Amino acid and glucose metabolism in the postabsorptive state and following amino acid ingestion in the dog

Amino acid and glucose metabolism was studied in nine awake 18-hour fasted dogs with chronic portal, arterial, and hepatic venous catheters before and for three hours after oral ingestion of amino acids. The meal was composed of a crystalline mixture of free amino acid, containing neither carbohydrate nor lipid. Following the amino acid meal, plasma glucose concentration declined slowly and this occurred despite a rise in hepatic glucose release. Portal plasma insulin rose transiently (30 ± 7 to 50 ± 11 μU/mL, P < 0.05) while the increase in portal glucagon was more striking and persisted throughout the study (162 ± 40 to 412 ± 166 pg/mL). Over the three hours following amino acid ingestion, the entire ingested load of glycine, serine, phenylalanine, proline, and threonine was recovered in portal blood as was 80% of the ingested branched chain amino acids (BCAA). The subsequent uptake of these glucogenic amino acids by the liver was equivalent to the amount ingested, while hepatic removal of BCAA could account for disposal of 44% of the BCAA absorbed; the remainder was released by the splanchnic bed. During this time, ongoing gut production of alanine was observed and the liver removed 1,740 ± 170 μmol/kg of alanine, which was twofold greater than combined gut output of absorbed and synthesized alanine. In the postcibal state, the total net flux of alanine and five other glucogenic amino acids from peripheral to splanchnic tissues (1,480 μmol/kg 3 h) exceeded the net movement of branched chain amino acids from splanchnic to peripheral tissues (590 μmol / kg /3 h). We concluded that, in the dog, following ingestion of a mixture of amino acids; (1) gut tissues quantitatively transfer the ingested amino acids to portal blood, and produce alanine at a rate similar to that observed in the basal state, (2) the liver is a major site of disposal of ingested glucogenic and branched chain amino acids, and (3) only BCAAs demonstrate a net release from splanchnic tissues, and this is exceeded by an oppositely directed net flow of glucogenic amino acids from peripheral tissues to the splanchnic bed, indicating that net repletion of muscle nitrogen cannot be accounted for by the escape of BCAAs alone from the splanchnic bed following amino acid ingestion.