Fiber-type composition of nine rat muscles. II. Relationship to protein turnover.

Rates of protein synthesis in vivo and fiber-type composition were measured in nine limb muscles of female rats at ages ranging from weaning to 1 yr. In all muscles, there was a decline in protein synthesis with increasing age, mostly as a result of a fall in the RNA content. Rates of protein breakdown and growth were determined in six muscles and these also declined with age. Regression analysis of the data for all ages showed that protein synthesis was correlated with the content of slow oxidative fibers but not with the relative proportions of fast glycolytic to fast oxidative glycolytic fibers.