Myofibrillar disruption in the rabbit soleus muscle after one-week hindlimb suspension

The Structural Adaptations That Mediate Disuse-Induced Atrophy of Skeletal Muscle

The maintenance of skeletal muscle mass plays a fundamental role in health and issues associated with quality of life. Mechanical signals are one of the most potent regulators of muscle mass, with a decrease in mechanical loading leading to a decrease in muscle mass. This concept has been supported by a plethora of human- and animal-based studies over the past 100 years and has resulted in the commonly used term of 'disuse atrophy'. These same studies have also provided a great deal of insight into the structural adaptations that mediate disuse-induced atrophy. For instance, disuse results in radial atrophy of fascicles, and this is driven, at least in part, by radial atrophy of the muscle fibers. However, the ultrastructural adaptations that mediate these changes remain far from defined. Indeed, even the most basic questions, such as whether the radial atrophy of muscle fibers is driven by the radial atrophy of myofibrils and/or myofibril hypoplasia, have yet to be answered. In this review, we thoroughly summarize what is known about the macroscopic, microscopic, and ultrastructural adaptations that mediated disuse-induced atrophy and highlight some of the major gaps in knowledge that need to be filled.

Predominant synthesis of giant myofibrillar proteins in striated muscles of the long-tailed ground squirrel Urocitellus undulatus during interbout arousal

Molecular mechanisms underlying muscle-mass retention during hibernation have been extensively discussed in recent years. This work tested the assumption that protein synthesis hyperactivation during interbout arousal of the long-tailed ground squirrel Urocitellus undulatus should be accompanied by increased calpain-1 activity in striated muscles. Calpain-1 is known to be autolysed and activated in parallel. Western blotting detected increased amounts of autolysed calpain-1 fragments in the heart (1.54-fold, p < 0.05) and m. longissimus dorsi (1.8-fold, p < 0.01) of ground squirrels during interbout arousal. The total protein synthesis rate determined by SUnSET declined 3.67-fold in the heart (p < 0.01) and 2.96-fold in m. longissimus dorsi (p < 0.01) during interbout arousal. The synthesis rates of calpain-1 substrates nebulin and titin in muscles did not differ during interbout arousal from those in active summer animals. A recovery of the volume of m. longissimus dorsi muscle fibres, a trend towards a heart-muscle mass increase and a restoration of the normal titin content (reduced in the muscles during hibernation) were observed. The results indicate that hyperactivation of calpain-1 in striated muscles of long-tailed ground squirrels during interbout arousal is accompanied by predominant synthesis of giant sarcomeric cytoskeleton proteins. These changes may contribute to muscle mass retention during hibernation.

New titin (connectin) isoforms and their functional role in striated muscles of mammals: facts and suppositions

This review summarizes results of our studies on titin isoform composition in vertebrate striated muscles under normal conditions, during hibernation, real and simulated microgravity, and under pathological conditions (stiff-person syndrome, post-apoplectic spasticity, dilated cardiomyopathy, cardiac hypertrophy). Experimental evidence for the existence in mammalian striated muscles of higher molecular weight isoforms of titin (NT-isoforms) in addition to the known N2A-, N2BA-, and N2B-titin isoforms was obtained. Comparative studies of changes in titin isoform composition and structure-functional properties of human and animal striated muscles during adaptive and pathological processes led to a conclusion about the key role of NT-isoforms of titin in maintenance of sarcomere structure and contractile function of these muscles.

Difference in stretching of sarcomeres between medial gastrocnemius and tibialis anterior by tibial lengthening: an experiment in rabbits

PURPOSE

To compare the degree of stretching of sarcomeres--in the medial gastrocnemius versus the tibialis anterior--acutely extended by tibial lengthening.

METHODS

The right lower legs of 6 New Zealand White rabbits were acutely distracted by 6.3 mm using an external fixator, whereas the left lower legs served as controls. Immediately after distraction, the ankle was dorsiflexed at 50 degrees and the knee flexed at 140 degrees, with respect to the posture of resting on the ground. Both legs were immersed in 10% buffered formalin for fixation, and the medial gastrocnemius and tibialis anterior were removed. Length of the 2 sarcomeres was measured by a laser diffraction technique using isolated muscle fibre bundles.

RESULTS

The mean lengths of sarcomeres in the medial gastrocnemius and tibialis anterior were 3.0 microm and 2.3 microm in the lengthened legs, and 2.4 microm and 2.1 microm in control legs, respectively. The degree of stretching of corresponding sarcomeres was therefore 25% and 10%, this difference being significant (p<0.01).

CONCLUSION

Sarcomeres of the medial gastrocnemius are more stretchable than those of the tibialis anterior following tibial lengthening.

Passive stretch inhibits central corelike lesion formation in the soleus muscles of hindlimb-suspended unloaded rats

Hindlimb suspension unloading (HSU) is a ground-based model simulating the effects of microgravity unloading on the musculoskeletal system. In this model, gravity causes the hind foot of the rat to drop, opening the front of the ankle to 90–105° plantar flexion at rest. As HSU proceeds, the normal weight-bearing angle of 30° dorsiflexion is achieved progressively less, and the contraction range of soleus is abbreviated. Our laboratory reported that 12 days of HSU caused central corelike lesions (CCLs) of myofibril breakdown (Riley DA, Slocum GR, Bain JL, Sedlak FR, Sowa TE, and Mellender JW. J Appl Physiol. 69: 58–66, 1990). The present study investigated whether daily stretch of the calf muscles prevents CCL formation. The soleus muscles of HSU Sprague-Dawley male rats (∼287 g) were lengthened by unilateral ankle splinting at 30°. Compared with the nonsplinted side, splinting for 10 or 20 min per day in awake rats significantly decreased CCLs in soleus by 88 and 91%, respectively ( P < 0.01). Compared with control muscle wet weight, 20-min splinting reduced atrophy by 33%, whereas 10-min splinting ameliorated atrophy by 17% ( P < 0.01). Bilateral soleus electromyograph recording revealed higher levels of contractile activity on the splinted side during splinting. To isolate the effects of stretch from isometric contractile activity, contractions were eliminated by whole animal anesthesia with isoflurane during 10-min daily splinting. The percentage of fibers with CCLs was reduced by 57%, and the average lesion size was 29% smaller in the stretched muscle ( P < 0.05). Soleus muscle wet weight and fiber area were unaltered by stretch alone. Loaded contractions during splinting are necessary to prevent muscle fiber atrophy. Passive muscle stretch acts to maintain myofibril structural integrity.

Effect of hindlimb suspension on the functional properties of slow and fast soleus fibers from three strains of mice

Cross-sectional area (CSA), peak Ca2+-activated force (Po), fiber specific force (Po/CSA), and unloaded shortening velocity (Vo) were measured in slow-twitch [containing type I myosin heavy chain (MHC)] and fast-twitch (containing type II MHC) chemically skinned soleus muscle fiber segments obtained from three strains of weight-bearing and 7-day hindlimb-suspended (HS) mice. HS reduced soleus slow MHC content (from approximately 50 to approximately 33%) in CBA/J and ICR strains without affecting slow MHC content in C57BL/6 mice ( approximately 20% of total MHC). Two-way ANOVA revealed HS-induced reductions in CSA, Po, and Po/CSA of slow and fast fibers from all strains. Fiber Vo was elevated post-HS, but not consistently across strains. No MHC x HS treatment interactions were observed for any variable for C57BL/6 and CBA/J mice, and the two significant interactions found for the ICR strain (CSA, Po) appeared related to inherent pre-HS differences in slow vs. fast fiber CSA. In the mouse HS models studied here, fiber atrophy and contractile dysfunction were partially dependent on animal strain and generally independent of fiber MHC isoform content.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

Abnormal mitochondria and sarcoplasmic changes in rabbit skeletal muscle induced by immobilization

Immobilization of the rabbit knee in extended position results in damage to the vastus intermedius profundus (VIP) muscle. To examine the mechanisms involved in initiation of the injury, we studied the light and electron microscopic morphology of the VIP muscle, as well as the activity and distribution of NADH tetrazolium reductase (NADH-TR) in the affected muscle, and determined serum total creatine kinase (CK) activity in immobilized rabbits. The VIP muscle of the immobilized right hindlimb was removed at various time points (10 h, 24 h, 36 h and 48-72 h, n=5 for each time point). The nonimmobilized left hindlimb and five nonimmobilized animals served as controls. No morphological changes were observed by light microscopy within 48-72 h in routine stainings. Transient ultrastructural abnormalities, including abnormal cristae, matrix lucencies and mild swelling of mitochondria, were observed between 10 h and 36 h of immobilization, subsiding by 48-72 h. On the other hand, progressive disorganization of myofibrils with breaking-up of Z-bands and an increase in the number and size of sarcoplasmic lipid vacuoles was seen with increasing duration of immobilization. NADH-TR activity at subsarcolemmal locations had decreased by 10 h and disappeared by 24 h of immobilization, while the intermyofibrillar mitochondria remained unaltered. Serum total CK activity began to increase by 2 h of immobilization and reached a peak by 24 h. The results indicate that already a few hours of immobilization of the rabbit knee in extension leads to signs of metabolic disturbance of the VIP muscle and sarcolemmal leakage. The simultaneous occurrence of transient mitochondrial abnormalities, transient CK efflux and progressive myofibrillar damage suggests the operation of multiple adverse mechanisms already at the onset of disuse muscle atrophy.

Vulnerability to dysfunction and muscle injury after unloading

OBJECTIVE

To test whether unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury.

DESIGN

Before-after trial.

SETTING

General community.

PATIENTS OR OTHER PARTICIPANTS

Two women and 5 men (73 +/- 3kg [mean +/- SE]) who were active college students but were not trained in lower body resistance exercise volunteered.

INTERVENTION

Five weeks of unilateral lower limb suspension (ULLS), which has been shown to decrease strength and size of the unloaded, left, but not load-bearing, right quadriceps femoris muscle group (QF) by 20% and 14%, respectively; performance of 10 sets of ten eccentric actions with each QF immediately after the ULLS strength tests with a load equivalent to 65% of the post-ULLS eccentric 1-repetition maximum.

MAIN OUTCOME MEASURE(S)

Concentric and eccentric 1-repetition maximum for the left, unloaded and the right, load-bearing QF measured immediately after ULLS and 1,4,7,9, and 11 days later; cross-sectional area and spin-spin relaxation time (T2) of each QF as determined by magnetic resonance imaging and measured the last day of ULLS and 3 days later.

RESULTS

The mean load used for eccentric exercise was 23 +/- 2 and 30 +/- 3kg for the left, unloaded and right, load-bearing QF, respectively. The concentric and eccentric 1-repetition maximum for the unloaded and already weakened left QF was further decreased by 18% (p = .000) and 27% (p = .000), respectively, 1 day after eccentric exercise. Strength did not return to post-ULLS levels until 7 days of recovery. The right, load-bearing QF showed a 4% decrease (p = .002) in the eccentric 1-repetition maximum 1 day after eccentric exercise. The left, unloaded QF showed an increase in T2 (p = .002) in 18% of its cross-sectional area 3 days after the eccentric exercise, thus indicating muscle injury. The right, load-bearing QF showed no elevation in T2 (p = .280).

CONCLUSION

Unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury, even at relatively light loads.

Mechanical regulation of cardiac myocyte protein turnover and myofibrillar structure

Mechanical forces play an essential role in regulating the synthesis and assembly of contractile proteins into the sarcomeres of cardiac myocytes. To examine if physical forces might also regulate the turnover of contractile proteins at a posttranslational site of control, beating and nonbeating neonatal cardiac myocytes (NCM) were subjected to a 5% static stretch. The L-type calcium channel blocker nifedipine (12 microM) was used to inhibit contraction. Pulse-chase biosynthetic labeling experiments demonstrated that contractile arrest accelerated the loss of isotopic tracer from the total myofibrillar protein fraction, myosin heavy chain (MHC), and actin, but not desmin. Myofibrillar abnormalities developed in parallel with these metabolic changes. A 5% static load appeared to partially stabilize myofibrillar structure in nonbeating NCM and suppressed the loss of isotopic tracer from the total myofibrillar protein fraction, MHC, and actin in beating and nonbeating NCM. Contractile activity and/or a static stretch promoted the accumulation of MHC, actin, and desmin. Applying a static load to myocytes that lacked preexisting myofibrils did not promote the assembly of sarcomeres or alter protein turnover. These data indicate that the turnover of MHC and actin is correlated with the organizational state of the myofibrillar apparatus.

Active muscle length reduction progressively damages soleus in hindlimb-suspended rabbits

This study describes the morphologic changes in rabbit soleus muscle following hindlimb suspension (HS) for 1 to 4 weeks (group A); or following HS with hindfeet passively dorsiflexed, by means of an elastic band, for 1 to 2 weeks (group B). In the latter, elastic band use allowed phasic contractions of foot extensor muscles against resistance and prevented 35% chronic soleus shortening, which occurred in group A animals. In group A, the soleus revealed progressive muscle atrophy and myofibrillar damage. Myofibrils underwent dissolution, muscle regeneration was ineffective, and adipose tissue developed from about 2-week suspension onward. Conversely, passive dorsiflexion of unloaded hindfeet was essential in maintaining mass and structural muscle integrity in the soleus of group B. It is hereby demonstrated that HS-induced soleus damage in the rabbit is progressive, and can be prevented, avoiding long-term shortening of soleus and its phasic unloaded contractions. Soleus sensitivity to unloading conditions, such as HS, tenotomy, and hypogravity, may depend on the particular physiology of this tonic antigravity muscle, engaged mainly in developing long-lasting isometric contractions in a stretched length.