Differential effects of insulin on peripheral and visceral tissue protein synthesis in neonatal pigs

We recently demonstrated in neonatal pigs that, with amino acids and glucos e maintained at fasting levels, the stimulation of protein synthesis in lon gissimus dorsi muscle with feeding can be reproduced by a physiological ris e in insulin alone. In the current report, we determine whether the respons e of protein synthesis to insulin in the neonatal pig is 1) present In musc les of different fiber types, 2) proportional in myofibrillar and sarcoplas mic proteins, 3) associated with increased translational efficiency and rib osome number, and 4) present in other peripheral tissues and in viscera. Hy perinsulinemic-euglycemic-amino acid clamps were performed in 7- and 26-day -old pigs infused with 0, 30, 100, or 1,000 ng.kg(-0.66).min(-1) of insulin to reproduce insulin levels present in fasted, fed, refed, and supraphysio logical conditions, respectively. Tissue protein synthesis was measured usi ng a flooding dose of L-[4-H-3]phenylalanine. Insulin increased protein syn thesis in gastrocnemius muscle and, to a lesser degree, masseter muscle. Th e degree of stimulation of protein synthesis by insulin was similar in myof ibrillar and sarcoplasmic proteins. Insulin increased translational efficie ncy but had no effect on ribosome number in muscle. AU of these insulin-ind uced changes in muscle protein synthesis decreased with age. Insulin also s timulated protein synthesis in cardiac muscle and skin but not in liver, in testine, spleen, pancreas, or kidney. The results support the hypothesis th at insulin mediates the feeding-induced stimulation of myofibrillar and sar coplasmic protein synthesis in muscles of different fiber types in the neon ate by increasing the efficiency of translation. However, insulin does not appear to be involved in the feeding-induced stimulation of protein synthes is in visceral tissues. Thus different mechanisms regulate the growth of pe ripheral and visceral tissues in the neonate.