GLUTAMINE UPTAKE AND METABOLISM BY THE ISOLATED TOAD BRAIN: EVIDENCE PERTAINING TO ITS PROPOSED ROLE AS A TRANSMITTER PRECURSOR

Abstract— Hemisections of toad brains, when incubated in a physiological medium containing no glutamine. released considerable amounts of this amino acid into the medium. When glutamine was included in the medium at a concentration of 0.2 mm the net efflux from the tissue was reduced but not totally prevented. Although there was no net uptake of glutamine, the tissue did accumulate [U‐14C]glu‐tamine and some of this labelled glutamine was rapidly metabolized to glutamate, GABA and aspartate. The precursor‐product relationship for the metabolism of glutamine to glutamate differed from the classic single compartment model in that the specific radioactivity of glutamate rose very quickly to approx one‐tenth that of glutamine, but increased slowly thereafter. These data suggest that the [14C]glutamine was taken up into two metabolically distinct compartments and/or that some of the [14C]glutamine was converted to [14C]glutamate during the uptake process. The uptake of [14C]glutamine was diminished when the tissue was incubated in a non‐oxygenated medium or when Na+ was omitted (substituted with sucrose) and K+ was concomitantly elevated. However, on a relative basis, the incorporation of radioactivity into glutamate and GABA was increased by these incubation conditions. The metabolism of glutamine to aspartate was greatly depressed when the tissue was not oxygenated. The glutamate formed from [U‐14C]glutamine taken up by the tissue was converted to GABA at a faster rate than was glutamate derived from [U‐14C]glucose. [U‐14C]gly‐cerol or exogenous [U‐14C]glutamate. This suggests that glutamine was metabolized to GABA selectively; i.e. on a relative basis, glutamine served as a better source of carbon for the synthesis of GABA than did glucose, glycerol or exogenous glutamate. When the brain hemisections were incubated in the normal physiological medium with or without glutamine. there was very little efflux of glutamate, GABA or aspartate from the tissue. However when NaCl was omitted from the medium (substituted with sucrose) and K+ was elevated to 29 miu. a marked efflux of these three amino acids into the medium did occur, and over a period of 160min, the content of each amino acid in the tissue was depleted considerably. When glutamine (0.2 mm) was included in the Na+ deficient‐high K.+ medium, the average amount of glutamate, GABA and aspartate in the tissue plus the medium was greater than when glutamine was not included in the medium. Such data indicate that CNS tissues can utilize glutamine for a net synthesis of glutamate, GABA and aspartate. The results of this study provide further evidence in support of the concept that the functional (transmitter) pools of glutamate and GABA are maintained and regulated in part via biosynthesis from glutamine. One specific mechanism instrumental in regulating the content of glutamate in nerve terminals may be a process of glutamine uptake coupled to deamidation.