Splanchnic Amino Acid and Glucose Metabolism During Amino Acid Infusion in Dogs

With the organ-balance technique, we studied amino acid and glucose metabolism by hepatic and extrahepatic splanchnic tissues in awake dogs in the postabsorptive state and during a 3-h intravenous amino acid infusion. Dogs received a high (1.4 g/kg body wt, n = 5) or low (0.7 g/kg body wt, n = 8) dose of amino acids. In four of the latter dogs, the dose was delivered into a mesenteric vein. During the basal period there was a net removal of gluconeogenic amino acids (particularly alanine), but not branched-chain amino acids, and a net production of glucose by the liver in all dogs. During this time there was a net removal of glucose and production of alanine by the extrahepatic splanchnic tissues. During either high- or low-dose amino acid infusion, net hepatic glucose release increased; despite this, arterial plasma glucose declined due to an increase in tissue glucose uptake at extrasplanchnic sites. The net amount of glucogenic amino acids removed by the liver during high-dose (9.1 ± 1.0 mmol · kg⁻¹ · 3 h⁻¹) and low-dose (4.8 ± 0.6 mmol · kg⁻¹ · 3 h⁻¹) infusion equaled or exceeded the infused load of these amino acids. In addition, the liver contributed to the net disposal of branched-chain amino acids during high-dose (536 ± 147 μmol · kg⁻¹ · 3 h⁻¹) and low-dose (341 ± 70 μmol · kg⁻¹ · 3 h⁻¹) infusion. During high-dose infusion, extrahepatic splanchnic tissues participated in the net removal of branched-chain amino acids (436 ± 162 ¼mol · kg⁻¹ · 3 h⁻¹) but not glucogenic amino acids, and net alanine production continued (410 ± 91 μmol · kg⁻¹ · 3 h⁻¹). The route (peripheral or mesenteric) did not appear to influence the subsequent disposal of the amino acid load. We conclude that during intravenous amino acid administration in the dog, the splanchnic bed is the major site of net amino acid disposal. The liver contributes to the net disposal of both glucogenic and branched-chain amino acids, whereas extrahepatic splanchnic tissue contributes to the net disposal of only the branched-chain amino acids. Overall, only 10–15% of the total infused amino acids was removed by extrasplanchnic (peripheral) sites, despite increased concentrations of plasma amino acids and elevated plasma insulin concentrations. These tissues may replete nitrogen stores during feeding by mechanisms other than removal of circulating amino acids.