Time-resolved X-ray diffraction studies on the effect of slow length changes on tetanized frog skeletal muscle

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I_1,1/I_1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I_1,1/I_1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Reduced preload increases Mechanical Control (strain-rate dependence) of Relaxation by modifying myosin kinetics

Rapid myocardial relaxation is essential in maintaining cardiac output, and impaired relaxation is an early indicator of diastolic dysfunction. While the biochemical modifiers of relaxation are well known to include calcium handling, thin filament activation, and myosin kinetics, biophysical and biomechanical modifiers can also alter relaxation. We have previously shown that the relaxation rate is increased by an increasing strain rate, not a reduction in afterload. The slope of the relaxation rate to strain rate relationship defines Mechanical Control of Relaxation (MCR). To investigate MCR further, we performed in vitro experiments and computational modeling of preload-adjustment using intact rat cardiac trabeculae. Trabeculae studies are often performed using isometric (fixed-end) muscles at optimal length (Lo, length producing maximal developed force). We determined that reducing muscle length from Lo increased MCR by 20%, meaning that reducing preload could substantially increase the sensitivity of the relaxation rate to the strain rate. We subsequently used computational modeling to predict mechanisms that might underlie this preload-dependence. Computational modeling was not able to fully replicate experimental data, but suggested that thin-filament properties are not sufficient to explain preload-dependence of MCR because the model required the thin-filament to become more activated at reduced preloads. The models suggested that myosin kinetics may underlie the increase in MCR at reduced preload, an effect that can be enhanced by force-dependence. Relaxation can be modified and enhanced by reduced preload. Computational modeling implicates myosin-based targets for treatment of diastolic dysfunction, but further model refinements are needed to fully replicate experimental data.

X-ray Diffraction Studies on the Structural Origin of Dynamic Tension Recovery Following Ramp-Shaped Releases in High-Ca Rigor Muscle Fibers

It is generally believed that during muscle contraction, myosin heads (M) extending from myosin filament attaches to actin filaments (A) to perform power stroke, associated with the reaction, A-M-ADP-Pi → A-M + ADP + Pi, so that myosin heads pass through the state of A-M, i.e., rigor A-M complex. We have, however, recently found that: (1) an antibody to myosin head, completely covering actin-binding sites in myosin head, has no effect on Ca²⁺-activated tension in skinned muscle fibers; (2) skinned fibers exhibit distinct tension recovery following ramp-shaped releases (amplitude, 0.5% of Lo; complete in 5 ms); and (3) EDTA, chelating Mg ions, eliminate the tension recovery in low-Ca rigor fibers but not in high-Ca rigor fibers. These results suggest that A-M-ADP myosin heads in high-Ca rigor fibers have dynamic properties to produce the tension recovery following ramp-shaped releases, and that myosin heads do not pass through rigor A-M complex configuration during muscle contraction. To obtain information about the structural changes in A-M-ADP myosin heads during the tension recovery, we performed X-ray diffraction studies on high-Ca rigor skinned fibers subjected to ramp-shaped releases. X-ray diffraction patterns of the fibers were recorded before and after application of ramp-shaped releases. The results obtained indicate that during the initial drop in rigor tension coincident with the applied release, rigor myosin heads take up applied displacement by tilting from oblique to perpendicular configuration to myofilaments, and after the release myosin heads appear to rotate around the helical structure of actin filaments to produce the tension recovery.

Current Understanding of Residual Force Enhancement: Cross-Bridge Component and Non-Cross-Bridge Component

Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by cross-bridge cycling, which is governed by the theoretical framework of the cross-bridge theory. The cross-bridge theory explains well a number of mechanical responses, such as isometric and concentric contractions. However, some experimental observations cannot be explained with the cross-bridge theory; for example, the increased isometric force after eccentric contractions. The steady-state, isometric force after an eccentric contraction is greater than that attained in a purely isometric contraction at the same muscle length and same activation level. This well-acknowledged and universally observed property is referred to as residual force enhancement (rFE). Since rFE cannot be explained by the cross-bridge theory, alternative mechanisms for explaining this force response have been proposed. In this review, we introduce the basic concepts of sarcomere length non-uniformity and titin elasticity, which are the primary candidates that have been used for explaining rFE, and discuss unresolved problems regarding these mechanisms, and how to proceed with future experiments in this exciting area of research.

Basic science and clinical use of eccentric contractions: History and uncertainties

The peculiar attributes of muscles that are stretched when active have been noted for nearly a century. Understandably, the focus of muscle physiology has been primarily on shortening and isometric contractions, as eloquently revealed by A.V. Hill and subsequently by his students. When the sliding filament theory was introduced by A.F. Huxley and H.E. Huxley, it was a relatively simple task to link Hill's mechanical observations to the actions of the cross bridges during these shortening and isometric contractions. In contrast, lengthening or eccentric contractions have remained somewhat enigmatic. Dismissed as necessarily causing muscle damage, eccentric contractions have been much more difficult to fit into the cross-bridge theory. The relatively recent discovery of the giant elastic sarcomeric filament titin has thrust a previously missing element into any discussion of muscle function, in particular during active stretch. Indeed, the unexpected contribution of giant elastic proteins to muscle contractile function is highlighted by recent discoveries that twitchin-actin interactions are responsible for the "catch" property of invertebrate muscle. In this review, we examine several current theories that have been proposed to account for the properties of muscle during eccentric contraction. We ask how well each of these explains existing data and how an elastic filament can be incorporated into the sliding filament model. Finally, we review the increasing body of evidence for the benefits of including eccentric contractions into a program of muscle rehabilitation and strengthening.

Muscle residual force enhancement: a brief review

Muscle residual force enhancement has been observed in different muscle preparations for more than half a century. Nonetheless, its mechanism remains unclear; to date, there are three generally accepted hypotheses: 1) sarcomere length non-uniformity, 2) engagement of passive elements, and 3) an increased number of cross-bridges. The first hypothesis uses sarcomere non-homogeneity and instability to explain how "weak" sarcomeres would convey the higher tension generated by an enhanced overlap from "stronger" sarcomeres, allowing the whole system to produce higher forces than predicted by the force-length relationship; non-uniformity provides theoretical support for a large amount of the experimental data. The second hypothesis suggests that passive elements within the sarcomeres (i.e., titin) could gain strain upon calcium activation followed by stretch. Finally, the third hypothesis suggests that muscle stretch after activation would alter cross-bridge kinetics to increase the number of attached cross-bridges. Presently, we cannot completely rule out any of the three hypotheses. Different experimental results suggest that the mechanisms on which these three hypotheses are based could all coexist.

Crossbridge properties during force enhancement by slow stretching in single intact frog muscle fibres

The mechanism of force enhancement during lengthening was investigated on single frog muscle fibres by using fast stretches to measure the rupture tension of the crossbridge ensemble. Fast stretches were applied to one end of the activated fibre and force responses were measured at the other. Sarcomere length was measured by a striation follower device. Fast stretching induced a linear increase of tension that reached a peak and fell before the end of the stretch indicating that a sudden increase of fibre compliance occurred due to forced crossbridge detachment induced by the fast loading. The peak tension (critical tension, Pc) and the sarcomere length needed to reach Pc (critical length, Lc) were measured at various tensions during the isometric tetanus rise and during force enhancement by slow lengthening. The data showed that Pc was proportional to the tension generated by the fibre under both isometric and slow lengthening conditions. However, for a given tension increase, Pc was 6.5 times greater during isometric than during lengthening conditions. Isometric critical length was 13.04 +/- 0.17 nm per half-sarcomere (nm hs(-1)) independently of tension. During slow lengthening critical length fell as the force enhancement increased. For 90% enhancement, Lc reduced to 8.19 +/- 0.039 nm hs(-1). Assuming that the rupture force of the individual crossbridge is constant, these data indicate that the increase of crossbridge number during lengthening accounts for only 15.4% of the total force enhancement. The remaining 84.6% is accounted for by the increased mean strain of the crossbridges.

Does the speed of shortening affect steady-state force depression in cat soleus muscle?

It has been stated repeatedly for the past 50 years that the steady-state force depression following shortening of an activated muscle depends on the speed of shortening. However, these statements were based on results from experiments in which muscles were shortened at different speeds but identical activation levels. Therefore, the force during shortening was changed in accordance with the force-velocity relationship of muscles: that is, increasing speeds of shortening were associated with decreasing forces, and vice versa. Consequently, it is not possible at present to distinguish whether force depression is caused by the changes in speed, as frequently stated, or the associated changes in force, or both. The purpose of this study was to test if force depression depends on the speed of shortening. We hypothesized that force depression was dependent on the force but not the speed of contraction. Our prediction is that the amount of force depression after shortening contractions at different speeds could be similar if the force during contraction was controlled at a similar level. Cat soleus muscles (n=7) were shortened by 9 or 12 mm at speeds of 3, 9, and 27 mm/s, first with a constant activation during shortening (30Hz), then with activation levels that were reduced (<30Hz) for the slow speeds (3 and 9 mm/s) to approximate the shortening forces of the fast speed contractions (27 mm/s). If done properly, force depression could be precisely matched at the three different speeds, indicating that force depression was related to the force during the shortening contraction but not to the speed. However, in order to match force depression, the forces during shortening had to be systematically greater for the slow compared to the fast speeds of shortening, suggesting that force depression also depends on the level of activation, as force depression at constant activation levels can only be matched if the force during shortening, evaluated by the mechanical work, is identical. Therefore, we conclude that force depression depends on the force and activation level during shortening, but does not depend on the speed of shortening as has been assumed for half a century. These results support, but do not prove, the current hypothesis that force depression is caused by a stress-related cross-bridge inhibition in the actin-myosin overlap zone that is newly formed during muscle shortening.

Considerations on the history dependence of muscle contraction

When a skeletal muscle that is actively producing force is shortened or stretched, the resulting steady-state isometric force after the dynamic phase is smaller or greater, respectively, than the purely isometric force obtained at the corresponding final length. The cross-bridge model of muscle contraction does not readily explain this history dependence of force production. The most accepted proposal to explain both, force depression after shortening and force enhancement after stretch, is a nonuniform behavior of sarcomeres that develops during and after length changes. This hypothesis is based on the idea of instability of sarcomere lengths on the descending limb of the force-length relationship. However, recent evidence suggests that skeletal muscles may be stable over the entire range of active force production, including the descending limb of the force-length relationship. The purpose of this review was to critically evaluate hypotheses aimed at explaining the history dependence of force production and to provide some novel insight into the possible mechanisms underlying these phenomena. It is concluded that the sarcomere nonuniformity hypothesis cannot always explain the total force enhancement observed after stretch and likely does not cause all of the force depression after shortening. There is evidence that force depression after shortening is associated with a reduction in the proportion of attached cross bridges, which, in turn, might be related to a stress-induced inhibition of cross-bridge attachment in the myofilament overlap zone. Furthermore, we suggest that force enhancement is not associated with instability of sarcomeres on the descending limb of the force-length relationship and that force enhancement has an active and a passive component. Force depression after shortening and force enhancement after stretch are likely to have different origins.

Force enhancement following an active stretch in skeletal muscle

When skeletal muscle is stretched during a tetanic contraction, the resulting force is greater than the purely isometric force obtained at the corresponding final length. Several mechanisms have been proposed to explain this phenomenon, but the most accepted mechanism is the sarcomere length non-uniformity theory. This theory is associated with the notion of instability of sarcomeres on the descending limb of the force-length relationship. However, recent evidence suggests that this theory cannot account solely for the stretch-induced force enhancement. Some of this evidence is presented in this paper, and a new mechanism for force enhancement is proposed: one that is associated with the engagement of a passive force during stretch. We speculate that this passive force enhancement may be caused by titin, a protein associated with passive force production at long sarcomere lengths.

Evidence for two distinct cross-bridge populations in tetanized frog muscle fibers stretched with moderate velocities

When a tetanized frog skeletal muscle fiber is stretched with moderate velocities (< 1 L0/S), the tension developed above the level of isometric tension starts to decay after a sudden reduction of stretch velocity by more than 40-50%, though the fiber is still being stretched. We analyzed the decay of tension responses caused by the sudden reduction of stretch velocity, by applying three different types of stretch, i.e. a 1.5% stretch with velocity V1 (stretch 1), a 1.5% stretch with velocity V2 < V1 (stretch 2), and a 3% stretch consisting of stretches 1 and 2 applied in succession (stretch 3) and comparing the corresponding tension responses, TR 1, TR 2 and TR 3. It was found that TR 3 to stretch 3 was equal to algebraical sum of TR 1 to the preceding stretch 1 and TR 2 to the subsequent stretch 2. In other words, TR 2 started on the falling tension baseline equal to the decay of TR 1 after completion of stretch 1, These results can be explained by assuming two distinct cross-bridge populations mechanically acting in parallel with each other.

Evidence for two distinct cross-bridge populations in tetanized frog muscle fibers stretched with moderate velocities

When tetanized frog skeletal muscle fibers are subjected to moderate-velocity stretches (< 1 L0/s), the tension developed above the level of isometric tension starts to decay after a sudden reduction of stretch velocity by more than 40-50%, though the fibers are still being stretched. We analysed the decay of tension response caused by the sudden reduction of stretch velocity, by applying three different types of stretch to a tetanized fiber, i.e., a 1.5% stretch with velocity V1 (stretch-1), a 1.5% stretch with velocity V2 < V1 (stretch-2), and a 3% stretch consisting of stretch-1 and stretch-2 applied in succession (stretch-3) and comparing the corresponding tension responses, TR-1, TR-2, and TR-3. It was found that TR-3 to stretch-3 resulted from algebraical summation of TR-1 to the preceding stretch-1 and TR-2 to the subsequent stretch-2. These results can be accounted for by assuming two distinct cross-bridge populations in stretched fibers.

The filament lattice of striated muscle

The filament lattice of striated muscle is an overlapping hexagonal array of thick and thin filaments within which muscle contraction takes place. Its structure can be studied by electron microscopy or X-ray diffraction. With the latter technique, structural changes can be monitored during contraction and other physiological conditions. The lattice of intact muscle fibers can change size through osmotic swelling or shrinking or by changing the sarcomere length of the muscle. Similarly, muscle fibers that have been chemically or mechanically skinned can be compressed with bathing solutions containing very large inert polymeric molecules. The effects of lattice change on muscle contraction in vertebrate skeletal and cardiac muscle and in invertebrate striated muscle are reviewed. The force developed, the speed of shortening, and stiffness are compared with structural changes occurring within the lattice. Radial forces between the filaments in the lattice, which can include electrostatic, Van der Waals, entropic, structural, and cross bridge, are assessed for their contributions to lattice stability and to the contraction process.

Strain of passive elements during force enhancement by stretch in frog muscle fibres

1. The force enhancement during and after stretch (0.15 micron per sarcomere) was studied during fused tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (0.5-3.6 degrees C; sarcomere length, 2.05-2.65 microns). Changes in length were recorded simultaneously from the fibre as a whole (puller movement) and from marked segments (approximately 0.5 mm in length) of the same fibre. 2. The residual force enhancement after stretch (recorded at the end of a long tetanus) was found to be linearly related to the slow component of tension rise during the stretch ramp. 3. The fibres were released to shorten against a very small load at different times after stretch (load clamp). The shortening records derived after a preceding stretch exhibited a larger and steeper initial transient than that recorded in an isometric tetanus without stretch. The excess length change (LS; nanometres per half-sarcomere) recorded during the initial transient increased with the amplitude of stretch and was linearly related to the force enhancement produced by the stretch (FE; % of maximum tetanic tension) according to the following regression: LS = 0.200 FE + 8.65 (P < 0.001). The length changes recorded from the whole fibre agreed well with measurements from individual segments. 4. Slack-test measurements confirmed the existence of a large initial transient phase when the fibre was released to shorten after a preceding stretch. The excess length change determined from the slack tests agreed closely with the values derived from load-clamp recordings. 5. The results support the view that stretching a muscle fibre during tetanus leads to strain of elastic elements and, presumably, to variation of filament overlap due to non-uniform distribution of the length change within the fibre volume. Regions with greater filament overlap are likely to generate the long-lasting extra force referred to as 'residual force enhancement after stretch'. The elastic elements recruited during stretch can be presumed to play an essential part in this process by supporting regions in which the filament overlap has been reduced during the stretch ramp. Recoil of these elastic elements is responsible for the excess length change that is recorded during the initial transient after release as described under point 3.

Structural changes in myosin cross-bridges during shortening of frog skeletal muscle

X-ray diffraction patterns from frog sartorius muscle were recorded during steady shortening with various loads. The intensity of the third meridional reflection from the thick filament decreased on shortening to an extent proportional to the drop in tension. The intensity correlated more closely with the tension than with the shortening velocity. The Bragg spacing of the third meridional reflection decreased in proportion to the decrease in tension. The intensity decrease of the actin layer lines at 1/5.1 and 1/5.9 nm-1 was roughly proportional to the decrease in the load, indicating that the number of cross-bridges decreases similarly. The intensity of the (1,1) equatorial reflection showed a significant decrease only with low loads. Assuming that a steady structural state is attained during steady shortening, the results are consistent with the cross-bridge model in which the number of myosin cross-bridges decreases during shortening.

Effect of stretch and release on equatorial X-ray diffraction during a twitch contraction of frog skeletal muscle

Time-resolved intensity measurements of the x-ray equatorial reflections were made during twitch contractions of frog skeletal muscles, to which stretches or releases were applied at various times. A ramp stretch applied at the onset of a twitch (duration, 15 ms; amplitude, approximately 3% of muscle length) caused a faster and larger development of contractile force than in an isometric twitch. The stretch accelerated the decrease of the 1.0 reflection intensity (I1,0). The magnitude of increase of the 1,1 reflection intensity (I1,1) was reduced by the stretch, but its time course was also accelerated. A release applied at the peak of a twitch or later (duration, 5 ms; amplitude, approximately 1.5%) caused only a partial redevelopment of tension. The release produced clear reciprocal changes of reflections toward their relaxed levels, i.e., the I1,0 increased and the I1,1 decreased. A release applied earlier than the twitch peak had smaller effects on the reflection intensities. The results suggest that a strength applied at the onset of a twitch causes a faster radial movement of the myosin heads toward actin, whereas a release applied at or later than the peak of a twitch accelerates their return to the thick filament backbone. The results are discussed in the context of the regulation of the myosin head attachment by calcium.

Tetragonal deformation of the hexagonal myofilament matrix in single skinned skeletal muscle fibres owing to change in sarcomere length

Single skinned skeletal muscle fibres were immersed in solutions containing two different levels of activator calcium (pCa: 4.4; 6.0). Sarcomere length was varied from 1.6 to 3.5 microns and recorded by laser diffraction. Slack length was 2.0 microns. Small-angle equatorial X-ray diffraction patterns of relaxed and activated fibres at different sarcomere lengths were recorded using synchrotron radiation. The position and amplitude of the diffraction peaks were calculated from the spectra based on the hexagonal arrangement of the myofilament matrix, relating the position of the (1.0)- and (1.1)-diffraction peaks in this model by square root of 3. The diffraction peaks were fitted by five Gaussian functions (1.0, 1.1, 2.0, 2.1 and Z-line) and residual background was corrected by means of a hyperbola. The coupling of the position of the (1.0)- and (1.1)-peak was expressed as a factor: FAC = [d(1.0)/d(1.1)]/square root 3. In the relaxed state this coupling factor decreased at increasing sarcomere length (0.9880 +/- 0.002 at 2.0 microns; 0.900 +/- 0.01 at 3.5 microns). The coupling factor tends toward the one that will be obtained from the squared structure of actin filaments near the Z-discs. At shorter sarcomere lengths a decrease of the coupling factor has also been seen (0.9600 +/- 0.005 at 1.6 microns), giving rise to an increased uniform deformation of the hexagonal matrix, when sarcomere length is changed from slack length. From these experiments we conclude that a change in sarcomere length (from slack length) increases the deformation of the actin-myosin matrix to a tetragonal lattice.

Stiffness changes during enhancement and deficit of isometric force by slow length changes in frog skeletal muscle fibres

1. The mechanism of the enhancement and the deficit of isometric force by slow length changes in frog fast muscle fibres was studied by recording muscle fibre stiffness changes as measured with sinusoidal vibrations (0.5-1.9 kHz, peak-to-peak amplitude 0.1% of slack length, L0). 2. When a tetanized fibre was slowly stretched by 5-9% from sarcomere lengths 2.4-2.6 microns, the force rose to a peak during the stretch and then decreased towards a steady level higher than that during the ordinary isometric tetanus at the same sarcomere length. 3. The stiffness of the fibre first rose abruptly in response to stretch and then started to decrease linearly while the stretch went on; after the completion of stretch the stiffness decreased towards a steady value which was equal to that during the isometric tetanus at the same sarcomere length, indicating that the enhancement of isometric force is associated with decreased stiffness. 4. If a tetanized fibre was slowly released by 4-12% from sarcomere lengths 2.55-2.7 microns, the steady force attained after the completion of release was lower than that during an isometric tetanus at the same sarcomere length. 5. The stiffness of the fibre changed in parallel with the force both during and after the applied release. 6. Recordings of the segmental length changes along the fibre with a high-speed video system (200 frames/s) indicated that all segments lengthened in response to the applied stretch. 7. The segmental length changes in response to the applied release were markedly non-uniform; the length of a segment located at the centre of the fibre did not change appreciably both during and after the release. 8. These results are discussed in terms of cross-bridge performance and structure of the myofilament lattice.