Pre- and postnatal growth and protein turnover in four muscles of the rat.

Developmental growth and associated changes in protein turnover and nucleic acid concentrations have been studied in four individual skeletal muscles. They have also been related to changes in the whole animal. The growth rates of both fast and slow muscle types progressively diminished from the fetus to old age. Similarly, the fractional rates of protein synthesis (measured in vivo) and breakdown in each muscle type declined with age; the changes in the former correlating with decreases in the ribosomal capacities of the muscles. Throughout, fast muscles possessed lower turnover rates. The mean half-lives of mixed proteins were 12.0, 14.4, 13.5, and 7.2 days in the extensor digitorum longus (EDL), gastrocnemius, diaphragm, and soleus muscles, respectively, 310 days postpartum. Muscle atrophy was found at 735 days, at which stage the decreased protein synthetic rate in the soleus was due to a fall in the ribosomal capacity, while that in the EDL was attributable to a decreased synthetic rate per ribosome.