Cerebral ammonia metabolism in normal and hyperammonemic rats

Abstract

Brain ammonia is generated from many enzymatic reactions, including glutaminase, glutamate dehydrogenase, and the purine nucleotide cycle. In contrast, the brain possesses only one major enzyme for the removal of exogenous ammonia, i.e., glutamine synthetase. Thus, following administration of [13N]ammonia to rats [via either the carotid artery or cerebrospinal fluid (csf)], most metabolized label was in glutamine (amide) and little was in glutamate (plus aspartate). Since blood-and csf-borne ammonia are converted to glutamine largely, if not entirely, in the astrocytes, it is not possible from these types of experiments to predict with certainty the metabolic fate of the bulk of endogenously produced ammonia. By comparing the specific activity of L-[13N]glutamate to that of L-[amine-13N]glutamine following intracarotid [13N]ammonia administration it was concluded that metabolic compartmentation is no longer intact in the brains of rats treated with the glutamine synthetase inhibitor L-methionine-SR-sulfoximine (MSO) and that blood and brain ammonia pools mix in such animals. In MSO-treated animals, recovery of label in brain was low (approximately 20% of controls), and of the label remaining, a prominent portion was in glutamine (amide) (despite an 87% decrease in brain glutamine synthetase activity). These data are consistent with the hypothesis that glutamine synthetase is the major enzyme for metabolism of endogenously--as well as exogenously--produced ammonia. The rate of turnover of blood-derived ammonia to glutamine in normal rat brain is extremely rapid (t1/2 less than or equal to 3 s), but is slowed in the brains of chronically (12-14-wk portacaval-shunted) or acutely (urease-treated) hyperammonemic rats (t1/2 less than or equal to 10 s). The slowed turnover rate may be caused by an increased astrocytic ammonia, decreased glutamine synthetase activity, or both. In the hyperammonemic rat brain, glutamine synthetase is still the only important enzyme for the removal of blood-borne ammonia. Hyperammonemia causes an increase in brain lactate/pyruvate ratios and decreases in brain glutamate and brainstem ATP, consistent with an interference with the malate-aspartate shuttle. In vitro, pathological levels of ammonia also inhibit brain alpha-ketoglutarate dehydrogenase complex and, less strongly, pyruvate dehydrogenase complex. The rat brain does not adapt to prolonged hyperammonemia by increasing its glutamine synthetase activity.(ABSTRACT TRUNCATED AT 400 WORDS)