Hypokinesia in the hindlimbs of rats were induced by suspension; a newly developed harness system was used. The animal was able to use its forelimbs to maneuver, within a 140 degrees arc, to obtain food and water and to permit limited grooming of the forequarters. The hindlimbs were nonload bearing for 7 days; following a 7-day period of hypodynamia, selected animals were placed in metabolic cages for 7 days to study recovery from hypokinesia. During the 7-day period of hypokinesia there was evidence of muscle atrophy. Gastrocnemius weight decreased, renal papillary urea content increased, and daily urinary losses of urea, NH3, and 3-methylhistidine increased. During the 7-day recovery period muscle mass and excretion rate of urea, NH3 and 3-methylhistidine returned to control levels. Calcium balance was positive throughout the 7-day period of hypokinesia. Hypertrophy of the adrenals suggested the occurrence of some level of stress despite the apparent behavioral adjustment to the suspension harness. It was concluded that significant muscle atrophy and parallel changes in nitrogen metabolism occur in suspended rats and these changes are readily reversible.
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.
Weightlessness is associated with a differential atrophic effect on skeletal muscle that has been attributed to both hypokinesia and hypodynamia. A suspension technique was developed to simulate this atrophic effect by inducing hypokinesia/hypodynamia (H/H) in the rat hindlimb. The purpose of the present studies was to assess the effects of H/H on protein, RNA, and DNA contents in hindlimb skeletal muscles. Suspended animals exhibited a differential reduction in absolute muscle protein content with some alterations in protein concentration. Absolute DNA levels did not change in atrophic muscles. There were pronounced effects of suspension on DNA concentration and on protein/DNA, which suggests that muscle atrophy was accompanied by a reduction in muscle cell size. Hindlimb H/H was associated with a decreased absolute content of RNA in atrophic muscles as well as reductions in RNA/DNA. RNA concentration was reduced 16-21% in atrophic muscles. These findings document a pronounced and differential effect of suspension on hindlimb muscle protein and suggest the potential for effects on the capacity for protein synthesis in muscles from suspended animals.
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