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. Design: 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 [ ¹⁴C]leucine infusion. Results: Net hepatic glucose uptake in the PoAA condition was ≤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 [¹⁴C]leucine utilization were ≈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.