Effects of stretching and disuse on amino acids in muscles of rat hind limbs

Effects of stretching on muscle amino acids were tested in unloaded soleus by casting the foot in dorsiflexion on one limb of tail-casted, hindquarter-suspended rats. For comparison with unloading, amino acids also were measured in shortened extensor digitorum longus (EDL) in the same casted limb and in denervated leg muscles. Concentrations of tyrosine and glutamate were lower, while aspartate, ammonia, and the ratio of glutamine to glutamate were greater in the stretched than in the freely moving, unloaded soleus, but stretched did not differ from weight-bearing, control muscle. Therefore, stretching the soleus muscle prevented changes in certain amino acids due to unloading. Aspartate, ammonia, glutamine, and the ratio of glutamine to glutamate were lower in the shortened EDL than in the freely moving muscle of the contralateral limb, or in the control muscle. When denervated, these leg muscles also showed lower aspartate, ammonia, and ratio of glutamine to glutamate relative to innervated muscles. Since muscle shortening or denervation produced amino acid changes that mimicked the effects of unloading on the soleus, these responses must reflect the effect of muscle disuse. These data suggested that lower ammonia might cause the lower ratio of glutamine to glutamate with disuse. Because the fresh muscle energy charge, one factor which controls AMP deaminase, generally was not affected by disuse, altered deamination of glutamate via glutamate dehydrogenase may explain the variations in muscle ammonia. The in vitro production of glutamine, ratio of glutamine to glutamate and use of aspartate were less in the unloaded than in the stretched soleus, and were less in the shortened EDL than in the contralateral freely moving muscle. Since the addition to the medium of ammonium chloride abolished these differences, reduced availability of ammonia in disused muscles may explain their altered metabolism of glutamine and aspartate.