Insulin Stimulates Human Skeletal Muscle Protein Synthesis via an Indirect Mechanism Involving Endothelial-Dependent Vasodilation and Mammalian Target of Rapamycin Complex 1 Signaling

• Published in: The journal of clinical endocrinology and metabolism Vol. 95; no. 8; pp. 3848 - 3857
• Main Authors: Timmerman, Kyle L., Lee, Jessica L., Dreyer, Hans C., Dhanani, Shaheen, Glynn, Erin L., Fry, Christopher S., Drummond, Micah J., Sheffield-Moore, Melinda, Rasmussen, Blake B., Volpi, Elena
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 01.08.2010; The Endocrine Society
• Subjects: Adult; AKT protein; Amino acids; Analysis of Variance; Arginine; Blood flow; Blood Glucose - metabolism; Blotting, Western; Female; Femoral Vein - metabolism; Glucose metabolism; Humans; Initiation factor eIF-4E; Insulin; Insulin - metabolism; Insulin - pharmacology; Intracellular Signaling Peptides and Proteins - metabolism; Kinases; Male; Metabolism; Monomethyl-L-arginine; Muscle, Skeletal - drug effects; Muscle, Skeletal - metabolism; Musculoskeletal system; Nitric oxide; Nitric-oxide synthase; omega-N-Methylarginine - pharmacology; Original; Phosphorylation; Protein biosynthesis; Protein Biosynthesis - drug effects; Protein Serine-Threonine Kinases - metabolism; Protein synthesis; Protein turnover; Proteins; Proteolysis; Rapamycin; Ribosomal Protein S6 Kinases - metabolism; Signal Transduction - drug effects; Skeletal muscle; TOR protein; TOR Serine-Threonine Kinases; Tracers; Vasodilation; Vasodilation - drug effects; Young adults
• ISSN: 0021-972X; 1945-7197; 1945-7197
• DOI: 10.1210/jc.2009-2696

Objective: Our objective was to determine whether endothelial-dependent vasodilation is an essential mechanism by which insulin stimulates human skeletal muscle protein synthesis and anabolism. Subjects: Subjects were healthy young adults (n = 14) aged 31 ± 2 yr. Design: Subjects were studied at baseline and during local leg infusion of insulin alone (control, n = 7) or insulin plus the nitric oxide synthase inhibitor NG-monomethyl-l-arginine (l-NMMA, n = 7) to prevent insulin-induced vasodilation. Methods: We measured skeletal muscle protein metabolism with stable isotope tracers, blood flow with indocyanine green, capillary recruitment with contrast enhanced ultrasound, glucose metabolism with stable isotope tracers, and phosphorylation of proteins associated with insulin (Akt) and amino acid-induced mammalian target of rapamycin (mTOR) complex 1 (mTORC1) signaling (mTOR, S6 kinase 1, and eukaryotic initiation factor 4E-binding protein 1) with Western blot analysis. Results: No basal differences between groups were detected. During insulin infusion, blood flow and capillary recruitment increased in the control (P < 0.05) group only; Akt phosphorylation and glucose uptake increased in both groups (P < 0.05), with no group differences; and mTORC1 signaling increased more in control (P < 0.05) than in l-NMMA. Phenylalanine net balance increased (P < 0.05) in both groups, but with opposite mechanisms: increased protein synthesis (basal, 0.051 ± 0.006 %/h; insulin, 0.077 ± 0.008 %/h; P < 0.05) with no change in proteolysis in control and decreased proteolysis (P < 0.05) with no change in synthesis (basal, 0.061 ± 0.004 %/h; insulin, 0.050 ± 0.006 %/h; P value not significant) in l-NMMA. Conclusions: Endothelial-dependent vasodilation and the consequent increase in nutritive flow and mTORC1 signaling, rather than Akt signaling, are fundamental mechanisms by which insulin stimulates muscle protein synthesis in humans. Additionally, these data underscore that insulin modulates skeletal muscle proteolysis according to its effects on nutritive flow.