Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased elF4F formation

We investigated the protein synthetic response of skeletal muscle to an orally administered dose of leucine given alone or in combination with carbohydrate. Male rats were freely fed (F) or food deprived for 18 h; food- deprived rats were then administered saline (S), carbohydrate (CHO), leucine (L) or a combination of carbohydrate plus leucine (CL). CHO and CL meals were isocaloric and provided 15% of daily energy requirements. L and CL meals each delivered 270 mg leucine. Muscle protein synthesis in S was 65% of F (P < 0.01) 1 h after meal administration. Concomitant with lower rates of protein synthesis, phosphorylation of the translational repressor, eukaryotic initiation factor (elF)4E-binding protein 1 (4E-BP1), was less in S, leading to greater association of 4E-BP1·elF4E, and reduced formation of the active elF4G·elF4E complex compared with F (P < 0.01). Oral administration of leucine (L or CL), but not CHO, restored protein synthesis equal to that in F and resulted in 4E-BP1 phosphorylation that was threefold greater than that of S (P < 0.01). Consequently, formation of 4E-BP1·elF4E was inhibited and elF4G·elF4E was not different from F. The amount of elF4E in the phosphorylated form was greater in S and Clio (P < 0.01) than in all other groups. In contrast, no differences in the phosphorylation state of elF2α or the activity of elF2B were noted among treatment groups. Serum insulin was elevated 2.6- and 3.7-fold in Clio and CL, respectively, but was not different in L, compared with S (P < 0.05). These results suggest that leucine stimulates protein synthesis in skeletal muscle by enhancing elF4F formation independently of increases in serum insulin.