Myofibrillar protein turnover. Synthesis rates of myofibrillar and sarcoplasmic protein fractions in different muscles and the changes observed during postnatal development and in response to feeding and starvation.

Measurement of rates of synthesis of skeletal-muscle proteins in adult rats shows that the faster overall rate of turnover in diaphragm and soleus muscles compared with several other, more glycolytic, muscles is also exhibited by the myofibrillar proteins, since the ratio of sarcoplasmic to myofibrillar protein synthesis is similar for all muscles. Further, throughout postnatal development, when the overall turnover rate falls with age, parallel changes occur for the myofibrillar proteins, as indicated by a constant ratio of sarcoplasmic to myofibrillar protein synthesis (2.06) in the steady state after overnight starvation. Only in the youngest (4 weeks old) rats is a slightly lower ratio observed (1.72). These results indicate that, when changes in the overall turnover rate of muscle proteins occur, the relative turnover of the two major protein fractions stays constant. However, measurements in the non-steady state during growth and after starvation for 4 days show that the relative synthesis rates of the two fractions change as a result of a disproportionate increase in myofibrillar protein synthesis during growth and decrease during starvation. Thus the synthesis rate of the slower-turning-over myofibrillar protein fraction is more sensitive to nutritional state than is that of the sarcoplasmic protein. It is suggested that such responses may help to maintain constant tissue composition during non-steady-state conditions of growth and atrophy.