The metabolic fate of 13N-labeled ammonia in rat brain

13N-labeled ammonia was used to study the cerebral uptake and metabolism of ammonia in conscious rats. After infusion of physiological concentrations of [13N]ammonia for 10 min via one internal carotid artery, the relative specific activities of glutamate, glutamine (alpha-amino), and glutamine (amide) in brain were approximately 1:5:400, respectively. The data are consistent with the concept that ammonia, entering the brain from the blood, is metabolized in a small pool of glutamate that is both rapidly turning over and distinct from a larger tissue glutamate pool (Berl, S., Takagaki, G., Clarke, D.D., and Waelsch, H. (1962) J. Biol. Chem. 237, 2562-2569). Analysis of 13N-metabolites, after infusion of [13N]ammonia into one lateral cerebral ventricle, indicated that ammonia entering the brain from the cerebrospinal fluid is also metabolized in a small glutamate pool. Pretreatment of rats with methionine sulfoximine led to a decrease in the label present in brain glutamine (amide) following carotid artery infusion of [13N]ammonia. On the other hand, 13N activity in brain glutamate was greater than that in the alpha-amino group of glutamine, i.e. following methionine sulfoximine treatment the expected precursor-product relationship was observed, indicating that the two pools of glutamate in the brain were no longer metabolically distinct. The amount of label recovered in the right cerebral hemisphere, 5 s after a rapid bolus injection of [13N]ammonia via the right common carotid artery, was found to be independent of ammonia concentration within the bolus over a 1000-fold range. This finding indicates that ammonia enters the brain from the blood largely by diffusion. In normal rats that were killed by a freeze-blowing technique 5 s after injection of an [13N]ammonia bolus, approximately 60% of the label recovered in brain had already been incorporated into glutamine, indicating that the t1/2 for conversion of ammonia to glutamine in the small pool is in the range of 1 to 3 s or less. The data emphasize the importance of the small pool glutamine synthetase as a metabolic trap for the detoxification of blood-borne and endogenously produced brain ammonia. The possibility that the astrocytes represent the anatomical site of the small pool is considered.