HORMONAL REGULATION OF NEONATAL MUSCLE PROTEIN SYNTHESIS

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The long-term objectives are to identify the mechanisms by which hormones and nutrients interact to regulate the high rate of skeletal muscle protein deposition in the neonate, and to determine how these mechanisms are altered by sepsis. The goal is to identify new strategies to optimize nutrient utilization for growth in growth-retarded and septic infants. Rapid accretion of skeletal muscle mass is a particularly important characteristic of neonatal growth. Previous work has shown that the feeding-induced stimulation of muscle protein synthesis in normal neonates involves regulation by both insulin and amino acids. However, because of the exquisite sensitivity of muscle protein synthesis to insulin and amino acids in the normal neonate, it could not be discerned whether both anabolic agents interact to regulate muscle protein synthesis. Sepsis contributes substantially to the morbidity and mortality of neonates, a population with limited muscle protein stores. Hormonal changes and substrate deficiency likely contribute to the muscle wasting that occurs with sepsis. The hypotheses are: 1) amino acids enhance the sensitivity of muscle protein synthesis to insulin levels in neonates, 2) the interaction of amino acids and insulin reflects their independent effects on the activity of eukaryotic initiation factors (eIFs) and signaling kinases, and 3) sepsis inhibits muscle protein synthesis in neonates via an interaction of tumor necrosis factor alpha (TNFalpha) with insulin-mediated protein synthesis, an effect that can be circumvented by amino acids. The following aims are proposed to address these hypotheses: In Aim 1, the dose response of muscle protein synthesis to insulin and the effect of amino acids on this response will be determined in normal neonates. To achieve this, studies will be performed in 7-d-old pigs using direct kinetic measurement of muscle protein synthesis and refinements of a novel hormone-substrate clamp technique. In Aim 2, the effects of insulin and amino acids on the activity of key eIFs will be determined. To achieve this, muscles from the experiments in Aim 1 will be used to determine the quantitative relationships among protein synthesis rates, the activity of the eIFs that are involved in the binding of Met-tRNA and mRNA to the 40S ribosomal subunit, and the activation of signaling kinases. This will identify the specific translational processes regulated by insulin and amino acids in vivo. In Aim 3, the response of muscle protein synthesis to insulin following administration of endotoxin, TNFalpha, anti-cytokine therapy, and enhanced amino acid supply will be determined. These studies will identify the role of hormones, cytokines, and nutrients in the sepsis-induced muscle wasting in neonates. The practical efficacy of amino acid administration to circumvent the sepsis-induced insulin resistance of muscle protein synthesis will be identified.