Arens J, Stabel J, Heinemann U
Institute of Neurophysiology, University at Cologne, Köln, Germany.
Can J Physiol Pharmacol. 1992;70 Suppl:S194-205. doi: 10.1139/y92-263.
We have studied extracellular ionic changes induced by iontophoretic application of excitatory amino acids in rat hippocampal slices. In contrast to kinetics of changes in [Ca2+]o, kinetics of changes in [K+]o, [Na+]o, [Cl-]o as well as in extracellular space size were comparable for different glutamate receptor agonists. Thus, alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA), quisqualate (quis), and kainate caused reductions in [Ca2+]o followed by an increase of [Ca2+]o above baseline, whereas glutamate, aspartate, N-methyl-D-aspartate (NMDA), and DL-homocysteic acid caused only reductions in [Ca2+]o. After blocking the NMDA receptors with ketamine and 2-amino-5- phosphonovaleric acid (2-APV), glutamate-induced decreases in [Ca2+]o were followed by an overshoot. Reduction of the transmembrane Na+ gradient by lowering [Na+]o, blocking of the Na(+)-K+ ATPase by lowering [K+]o, and application of ouabain blocked the overshoots after quis application, whereas vanadate, a blocker of the Ca(2+)-Mg2+ ATPase, had no effects. Lithium enhanced the reductions in [Ca2+]o and blocked the overshoots. Amiloride also reduced the overshoots. All organic Ca2+ entry blockers diminished reductions of [Ca2+]o but increased the overshoots. Inorganic Ca2+ antagonists had variable effects. Ni2+ had similar effects as the organic Ca2+ entry blockers while Cd2+ reduced both the [Ca2+]o decreases as well as the subsequent overshoots. Co2+ had initially a similar action as Ni2+. With prolonged application, [Ca2+]o decreases became augmented and, during wash, overshoots could no longer be elicited. We suggest that the overshoots in [Ca2+]o are due to a combined effect of extracellular space shrinkage and activation of the Na+/Ca2+ exchangers. This would imply that NMDA receptor activation blocks extrusion of Ca2+ from the cells. We tested the hypothesis that quis-induced intracellular Ca2+ release and extrusion of Ca2+ from the cells contributed to the overshoots. Dantrolene was without effect on the quis-induced signals, while ryanodine reduced the overshoots. Caffeine on the other hand diminished the [Ca2+]o decreases with no effects on the overshoots. To test for possible second messenger routes by which NMDA receptor activation might slow Ca2+ extrusion from cells, we investigated the effects of arachidonic acid and N-monomethyl-D- arginine on the quis-induced signals. While these agents reduced decreases in [Ca2+]o, they had no clear effects on the overshoots. Thus a possible route by which NMDA receptor activation may affect Ca2+ extrusion from cells has still to be elucidated.
我们研究了离子电渗法施加兴奋性氨基酸在大鼠海马切片中诱导的细胞外离子变化。与[Ca2+]o变化的动力学不同，不同谷氨酸受体激动剂引起的[K+]o、[Na+]o、[Cl-]o以及细胞外空间大小变化的动力学是可比的。因此，α-氨基-3-羟基-5-甲基异恶唑丙酸（AMPA）、quisqualate（quis）和海人藻酸导致[Ca2+]o降低，随后[Ca2+]o升高至基线以上，而谷氨酸、天冬氨酸、N-甲基-D-天冬氨酸（NMDA）和DL-高胱氨酸仅导致[Ca2+]o降低。用氯胺酮和2-氨基-5-磷酸戊酸（2-APV）阻断NMDA受体后，谷氨酸诱导的[Ca2+]o降低后出现超调。通过降低[Na+]o降低跨膜Na+梯度、通过降低[K+]o阻断Na(+)-K+ ATP酶以及应用哇巴因可阻断quis应用后的超调，而钒酸盐（一种Ca(2+)-Mg2+ ATP酶的阻断剂）则无作用。锂增强了[Ca2+]o的降低并阻断了超调。氨氯地平也减少了超调。所有有机Ca2+进入阻断剂减少了[Ca2+]o的降低，但增加了超调。无机Ca2+拮抗剂有不同的作用。Ni2+与有机Ca2+进入阻断剂有相似的作用，而Cd2+既降低了[Ca2+]o的降低，也降低了随后的超调。Co2+最初与Ni2+有相似的作用。随着长时间应用，[Ca2+]o降低加剧，在冲洗过程中，不再能引发超调。我们认为[Ca2+]o的超调是由于细胞外空间收缩和Na+/Ca2+交换体激活的联合作用。这意味着NMDA受体激活会阻断Ca2+从细胞中挤出。我们测试了quis诱导的细胞内Ca2+释放和Ca2+从细胞中挤出导致超调的假设。丹曲林对quis诱导的信号无作用，而ryanodine减少了超调。另一方面，咖啡因减少了[Ca2+]o的降低，对超调无作用。为了测试NMDA受体激活可能减缓Ca2+从细胞中挤出的可能的第二信使途径，我们研究了花生四烯酸和N-单甲基-D-精氨酸对quis诱导的信号的影响。虽然这些试剂减少了[Ca2+]o的降低，但它们对超调没有明显影响。因此，NMDA受体激活可能影响Ca2+从细胞中挤出的可能途径仍有待阐明。
