1 Departments of Orthopaedics, Bioengineering, and Applied Mechanics and Engineering Sciences, Biomedical Sciences Graduate Group, University of California, San Diego and Veterans Administration Medical Centers, San Diego, California 92161; 2 Department of Hand Surgery, Sahlgrenska University Hospital, S-413 45 Göteborg, Sweden; and 3 Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The functional role of the skeletal muscle intermediate filament system was investigated by measuring the magnitude of muscle force loss after cyclic eccentric contraction (EC) in normal and desmin null mouse extensor digitorum longus muscles. Isometric stress generated was significantly greater in wild-type (313 ± 8 kPa) compared with knockout muscles (276 ± 13 kPa) before EC (P < 0.05), but 1 h after 10 ECs, both muscle types generated identical levels of stress (~250 kPa), suggesting less injury to the knockout. Differences in injury susceptibility were not explained by the different absolute stress levels imposed on wild-type versus knockout muscles (determined by testing older muscles) or by differences in fiber length or mechanical energy absorbed. Morphometric analysis of longitudinal electron micrographs indicated that Z disks from knockout muscles were more staggered (0.36 ± 0.03 µm) compared with wild-type muscles (0.22 ± 0.03 µm), which may indicate that the knockout cytoskeleton is more compliant. These data demonstrate that lack of the intermediate filament system decreases isometric stress production and that the desmin knockout muscle is less vulnerable to mechanical injury.
intermediate filaments; cytoskeletal; muscle injury; biomechanics; aging
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
THE DESMIN INTERMEDIATE FILAMENT protein network is suggested to mechanically integrate the myofibrillar matrix in the radial and, to a lesser extent, in the longitudinal direction (7). A previous study reported that desmin knockout muscles showed greater evidence of mechanical injury and lower isometric force production relative to wild types when subjected to mechanical stress (9). Increased susceptibility to mechanical injury was also reported in the murine mdx model of muscular dystrophy (16) in which the subsarcolemmal protein dystrophin is absent. On the basis of these studies and the presumed role of desmin in normal muscle, we hypothesized that desmin knockout muscles would demonstrate a greater injury response to high-stress eccentric contractions (EC) where the muscle is forcibly lengthened while activated. This treatment could be used to probe the mechanical integrity of muscle. The fact that muscle injury occurs after cyclic EC is already well documented in a variety of experimental models (1, 12). Cyclic EC may result in loss of isometric force-generating capacity, disruption of the excitation-contraction coupling mechanism (21), and structural disruption of the myofibrillar apparatus (4). One of the earliest structural changes noted in rabbit fast muscles subjected to cyclic EC is the rapid and widespread loss of immunostaining for the muscle-specific intermediate filament protein desmin (11). Because the desmin intermediate filament protein network mechanically stabilizes myofibrils, loss of desmin due to EC may mechanically destabilize myofibrils, resulting in greater functional and morphological injury. Because the specific role of desmin in normal muscle is still not completely understood and has been implicated in a muscle injury mechanism, the purpose of this work was to quantify the magnitude of muscle injury that occurs in both wild-type and desmin knockout extensor digitorum longus (EDL) muscles subjected to cyclic ECs. We hypothesized that a lack of desmin in the desmin knockout muscles would render them more susceptible to mechanical injury compared with wild-type muscles.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Experimental model and design. The model used for this study was the fifth toe muscle of the multibellied mouse EDL. This belly was chosen on the basis of its fiber length homogeneity, distinct origin and insertion tendons, mixed fiber-type distribution, and relatively small number of muscle fibers (3). Mice were anesthetized with a cocktail composed of (in mg/kg) 10 ketamine, 5 rompum, and 11 acepromazine given by intraperitoneal injection. All procedures were performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the University of California and Veterans Affairs Committees on the Use of Animal Subjects. Muscles were dissected in a Ringer solution composed of (in mM): 137 NaCl, 5 KCl, 1 NaH2PO4, 24 NaHCO3, 2 CaCl2, 1 MgSO4, and 11 glucose containing 10 mg/l curare. Ringer solution was equilibrated with a 95% O2-5% CO2 gas introduced by bubbling, and the pH was adjusted to 7.5 at 25°C. After muscles were removed, mice were euthanized by intracardiac injection of pentobarbital sodium.
To test the role of desmin in normal and desmin knockout muscles, we obtained EDL muscles from one of the following four experimental groups: 1) young wild-type mice (n = 7, age 8-12 wk), 2) young desmin homozygous knockout mice (n = 7, age 8-12 wk) in which the desmin gene had been deleted by homologous recombination (14), 3) adult wild-type mice (n = 7, age 37-41 wk), and 4) adult desmin knockout mice (n = 7, age 37-41 wk). In terms of muscle age, the 50% survival point is often considered as the onset of the senescent period (19). For mice, this point is somewhat dependent on strain and generally occurs at ~24 mo of age. Thus the young mice examined in this experiment should be considered "adolescents," and the adult wild-type mice should be considered "mature adults." However, because the life span of the knockout animals is only ~1 yr (14), the knockouts are relatively older compared with wild types despite the fact that they are chronologically age matched.Eccentric injury protocol. After dissection, the fifth toe muscle belly was transferred to a custom muscle chamber filled with Ringer solution that was superfused with the O2-CO2 mixture throughout the experiment. With the use of 8-0 black-braided silk suture (Deknatel, Fall River, MA), the muscle origin was secured to a rigid post and the insertion was secured to the arm of a dual-mode ergometer (model 300B; Aurora Scientific, Richmond Hill, ON, Canada) that could impose rapid length changes. At both the origin and insertion sites, the muscle was tied as closely as possible to the fiber insertions to minimize series compliance of tendons and ensure that deformation applied to the muscle-tendon unit was indeed delivered to the muscle fibers. System compliance, including the ergometer, was ~5 µm/g. Muscle activation was provided by an electrical stimulator (model S88; Astro-Med, West Warwick, RI) via platinum plate electrodes that extended the length of the muscle. Ergometer arm movement was induced by a computer-controlled function generator (model 3314A; Hewlett-Packard, Cupertino, CA), and the entire experiment was synchronized and recorded by an acquisition program written in the LabWindows environment (National Instruments, Austin, TX) for the particular data acquisition board used (model 512; Gage, Montreal, PQ, Canada).
Muscle belly length was adjusted to a length that was barely taught (slack length), and this length was measured by using a calibrated reticule mounted onto the eyepiece of a dissecting microscope (model 8 Wild; Leitz, New York, NY). The muscle was elongated by 15% of its slack length, and fiber length was measured again. This length was used as the starting fiber length (Lf) for all experiments and corresponded to a resting sarcomere length of ~3 µm (3.02 ± 0.03 µm) as measured by laser diffraction. Approximately 30 min after the muscle was mounted in the chamber, passive mechanical properties were measured by imposing a 10% Lf stretch at a rate of ~0.7 Lf/s (0.72 ± 0.011 Lf/s). This stretch was repeated three times at 3-min intervals. Maximum isometric tension was then measured by applying a 400-ms train of 0.3-ms pulses delivered at 100 Hz while muscle length was held constant. This measurement was repeated twice at 10-min intervals. Each muscle then underwent a series of 10 ECs, one every 3 min, to minimize the effects of fatigue. For each EC, the muscle was first activated isometrically until tension stabilized (~200 ms), and then a 15% Lf change was imposed at a rate of 2 Lf/s, resulting in a rapid tension rise (Fig. 1). Tension increased in two phases, a reflection of the muscle short-range stiffness (5) and low system compliance. Muscle length was held fixed for a time during which tension declined due to active stress relaxation. Stimulation was then ceased and muscle length was returned to its starting value.
|
Tissue preparation for electron microscopy.
Three muscles from each experimental group were randomly selected for
electron microscopy. After the experiment was completed, muscles were
pinned on cork along with their contralateral counterparts and
submerged into phosphate-buffered Karnovsky's fixative, where they
were allowed to incubate overnight at 4°C. Specimens were washed
three times in cold (4°C) sodium cacodylate buffer (0.1 M adjusted to
pH 7) and then incubated in 2% osmium tetroxide for 1 h at room
temperature. After three buffer washes of 5 min each, muscles were
dehydrated in graded ethanols and propylene oxide. Specimens were then
cut into approximately equal pieces and oriented to obtain longitudinal
views embedded in Scipoxy 812 Resin (Energy Beam Sciences, Agawam, MA).
Micrographs were photographed from longitudinal sections by using a
diagonal random sampling algorithm with a grid-fiber orientation angle
of ~40° (22). The end points of all Z disks from each
of 10 micrographs from each muscle were digitized between adjacent
myofibrils to quantify Z-disk angle as well as the horizontal
displacement (x) of adjacent Z disks. The 10 micrographs
from each muscle were obtained in different regions of different
sections and were thus presumably from different muscle fibers.
Data analysis.
A computer algorithm was written to process the force-time records from
each contraction of each muscle specimen (version 40, Mathematica;
Wolfram Research, Champaign, IL). This algorithm used a combination of
linear regression, breakpoint analysis, and the known timings of
activation and deformation to yield the following mechanical parameters
(where applicable): initial baseline force, maximum isometric tetanic
tension, peak eccentric tension, mechanical energy absorbed (phase II
slope), and yield point during EC (as defined in Ref. 23) (Fig.
1). For isometric and passive stretch experiments, baseline and
peak force were the only relevant parameters obtained from the
force-time record. Muscle stress was calculated by normalizing muscle
force to muscle physiological cross-sectional area (PCSA), calculated
by using the equations provided by Sacks and Roy (17) and
assuming a muscle density of 1.056 g/cm3 (13).
PCSA was calculated individually for each muscle on the basis of
dissection of small fiber bundles from fixed tissue as previously
reported (2). Mechanical energy absorbed was calculated according to the equation
![]() |
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Animal and muscle masses varied by age and muscle type.
Animal and muscle masses were significantly lower in desmin knockout
compared with wild-type specimens for both age groups (P < 0.05, Table 1).
Two-way ANOVA revealed a significant effect of type (P < 0.01) and age (P < 0.05) on animal mass with no
significant interaction (P > 0.3). Two-way ANOVA also
revealed a significant effect of type (P < 0.0001) but
no significant effect of age (P > 0.2) on muscle mass
with no significant type × age interaction (P > 0.5). These measurements suggest that lack of desmin is a greater
determinant of animal and muscle mass than is age and that the age
effect on these properties is independent of the muscle type. It is
important, however, to consider the difference in muscle mass, and thus
PCSA, as a normalizing factor when comparing muscles across types.
Architectural analysis of the muscles revealed a significant decrease
in fiber length and muscle PCSA in knockout compared with wild-type
muscles (P < 0.02) but no significant effect of age
(P > 0.5) or age × type interaction
(P > 0.3; Table 1).
|
Isometric stress differences among groups before EC treatment. Absolute isometric tension generated before EC treatment was significantly different between mature and young specimens (P < 0.0001) and wild-type and knockout specimens (P < 0.0001) with no significant interaction (P > 0.5). Isometric force before EC treatment was greatest in the young wild-type EDLs (14.6 ± 0.4 g), followed by that in the mature wild-type EDLs (13.4 ± 0.9 g), young knockouts (11.0 ± 0.6 g), and mature knockouts (9.4 ± 0.5 g). When data were normalized to stress, young wild-type EDLs still generated the greatest stress (313 ± 8 kPa) and mature knockouts generated the lowest stress (227 ± 17 kPa), but mature wild types and young knockouts generated isometric stresses that were not significantly different (281 ± 13 and 276 ± 13 kPa, respectively, P > 0.4). This fortuitous result permitted testing of the effects of stress on muscle injury independent of the presence of intermediate filaments. No change in isometric force-generating capacity or peak passive stress was observed for control muscles undergoing cyclic isometric activation or cyclic passive stretch (data not shown).
Unique time course of isometric stress for each experimental group.
Isometric stress measured just before each EC demonstrated a unique
time course as a function of muscle type and age (Fig. 2A). All four experimental
groups significantly decreased isometric stress-generating capacity as
a function of time (P < 0.001), but the magnitude of
the decrease was characteristic of each muscle age and type. For
example, whereas mature wild-type and mature knockout muscles generated
isometric stresses that were significantly different before eccentric
exercise (Fig. 2A), after 10 ECs, the isometric stress
generated by the two groups was nearly identical (184 ± 11 and
186 ± 21 kPa, respectively, P > 0.8). Similarly, whereas young wild-type and young knockout muscles also generated isometric stresses that were significantly different before eccentric exercise (Fig. 2A), after 10 ECs, the isometric stress
generated by these two groups was also nearly identical (246 ± 6 and 252 ± 8 kPa, respectively, P > 0.8), and
both were significantly greater than those of their mature
counterparts. Thus the presence of the intermediate filament system
significantly affected maximum isometric stress before any EC
treatment, but after 10 ECs, no difference in isometric stress
generated was seen among muscle types, a finding that may reflect
preferential damage to the wild-type intermediate filament system. This
effect was independent of the generally increased vulnerability of the
aged muscles from both types. These data provide strong support for the
physiological importance of the intermediate filament network for
normal muscle force generation and also in response to injury. No sign
of recovery after the end of the eccentric period was seen, because
only a small transient isometric force change after EC was observed
over a 75-min recovery period (Fig. 2A). Cyclic isometric
contraction alone had no effect on either the wild-type or desmin
knockout muscle stress (Fig. 2B), demonstrating that the
force changes observed were not confounded by differences in
fatigability among muscle types. We thus attribute the force changes
measured during EC to impairment of the contractile apparatus rather
than simple consequences of metabolic fatigue.
|
|
Passive mechanical properties change after EC treatment. Whereas no significant differences in passive stress were observed between wild-type and knockout specimens (P > 0.2), a significant difference was observed among specimens as a function of age (P < 0.0005) and timing relative to EC treatment (P < 0.01; Fig. 3B). Specifically, passive stress decreased significantly for the older specimens, and there was a tendency for a passive stress decrease after EC treatment in the young knockout specimens. These results were not simply due to stress-relaxation of the tissues involved because no such passive stress decreases were observed in muscles subjected to either cyclic isometric contraction or cyclic passive stress. In addition, no correlation was observed between peak eccentric stress and change in passive stress (P > 0.6, r2 = 0.11) after EC treatment, which would be expected if stress-relaxation were a significant factor.
Energy absorbed during EC treatment does not explain knockout
effect.
The decreased vulnerability of knockout muscles compared with wild
types could not be explained on the basis of differences in energy
absorbed during EC treatment (calculated as described in
METHODS; Table 2). For both
mature and young specimens, as expected on the basis of simple size
differences, the wild-type muscles absorbed more absolute energy than
their knockout counterparts because the absolute force levels were
higher. However, when energy was expressed as energy absorbed per unit
muscle mass, there was no significant difference between energy
absorbed by wild-type and knockout specimens of matched ages, with the
younger muscles absorbing ~600 J/kg and the mature muscles absorbing
~500 J/kg (Table 2).
|
Passive stretch or activation alone does not explain differential effect. As a control for the effect of activation or stretch alone, isometric contraction and passive stretch groups were also studied (n = 3 experimental subjects per group). Because the drop in muscle force can often be explained, at least in part, by muscle fatigue (10), fatigability due to repetitive activation was measured across the same time period but with 10 isometric contractions instead of ECs. Average maximum tetanic tension after 10 isometric contractions was 99.4 ± 0.9% of the pre-isometric contraction level in knockout muscles and 99.7 ± 0.3% in wild-type muscles (Fig. 2B). Similarly, the 10-passive contraction protocol resulted in an average maximum tetanic tension of 100.5 ± 0.9% of the pre-isometric contraction level in knockout muscles and 99.8 ± 0.6% in wild-type muscles. None of these values were significantly different from 100% (P > 0.4). Because force decline did not result from either passive stretch or isometric contraction, the effects observed were not confounded by potential differences in passive properties or metabolic fatigability of muscles from knockout animals.
Morphological appearance of Z disks.
Displacement of adjacent Z disks was significantly greater in the
injured knockout muscles (0.36 ± 0.03 µm) compared with wild-type muscles (0.22 ± 0.03 µm, P < 0.05),
and a significantly greater proportion of Z disks was slanted >30°
in wild-type (10.4 ± 1.5%) compared with knockout muscles
(5.8 ± 0.9%, P < 0.05; Fig.
4).
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The intermediate filament system of skeletal muscle is believed to be responsible for the mechanical integration of the myofibrillar lattice in both the longitudinal and radial directions (7). The only mechanical studies available on this system come from partial extraction experiments of rabbit skeletal muscle demonstrating that longitudinal intermediate filaments bear significant passive tension only at sarcomere lengths above ~5 µm (20). Because this was not in the physiological range, the finding seemed to imply that the intermediate filament network was not important for bearing normal loads in skeletal muscle. However, we have demonstrated that the intermediate filament system plays a significant role in the development of isometric stress under physiological conditions, independent of mechanisms that may be related to maturation of the system. In addition, the lack of intermediate filaments actually rendered the mouse EDL muscles less susceptible to mechanical disruption, a result in contrast to previous reports in another desmin knockout system (9) and to reports from studies of the naturally occurring mutant in which the subsarcolemmal cytoskeletal protein dystrophin is absent (16). Thus the simple lack of a particular cytoskeletal protein should not be considered an a priori reason to expect increased mechanical vulnerability to injury.
Stereological analysis of wild-type and knockout muscle revealed no significant difference in volume fraction of contractile filaments, which could have provided an anatomic basis for the differences in isometric stress development. Longitudinal images of knockout muscle typically revealed a striking regularity of myofibrillar structure along the muscle fiber length in noninjured muscles. This is in contrast to previous micrographs demonstrating muscle derangement in the diaphragm, heart, and soleus muscles from desmin knockout animals (8, 18). We conjecture that disparity among these findings represents true differences related to the variability in the amount and type of use experienced by the mouse diaphragm, heart, and soleus muscles compared with the less frequently used EDL muscle studied here.
Morphological measurements demonstrating increased Z-disk displacement and a decreased percentage of slanted Z disks support the mechanical concept that adjacent myofibrils in knockout muscles were allowed to "slide" relative to one another in response to the ECs, perhaps placing less stress on adjacent myofibrils. Conversely, the increased stress of adjacent desmin-based interconnections may be responsible for the increased area fraction of slanted disks in the wild-type muscles. This finding demonstrates that the muscle intermediate filament system does not protect a fiber from muscle to injury but may even permit the injury and associated damage to be more severe (Fig. 4).
A previous report of desmin playing a critical role in maintaining muscle tensile strength and integrity (9) was partly confounded on the basis of the significant size difference between muscles that occurs secondary to the homologous recombination itself (Table 1). The absolute strength differences reported can easily be explained on the basis of size differences between the knockout and wild-type muscles. These limitations were overcome in the current study by explicitly including muscle dimensions in all stress calculations and matching stresses among groups by using mature muscles in which isometric stress drops due to the aging process itself. The decreased vulnerability of knockout muscles to mechanical injury could not be explained on the basis of simple size differences because mechanical energy absorbed was the same between wild-type and knockout muscles (Table 2). In addition, when muscles of similar stress-generating capacity (young knockout vs. mature wild type, Fig. 2A) were compared, injury was much greater to the wild-type muscle compared with the knockout muscle. Support for the concept that stress and injury are not associated is provided by the finding that there was no significant correlation between peak muscle stress and percentage drop in maximum tetanic tension after EC (r2 = 0.0005, P > 0.7).
Whereas a significant change in passive stress was observed after EC treatment for all but the young wild-type experimental groups (Fig. 2B), the underlying basis for this result is not clear. A trivial explanation for passive stress differences could be that the resting sarcomere lengths of the experimental groups were different as a function of age and type. Thus passive stress differences would simply be due to muscle fibers operating on different regions of their sarcomere length-passive stress relations. However, this was not the case. One-way ANOVA among the four experimental groups revealed no significant difference among groups in resting sarcomere length (Table 1; P > 0.5). In fact, resting sarcomere among all groups demonstrated a coefficient of variation of only 7.5%, probably indicating similar splice variants of the intrasarcomeric protein titin (6). The structural basis for the selective decrease in passive force in three of the four experimental groups probably represents a complex interaction among the titin protein, other cytoskeletal proteins, and extracellular collagen, all of which may change in skeletal muscle as a function of age and in response to creation of the desmin null experimental model. Detailed reorganization of such proteins has not been studied in great detail.
On the basis of these findings, we hypothesize that normal interconnections between sarcomeres along and across the muscle fiber permit efficient transfer of mechanical stress from the myofibril to the fiber exterior. In the absence of desmin, the intermyofibrillar connections may be more compliant, permitting a greater degree of internal sarcomere shortening and motion and thereby decreasing the efficiency of force transfer across and along the fiber. Similarly, on the basis of the lack of these connections, the opposite situation, in which strain is transmitted from the fiber exterior to the myofibrils, could also be less efficient. The result would be a decreased sarcomere strain during a fixed degree of lengthening in knockout compared with wild-type muscles and, thus, less injury despite the fact that the energy delivered to the desmin knockout muscle is identical. This particular argument is predicated on the observation that sarcomere strain is directly related to muscle injury, whereas fiber stress is not. We recently reported real-time sarcomere length measurements in frog skeletal muscle using a paradigm similar to that used here and demonstrated a strong correlation between sarcomere strain and muscle injury with no significant correlation between muscle fiber stress and injury (15).
Of course, an alternative interpretation of these data is that the muscles from knockout animals were already injured simply due to normal animal activity. If this were the case, the decreased stress generated by muscles from knockout animals would reflect their injury, and the lack of further decrease would reflect their already injured state. However, we do not believe that this was the case because muscles from knockout animals did not demonstrate morphological abnormality or disruption that would imply prior injury.
Finally, these data have implications for understanding the mechanism of muscle injury in skeletal muscle. The fact that the age-matched muscles generated the same amount of stress after the injury protocol, independent of the presence or absence of the intermediate filament system, provides support for the selective functional loss of desmin in wild-type muscle and its functional importance. At the end of the EC protocol, the younger muscles generate significantly greater stress compared with the mature muscles (~250 vs. ~200 kPa), but there was no difference between knockout and wild-type muscles. These data support the proposal that the time course of loss in desmin immunostaining previously observed (11) would have functional consequences and might even provide a protective effect on the muscle to further mechanical injury. Further studies are required to resolve the precise mechanical events that accompany EC and the loss of stress in wild-type and knockout muscles.
![]() |
ACKNOWLEDGEMENTS |
---|
This work was supported by the Department of Veterans Affairs, National Institute of Arthritis and Musculoskeletal and Skin Diseases Grant AR-40050, and Swedish Medical Research Council Grant 11200.
![]() |
FOOTNOTES |
---|
Address for reprint requests and other correspondence: R. L. Lieber, Dept. of Orthopaedics (9151), Univ. of California, San Diego School of Medicine and Veterans Affairs Medical Center, San Diego, CA 92161 (E-mail: rlieber{at}ucsd.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 21 January 2000; accepted in final form 9 May 2000.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Armstrong, RB,
Ogilvie RW,
and
Schwane JA.
Eccentric exercise-induced injury to rat skeletal muscle.
J Appl Physiol
54:
80-93,
1983
2.
Burkholder, TJ,
Fingado B,
Baron S,
and
Lieber RL.
Relationship between muscle fiber types and sizes and muscle architectural properties in the mouse hindlimb.
J Morphol
220:
1-14,
1994[ISI].
3.
Chleboun, GS,
Patel TJ,
and
Lieber RL.
Skeletal muscular architecture and fiber type distribution with the multiple bellies of the mouse extensor digitorum longus muscle.
Acta Anat (Basel)
159:
147-155,
1988.
4.
Fridén, J,
Sjöström M,
and
Ekblom B.
Myofibrillar damage following intense eccentric exercise in man.
Int J Sports Med
4:
170-176,
1983[ISI][Medline].
5.
Hill, DK.
Tension due to interaction between the sliding filaments in resting striated muscle. The effect of stimulation.
J Physiol (Lond)
199:
637-684,
1968[ISI][Medline].
6.
Labeit, S,
and
Kolmerer B.
Titins: giant proteins in charge of muscle ultrastructure and elasticity.
Science
270:
293-296,
1995[Abstract].
7.
Lazarides, E.
Intermediate filaments as mechanical integrators of cellular space.
Nature
283:
249-256,
1980[ISI][Medline].
8.
Li, Z,
Colucci-Guyon E,
Picon-Raymond M,
Mericskay M,
Pourmin S,
Paulin D,
and
Babinet C.
Cardiac lesions and skeletal myopathy in mice lacking desmin.
Dev Biol
175:
362-366,
1996[ISI][Medline].
9.
Li, Z,
Mericskay M,
Agbulut O,
Butler-Browne G,
Carlsson L,
Thornell LE,
Babinet C,
and
Paulin D.
Desmin is essential for the tensile strength and integrity of myofibrils but not for myogenic commitment, differentiation, and fusion of skeletal muscle.
J Cell Biol
139:
129-144,
1997
10.
Lieber, RL,
McKee-Woodburn T,
and
Fridén J.
Muscle damage induced by eccentric contractions of 25% strain.
J Appl Physiol
70:
2498-2507,
1991
11.
Lieber, RL,
Thornell LE,
and
Fridén J.
Muscle cytoskeletal disruption occurs within the first 15 minutes of cyclic eccentric contraction.
J Appl Physiol
80:
278-284,
1996
12.
McCully, KK,
and
Faulkner JA.
Injury to skeletal muscle fibers of mice following lengthening contractions.
J Appl Physiol
59:
119-126,
1985
13.
Mendez, J,
and
Keys A.
Density and composition of mammalian muscle.
Metabolism
9:
184-188,
1960[ISI].
14.
Milner, DJ,
Weitzer G,
Tran D,
Bradley A,
and
Capetanaki Y.
Disruption of muscle architecture and myocardial degeneration in mice lacking desmin.
J Cell Biol
134:
1255-1270,
1996[Abstract].
15.
Patel, TJ,
Fridén J,
and
Lieber RL.
Sarcomere strain causes muscle injury during eccentric contracts (Abstract).
Trans Orthop Res Soc
24:
141,
1999.
16.
Petrof, BJ,
Shrager JB,
Stedman HH,
Kelly AM,
and
Sweeney HL.
Dystrophin protects the sarcolemma from stresses developed during muscle contraction.
Proc Natl Acad Sci USA
90:
3710-3714,
1993[Abstract].
17.
Sacks, RD,
and
Roy RR.
Architecture of the hindlimb muscles of cats: functional significance.
J Morphol
173:
185-195,
1982[ISI][Medline].
18.
Thornell, L,
Carlsson L,
Li Z,
Mericskay M,
and
Paulin D.
Null mutation in the desmin gene gives rise to a cardiomyopathy.
J Mol Cell Cardiol
29:
2107-2124,
1997[ISI][Medline].
19.
Walford, RL.
When is a mouse "old"?
J Immunol
117:
352-353,
1976[ISI][Medline].
20.
Wang, K,
McCarter R,
Wright J,
Beverly J,
and
Ramirez-Mitchell R.
Viscoelasticity of the sarcomere matrix of skeletal muscles. The titin-myosin composite filament is a dual-stage molecular spring.
Biophys J
64:
1161-1177,
1993[Abstract].
21.
Warren, GL,
Lowe DA,
Hayes DA,
Karwoski CJ,
Prior BM,
and
Armstrong RB.
Excitation failure in eccentric contraction-induced injury of mouse soleus muscle.
J Physiol (Lond)
468:
487-499,
1993[Abstract].
22.
Weibel, ER.
Point Counting Methods: Practical Methods for Biological Morphometry. London: Academic, 1979.
23.
Wood, SA,
Morgan DL,
and
Proske U.
Effects of repeated eccentric contractions on structure and mechanical properties of toad sartorius muscle.
Am J Physiol Cell Physiol
265:
C792-C800,
1993