TI - Contribution of insulin to the translational control of protein synthesis in skeletal muscle by leucine . AB - Enhanced protein synthesis in skeletal muscle after ingestion of a balanced meal in postabsorptive rats is mimicked by oral leucine administration . To assess the contribution of insulin to the protein synthetic response to leucine , food-deprived ( 18 h ) male rats ( approximately 200 g ) were intravenously administered a primed-constant infusion of somatostatin ( 60 microg + 3 microg.kg ( -1 ) .h ( -1 ) ) or vehicle beginning 1 h before administration of leucine ( 1.35 g L-leucine/kg ) or saline ( control ) . Rats were killed 15 , 30 , 45 , 60 , or 120 min after leucine administration . Compared with controls , serum insulin concentrations were elevated between 15 and 45 min after leucine administration but returned to basal values by 60 min . Somatostatin maintained insulin concentrations at basal levels throughout the time course . Protein synthesis was increased between 30 and 60 min , and this effect was blocked by somatostatin . Enhanced assembly of the mRNA cap-binding complex ( composed of eukaryotic initiation factors eIF4E and eIF4G ) and hyperphosphorylation of the eIF4E -binding protein 1 (4E-BP1) , the 70-kDa ribosomal protein S6 kinase ( S6K1 ) , and the ribosomal protein S6 ( rp S6 ) were observed as early as 15 min and persisted for atleast 60 min . Somatostatin attenuated the leucine-induced changes in 4E-BP1 and S6K1 phosphorylation and completely blocked the change in rp S6 phosphorylation but had no effect on eIF4G small middle dot eIF4E assembly . Overall , the results suggest that the leucine-induced enhancement of protein synthesis and the phosphorylation states of 4E-BP1 and S6K1 are facilitated by the transient increase in serum insulin . In contrast , assembly of the mRNA cap-binding complex occurs independently of increases in insulin and , by itself , is insufficient to stimulate rates of protein synthesis in skeletal muscle after leucine administration .