EFFECT OF AGING ON THE RATE OF MUSCLE PROTEIN-TURNOVER IN RAT

The urinary excretion of N-t-methylhistidine (3-methylhistidine, 3-MH, an index related to myofibrillar protein breakdown), as well as the r ate of L-[C-14]-tyrosine incorporation into gastrocnemius muscle (k(s) , fractional rate of muscle protein synthesis, evaluated by the consta nt infusion method) have been measured to assess the effect of aging o n the rate of skeletal muscle protein turnover. In addition, nucleic a cids, muscle protein and serum corticosterone levels were determined. Weaning rats were fed a 10% lactalbumin diet and killed in groups of s even when they were 35, 60, 120 and 300 days old. Apart from the rate of growth, no major differences were found between 35- and 60-day old animals. However, as compared to the youngest rats, 120-day old rats s howed a significant reduction in the relative weight of the four muscl es excised. Plasma corticosterone levels increased as the animals beca me older. Finally, in the 300-day old rats, the reduced rate of growth was accompanied by a significant reduction in the relative organ weig ht (with the exception of soleus), 3-MH and k(s). It is concluded that aging caused a reduction in the rates of both protein breakdown and s ynthesis. The reduced muscle breakdown may not be due to a relative re duced muscle mass in elder rats since urinary 3-MH remained low even w hen expressed per creatinine output.