Insulin and amino acids act independently to stimulate protein synthesis in 
skeletal muscle of neonatal pigs, and the responses decrease with development. 
The purpose of this study was to compare the separate effects of fed levels of 
INS and AA on the activation of signaling components leading to translation 
initiation and how these responses change with development. Overnight-fasted 6- 
(n = 4/group) and 26-day-old (n = 6/ group) pigs were studied during 1) 
euinsulinemic-euglycemiceuaminoacidemic conditions (controls), 2) 
euinsulinemic-euglycemichyperaminoacidemic clamps (AA), and 3) 
hyperinsulinemic-euglycemic-euaminoacidemic clamps (INS). INS, but not AA, 
increased the phosphorylation of protein kinase B (PKB) and tuberous sclerosis 2 
(TSC2). Both INS and AA increased protein synthesis and the phosphorylation of 
mammalian target of rapamycin (mTOR), ribosomal protein S6 kinase-1, and 
eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1), and these 
responses were higher in 6-day-old compared with 26-day-old pigs. Both INS and 
AA decreased the binding of 4E-BP1 to eIF4E and increased eIF4E binding to 
eIF4G; these effects were greater in 6-day-old than in 26-day-old pigs. Neither 
INS nor AA altered the composition of mTORC1 (raptor, mTOR, and GbetaL) or 
mTORC2 (rictor, mTOR, and GbetaL) complexes. Furthermore, neither INS, AA, nor 
age had any effect on the abundance of Rheb and the phosphorylation of 
AMP-activated protein kinase and eukaryotic elongation factor 2. Our results 
suggest that the activation by insulin and amino acids of signaling components 
leading to translation initiation is developmentally regulated and parallels the 
developmental decline in protein synthesis in skeletal muscle of neonatal pigs.