Szerb J C, O'Regan P A
Department of Physiology and Biophysics, Dalhousie University, Halifax, Canada.
Synapse. 1987;1(3):265-72. doi: 10.1002/syn.890010308.
It is known that low glucose concentrations increase the aspartate and decrease the glutamate content of brain tissue both in vivo and in vitro. To see whether these changes occur in the transmitter compartment or not, the release of aspartate and glutamate evoked by electrical-field stimulation or by high K+ was followed in slices of rat hippocampus superfused with 5 or 0.2 mM glucose. Superfusion with 0.2 mM glucose increased the evoked release of aspartate about ten times and that of glutamate about threefold. This shift in the ratio of aspartate to glutamate released was accompanied by a similar increase in the relative amount of aspartate contained in the slices. The high evoked release of aspartate and glutamate was well maintained, provided 0.5 mM glutamine was added to the medium. Changing the concentration of glucose after the first period of stimulation rapidly altered the relative amounts of aspartate and glutamate released but not the enhanced release of glutamate. The large evoked release of both aspartate and glutamate in 0.2 mM glucose was almost entirely Ca2+-dependent. The relative amounts of aspartate and glutamate released by 50 mM K+ also changed when the glucose concentration was reduced. Results suggest two effects of low glucose concentrations: an increase in the overflow of synaptically released glutamate due to a decreased uptake and an increase in the proportion of aspartate to glutamate formed and released from the transmitter pool. These observations are consistent with the interpretation that these two transmitters can be released in different proportions from the same terminals.
已知低葡萄糖浓度在体内和体外均会增加脑组织中天冬氨酸的含量并降低谷氨酸的含量。为了探究这些变化是否发生在递质区室，在分别用5 mM或0.2 mM葡萄糖进行灌流的大鼠海马切片中，监测电场刺激或高钾诱发的天冬氨酸和谷氨酸的释放情况。用0.2 mM葡萄糖灌流可使诱发的天冬氨酸释放量增加约十倍，谷氨酸释放量增加约三倍。天冬氨酸与谷氨酸释放比例的这种变化伴随着切片中天冬氨酸相对含量的类似增加。如果向培养基中添加0.5 mM谷氨酰胺，天冬氨酸和谷氨酸的高诱发释放可得到良好维持。在第一阶段刺激后改变葡萄糖浓度会迅速改变天冬氨酸和谷氨酸的相对释放量，但不会改变谷氨酸的增强释放。在0.2 mM葡萄糖中，天冬氨酸和谷氨酸的大量诱发释放几乎完全依赖于Ca2+。当葡萄糖浓度降低时，50 mM钾诱发释放的天冬氨酸和谷氨酸的相对量也会发生变化。结果表明低葡萄糖浓度有两种作用：由于摄取减少导致突触释放的谷氨酸溢出增加，以及从递质池中形成和释放的天冬氨酸与谷氨酸的比例增加。这些观察结果与以下解释一致，即这两种递质可以从相同的终末以不同比例释放。
