Energetics of the actomyosin bond in the filament array of muscle fibers

Pate, Edward; Cooke, Roger

doi:10.1016/s0006-3495(88)83136-9

Cited by 57 publications

(76 citation statements)

References 47 publications

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“…The instantaneous stiffness of these MSL-labeled fibers during contraction is 75-85% of the rigor stiffness, as previously reported for unlabeled fibers (21). If the small fraction of heads with rigor-like orientation is responsible for this high stiffness, they must have much higher stiffness than in rigor (10). However, ifwe assume that the fraction of rigor stiffness (75-85%) is the fraction of attached heads, then at most 17% (13%/75%) of the attached heads exhibit rigor-like orientation, since all the heads are bound in rigor (22,23).…”

Section: Methodsmentioning

confidence: 99%

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP.

Fajer

Thomas

1990

Proc. Natl. Acad. Sci. U.S.A.

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To study the orientation of spin-labeled myosin heads in the first few seconds after the production of saturating ATP, we have used a laser flash to photolyze caged ATP during EPR data acquisition. Rabbit psoas muscle fibers were labeled with maleinide spin label, modifying 60% of the myosin heads without impairing muscle fiber biochemical and physiological activity (ATPase and force). The muscle bundles were incubated for 30 min with 5 mM caged ATP prior to the UV flash. The flash, from an excimer laser, liberated 2-3 mM ATP, generating maximum force in the presence of Ca2l and relaxing fully in the absence of Ca2". Control experiments, using fibers decorated with labeled myosin subfragment, showed that the flash liberates sufficient ATP to saturate myosin active sites in all regions of the muscle bundles. To increase the time resolution, and to minimize the time of the contraction, we followed in time the intensity at a single spectral position (P2)~which is associated with the high degree of orientational order in rigor. ATP liberation produced a rapid decrease of P2 with liberation of ATP, indicating a large decrease in orientational order in both relaxation and contraction. This transient was absent when caged AMP was used, ruling out nonspecific effects of the UV flash and subsequent photochemistry. The steady-state level of P2 during contraction was almost as low as that reached in relaxation, although the duration of the steady state was much more brief in contraction. Upon depletion of ATP in contraction, the P2 intensity reverted to the original rigor level, accompanied by development of rigor tension. The steady-state results obtained in the brief contractions induced by caged ATP are quantitatively consistent with those obtained in longer contractions by continuously perfusing fibers with ATP. In isometric contraction, most (88% ± 4%) of the heads are in a population characterized by a high degree of axial disorder, comparable to that observed for all heads in relaxation. Since the stiffness of these fibers in contraction is 80% of the stiffness in rigor, it is likely that most of the heads in this highly disoriented population are attached to actin in contraction and that most actin-attached heads in contraction are in this disoriented population.Electron microscopic observation of different orientations of muscle cross-bridges with respect to the muscle fiber axis in rigor and relaxation spawned the widely discussed "rotating head" hypothesis of muscle contraction (1). It has been postulated that the myosin head forms a cross-bridge by binding to the actin filament at a 900 angle; it then undergoes a conformational change that results in a pivoting motion of the head to a 45°orientation (2). Strain in the cross-bridge is then relieved by a sliding motion of actin and myosin filaments (2, 3). A rigorous confirmation of this hypothesis requires direct evidence for a large (-45°) tilting motion ofthe entire actin-attached myosin head with respect to the muscle fiber axis. Despite a wide range ...

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Section: Methodsmentioning

confidence: 99%

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP.

Fajer

Thomas

1990

Proc. Natl. Acad. Sci. U.S.A.

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“…These assumptions have all been questioned (25)(26)(27), but their validity is not required for the main conclusions in the Discussion. Determination of Myosin Head Concentration.…”

Section: [2]mentioning

confidence: 99%

“…First, the number of attached crossbridges may have been overestimated by comparing stiffness relative to rigor (e.g., as mentioned earlier, in the rigor fiber, all of the myosin heads may not be contributing to stiffness) (25,26), and this would have led to an underestimation of the calculated value of g (see Eq. 1 in Methods).…”

mentioning

confidence: 99%

Trading force for speed: Why superfast crossbridge kinetics leads to superlow forces

Rome

Cook

Syme

et al. 1999

Proc. Natl. Acad. Sci. U.S.A.

127

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Superfast muscles power high-frequency motions such as sound production and visual tracking. As a class, these muscles also generate low forces. Using the toadfish swimbladder muscle, the fastest known vertebrate muscle, we examined the crossbridge kinetic rates responsible for high contraction rates and how these might affect force generation. Swimbladder fibers have evolved a 10-fold faster crossbridge detachment rate than fast-twitch locomotory fibers, but surprisingly the crossbridge attachment rate has remained unchanged. These kinetics result in very few crossbridges being attached during contraction of superfast fibers (only Ϸ1͞6 of that in locomotory fibers) and thus low force. This imbalance between attachment and detachment rates is likely to be a general mechanism that imposes a tradeoff of force for speed in all superfast fibers.The superfast fiber type is found where high-frequency contractions are required, such as in vertebrate eye muscles and in both vertebrate and invertebrate synchronous soundproducing muscles. These muscles have a series of modifications for speed, including a large volume of sarcoplasmic reticulum (SR) (1-7) to produce very rapid calcium transients (8) and low-affinity troponin to speed myofilament deactivation after [Ca 2ϩ

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“…A third possibility is that axial stiffness is a measure of the fraction of myosin molecules interacting with actin, regardless of whether the interaction involves one or both heads of the myosin molecule, while the radial stiffness is a measure of the fraction of myosin heads attached. Fajer et al (1988) and Pate & Cooke (1988), based on EPR studies, suggested that in the axial direction, a single-headed nucleotide-free attached cross-bridge was as stiff as a double-headed nucleotide-free one. Finally and quite likely, the overall elastic properties.…”

Section: Radial Elasticity Of Cross-bridges In Musclementioning

confidence: 99%

State‐dependent radial elasticity of attached cross‐bridges in single skinned fibres of rabbit psoas muscle

Brenner

1993

The Journal of Physiology

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SUMMARY1. In a single skinned fibre of rabbit psoas muscle, upon attachment of crossbridges to actin in the presence of ADP or pvrophosphate (PPi), the separation between the contractile filaments, as determined by equatorial X-ray diffraction, is found to decrease, suggesting that force is generated in the radial direction.2. The single muscle fibres were subjected to compression by 0-8 % of dextran T500. The changes in lattice spacings by dextran compression were compared with changes induced by cross-bridge attachment to actin. Based on this comparison, the magnitude and the direction of the radial force generated by the attached crossbridges were estimated. The radial cross-bridge force varied with filament separation, and the magnitude of the radial cross-bridge force reached as high as the maximal axial force produced during isometric contraction. 3. One key parameter of the radial elasticity, i.e. the equilibrium spacing where the radial force is zero, was found to depend on the ligand bound to the myosin head. In the presence of ADP, the equilibrium spacing was 36 nm. In the presence of MgPPi the equilibrium spacing shifted to 35 nm and Ca2" had little effect on the equilibrium spacing.4. The equilibrium spacing was independent of the fraction of cross-bridges attached to actin. The fraction of cross-bridges attached in rigor was modulated from 100 % to close to 0 % by adding up to 10 mm of ATPyS in the rigor solution. The lattice spacing remained at 38 nm. the equilibrium spacing for nucleotide-free crossbridges at ,u = 170 mm.5. Radial force generated by cross-bridges in rigor at large lattice spacings (38 nm < d0o < 46 nm) appeared to vary linearly with lattice spacing.6. The titration of ATPyS to fibres in rigor provided a correlation between the radial stiffness of the nucleotide-free cross-bridges and the equatorial intensities. The relation between the equatorial intensity ratio I1,/I1o and radial stiffness appeared to be approximately linear.7. The fibres under different conditions showed a wide range of radial stiffness, which was not proportional to the apparent axial stiffness of the fibre. If the apparent axial stiffness is a measure of the fraction of cross-bridges bound to actin, it follows that the radial elastic constant is state dependent; or vice versa.t To whom correspondence should be addressed. MS 9894S. XL, B. BRENNE-R AND L. C YUT 8. Differences in equilibrium lattice spacing and in radial elastic constant, most probably reflect differences in the molecular structure of the acto-myosin complex and there is more than one single conformation of the various strongly bound crossbridge states.9. Determining equilibrium spacings of the radial elasticity appears to be an effective new approach in detecting structural differences among the attached crossbridges, since this approach is independent of the fraction of cross-bridges attached, a factor that frequently encumbers the interpretation of structural studies of attached cross-bridge states.

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Energetics of the actomyosin bond in the filament array of muscle fibers

Cited by 57 publications

References 47 publications

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP.

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP.

Trading force for speed: Why superfast crossbridge kinetics leads to superlow forces

State‐dependent radial elasticity of attached cross‐bridges in single skinned fibres of rabbit psoas muscle

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