Sibson N R, Mason G F, Shen J, Cline G W, Herskovits A Z, Wall J E, Behar K L, Rothman D L, Shulman R G
Departments of Molecular Biophysics, Yale University School of Medicine, New Haven, Connecticut, USA.
J Neurochem. 2001 Feb;76(4):975-89. doi: 10.1046/j.1471-4159.2001.00074.x.
The aims of this study were twofold: (i) to determine quantitatively the contribution of glutamate/glutamine cycling to total astrocyte/neuron substrate trafficking for the replenishment of neurotransmitter glutamate; and (ii) to determine the relative contributions of anaplerotic flux and glutamate/glutamine cycling to total glutamine synthesis. In this work in vivo and in vitro (13)C NMR spectroscopy were used, with a [2-(13)C]glucose or [5-(13)C]glucose infusion, to determine the rates of glutamate/glutamine cycling, de novo glutamine synthesis via anaplerosis, and the neuronal and astrocytic tricarboxylic acid cycles in the rat cerebral cortex. The rate of glutamate/glutamine cycling measured in this study is compared with that determined from re-analysis of (13)C NMR data acquired during a [1-(13)C]glucose infusion. The excellent agreement between these rates supports the hypothesis that glutamate/glutamine cycling is a major metabolic flux ( approximately 0.20 micromol/min/g) in the cerebral cortex of anesthetized rats and the predominant pathway of astrocyte/neuron trafficking of neurotransmitter glutamate precursors. Under normoammonemic conditions anaplerosis was found to comprise 19-26% of the total glutamine synthesis, whilst this fraction increased significantly during hyperammonemia ( approximately 32%). These findings indicate that anaplerotic glutamine synthesis is coupled to nitrogen removal from the brain (ammonia detoxification) under hyperammonemic conditions.
（i）定量确定谷氨酸/谷氨酰胺循环对星形胶质细胞/神经元底物转运以补充神经递质谷氨酸的总贡献；（ii）确定回补通量和谷氨酸/谷氨酰胺循环对总谷氨酰胺合成的相对贡献。在这项体内和体外研究中，使用[2-(13)C]葡萄糖或[5-(13)C]葡萄糖输注的(13)C NMR光谱法，来测定大鼠大脑皮层中谷氨酸/谷氨酰胺循环速率、通过回补途径的从头合成谷氨酰胺速率以及神经元和星形胶质细胞的三羧酸循环速率。将本研究中测得的谷氨酸/谷氨酰胺循环速率与对[1-(13)C]葡萄糖输注期间获取的(13)C NMR数据重新分析所确定的速率进行比较。这些速率之间的高度一致性支持了以下假设：谷氨酸/谷氨酰胺循环是麻醉大鼠大脑皮层中的主要代谢通量（约0.20微摩尔/分钟/克），并且是星形胶质细胞/神经元转运神经递质谷氨酸前体的主要途径。在正常氨血症条件下，发现回补途径占总谷氨酰胺合成的19 - 26%，而在高氨血症期间这一比例显著增加（约32%）。这些发现表明，在高氨血症条件下，回补性谷氨酰胺合成与大脑中的氮清除（氨解毒）相关联。
