Effects of 9-week hindlimb suspension and 8-week recovery on locomotor performance and electromyogram (EMG) activities of soleus (Sol), plantaris (Pl), lateral gastrocnemius (LG), and tibialis anterior (TA), were studied in adult rats. Hyperextension of knee and ankle joints, noted after nine weeks of suspension, did not recover during 8-week ambulation. Growth of Sol was fully inhibited by suspension and did not recover completely within 8 weeks of ambulation. The EMG levels in Sol, Pl, and LG 1 day after suspension were 52-76% less than the pre-suspension level (resting on the floor). These activities were recovered to or near the pre-suspension level around 1 week, but decreased again to 10-29% of controls from 7 to 9 weeks. The integrated EMG of TA was elevated during the first week of suspension but then gradually declined to control levels within four weeks. At the end of suspension, the Sol and Pl, not the LG, EMGs remained reduced and the TA EMG remained hyperactive. Co-activation of dorsi- and plantar- flexors occurred often during quadripedal walking following suspension. Such a phenomenon was not observed in the control rats. These phenomena were recovered within 1 week. It is suggested that the unloading-related alterations of neuromuscular activities and/or locomotion, but not the hyperextension of knee and ankle joints, in rats are reversible.
The effect of microgravity on skeletal muscles has so far been examined in rat and mice only after short-term (5–20 day) spaceflights. The mice drawer system (MDS) program, sponsored by Italian Space Agency, for the first time aimed to investigate the consequences of long-term (91 days) exposure to microgravity in mice within the International Space Station. Muscle atrophy was present indistinctly in all fiber types of the slow-twitch soleus muscle, but was only slightly greater than that observed after 20 days of spaceflight. Myosin heavy chain analysis indicated a concomitant slow-to-fast transition of soleus. In addition, spaceflight induced translocation of sarcolemmal nitric oxide synthase-1 (NOS1) into the cytosol in soleus but not in the fast-twitch extensor digitorum longus (EDL) muscle. Most of the sarcolemmal ion channel subunits were up-regulated, more in soleus than EDL, whereas Ca2+-activated K+ channels were down-regulated, consistent with the phenotype transition. Gene expression of the atrophy-related ubiquitin-ligases was up-regulated in both spaceflown soleus and EDL muscles, whereas autophagy genes were in the control range. Muscle-specific IGF-1 and interleukin-6 were down-regulated in soleus but up-regulated in EDL. Also, various stress-related genes were up-regulated in spaceflown EDL, not in soleus. Altogether, these results suggest that EDL muscle may resist to microgravity-induced atrophy by activating compensatory and protective pathways. Our study shows the extended sensitivity of antigravity soleus muscle after prolonged exposition to microgravity, suggests possible mechanisms accounting for the resistance of EDL, and individuates some molecular targets for the development of countermeasures.
Roles of mechanical load and satellite cells in the regulation of morphological properties of soleus muscle fibers were reviewed. Gravitational unloading by exposure to microgravity and/or by hindlimb suspension causes passive plantarflexion of ankle joints, which then shortens the length of muscle fibers and sarcomeres. Such phenomena cause the decrease of tension development. Atrophy of muscle fibers caused by inhibited protein synthesis is induced in association with decreased number and increased size of myonuclei. Distribution of satellite cells, which serve as a source of new myonuclei during regeneration after a muscle injury and/or atrophy, also decreases in response to lowered mechanical load. However, these responses are generally reversible, when the mechanical load applied to muscle fibers is increased, suggesting that satellite cells play important role(s) in the regulation of muscle fiber properties. The data also indicated that one of the satellite cell-related regulations of muscle fiber was mechanical load-dependent, which was influenced by the sarcomere length.
The mechanisms responsible for the morphological and metabolic adaptation of skeletal muscles to the removal of antigravity activity were investigated in rats. Significant atrophy relative to the levels before suspension was induced in ankle plantarflexsors, may be due to a reduced tension production caused by decreased muscle length and electromyogram activity. Growth failure was significant in ankle dorsiflexors, although these muscles did not atrophy. Forced muscle contraction through electrical stimulation at 1 or 100 Hz during hind limb suspension generally had detrimental effects. The percent contribution of water loss to the suspension-related change in weight was 85, 88, and 93% in soleus, plantaris, and extensor digitorum longus, respectively. The total levels of both beta-hydroxyacyl CoA dehydrogenase (HAD) and lactate dehydrogenase (LDH) were less in the suspended muscles than in the controls, having high positive correlations with the total protein content. The specific activity of HAD, but not of LDH, of the suspended muscles was lower than in the controls (25-61%). These data suggest that the cause of muscle atrophy and changes in metabolic properties may be a decreased tension development, not necessarily the reduction of electrical or contractile activity. Further, it is clearly suggested that electrical stimulation of a muscle group with different composition of fiber phenotype at a certain pattern or frequency is not suitable for the countermeasure. It is also suggested that the major cause of the decreased muscle weight was loss of water, even though protein content was also lowered after suspension. Moreover, the data suggest that the HAD level was affected more than the total protein content and LDH.
Effects of hindlimb unloading during the first 3 months after birth on the development of soleus muscle fibers were studied in rats. The mean absolute weigh and cross-sectional area of whole soleus muscle in the unloaded rats were -1/3 and 1/4 of those in the controls, respectively. But the unloading did not affect the lengths of muscle, at 90 degrees of ankle joint angle, and of muscle fibers sampled from tendon to tendon, and the total sarcomere number. Since the total number of fibers in soleus was not affected either, the inhibited increase of muscle mass following unloading was mainly due to the smaller CSA of individual fibers. Numbers of both myonuclei and satellite cells were significantly less in unloaded than control rats. The % distribution of fibers expressing pure type I myosin heavy chain was significantly less in unloaded than controls (-23 %). Further, muscle fibers with multiple innervation were noted in the unloaded rats. It is suggested that the development and/or differentiation of soleus muscle fibers are closely associated with gravitational loading and that the growth-associated increase in fiber number may be genetically programmed.
Antigravity function plays an important role in determining the morphological and physiological properties of the neuromuscular system. Inhibition of the normal development of the neuromuscular system is induced by hindlimb unloading during the neonatal period in rats. However, the role of gravitational loading on the development of skeletal muscle in rats is not well understood. It could be hypothesized that during the early postnatal period, i.e. when minimal weight-supporting activity occurs, the activity imposed by gravity would be of little consequence in directing the normal development of the skeletal musculature. We have addressed this issue by limiting the amount of postnatal weight-support activity of the hindlimbs of rats during the lactation period. We have focused on the development of three characteristics of the muscle fibers, i.e. size, myonuclear number and myosin heavy chain expression.
