Portal infusion of amino acids is more efficient than peripheral infusion in stimulating liver protein synthesis at the same hepatic amino acid load in dogs.
BACKGROUND
Hepatic glucose uptake is enhanced by the portal delivery of glucose, which creates a negative arterioportal substrate gradient. Hepatic amino acid (AA) utilization may be regulated by the same phenomenon, but this has not been proven.


OBJECTIVE
We aimed to assess hepatic AA balance and protein synthesis with or without a negative arterioportal AA gradient.


METHODS
Somatostatin was infused intravenously, and insulin and glucagon were replaced intraportally at 4- and 3-fold basal rates, respectively, in 3 groups (n = 9 each) of conscious dogs with catheters for hepatic balance measurement. Arterial glucose concentrations were clamped at 9 mmol/L. An AA mixture was infused intravenously to maintain basal concentrations (EuAA), intraportally to mimic the postmeal AA increase (PoAA), or intravenously (PeAA) to match the hepatic AA load in PoAA. Protein synthesis was assessed with a primed, continuous [(14)C]leucine infusion.


RESULTS
Net hepatic glucose uptake in the PoAA condition was < or =50% of that in the EuAA and PeAA conditions (P < 0.05). In the PoAA and PeAA conditions, hepatic intracellular leucine concentrations were 2- to 2.5-fold those in the EuAA condition (P < 0.05); net hepatic leucine uptake and [(14)C]leucine utilization were approximately 2-fold greater (P < 0.05) and albumin synthesis was 30% greater (P < 0.05) in the PoAA condition than in the EuAA and PeAA conditions. Phosphorylation of ribosomal protein S6 [downstream of the mammalian target of Rapamycin complex 1 (mTORC1)] was significantly higher in the PoAA, but not PeAA, condition than in the EuAA condition.


CONCLUSIONS
Portal, but not peripheral, AA delivery significantly enhanced hepatic protein synthesis under conditions in which AAs, glucose, insulin, and glucagon did not differ at the liver, an effect apparently mediated by mTORC1 signaling.
Dardevet
Dominique
D
Department of Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN, USA. dardevdp@clermont.inra.fr
Kimball
Scot R
SR
Jefferson
Leonard S
LS
Cherrington
Alan D
AD
Rémond
Didier
D
DiCostanzo
Catherine A
CA
Moore
Mary Courtney
MC
D000596:Amino Acids / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics*; D000818:Animals; D001786:Blood Glucose / Q000032:analysis / Q000378:metabolism*; D002247:Carbon Isotopes; D002406:Catheterization, Peripheral / Q000379:methods; D018592:Cross-Over Studies; D004285:Dogs; D005934:Glucagon / Q000378:metabolism; D007262:Infusions, Intravenous / Q000379:methods; D007263:Infusions, Parenteral / Q000379:methods*; D007328:Insulin / Q000097:blood; D008099:Liver / Q000378:metabolism*; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes; D011169:Portal Vein; D014176:Protein Biosynthesis / Q000187:drug effects* / Q000502:physiology; D011506:Proteins; D011897:Random Allocation; D013004:Somatostatin / Q000008:administration & dosage / Q000378:metabolism / Q000493:pharmacokinetics*; D058570:TOR Serine-Threonine Kinases; D014157:Transcription Factors
D000596:Amino Acids; D001786:Blood Glucose; D002247:Carbon Isotopes; D007328:Insulin; D046912:Multiprotein Complexes; D011506:Proteins; D014157:Transcription Factors; D013004:Somatostatin; D005934:Glucagon; D000076222:Mechanistic Target of Rapamycin Complex 1; D058570:TOR Serine-Threonine Kinases