Characteristics of skeletal muscle growth and protein turnover in a fast-growing rat strain.

1. Protein turnover and muscle composition has been studied in rat skeletal muscle throughout development in a relatively-fast-growing rat strain. 2. Muscle growth involved an increase in the total DNA and in the DNA-unit size as indicated by proteins: DNA. As a result of the latter increase together with no change in RNA: DNA, the RNA concentration fell throughout development. 3. Rates of protein synthesis measured in vivo by the continuous intravenous infusion method fell throughout development from 15.6%/d at 25 d to 4.46%/d at 320 d, and these changes reflected mainly the fall in RNA concentration, since there was no marked change in the rate of protein synthesis per unit RNA. 4. The rate of protein degradation, measured as the difference between rates of protein synthesis and growth, fell from 9.82%/d at 25 d to 4.46%/d at 320 d. 5. When these changes in protein turnover throughout development are compared with measurements made previously in a slow-growing strain it would appear that the faster growth was achieved as a result of increased efficiency of protein synthesis (defined as net synthesis: over-all synthesis) and this occurred mainly because of lower rates of protein degradation.