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Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033
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 (~200 g) were intravenously administered a
primed-constant infusion of somatostatin (60 µg + 3 µg · kg
1 · h1) 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 at least 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 · 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.
amino acids; eukaryotic initiation factors; rats; somatostatin; time course
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