Stys P K, Waxman S G, Ransom B R
Department of Neurology, Yale University School of Medicine, New Haven, Connecticut 06510.
J Neurosci. 1992 Feb;12(2):430-9. doi: 10.1523/JNEUROSCI.12-02-00430.1992.
White matter of the mammalian CNS suffers irreversible injury when subjected to anoxia/ischemia. However, the mechanisms of anoxic injury in central myelinated tracts are not well understood. Although white matter injury depends on the presence of extracellular Ca2+, the mode of entry of Ca2+ into cells has not been fully characterized. We studied the mechanisms of anoxic injury using the in vitro rat optic nerve, a representative central white matter tract. Functional integrity of the nerves was monitored electrophysiologically by quantitatively measuring the area under the compound action potential, which recovered to 33.5 +/- 9.3% of control after a standard 60 min anoxic insult. Reducing Na+ influx through voltage-gated Na+ channels during anoxia by applying Na+ channel blockers (TTX, saxitoxin) substantially improved recovery; TTX was protective even at concentrations that had little effect on the control compound action potential. Conversely, increasing Na+ channel permeability during anoxia with veratridine resulted in greater injury. Manipulating the transmembrane Na+ gradient at various times before or during anoxia greatly affected the degree of resulting injury; applying zero-Na+ solution (choline or Li+ substituted) before anoxia significantly improved recovery; paradoxically, the same solution applied after the start of anoxia resulted in more injury than control. Thus, ionic conditions that favored reversal of the normal transmembrane Na+ gradient during anoxia promoted injury, suggesting that Ca2+ loading might occur via reverse operation of the Na+)-Ca2+ exchanger. Na(+)-Ca2+ exchanger blockers (bepridil, benzamil, dichlorobenzamil) significantly protected the optic nerve from anoxic injury. Together, these results suggest the following sequence of events leading to anoxic injury in the rat optic nerve: anoxia causes rapid depletion of ATP and membrane depolarization leading to Na+ influx through incompletely inactivated Na+ channels. The resulting rise in the intracellular [Na+], coupled with membrane depolarization, causes damaging levels of Ca2+ to be admitted into the intracellular compartment through reverse operation of the Na(+)-Ca2+ exchanger. These observations emphasize that differences in the pathophysiology of gray and white matter anoxic injury are likely to necessitate multiple strategies for optimal CNS protection.
哺乳动物中枢神经系统的白质在遭受缺氧/缺血时会受到不可逆损伤。然而，中枢有髓神经纤维束缺氧损伤的机制尚未完全明确。尽管白质损伤依赖于细胞外Ca2+的存在，但Ca2+进入细胞的方式尚未完全阐明。我们利用体外大鼠视神经这一典型的中枢白质束研究缺氧损伤机制。通过定量测量复合动作电位下的面积，以电生理方式监测神经的功能完整性，在标准的60分钟缺氧损伤后，其恢复至对照的33.5±9.3%。在缺氧期间应用Na+通道阻滞剂（TTX，石房蛤毒素）减少通过电压门控Na+通道的Na+内流，可显著改善恢复情况；即使在对对照复合动作电位影响很小的浓度下，TTX也具有保护作用。相反，在缺氧期间用藜芦碱增加Na+通道通透性会导致更严重的损伤。在缺氧之前或期间的不同时间操纵跨膜Na+梯度，会极大地影响最终损伤程度；在缺氧前应用零Na+溶液（用胆碱或Li+替代）可显著改善恢复情况；矛盾的是，在缺氧开始后应用相同溶液会导致比对照更严重的损伤。因此，有利于缺氧期间正常跨膜Na+梯度逆转的离子条件会促进损伤，这表明Ca2+内流可能通过Na+ - Ca2+交换体的反向运作发生。Na+ - Ca2+交换体阻滞剂（苄普地尔、苄amil、二氯苄amil）可显著保护视神经免受缺氧损伤。总之，这些结果提示了大鼠视神经缺氧损伤的以下事件序列：缺氧导致ATP迅速耗竭和膜去极化，进而导致Na+通过未完全失活的Na+通道内流。由此导致的细胞内[Na+]升高，加上膜去极化，通过Na+ - Ca2+交换体的反向运作使细胞内Ca2+水平升高到损伤程度。这些观察结果强调，灰质和白质缺氧损伤病理生理学的差异可能需要多种策略来实现最佳的中枢神经系统保护。
