The high rate of myofibrillar protein synthesis in immature skeletal muscle is a key contributor to the rapid rate of muscle growth in the perinatal period. The objective of these studies is to identify the factors that confer this highly anabolic characteristic on the immature muscle, and to establish their contribution to the muscle's capacity for "catch-up" growth following undernutrition. Hypothesis I proposes that myofibrillar protein synthesis rates in the immature muscle are regulated by myofibrillar mRNA abundances, and that muscle- specific regulatory factors (MRFs) contribute quantitatively to this enhanced synthesis rate, by maintaining a high steady-state abundance of myofibrillar mRNAs. The fall in MRF activity underlies the developmental decrease in synthesis specific to myofibrillar proteins. This hypothesis will be tested in Aim 1 using transgenic mice (Mid) to determine the extent to which attenuation of MRF activity impairs muscle growth by reducing, specifically, myofibrillar mRNA abundances and synthesis rates. Age comparisons will reveal whether the contribution of mRNA abundance to total synthesis rates diminishes when ribosome abundance becomes limiting. Hypothesis II proposes that the insulin-like growth factors (IGFs) in the immature muscle promote muscle protein synthesis by MRF-independent mechanisms, specifically, by stimulating satellite cell replication, and enhancing the responsiveness of muscle protein synthesis to insulin. This hypothesis (Aim 2) will be tested by studying a bigenic mouse produced by crossing the Mid mouse with a transgenic mouse (SK-IGF) that has increased muscle growth due to targeted overexpression of IGF-I. Thus, IGF-I will be expressed in muscle against a background of inhibited MRF function. Total and myofibrillar protein synthesis rates and mitotic activity will be compared among genotypes, and the contribution of MRFs identified by the mitigation of IGF-stimulated activity in the bigenic mice. In Aim 3, the insulin sensitivity of muscle protein synthesis in SK-IGF mice will be established, by comparing the protein synthetic response to refeeding after an acute fast in the presence or absence of insulin antibody. Hypothesis III postulates that the efficiency of catch-up growth on refeeding is high in the immature muscle because protein synthesis is optimized by the high tissue levels of MRFs and IGFs. This will be tested in Aim 4 by comparing the protein synthetic responses to nutritional rehabilitation of immature and mature muscles in nontransgenic mice, Mid and SK-IGF transgenic mice. This work will identify the developmental window during which aggressive nutritional rehabilitation will restore muscle mass most effectively; intervention strategies, therefore, can be directed toward those populations that would most benefit. Additionally, identification of the pathways that regulate myofibrillar protein synthesis will enable the development of rational therapies to enhance muscle growth in other conditions of disordered muscle growth.