Nakamura Y, Kataoka K
Department of Physiology, Ehime University, School of Medicine, Japan.
J Mol Neurosci. 1993 Winter;4(4):255-62. doi: 10.1007/BF02821557.
Recently, we isolated a novel subcellular fraction of glial plasmalemmal vesicles (GPV), which showed a higher activity of Na(+)-dependent glutamate transport than synaptosomes (Nakamura et al., 1993). In order to study kinetically the glutamate transport mechanism, we measured the reaction under various ionic conditions both inside and outside the vesicles. The vesicles treated hypotonically and preloaded with KCl could take up glutamate in the presence of external Na+. The level of glutamate uptake was dependent on external concentrations of NaCl ([NaCl]o) and competitively inhibited by [KCl]o. However, it was dependent on [KCl]i, and competitively inhibited by [NaCl]i. The activation and inhibition constants of K+ were about 30 mM inside and 20 mM outside, respectively, whereas those of Na+ were 140 mM outside and 4 mM inside, respectively. These results suggest that the transport carrier molecules work asymmetrically to the membranes. Nigericin and monensin, acidic ionophores for K+ and Na+, respectively, inhibited the glutamate uptake. On the other hand, valinomycin, a neutral ionophore for K+, elevated the uptake level, suggesting that the inside-negative membrane potential induced by K+ diffusion enhances the uptake activity. We conclude that glutamate transport by glial cells requires both external Na+ and internal K+ and is regulated by the membrane potential.
最近，我们分离出一种新型的神经胶质细胞质膜囊泡（GPV）亚细胞组分，其显示出比突触体更高的钠依赖性谷氨酸转运活性（中村等人，1993年）。为了从动力学角度研究谷氨酸转运机制，我们在囊泡内外的各种离子条件下测量了反应。经低渗处理并预加载氯化钾的囊泡在外部存在钠离子的情况下能够摄取谷氨酸。谷氨酸摄取水平取决于外部氯化钠浓度（[NaCl]o），并受到[KCl]o的竞争性抑制。然而，它取决于[KCl]i，并受到[NaCl]i的竞争性抑制。钾离子的激活和抑制常数分别在囊泡内部约为30 mM，在外部约为20 mM，而钠离子的激活和抑制常数分别在外部为140 mM，在内部为4 mM。这些结果表明，转运载体分子相对于膜不对称起作用。尼日利亚菌素和莫能菌素分别是钾离子和钠离子的酸性离子载体，它们抑制谷氨酸摄取。另一方面，缬氨霉素是钾离子的中性离子载体，它提高了摄取水平，表明钾离子扩散诱导的膜内负电位增强了摄取活性。我们得出结论，神经胶质细胞的谷氨酸转运需要外部钠离子和内部钾离子，并且受膜电位调节。
