Effects of acute hepatic encephalopathy and in vitro treatment with ammonia on glutamate oxidation in bulk-isolated astrocytes and mitochondria of the rat brain.

The metabolism of [1-14C] glutamate to 14CO2 and the glutamate dehydrogenase (GLDH) activity towards alpha-ketoglutarate (alpha-KG) formation were measured in bulk isolated astrocytes derived from control rats and rats with acute hepatic encephalopathy (HE) induced with thioacetamide. In addition, the effects of in vitro treatment of control and HE astrocytes and non-synaptic mitochondria with toxic (3mM) NH4Cl concentration were followed. [1-14C] glutamate oxidation measured as a whole was identical in control and HE astrocytes and was inhibited by ammonia to the same degree in either fraction. In the presence of a glutamate transamination inhibitor--3mM aminooxyacetic acid (AOA), when only the GLDH-mediated part (25% of total) of the glutamate oxidation remained active, the inhibitory effect of ammonia treatment was much more pronounced in HE astrocytes than in control astrocytes. The ability of non-synaptic mitochondria to utilize glutamate to CO2 was not changed in presence of 3mM NH4Cl, whereas a substantial decrease of CO2 production (about 80%) in both the control and HE preparations was observed in the presence of 3mM AOA. GLDH activity was not at all affected by either of the experimental conditions, both in astrocytes and purified non-synaptic mitochondria. Thus, the inhibition of glutamate oxidation in astrocytes by ammonia and the compounded inhibitory effect of HE, ammonia and AOA appeared to be located beyond the glutamate dehydrogenation step within the tricarboxylic acid cycle.