Vaquero et al.-AASLD2008

Ammonia inhibits in vitro the tricarboxylic acid (TCA) cycle enzyme alpha-ketoglutarate dehydrogenase, potentially explaining the impairment of brain energy metabolism and the increase of brain lactate observed in acute liver failure. It has been hypothesized that the branched-chain amino acid isoleucine has cerebroprotective effects in liver failure, as isoleucine can by-pass the TCA cycle inhibition by providing both acetylCoA and succinylCoA. To test this hypothesis in vivo, 4 groups of male SD rats (300-450 gr) underwent surgical por- tacaval anastomoses (PCA) followed 24 h later by a) sham sur- gery (PCA-Sham) or hepatic artery ligation (PCA-HAL), and b) continuous i.v. infusion of either physiologic saline (SAL) or an isosmolar solution of 135 mM isoleucine (ILE) (both started 6 h after surgery). Cerebral microdialysis (flow: 2 µL/min, mem- brane cut-off: 20.000 Da) was performed in frontal cortex. Brain water content was measured by a gravimetric method, and lactate and ammonia by enzymatic assays. Body temper- ature and glycemia were periodically monitored and maintain within normal values. Precoma and coma were defined as loss of righting and corneal reflexes, respectively. Data were ana- lyzed using 2-way ANOVA. Compared with saline, adminis- tration of isoleucine did not delay the onset of precoma (11.9±3 vs. 11.1±2.4 h, ns) or coma (19.7±1.3 vs. 16.9±5.7 h, ns) in PCA-HAL rats. Brain water content increased in PCA- HAL rats compared to PCA-Sham rats (p<.01) rega^rdless of the infusion of saline or isoleucine. Plasma ammonia increased in PCA-HAL rats compared to PCA-Sham rats (p<.001), and it was not affected by isoleucine administration (assessed at 5 h of infusion and at the time of coma). Extracellular brain ammo- nia and extracellular brain lactate progressively increased in PCA-HAL compared to PCA-Sham rats (p<.01) rega^rdless of saline or isoleucine infusion, peaking at the time of coma. The concentration of isoleucine in cerebral microdialysate decreased in the groups infused with saline (p<.05), was unchanged in PCA-Sham rats infused with isoleucine, and increased significantly in PCA-HAL rats infused with isoleucine. Glucose supplementation requirements were similar in saline- and isoleucine-treated groups. In summary, isoleucine adminis- tration did not prevent encephalopathy, brain edema or the increase of brain lactate in rats with acute liver failure, despite achieving increased cerebral concentrations of the amino acid. These findings suggest that isoleucine cannot by-pass or com- pensate in vivo the potential inhibition of the TCA cycle by ammonia in the brain.