Atrophy and growth failure of rat hindlimb muscles in tail-cast-suspension

Atrophy and growth failure of muscle in a tail-cast suspension model were evaluated in hindlimbs of female Sprague-Dawley rats. Based on measurements of food consumption, animal growth rate, urinary excretion of urea and ammonia, and muscle size, 6 days seemed to be the optimum duration of suspension for studying muscle unloading. After 6 days, the soleus, plantaris, and gastrocnemius muscles from suspended animals were 27, 10, and 11% smaller (P<0.05), respectively, than those from tail-casted weight-bearing animals. The extensor digitorum longus and tibialis anterior muscles were unaffected by suspension (≤ 6 days) while the triceps brachii hypertrophied (8%, P<0.05). Wet weight-to-dry weight ratios were smaller in the plantaris (-0.19, P<0.05) and gastrocnemius (-0.19, P<0.05) muscles from suspended rats. In the plantaris, this difference coincided with a higher protein concentration (+12 mg/g, P<0.001). In vitro measurements of protein metabolism in the soleus muscles of suspended rats showed both slower protein synthesis (P<0.05) and faster protein degradation (P<0.05), whereas these processes were unaltered in the extensor digitorum longus muscles.