Regulation of myofibrillar protein turnover during maturation in normal and undernourished rat pups

The study tested the hypothesis that a higher rate of myofibrillar than sar coplasmic protein synthesis is responsible for the rapid postdifferentiatio n accumulation of myofibrils and that an inadequate nutrient intake will co mpromise primarily myofibrillar protein synthesis. Myofibrillar (total and individual) and sarcoplasmic protein synthesis, accretion, and degradation rates were measured in vivo in well-nourished (C) rat pups at 6, 15, and 28 days of age and compared at 6 and 15 days of age with pups undernourished (UN) from birth. In g-day-old C pups, a higher myofibrillar than sarcoplasm ic protein synthesis rate accounted for the greater deposition of myofibril lar than sarcoplasmic proteins. The fractional synthesis rates of both prot ein compartments decreased with age, but to a greater degree for myofibrill ar proteins (-54 vs. -42%). These decreases in synthesis rates were partial ly offset by reductions in degradation rates, and from 15 days, myofibrilla r and sarcoplasmic proteins were deposited in constant proportion to one an other. Undernutrition reduced both myofibrillar and sarcoplasmic protein sy nthesis rates, and the effect was greater at 6 (-25%) than 15 days (-15%). Decreases in their respective degradation rates minimized the effect of und ernutrition on sarcoplasmic protein accretion from 4 to 8 days and on myofi brillar proteins from 13 to 17 days. Although these adaptations in protein turnover reduced overall growth of muscle mass, they mitigated the effects of undernutrition on the normal maturational changes in myofibrillar protei n concentration.