Extracellular brain glutamate during acute liver failure and during acute hyperammonemia simulating acute liver failure: An experimental study based on in vivo brain dialysis

Hyperammonemia is thought to be important in the pathogenesis of hepatic encephalopathy. However, the mechanism leading to ammonia toxicity is still not known. Since the metabolism of the most important excitatory neurotransmitter, glutamate, is closely linked to that of ammonia, it has been postulated that hyperammonemia lowers the availability of the neurotransmitter glutamate. To study this hypothesis, we used brain dialysis to measure glutamate levels in extracellular cerebral fluid from rabbits with acute ischemic liver failure or acute hyperammonemia. The basal glutamate concentration was found to be increased during both acute liver failure (start of experiments 4.9±1.7 μmol/l; end of experiments 9.5±2.1 μmol/l, n=6, difference p<0.05) and acute hyperammonemia (start of experiments 4.4±1.2 μmol/l; end of experiments 7.3±1.8 μmol/l, n=7, difference p>0.05) (mean±SEM). Both the veratridine- and the potassium-evoked glutamate release were increased during acute liver failure but appeared normal during hyperammonemia. We conclude that during acute liver failure and acute hyperammonemia in the rabbit there is no decreased glutamate availability in the extracellular space of the cortical brain; on the contrary, we found evidence for increased extracellular glutamate concentrations in the cortical brain, which were more pronounced in acute liver failure. Experimental hepatic encephalophathy is thus not due to cerebral glutamate deficiency.