Effect of suspension hypokinesia/hypodynamia on tissue protein turnover in rats

The effect of suspension hypokinesia/hypodynamia on the protein metabolism in skeletal muscles (gastrocnemius and soleus) and visceral organs (liver and small intestine) was investigated in the rat. Suspension for 10 days resulted in atrophy of the visceral organs as well as the skeletal muscles, which was associated with decreases in the amounts of visceral and muscle protein. There was marked breakdown of muscle protein, which was reflected by increases in the urinary excretion of N7-methylhistidine and cathepsin D activity. Measurement of protein synthesis by the L-[4-3H]phenylalanine method revealed that the synthesis in the gastrocnemius, but not in the soleus, muscle was suppressed on suspension for 10 days. Thus, both the increased catabolism and decreased synthesis of protein in the muscles may be causally related to the muscle atrophy occurring in suspension. In the visceral organs, on the other hand, the protein synthesis was found to increase in hypokinetic rats. Moreover, the concentration of visceral protein was also increased, despite the decreased amount of muscle protein. These changes in protein metabolism in the liver and small intestine may explain, at least partly, the slightly positive nitrogen balance which was observed after long-term weightlessness.