Hepatic Uptake of Amino Acids Immediately after Liver Transplantation Is Well Preserved Despite Altered Plasma Profiles

Abstract

Background: The liver is one of the principal organs responsible for the uptake and release of amino acids in the body. The ability of the transplanted liver to clear plasma amino acids is associated with a functioning allograft. However, clinical assessment is limited by the inability to access the portal vein postoperatively. Therefore, using a porcine liver transplant model, we examined (1) the plasma levels of amino acids presented to the new hepatic allograft and (2) the capacity of the new allograft to clear these amino acids from the circulation.Materials and Methods: Two groups of commercially bred pigs were studied: a control group (n= 8) underwent laparotomy and a transplanted group (n= 6) underwent orthotopic liver transplantation (LT) using veno-venous bypass. All pigs had catheters placed in the carotid artery and portal and hepatic veins and ultrasonic transit time flow probes placed around the hepatic artery and portal vein. Plasma profiles of 23 amino acids were analyzed by high-pressure liquid chromatography. Hepatic balances of amino acids, using arteriovenous difference techniques coupled with hepatic blood flows, were also analyzed on postoperative day 1.Results: Neither portal vein blood flow (703 ± 74 ml/min vs 666 ± 82 ml/min) nor hepatic artery blood flow (322 ± 43 ml/min vs 209 ± 59 ml/min) was significantly different between the control and the transplanted groups, respectively. The transplanted group had significantly increased plasma levels of alanine (135 ± 13 μmol/l vs 382 ± 72 μmol/l), hydroxyproline (30 ± 5 μmol/l vs 60 ± 9 μmol/l), methionine (25 ± 2 μmol/l vs 55 ± 10 μmol/l), ornithine (36 ± 5 μmol/l vs 141 ± 33 μmol/l), phenylalanine (84 ± 5 μmol/l vs 120 ± 12 μmol/l), threonine (75 ± 9 μmol/l vs 159 ± 27 μmol/l), and tryptophan (17 ± 2 μmol/l vs 31 ± 4 μmol/l). The transplanted group also had significantly decreased plasma levels of isoleucine (122 ± 12 μmol/l vs 85 ± 8 μmol/l) and taurine (71 ± 7 μmol/l vs 35 ± 7 μmol/l). These individual amino acid changes were not accompanied by impairment in the net hepatic amino acid balance or the hepatic fractional extraction of amino acids between the two groups.Conclusion: These results suggest that the circumstances associated with liver transplantation alter the fasting amino acid profile immediately postoperatively. However, liver transplantation does not impair the normal hepatic allograft uptake of most plasma amino acids. Thus, the changes observed in the circulating levels of amino acids may represent alterations in nonhepatic production and/or utilization. Furthermore, altered plasma amino acid profiles following liver transplantation are not necessarily indicative of impaired hepatic allograft amino acid metabolism.