Skeletal muscle response to spaceflight, whole body suspension, and recovery in rats

Comparisons of soleus and extensor digitorum longus (EDL) muscles from male Sprague-Dawley rats (350-400 g) after 7 days of weightlessness, 7 and 14 days of whole body suspension (WBS), and 7 days of recovery from WBS and from vivarium controls were made. Muscle mass loss of approximately 30% was observed in soleus after 7 and 14 days of WBS. Measurement of slow- and fast-twitch fibers showed significant alterations. Reductions in cross-sectional areas and increases in fiber densities in soleus after spaceflight and WBS were related to previous findings of muscle atrophy during unloading. Capillary density also showed a marked increase with unloading. Seven days of weightlessness were sufficient to effect a 20 and 15% loss in absolute muscle mass in soleus and EDL, respectively. However, the antigravity soleus was more responsive in terms of cross-sectional area reductions. After 7 days of recovery from WBS, with normal ambulatory loading, the parameters studied showed a reversal to control levels. Muscle plasticity, in terms of fiber and capillary responses, indicated differences in responses in the two types of muscles and further amplified that antigravity posture muscles are highly susceptible to unloading. Studies of recovery from spaceflight for both muscle metabolism and microvascular modifications are further justified.     

Anatomy and histochemistry of hindlimb flight posture in birds. I. The extended hindlimb posture of shorebirds

Birds utilize one of two hindlimb postures during flight: an extended posture (with the hip and knee joints flexed, while the ankle joint is extended caudally) or a flexed posture (with the hip, knee, and ankle joints flexed beneath the body). American Avocets (Recurvirostra americana) and Black‐necked Stilts (Himantopus mexicanus) extend their legs caudally during flight and support them for extended periods. Slow tonic and slow twitch muscle fibers are typically found in muscles functioning in postural support due to the fatigue resistance of these fibers. We hypothesized that a set of small muscles composed of high percentages of slow fibers and thus dedicated to postural support would function in securing the legs in the extended posture during flight. This study examined the anatomy and histochemical profile of eleven hindlimb muscles to gain insight into their functional roles during flight. Contrary to our hypothesis, all muscles possessed both fast twitch and slow twitch or slow tonic fibers. We believe this finding is due to the versatility of dynamic and postural functions the leg muscles must facilitate, including standing, walking, running, swimming, and hindlimb support during flight. Whether birds use an extended or flexed hindlimb flight posture may be related to the aerodynamic effect of leg position or may reflect evolutionary history. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Rat cardiovascular responses to whole body suspension: head-down and non-head-down tilt.

The rat whole body suspension technique mimics responses seen during exposure to microgravity and was evaluated as a model for cardiovascular responses with two series of experiments. In one series, changes were monitored in chronically catheterized rats during 7 days of head-down tilt (HDT) or non-head-down tilt (N-HDT) and after several hours of recovery. Elevations of mean arterial (MAP), systolic, and diastolic pressures of approximately 20% (P < 0.05) in HDT rats began as early as day 1 and were maintained for the duration of suspension. Pulse pressures were relatively unaffected, but heart rates were elevated approximately 10%. During postsuspension (2-7 h), most cardiovascular parameters returned to presuspension levels. N-HDT rats exhibited elevations chiefly on days 3 and 7. In the second series, blood pressure was monitored in 1- and 3-day HDT and N-HDT rats to evaluate responses to rapid head-up tilt. MAP, systolic and diastolic pressures, and HR were elevated (P < 0.05) in HDT and N-HDT rats during head-up tilt after 1 day of suspension, while pulse pressures remained unchanged. HDT rats exhibited elevated pretilt MAP and failed to respond to rapid head-up tilt with further increase of MAP on day 3, indicating some degree of deconditioning. The whole body suspended rat may be useful as a model to better understand responses of rats exposed to microgravity.     

Functional and cellular adaptation to weightlessness in primates

Considerable data has been collected on the response of hindlimb muscles to unloading due to both spaceflight and hindlimb suspension. One generalized response to a reduction in load is muscle fiber atrophy, although not all muscles respond the same. For example, predominantly slow extensor muscles like the Sol exhibit a large reduction in fiber size to unloading, while fast extensors like the plantaris and fast flexors like the tibialis anterior show little, if any, atrophy. Our understanding of how muscles respond to microgravity, however, has come primarily from the examination of hindlimb muscles in the unrestrained rat in space. The non-human primate spaceflight paradigm differs considerably from the rodent paradigm in that the monkeys are restrained, usually in a sitting position, while in space. Recently, we examined the effects of microgravity on muscles of the Rhesus monkey by taking biopsies of selected hindlimb muscles prior to and following spaceflights of 14 and 12 day durations (Cosmos 2044 and 2229). Our results revealed that the monkey's response to microgravity differs from that of the rat. The apparent differences in the atrophic response of the hindlimb muscles of the monkey and rat to spaceflight may be attributed to 1) a species difference, 2) a difference in the manner in which the animals were maintained during the flight (i.e., chair restraint or "free-floating"), and/or 3) an ability of the monkeys to counteract the effects of spaceflight with resistive exercise.     

Skeletal muscle fiber, nerve, and blood vessel breakdown in space-flown rats

Histochemical and ultrastructural analyses were performed postflight on hind limb skeletal muscles of rats orbited for 12.5 days aboard the unmanned Cosmos 1887 biosatellite and returned to Earth 2 days before sacrifice. The antigravity adductor longus (AL), soleus, and plantaris muscles atrophied more than the non-weight-bearing extensor digitorum longus, and slow muscle fibers were more atrophic than fast fibers. Muscle fiber segmental necrosis occurred selectively in the AL and soleus muscles; primarily, macrophages and neutrophils infiltrated and phagocytosed cellular debris. Granule-rich mast cells were diminished in flight AL muscles compared with controls, indicating the mast cell secretion contributed to interstitial tissue edema. Increased ubiquitination of disrupted myofibrils implicated ubiquitin in myofilament degradation. Mitochondrial content and succinic dehydrogenase activity were normal, except for subsarcolemmal decreases. Myofibrillar ATPase activity of flight AL muscle fibers shifted toward the fast type. Absence of capillaries and extravasation of red blood cells indicated failed microcirculation. Muscle fiber regeneration from activated satellite cells was detected. About 17% of the flight AL end plates exhibited total or partial denervation. Thus, skeletal muscle weakness associated with spaceflight can result from muscle fiber atrophy and segmental necrosis, partial motor denervation, and disruption of the microcirculation.     

Activity dependent characteristics of fast and slow muscle: biochemical and histochemical considerations

The effects of denervation and hindlimb suspension induced disuse on concentrations of ATP, phosphocreatine (PC), and fiber type profile were investigated in slow twitch soleus and fast twitch extensor digitorum longus (EDL) muscles. The results show that the soleus and EDL muscles differ in their dependency on loadbearing as a stimulus for maintaining normal energy metabolism and the biochemical and morphological characteristics of muscle fibers. As determined by R-P methodology, suspension reduced ATP and PC concentrations of the soleus to 26% and 56%, respectively, while, in EDL only, PC is reduced to 71% of control with no change in ATP. Both muscles, however, show identical losses in ATP and PC following denervation. The energy charge, an indicator of Pi availability in muscle was reduced significantly in both denervated muscles to 82% and 85% in soleus and EDL, respectively. No significant reduction of the energy charge was seen in the muscles from suspended rats. Thus, in parallel with the indirect regulation through muscle loadbearing, the nerve can effectively modulate the levels of high-energy phosphates more directly by some regulatory mechanisms independent of muscle type. Denervation and suspension disuse increased the proportion of type 2 fibers in the soleus with a concomitant decrease in type 1 fibers and a relative rise in the number of very small diameter fibers. The EDL showed only variation in fiber size.     

The content of heat shock protein 47 (HSP47), a collagen-specific stress protein, changes with gravitational conditions in skeletal muscle.

It is well known that unloading of skeletal muscle with spaceflight or tail suspension leads rat soleus muscle atrophy. Previously, we reported that one of small heat shock protein (sHSP), alpha B-crystallin shows an early dramatic decrease in atrophied rat soleus muscle (Atomi et al, 1991). In this report, we focused to study the gravitational responses of another HSP, which may be reactive to the gravity. HSP47, a collagen-specific stress protein, has been postulated to be a collagen-specific molecular chaperone localized in the ER (Nagata et al, 1992). Western blot analysis revealed that HSP47 in slow skeletal muscle decreases at 5 days after tail suspension (TS) and increased at 5 days recovery after 10 days of TS as compared with the control level. Hypothetically, HSP47 in slow soleus muscle increases at 5 days after hypergravity (HG) induced by the centrifugation. The content of HSP47 in soleus muscle was strongly affected by gravity conditions.     

Anatomy and histochemistry of spread-wing posture in birds. 3. Immunohistochemistry of flight muscles and the "shoulder lock" in albatrosses

As a postural behavior, gliding and soaring flight in birds requires less energy than flapping flight. Slow tonic and slow twitch muscle fibers are specialized for sustained contraction with high fatigue resistance and are typically found in muscles associated with posture. Albatrosses are the elite of avian gliders; as such, we wanted to learn how their musculoskeletal system enables them to maintain spread-wing posture for prolonged gliding bouts. We used dissection and immunohistochemistry to evaluate muscle function for gliding flight in Laysan and Black-footed albatrosses. Albatrosses possess a locking mechanism at the shoulder composed of a tendinous sheet that extends from origin to insertion throughout the length of the deep layer of the pectoralis muscle. This fascial "strut" passively maintains horizontal wing orientation during gliding and soaring flight. A number of muscles, which likely facilitate gliding posture, are composed exclusively of slow fibers. These include Mm. coracobrachialis cranialis, extensor metacarpi radialis dorsalis, and deep pectoralis. In addition, a number of other muscles, including triceps scapularis, triceps humeralis, supracoracoideus, and extensor metacarpi radialis ventralis, were found to have populations of slow fibers. We believe that this extensive suite of uniformly slow muscles is associated with sustained gliding and is unique to birds that glide and soar for extended periods. These findings suggest that albatrosses utilize a combination of slow muscle fibers and a rigid limiting tendon for maintaining a prolonged, gliding posture.     

Functional properties of locust locomotor muscles

The ultrastructure of the muscle fibers and the electrical constants and responses of the membrane to microapplication of L-glutamate and acetylcholine were investigated in the longitudinal flight muscle and the flexor tibiae ofLocusta migratoria migratorioides. The twitch flight muscle differs from the slower leg muscle in the smaller size of its sarcomeres and the lower values of the space attenuation factor of the electrotonic potential, time constant, and resistance of the membrane. Microapplication of sodium L-glutamate at strictly definite points of the fibers of both muscles evoked depolarization responses of the membrane. In experiments on normal and denervated muscle, during microapplication of acetylcholine, changes in the level of the membrane potential were never observed. It is concluded that L-glutamic acid is the excitatory mediator of the twitch and slow muscle systems of insects.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 532–538, September–October, 1977.     

去负荷小鼠比目鱼肌收缩特性和纤维类型转化

目的:探讨去负荷后小鼠比目鱼肌的收缩特性与骨骼肌纤维类型转化之间的关系。方法:采用离体肌肉灌流技术和电刺激方法,在小鼠后肢去负荷28 d引起骨骼肌萎缩后,观察比目鱼肌单收缩、强直收缩能力和肌疲劳指标等收缩特性的改变,同时利用组织免疫荧光染色和实时定量聚合酶链式反应(real-time PCR)等技术检测去负荷后比目鱼肌快慢肌纤维组成和纤维类型转化的变化。结果:去负荷28 d后,小鼠比目鱼肌单收缩力、强直收缩能力和疲劳指数(fatigue index)均有显著性下降,同时伴有快肌纤维亚型的增加和慢肌纤维亚型的减少。结论:去负荷28 d后小鼠比目鱼肌收缩特性的改变和快慢肌纤维类型的转化有关。     

Age effects on rat hindlimb muscle atrophy during suspension unloading.

Disuse can induce numerous adaptive alterations in skeletal muscle. In the present study the effects of hindlimb unloading on muscle mass and biochemical responses were examined and compared in adult (450 g) and juvenile (200 g) rats after 1, 7, or 14 days of whole body suspension. Quantitatively and qualitatively the soleus (S), gastrocnemius (G), plantaris (P), and extensor digitorum longus (EDL) muscles of the hindlimb exhibited a differential sensitivity to suspension and weightlessness unloading in both adults and juveniles. The red slow-twitch soleus exhibited the most pronounced atrophy under both conditions, with juvenile responses being greater than adult. In contrast, the fast-twitch EDL hypertrophied during suspension and atrophied during weightlessness, with no significant difference between adults and juveniles. Determination of biochemical parameters (total protein, RNA, and DNA) indicated a less rapid rate of response in adult muscles. This was corroborated by assessment of muscle alpha-actin mRNA levels, which indicated a rapid (within 1 day) and significant (P less than 0.05) effect in juveniles but not in adults. The results of this investigation indicate 1) a qualitatively similar differential effect of unloading on muscles of adults and juveniles, 2) a quantitatively reduced and less rapid effect of suspension on adult muscles, and 3) a close similarity of adult and juvenile muscle responses during suspension and spaceflight, suggesting that this ground-based model simulates many of the unloading effects of weightlessness.     

Tibial Nerve Transection - A Standardized Model for Denervation-induced Skeletal Muscle Atrophy in Mice

The tibial nerve transection model is a well-tolerated, validated, and reproducible model of denervation-induced skeletal muscle atrophy in rodents. Although originally developed and used extensively in the rat due to its larger size, the tibial nerve in mice is big enough that it can be easily manipulated with either crush or transection, leaving the peroneal and sural nerve branches of the sciatic nerve intact and thereby preserving their target muscles. Thus, this model offers the advantages of inducing less morbidity and impediment of ambulation than the sciatic nerve transection model and also allows investigators to study the physiologic, cellular and molecular biologic mechanisms regulating the process of muscle atrophy in genetically engineered mice. The tibial nerve supplies the gastrocnemius, soleus and plantaris muscles, so its transection permits the study of denervated skeletal muscle composed of fast twitch type II fibers and/or slow twitch type I fibers. Here we demonstrate the tibial nerve transection model in the C57Black6 mouse. We assess the atrophy of the gastrocnemius muscle, as a representative muscle, at 1, 2, and 4 weeks post-denervation by measuring muscle weights and fiber type specific cross-sectional area on paraffin-embedded histologic sections immunostained for fast twitch myosin.     

Effect of hindlimb suspension on VO2 max and regional blood flow responses to exercise

Male rodents were studied before and after undergoing one of three treatment conditions for 9 days: 1) cage control (n = 15, CON), 2) horizontal suspension (n = 15, HOZ), and 3) head-down suspension (n = 18, HDT). Testing included measurements of maximal O2 uptake (VO2 max) and select cardiovascular responses to graded treadmill exercise. VO2 max expressed on an absolute basis (ml/min) was significantly decreased after HOZ (-14.1 +/- 2.5%) and HDT (-14.3 +/- 2.0%), while being essentially unchanged in CON (-1.0 +/- 3.3%). Significant reductions in body weight were observed after both HOZ (-10.1 +/- 4.2 g) and HDT (-22.5 +/- 3.3 g), whereas CON animals exhibited a significant increase in weight (10.4 +/- 3.8 g). As a result, when VO2 max was normalized for body weight, all groups exhibited similar significant reductions of 6-7%. Although no differences in heart rate and blood pressure response to graded exercise were observed, the HDT group exhibited greater increases in mesenteric resistance at the same absolute exercise intensity. Furthermore, both suspended groups had higher iliac resistance values during exercise at similar relative exercise conditions, suggesting that muscle blood flow during treadmill running may have been reduced after suspension. In general, the decrements associated with the HOZ and HDT conditions were similar. It was concluded that reduction in exercise capacity and altered cardiovascular responses to exercise observed after 6-9 days of suspension were attributable to a combination of hypokinesia, lack of hindlimb weight bearing, or restraint, rather than to hydrostatic influences associated with HDT.     

Inhibition and reversal of denervation-induced atrophy by theβ-agonist growth promoter,clenbuterol

Dietary administration of the growth promoter, clenbuterol, ameliorated denervation-induced atrophy in rat soleus muscles. In acutely denervated muscles the drug inhibited the appearance of atrophy, and in chronically denervated muscles the atrophy was almost fully reversed. Responses in slow twitch oxidative fibres were particularly marked.     

Skeletal muscle fiber atrophy: altered thin filament density changes slow fiber force and shortening velocity

Single skinned fibers from soleus and adductor longus (AL) muscles of weight-bearing control rats and rats after 14-day hindlimb suspension unloading (HSU) were studied physiologically and ultrastructurally to investigate how slow fibers increase shortening velocity (V₀) without fast myosin. We hypothesized that unloading and shortening of soleus during HSU reduces densities of thin filaments, generating wider myofilament separations that increase V₀ and decrease specific tension (kN/m²). During HSU, plantarflexion shortened soleus working length 23%. AL length was unchanged. Both muscles atrophied as shown by reductions in fiber cross-sectional area. For AL, the 60% atrophy accounted fully for the 58% decrease in absolute tension (mN). In the soleus, the 67% decline in absolute tension resulted from 58% atrophy plus a 17% reduction in specific tension. Soleus fibers exhibited a 25% reduction in thin filaments, whereas there was no change in AL thin filament density. Loss of thin filaments is consistent with reduced cross bridge formation, explaining the fall in specific tension. V₀ increased 27% in soleus but was unchanged in AL. The V₀ of control and HSU fibers was inversely correlated (R = –0.83) with thin filament density and directly correlated (R = 0.78) with thick-to-thin filament spacing distance in a nonlinear fashion. These data indicate that reduction in thin filament density contributes to an increased V₀ in slow fibers. Osmotically compacting myofilaments with 5% dextran returned density, spacing, and specific tension and slowed V₀ to near-control levels and provided evidence for myofilament spacing modulating tension and V₀. rat; soleus; adductor longus; fiber length; electron microscopy; hindlimb suspension unloading     

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 degrees plantar flexion at rest. As HSU proceeds, the normal weight-bearing angle of 30 degrees 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 (approximately 287 g) were lengthened by unilateral ankle splinting at 30 degrees. 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.     

Changes in fiber composition of soleus muscle during rat hindlimb suspension

Chronic reduction of gravitational load in the rear limbs of rats to simulate the influence of near-zero gravity in skeletal muscles has been shown previously to elicit atrophy in the soleus muscle. Use of this model by the present investigation indicates that soleus atrophy was characterized by a decline in the number of fibers in groups that contained the slow isoenzyme of myosin and which were classified as type I from intensity of staining to myofibrillar actomyosin adenosinetriphosphatase (ATPase) and to NADH tetrazolium reductase. Furthermore total fiber number was not changed, whereas fibers containing the intermediate isoenzyme and those classified as type IIa increased. There results could be explained by either a change in the composition within existing fibers or a simultaneous loss of slow fibers and de novo synthesis of intermediate and fast fibers. Evidence for transformation included an absence of embryonic or neonatal myosin in muscles from suspended rats and the constant fiber number that was unchanged by 4 wk of suspension. Furthermore although fiber areas of both groups of type I and IIa fibers declined during suspension, variability of the fiber areas within each group did not increase.     

Role of NO-synthase in regulation of protein metabolism of stretched rat m. soleus muscle during functional unloading

Gravitational unloading causes atrophy of muscle fibers and can lead to destruction of cytoskeletal and contractile proteins. Along with the atrophic changes, unloaded muscle frequently demonstrates significant shifts in the ratio of muscle fibers expressing fast and slow myosin heavy chain isoforms. Stretching of the m. soleus during hindlimb suspension prevents its atrophy. We supposed that neuronal NO-synthase (NOS) (which is attached to membrane dystrophin-sarcoglycan complex) can contribute to maintenance of protein metabolism in the muscle and prevent its atrophy when m. soleus is stretched. To test this hypothesis, we used Wistar rats (56 animals) in experiments with hindlimb suspension during 14 days. The group of hindlimb suspended rats with stretched m. soleus was injected with L-NAME to block NOS activity. We found that m. soleus mass and its protein content in hindlimb-suspended rats with stretched m. soleus were preserved due to prevention of protein degradation. NOS is involved in maintenance of expression of some muscle proteins. Proliferation of satellite cells in stretched m. soleus may be due to nNOS activity, but maintenance of muscle mass upon stretching is regulated not by NOS alone.     

Regulation of skeletal muscle dihydropyridine receptor gene expression by biomechanical unloading.

Biomechanical unloading of the rat soleus by hindlimb unweighting is known to induce atrophy and a slow- to fast-twitch transition of skeletal muscle contractile properties, particularly in slow-twitch muscles such as the soleus. The purpose of this study was to determine whether the expression of the dihydropyridine (DHP) receptor gene is upregulated in unloaded slow-twitch soleus muscles. A rat DHP receptor cDNA was isolated by screening a random-primed cDNA lambda gt10 library from denervated rat skeletal muscle with oligonucleotide probes complementary to the coding region of the rabbit DHP receptor cDNA. Muscle mass and DHP receptor mRNA expression were assessed 1, 4, 7, 14, and 28 days after hindlimb unweighting in rats by tail suspension. Isometric twitch contraction times of soleus muscles were measured at 28 days of unweighting. Northern blot analysis showed that tissue distribution of DHP receptor mRNA was specific for skeletal muscle and expression was 200% greater in control fast-twitch extensor digitorum longus (EDL) than in control soleus muscles. A significant stimulation (80%) in receptor message of the soleus was induced as early as 24 h of unloading without changes in muscle mass. Unloading for 28 days induced marked atrophy (control = 133 +/- 3 vs. unweighted = 62.4 +/- 1.8 mg), and expression of the DHP receptor mRNA in the soleus was indistinguishable from levels normally expressed in EDL muscles. These changes in mRNA expression are in the same direction as the 37% reduction in time to peak tension and 28% decrease in half-relaxation time 28 days after unweighting. Our results suggest that muscle loading necessary for weight support modulates the expression of the DHP receptor gene in the soleus muscle.     

Response to denervation of rabbit soleus and gastrocnemius muscles. Time-course study of postnatal changes in myosin isoforms,fiber types,and contractile properties

Summary— In contrast to general belief, the response of rabbit muscles to denervation is maturation to slow-like type muscles [7]. We report now an investigation by biochemical, morphological, and mechanical studies of the time course effects of muscle denervation on the slow-type soleus and fast-type gastrocnemius to help clucidate the mechanism of maturation of rabbit denervated muscles to slow-like muscles. In both muscles, denervation induced selective progressive atrophy of most fast fibers and hypertrophy of many slow fibers which displayed wide Z-lines; this was accompanied by the appearance of hybrid LC1F- and LC1E-associated slow myosins. The percentage of slow myosins increased with age similarly in the contralateral and denervated soleus. On the other hand, the percentage of slow myosins remained low in the contralateral gastrocnemius, whereas it increased to 95% in the denervated gastrocnemius; in the denervated gastrocnemius, the percentage of slow myosins reached 50% at about 35 days postnatal. At this age, the maximal shortening velocity of the denervated gastrocnemius and its twitch contraction time were already those of a slow-type muscle. This suggests that in addition to myosin, other proteins contributed to the mechanical properties of the denervated gastrocnemius. Transformation of rabbit denervated muscles to slow-like type muscles, which are associated with a lower energy requirement and higher muscle endurance than fast-type muscles, may constitute an adequate model for human neuromuscular pathology.