Research on intrinsic anti-ischemic mechanisms : From the standpoint of cell membrane and receptor channel mechanisms

在先前脑损伤的小鼠中，脑缺血后的存活率显著高于假手术小鼠，这表明某些针对缺血的保护因子是在机械损伤的大脑中内在产生的。为了阐明内在抗缺血活性的机制，我们从细胞膜和受体通道机制的角度进行了基础研究。缺氧缺糖(体外缺血)引起的小鼠海马脑片细胞内钙离子浓度升高，可被预先机械微损伤的小鼠抑制。在离损伤部位较近的区域，抑制作用明显。轻度酸中毒通过抑制细胞外钙离子的内流和细胞内钙离子的释放，抑制缺血引起的细胞内钙离子浓度的升高。缺血引起的细胞内Ca~(2+)浓度升高与…的升高有关细胞外谷氨酸浓度增加，说明谷氨酸是随着缺血引起的细胞内钙离子浓度升高引起细胞膜通透性改变而释放的。因此，缺血性神经元损伤的机制可能是异质性的。此外，通过在特定亚基中引入定点突变，研究了具有抗缺血作用的药物，如解离麻醉剂、MK-801、戊巴比妥和丁苯酮对谷氨酸受体通道的作用机制。这些研究表明，某个亚基的第二个通道形成片段对于构成抗缺血药物在谷氨酸受体通道上的阻断位置是至关重要的。在缺乏NMDA受体通道epsilon 1亚单位的小鼠中，通过丧失翻正反射来衡量氯胺酮的麻醉效果比野生型小鼠弱。因此，氯胺酮的麻醉作用可能是通过由epsilon 1亚基组成的NMDA受体通道来实现的。这些研究表明抗缺血机制的异质性和特定亚单位分子参与抗缺血活动。较少

Survival rate following brain ischemia was significantly higher in previously brain-injured mice than in sham-operated mice, suggesting that certain protective factors against ischemia are intrinsically produced in the mechanically injured brain. To elucidate the mechanisms of intrinsic anti-ischemic activities, we projected the basic research from the standpoint of cell membrane and receptor channel mechanisms. The increase in intracellular Ca^<2+> concentrations induced by oxygen/glucose deprivation (ischemia in vitro) was inhibited in hippocampal slices from mice with prior mechanical microinjury at the hippocampal CAl region. The inhibition was remarkable in the areas close to the injury site. Mild acidosis inhibited the ischemia-induced rise in intracellular Ca^<2+> concentrations by attenuating both Ca^<2+> influx from the extracellular space and Ca^<2+> release from intracellular sites. The ischemia-induced rise in intracellular Ca^<2+> concentrations was followed by the increas … More e in extracellular glutamate concentrations, indicating that glutamate was released following the change in the permeability of cell membrane caused by the ischemia-induced rise in intracellular Ca^<2+> concentrations. Thus, the mechanisms of ischemic neuronal damage were likely to be heterogeneous. Furthermore, action mechanisms of drugs having anti-ischemic effects, such as dissociative anesthetics, MK-801, pentobarbital and butyrophenones, on the glutamate receptor channels were investigated by site-directed mutagenesis introduced into a certain subunits. These studies revealed that the second channel-forming segment of a certain subunit was crucial to constitute the block sites of anti-ischemic drugs on the glutamate receptor channel. The anesthetic effects of ketamine measured by loss of righting reflex in mice lacking the epsilon1 subunit of the NMDA receptor channel were weaker than the wild-type mice. Thus, anesthetic effects of ketamine may be mediated by the NMDA recetpor channel composed of the epsilon1 subunit. These research indicated the heterogeneity of anti-ischemic mechanisms and the involvement of a specific subunit molecule in anti-ischemic activities. Less