Using tracer methods,
insulin stimulates
muscle protein synthesis in vitro, an effect not seen in vivo with physiological
insulin concentrations in adult animals or humans. To examine the action of physiological
hyperinsulinemia on
protein synthesis using a tracer-independent method in vivo and identify possible explanations for this discrepancy, we measured the phosphorylation of
ribosomal protein S6 kinase (P70(S6k)) and eIF4E-binding
protein (eIF4E-BP1), two key
proteins that regulate messenger
ribonucleic acid translation and
protein synthesis. Postabsorptive healthy adults received either a 2-h
insulin infusion (1 mU/min.kg; euglycemic
insulin clamp; n = 6) or a 2-h saline infusion (n = 5). Vastus lateralis muscle was biopsied at baseline and at the end of the infusion period. Phosphorylation of P70(S6k) and eIF4E-BP1 was quantified on Western blots after SDS-PAGE. Physiological increments in plasma
insulin (42 +/- 13 to 366 +/- 36 pmol/L; P: = 0.0002) significantly increased p70(S6k) (P: < 0.01), but did not affect eIF4E-BP1 phosphorylation in muscle. Plasma
insulin declined slightly during saline infusion (P: = 0.04), and there was no change in the phosphorylation of either p70(S6k) or eIF4E-BP1. These findings indicate an important role of physiological
hyperinsulinemia in the regulation of p70(S6k) in human muscle. This finding is consistent with a potential role for
insulin in regulating the synthesis of that subset of
proteins involved in ribosomal function. The failure to enhance the phosphorylation of eIF4E-BP1 may in part explain the lack of a stimulatory effect of physiological
hyperinsulinemia on bulk
protein synthesis in skeletal muscle in vivo.