RESPONSE OF SKELETAL-MUSCLE PROTEIN-SYNTHESIS TO INSULIN IN SUCKLING PIGS DECREASES WITH DEVELOPMENT

The elevated rate of muscle protein deposition in the neonate is large ly due to an enhanced stimulation of skeletal muscle protein synthesis by feeding. To examine the role of insulin in this response, hyperins ulinemic-euglycemic-amino acid clamps were performed in 7- and 26-day- old pigs. Pigs were infused with 0, 30, 100, or 1,000 ng.kg(-0.66).min (-1) of insulin to mimic the plasma insulin levels observed under fast ed, fed, refed, and supraphysiological conditions, respectively. Whole body amino acid disposal, was determined from the rate of infusion of an amino acid mixture necessary to maintain plasma essential amino ac id concentrations near their basal fasting levels. A flooding dose of L-[4-H-3]phenylalanine was used to measure skeletal muscle protein syn thesis. Whole body amino acid disposal increased progressively as the insulin infusion rate increased, and this response was greater in 7- t han in 26-day-old pigs. Skeletal muscle protein synthesis was stimulat ed by insulin, and this response was maximal at a law insulin infusion rate (30 ng.kg(-0.66).min(-1)). The stimulation of muscle protein syn thesis by insulin mas also greater in 7- than in 26-day-old pigs. Thes e data suggest that muscle protein synthesis is more sensitive to insu lin than whole body amino acid disposal. The results further suggest t hat insulin is a central regulatory factor in the elevated rate of mus cle protein deposition and the increased response of skeletal muscle p rotein synthesis to feeding in the neonate.