Amino acids were assayed in muscles from rats exposed to seven days of microgravity and twelve hours of gravity immediately postflight, and rats exposed to six days of suspension with and without twelve hours of loading. Overall changes in body weight, and changes in mass, protein, tyrosine, and glycogen in the hindlimb muscles were compared in SL-3 and ground control rats. Sliced muscles were placed in KOH for glycogen determination or cold perchloric acid for homogenization. Glucose was assayed enzymatically. Homogenates were then separated by centrifuge, protein was assayed spectrophotometrically, and the supernatant was analyzed fluorometrically for tyrosine and other amino acids.

Results:

Flight and ground control rats grew similarly, while unloaded suspension rats grew slower than loaded suspension rats. In flight, unloaded and loaded suspension rats, the soleus atrophied and the gastrocnemius, plantaris and extensor digitorum longus showed reduced growth. The tibialis anterior showed little response. Changes in mass and protein content correlated in these muscles. Muscles from the flight animals showed dramatic increases in glycogen, the soleus being most responsive. Tail-suspension rats without loading showed a greater glycogen concentration in the soleus only, with an even greater value in the loaded, tail-suspension soleus. Only in flight soleus was tyrosine greater than in the control, suggesting a more negative muscle protein balance. In this study, recovery from suspension decreased soleus tyrosine. These results suggest that the additional stress placed on the flight rats postflight by additional transport may have prevented the soleus from showing evidence of recovery from microgravity.