Insulin stimulates human skeletal muscle protein synthesis via an indirect mechanism involving endothelial-dependent vasodilation and mammalian target of rapamycin complex 1 signaling.
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.


METHODS
Subjects were healthy young adults (n=14) aged 31+/-2 yr.


METHODS
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.
Timmerman
Kyle L
KL
Sealy Center on Aging, Department of Internal Medicine, University of Texas Medical Branch, 301 University Boulevard, Galveston, Texas 77555-0460, USA.
Lee
Jessica L
JL
Dreyer
Hans C
HC
Dhanani
Shaheen
S
Glynn
Erin L
EL
Fry
Christopher S
CS
Drummond
Micah J
MJ
Sheffield-Moore
Melinda
M
Rasmussen
Blake B
BB
Volpi
Elena
E
D000328:Adult; D000704:Analysis of Variance; D001786:Blood Glucose / Q000378:metabolism; D015153:Blotting, Western; D005260:Female; D005268:Femoral Vein / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases; D014664:Vasodilation / Q000187:drug effects*; D019323:omega-N-Methylarginine / Q000494:pharmacology
D001786:Blood Glucose; D007328:Insulin; D047908:Intracellular Signaling Peptides and Proteins; D019323:omega-N-Methylarginine; C546842:MTOR protein, human; D017346:Protein Serine-Threonine Kinases; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases