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- Synchronous behavior of spontaneous oscillations of sarcomer...Synchronous behavior of spontaneous oscillations of sarcomeres in skeletal myofibrils under isotonic conditions
Biophysical Journal, Volume 70, Issue 4, 1 April 1996, Pages 1823-1829
K. Yasuda, Y. Shindo and S. Ishiwata
AbstractAn isotonic control system for studying dynamic properties of single myofibrils was developed to evaluate the change of sarcomere lengths in glycerinated skeletal myofibrils under conditions of spontaneous oscillatory contraction (SPOC) in the presence of inorganic phosphate and a high ADP-to-ATP ratio. Sarcomere length oscillated spontaneously with a peak-to-peak amplitude of about 0.5 microns under isotonic conditions in which the external loads were maintained constant at values between 1.5 x 10(4) and 3.5 x 10(4) N/m2. The shortening and yielding of sarcomeres occurred in concert, in contrast to the previously reported conditions (isomeric or auxotonic) under which the myofibrillar tension is allowed to oscillate. This synchronous SPOC appears to be at a higher level of synchrony than in the organized state of SPOC previously observed under auxotonic conditions. The period of sarcomere length oscillation did not largely depend on external load. The active tension under SPOC conditions increased as the sarcomere length increased from 2.1 to 3.2 microns, although it was still smaller than the tension under normal Ca2+ contraction (which is on the order of 10(5) N/m2). The synchronous SPOC implies that there is a mechanism for transmitting information between sarcomeres such that the state of activation of sarcomeres is affected by the state of adjacent sarcomeres. We conclude that the change of myofibrillar tension is not responsible for the SPOC of each sarcomere but that it affects the level of synchrony of sarcomere oscillations.
Abstract | PDF (833 kb) - Nonlinear Force-Length Relationship in the ADP-Induced Contr...Nonlinear Force-Length Relationship in the ADP-Induced Contraction of Skeletal Myofibrils
Biophysical Journal, Volume 93, Issue 12, 15 December 2007, Pages 4330-4341
Yuta Shimamoto, Fumiaki Kono, Madoka Suzuki and Shin’ichi Ishiwata
AbstractThe regulatory mechanism of sarcomeric activity has not been fully clarified yet because of its complex and cooperative nature, which involves both Ca and cross-bridge binding to the thin filament. To reveal the mechanism of regulation mediated by the cross-bridges, separately from the effect of Ca, we investigated the force-sarcomere length (SL) relationship in rabbit skeletal myofibrils (a single myofibril or a thin bundle) at >2.2m in the absence of Ca at various levels of activation by exogenous MgADP (4–20mM) in the presence of 1mM MgATP. The individual SLs were measured by phase-contrast microscopy to confirm the homogeneity of the striation pattern of sarcomeres during activation. We found that at partial activation with 4–8mM MgADP, the developed force nonlinearly depended on the length of overlap between the thick and the thin filaments; that is, contrary to the maximal activation, the maximal active force was generated at shorter overlap. Besides, the active force became larger, whereas this nonlinearity tended to weaken, with either an increase in [MgADP] or the lateral osmotic compression of the myofilament lattice induced by the addition of a macromolecular compound, dextran T-500. The model analysis, which takes into account the [MgADP]-and the lattice-spacing-dependent probability of cross-bridge formation, was successfully applied to account for the force-SL relationship observed at partial activation. These results strongly suggest that the cross-bridge works as a cooperative activator, the function of which is highly sensitive to as little as ≤1nm changes in the lattice spacing.
Abstract | Full Text | PDF (573 kb) - Spontaneous Oscillatory Contraction without Regulatory Prote...Spontaneous Oscillatory Contraction without Regulatory Proteins in Actin Filament-Reconstituted Fibers
Biophysical Journal, Volume 75, Issue 3, 1 September 1998, Pages 1439-1445
Hideaki Fujita and Shin’ichi Ishiwata
AbstractSkinned skeletal and cardiac muscle fibers exhibit spontaneous oscillatory contraction (SPOC) in the presence of MgATP, MgADP, and inorganic phosphate (P), but the molecular mechanism underlying this phenomenon is not yet clear. We have investigated the role of regulatory proteins in SPOC using cardiac muscle fibers of which the actin filaments had been reconstituted without tropomyosin and troponin, according to a previously reported method (Fujita et al., 1996. 71:2307–2318). That is, thin filaments in glycerinated cardiac muscle fibers were selectively removed by treatment with gelsolin. Then, by adding exogenous actin to these thin filament-free cardiac muscle fibers under polymerizing conditions, actin filaments were reconstituted. The actin filament-reconstituted cardiac muscle fibers generated active tension in a Ca-insensitive manner because of the lack of regulatory proteins. Herein we have developed a new solvent condition under which SPOC occurs, even in actin filament-reconstituted fibers: the coexistence of 2,3-butanedione 2-monoxime (BDM), a reversible inhibitor of actomyosin interactions, with MgATP, MgADP and P. The role of BDM in the mechanism of SPOC in the actin filament-reconstituted fibers was analogous to that of the inhibitory function of the tropomyosin-troponin complex (-Ca) in the control fibers. The present results suggest that SPOC is a phenomenon that is intrinsic to the actomyosin motor itself.
Abstract | Full Text | PDF (289 kb) - …more
Copyright © 2007 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 93, Issue 12, 4097-4098, 15 December 2007
doi:10.1529/biophysj.107.117762
New and Notable
The Regulation of Muscle Contraction: As in Life, It Keeps Getting More Complex
C.G. dos Remedios
, 
Muscle Research Unit, Bosch Institute, The University of Sydney, Sydney, Australia
Address reprint requests to C. G. dos Remedios.An article by Shimamoto et al. 1 in this issue examines a phenomenon called length-dependent activation (LDA). This is an important but unexplained anomaly in which contractile force increases as the overlap of thick and thin filaments decreases. It occurs in both cardiac and skeletal muscles although it is better known in the heart.
The Frank-Starling law tells us that increased filling of the heart (i.e., stretching the sarcomeres) produces increased ventricular pressure (i.e., stronger muscle contraction). How then does the sarcomere produce a greater force in the face of a decrease in the number of myosin “motor” units (myosin heads) able to interact with actin? The distance between the heads and the actin filaments may decrease, allowing the interaction to be more efficient, hence an increase in force as both Shimamoto et al. 1 and another independent article by Pearson et al. 2 in this issue demonstrate. LDA may also be explained by a cooperative enhancement of the binding of actin-myosin by the tropomyosin-troponin system as the sarcomere is stretched.
Pearson et al. 2 used x-ray diffraction to directly measure changes in the distance between the myosin motor and the actin filaments. Their method compelled them to use whole rat hearts, but wouldn’t it be better if they could have examined single cardiomyocytes or, even better, single myofibrils where sarcomere length and uniformity can be precisely measured? So, is there a technique that can reproducibly induce stretch activation of single sarcomeres while monitoring sarcomere dimensions and force?
In this issue, Shimamoto et al. 1 meet this challenge. They analyze optical microscopy images of single skeletal muscle myofibrils to determine the width (interfilament distance) and the length of each sarcomere with an accuracy of ±5nm (±0.2nm) and ±40nm, respectively. They also monitor contractile force (in the absence of membrane systems such as sarcolemma, sarcoplasmic reticulum, and mitochondria) using various concentrations of ATP, ADP and a wide range of free Ca2+. These conditions include those needed for partial activation (elevated concentrations of ADP in the absence of Ca2+).
The authors focus their attention on SPontaneous Oscillatory Contractions (SPOCs) that, once induced, travel back and forth along a single myofibril, thus providing a highly reproducible rate of contraction (at the rapid front edge) and relaxation (at the slower trailing edge) of the traveling wave of contractions). SPOCs can continue for several minutes, thus allowing precise averages of sarcomere length and diameter to be obtained under conditions where activation can be chemically controlled (see Movie 1 in the Supplementary Material of Shimamoto et al. 1).
To explain the molecular mechanism of LDA, Shimamoto et al. 1 used several key assumptions that they had previously proposed 3:
1. The interaction probability between myosin cross bridges in thick filaments and actin-troponin-tropomyosin in thin filaments depends on the distance between them.
2. The isovolumic nature of the sarcomere suggests that interfilament distance depends on sarcomere length.
3. Thin filament stiffness depends on both the Ca2+ concentration and cross-bridge formation.
4. The extension of myosin heads from the thick filaments depends on the ADP concentration.
2. The isovolumic nature of the sarcomere suggests that interfilament distance depends on sarcomere length.
3. Thin filament stiffness depends on both the Ca2+ concentration and cross-bridge formation.
4. The extension of myosin heads from the thick filaments depends on the ADP concentration.
As John Solaro points out in the prior New and Notable in this issue 4, hearts normally operate at submaximal Ca2+-activation and perhaps it is for this reason that LDA is more difficult to demonstrate in skeletal muscle that is either fully activated or completely relaxed. Surprisingly, LDA was first reported in skeletal muscle 5 more than 30 years ago, and SPOCs have been known for almost as long 6.
SPOC may not be as well known as twitches or tetanic contractions, but it is not new. The Ishiwata laboratory is certainly the best protagonist for the phenomenon 7, but several other laboratories 6,8,9,10 have reported SPOC for both skeletal and cardiac muscle, suggesting it is not merely a curious artifact. It can even be argued that SPOC is relevant to normal physiology. The frequency of the SPOC oscillations is linearly correlated with the resting heart rate of different animals (mouse, rat, guinea pig, rabbit, dog, pig, and cow, see 7,11) and the oscillations occur in Ca2+ concentration range (pCa <6 to >7) that occurs in vivo 11.
It is likely that SPOC will be useful for examining the effects of isoform switching and posttranslational modifications of specific myofibrillar proteins in striated muscles, or even for examining myofibrils isolated from biopsies failing human hearts 12.
References
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2. (2007). Effects of sustained length-dependent activation on in situ cross-bridge dynamics in rat hearts. Biophys. J. 93, 4319–4329. Abstract | Full Text | PDF (437 kb) | CrossRef | PubMed
3. (1974). A regulatory mechanism of muscle contraction based on the flexibility change of the thin filaments. J. Mechanochem. Cell Motil. 3, 9–17. PubMed
4. (2007). New and Notable. Mechanisms of the Frank-Starling law of the heart: the beat goes on. Biophys. J. 93, 4095–4096. Full Text | PDF (49 kb) | CrossRef | PubMed
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10. (2002). Isometric force kinetics upon rapid activation and relaxation of mouse, guinea pig and human heart muscle studied on the subcellular myofibrillar level. Bas. Res. Cardiol. 97, I127–I135. PubMed
11. (2006). Myocardial sarcomeres spontaneously oscillate with the period of heartbeat under physiological conditions. Biochem. Biophys. Res. Commun. 343, 1146–1152. CrossRef | PubMed
12. (1998). Changes in force and cytosolic Ca2+ after length changes in isolated rat ventricular trabeculae. J. Physiol. 506, 431–433. CrossRef | PubMed
Publication Information
Received: August 20, 2007
Accepted: September 18, 2007
