Metabolism of amino acids by the atrophied soleus of tail-casted, suspended rats

Amino acid metabolism was investigated in atrophied soleus muscle from rats subjected to six days of tail-cast, hindlimb suspension. The fresh-frozen unloaded muscle showed higher concentrations of tyrosine and glutamate but lower amounts of aspartate, glutamine, ammonia, and a lower ratio of glutamine to glutamate than normal muscle. The atrophied muscle also showed faster in vitro production of alanine and tyrosine, and slower utilization of glutamate and aspartate. Despite a greater activity of glutamine synthetase, synthesis of glutamine was slower in the soleus muscle of suspended rats than in control muscle. Provision of ammonium chloride and/or glutamate showed that this slower synthesis of glutamine in the atrophied soleus probably was due to limiting amounts of free ammonia and not of glutamate. Flux through AMP deaminase was probably slower as demonstrated by the maintenance of a greater pool of total adenine nucleotides and by the slower release of nucleosides by the incubated soleus muscle of suspended v control rats. The extensor digitorum longus muscles of suspended animals showed greater glutamine production, glutamine synthetase activity, and aspartate utilization than control muscles. Data from muscles of intact, adrenalectomized and adrenalectomized, cortisol-treated rats suggested that the greater glutamine synthetase activity was mediated possibly by higher circulating glucocorticoid hormones and a greater response of the soleus muscle to these hormones. Glutamine synthesis in skeletal muscle may be regulated primarily by the availability of ammonia, which is associated with the degradation of adenine nucleotides, and secondarily by the amount of glutamine synthetase and glutamate in the tissue.