Because of turnover,
protein synthesis and breakdown can each be involved in the regulation of the growth of tissue
protein. To investigate the regulation of skeletal-muscle-
protein growth we measured rates of
protein synthesis and breakdown in growing rats during development on a good diet, during development on a marginally
low-protein diet and during rehabilitation on a good diet after a period of severe
protein deficiency. Rates of
protein synthesis were measured in vivo with a constant
intravenous infusion of [14C]
tyrosine. The growth rate of
muscle protein was measured and the rate of breakdown calculated as breakdown rate=synthesis rate-growth rate. These measurements showed that during development on a good diet there was a fall with age in the rate of
protein synthesis resulting from a fall in capacity (
RNA concentration) and activity (synthesis rate per unit of
RNA). There was a fall with age in the breakdown rate so that the rate was highest in the weaning rats, with a half-life of 3 days. There was a direct correlation between the fractional growth and breakdown rates. During rehabilitation on the good diet, rapid growth was also accompanied by high rates of
protein breakdown. During growth on the inadequate diet
protein synthesis rates were lesss than in controls, but growth occurred because of decreased rates of
protein breakdown. This compression was not complete, however, since ultimate muscle size was only one-half that of controls. It is suggested that increased rates of
protein breakdown are a necessary accompaniment to muscle growth and may result from the way in which myofibrils proliferate.