At liver transplantation, the graft undergoes a degree of ischaemia followed by a reperfusion injury. As early detection of impaired graft function is essential, we have monitored amino acid changes in the extracellular compartment of the graft liver by implanting a microdialysis probe at the end of the recipient operation.

With local ethical committee approval, microdialysis probes were inserted into the graft liver of 18 patients at the falciform ligament, with the dialysis membrane placed in segment IV/V. The probe was perfused with an isotonic sterile solution at 0.3 µl/min for a period of 48h. The dialysate fluid was collected at 1 hour intervals, and, in selected samples, concentrations of alanine (ALA), arginine (ARG), citrulline (CIT), -amino-butyric acid (GABA), glutamate (GLU), glutamine (GLN), glycine (GLY) and taurine (TAU) were determined by HPLC with fluorimetric detection. As microdialysis samples were also being analysed for other metabolic markers, amino acid levels were determined in one sample from each of the following time windows (h, following implantation): 1, 2-6, 7-12, 13-18, 19-24, 25-30, 31-36, 37-42, 43-48. To establish time course changes of amino acids, data were analysed by one-way ANOVA, with post-hoc comparison to the value at 2-6h, using Dunnett's test, when significant differences were found (p<0.05). All values shown below are µM, mean ± s.e.mean, n=18.

For CIT, GABA, GLN and GLY, no significant changes were observed. GLU declined progressively (69.0 ±11.4 (2-6h) to 19.9 ±3.0 (43-48h)), reaching significantly lower values from 19h onwards. ALA levels also declined slowly (219.7 ±18.3 (2-6h) to 141.9 ±11.2 (43-48h)), although not reaching significance until 37h post-implantation. TAU also declined from initial high values (59.9 ±4.9 (2-6h)), but far more rapidly than GLU or ALA. Even during the 7-12h period following implantation, TAU levels were significantly lower than during the first 6h (31.7 ±3.2), and from 13h onwards they remained relatively constant (~25 µM). In contrast to the other amino acids studied, extracellular levels of ARG progressively increased, reaching significantly higher values from 19h onwards (19.9 ±2.9 (2-6h) to 48.3 ±4.2 (43-48h)).

Since all grafts in this study functioned well, this data may be seen to represent 'normal' changes in the post-transplant period. The gradual and progressive reduction of extracellular GLU and ALA may reflect recovery from damage produced by implantation of the microdialysis probe. In contrast, the faster kinetics of the reduction in taurine may reflect its role as an osmolyte in order to maintain cell volume homeostasis (Wettstein et al., 2000). Two of the most important metabolic fates of ARG in the liver are in the detoxification of ammonia via the urea cycle, and in the synthesis of nitric oxide. As intracellular ARG levels in the hepatocyte are only around 5 µM, our findings of progressively increasing extracellular ARG levels during the 48 hours after transplant may reflect restriction of arginine influx by the system y⁺ transporter (Inoue et al., 1993) as the organ stabilises.

Inoue Y et al., (1993) Annals of Surgery, 218, 350-363
Wettstein M et al., (2000) American Journal of Physiology, 278, G227-G233