

Muscle contraction is due to myosin motors that transiently attach with their globular head to an actin filament and generate force. After a sudden reduction of the load below the maximum isometric force (T0), the attached myosin heads execute an axial movement (the working stroke) that drives the sliding of the actin filament toward the center of the sarcomere by an amount that is larger at lower load and is 11 nm near zero load. Here, we show that an increase in temperature from 2 to 17 degrees C, which increases the average isometric force per attached myosin head by 60%, does not affect the amount of filament sliding promoted by a reduction in force from T0 to 0.7T0, whereas it reduces the sliding under low load by 2.5 nm. These results exclude the possibility that the myosin working stroke is due to the release of the mechanical energy stored in the initial endothermic force-generating process and show that, at higher temperatures, the working stroke energy is greater because of higher force, although the stroke length is smaller at low load. We conclude the following: (i) the working stroke is made by a series of state transitions in the attached myosin head; (ii) the temperature increases the probability for the first transition, competent for isometric force generation; and (iii) the temperature-dependent rise in work at high load can be accounted for by the larger free energy drop that explains the rise in isometric force.