Muscle Cross-Bridges: Do They Rotate?

Cooke, Roger; Crowder, M. S.; Wendt, Christine; Barnett, Vincent A.; Thomas, David D.

doi:10.1007/978-1-4684-4703-3_37

Cited by 39 publications

(32 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…12), indicating that the orientational distributions are not completely random in either relaxation or contraction. This is consistent with previous reports and conclusions from this laboratory (18,19), despite citations that interpret our data as implying random distribution in relaxation (20). DISCUSSION Interpretation of Results.…”

Section: Methodssupporting

confidence: 94%

See 1 more Smart Citation

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.

View full text Add to dashboard Cite

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 ...

show abstract

Section: Methodssupporting

confidence: 94%

“…The results of the present study are generally consistent with previous EPR studies of contracting MSL fibers (6,19). The present study confirms that most of the spin labels are highly disoriented in contraction, with the spectrum resembling that of relaxation much more than that of rigor (6,7,19).…”

Section: Methodssupporting

confidence: 92%

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.

View full text Add to dashboard Cite

show abstract

“…In this case the regeneration of working stroke in the staircase experiments is in a great part due to detachment and reattachment of cross-bridges already exerting force before each step. According to Lombardi et al (1992) (Cooke, Crowder, Wendt, Barnett & Thomas, 1984;Huxley & Kress, 1985). In this case, rw8 = 0-2 x 0 7 x 100 s-' = 14 s.…”

Section: Discussionmentioning

confidence: 99%

Comparison of energy output during ramp and staircase shortening in frog muscle fibres.

Linari

Woledge

1995

The Journal of Physiology

View full text Add to dashboard Cite

1. We compared the rates of work and heat production during ramp shortening with those during staircase shortening (sequence of step releases of the same amplitude, separated by regular time intervals). Ramp or staircase shortening was applied to isolated muscle fibres (sarcomere length, 2-2 ,sm; temperature, 1 00) at the plateau of an isometric tetanus. The total amount of shortening was no greater than 6 % of the fibre length. 2. During ramp shortening the power output showed a maximum at about 0-8 fibre lengths per second (Lo C1), which corresponds to 1/3 the maximum shortening velocity (VO). For the same average shortening velocity during staircase shortening (step size, -0 5% Lo) the power output was 40-60% lower. The rate of heat production for the same average shortening velocity was -45% higher during staircase shortening than during ramp shortening. 3. The relation between rate of total energy output and shortening velocity was well described by a second order regression line in the range of velocities used (0 1-2 3 Lo s-1). For any shortening velocity the rate of total energy output (power plus heat rate) was not statistically different for staircase (step size, 0 5 % Lo) and ramp shortening.4. The mechanical efficiency (the ratio of the power over the total energy rate) during ramp shortening had a maximum value of 0-36 at 1/5 VO; during staircase shortening, for any given shortening velocity, the mechanical efficiency was reduced compared with ramp shortening: with a staircase step of about 0 5 % Lo at 1/5 VO the efficiency was 0 2. 5.The results indicate that a cross-bridge is able to convert different quantities of energy into work depending on the different shortening protocol used. The fraction of energy dissipated as heat is larger during staircase shortening than during ramp shortening.

show abstract

“…According to the helix-coil mechanism, it would correspond to those myosin heads that are transiently coupled to melting in the hinge domain of the S-2 segments in a cross-bridge cycle (18). The fraction of heads estimated in the strongbinding rigor-like state, which Huxley and Kress (2) suggest may be the force-generating state, has been reported to be in the range 12-20%, based on steady-state (19)(20)(21) and timeresolved (22) EPR studies of actively contracting muscle. By using an independent method (proteolytic digestion), Duong and Reisler (23) also find only a small fraction of myosin heads (25%) form rigor-like bonds during ATP hydrolysis in myofibrils.…”

Section: Discussionmentioning

confidence: 99%

Contraction of myofibrils in the presence of antibodies to myosin subfragment 2.

Harrington

Karr

Busa

et al. 1990

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

View full text Add to dashboard Cite

In a muscle-based version of in vitro motility assays, the unloaded shortening velocity of rabbit skeletal myofibrils has been determined in the presence and absence of affinity-column-purified polyclonal antibodies directed against the subfragment-2 region of myosin. Contraction was initiated by photohydrolysis of caged ATP and the time dependence of shortening was monitored by an inverted microscope equipped with a video camera. Antibody-treated myofibrils undergo unloaded shortening in a fast phase with initial rates and half-times comparable to control (untreated) myofibrils, despite a marked reduction in the isometric force of skinned muscle fibers in the presence of the antibodies. In antibodytreated myofibrils, this process is followed by a much slower phase of contraction, terminating in elongated structures with well-defined sarcomere spacings (-1 ,um) in contrast to the supercontracted globular state of control myofibrils. These results suggest that although the unloaded shortening of myofibrils (and in vitro motility of actin filaments over immobilized myosin heads) can be powered by myosin heads, the subfragment-2 region as well as the myosin head contributes to force production in actively contracting muscle.The classical rotating cross-bridge model proposes that the contractile force in activated muscle originates from a structural transition within the myosin head [subfragment-1 (S-1) subunit] while it is attached to actin (1, 2). The helix-coil model proposes that melting within subfragment-2 (S-2; the a-helical tail segment of heavy meromyosin) is triggered and generates force when the actin-attached cross-bridge swings away from the thick filament surface (3, 4). In earlier work (5), it was shown that purified anti-S-2 antibody can markedly depress the isometric tension generated by skinned psoas fibers. The reduction in tension (to about 7% ofcontrol fibers) occurs in the absence of any direct effect on the ability of S-1 to undergo cyclic interaction with actin as measured by the ATPase of antibody-treated fibers. One interpretation of this experiment is that S-2 contributes to force production in vivo, possibly through a helix-coil transition in the S-2 hinge domain of the myosin molecule (for review, see ref. 6). However, it seems clear from recent in vitro model studies that S-1 alone, energized by ATP, can produce force (7) and slide actin filaments at speeds approaching those obtained with muscle fibers under no-load conditions (8,9). One possibility to reconcile these observations is that the force generated by S-1 in the model systems is sufficient to produce sliding motion but not sufficient for the expression of the full isometric force developed in working muscle. To test this explanation, we have measured the relative unloaded sliding velocity of overlapping thin and thick filaments in myofibrils, the intact basic structural units of skeletal muscle, in the presence and absence of polyclonal antibodies directed against the S-2 region of myosin. In these experiments, we have...

show abstract

Muscle Cross-Bridges: Do They Rotate?

Cited by 39 publications

References 19 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.

Comparison of energy output during ramp and staircase shortening in frog muscle fibres.

Contraction of myofibrils in the presence of antibodies to myosin subfragment 2.

Contact Info

Product

Resources

About