Gardner-Medwin A R, Nicholson C
J Physiol. 1983 Feb;335:375-92. doi: 10.1113/jphysiol.1983.sp014540.
Ion-selective micro-electrodes have been used to measure K+ and Ca2+ activity changes in extracellular space beneath the surface of the neocortex and cerebellar cortex during current flow across the tissue surface in anaesthetized rats. Inward currents produced decreases of [K+]o and outward currents produced increases, with insignificant changes in [Ca2+]o. Changes of [K+]o were largest just under the surface of the tissue, but were detectable down to depths of ca. 1 mm. With appropriate sitting of electrodes in the cerebellar cortex, currents of 22 microA mm-2 for 400 sec produced changes averaging -42% for inward current and +66% for outward current. The [K+]o changes near the surface were most rapid immediately after the onset of current and more gradual after some tens of seconds. Deeper within the tissue the rate of change was more uniform and after the end of stimulation the return to base line was slower. The amplitude, depth dependence and time course of the [K+]o changes were in reasonable agreement with the results calculated for a model in which K+ moves partly through extracellular space but primarily through membranes and cytoplasm within the tissue. The [K+]o changes were not attributable to variations in neuronal activity, although unit activity could be modified by current, since alternating currents failed to produce [K+]o changes and neither 0.1 mM-tetrodotoxin nor 5 mM-Mn2+ abolished the changes. The [K+]o changes were not abolished by topically applied ouabain (4 X 10(-4) M), 2,4-dinitrophenol (20 mM) or iodoacetate (10 mM), or by asphyxiation. Consequently the [K+]o changes are not dependent on metabolism. The data suggest that there is a selective mechanism for passive K+ transport in an electrochemical gradient within brain tissue that results in higher K+ fluxes than could be supported by ionic mobility in the extracellular fluid. This mechanism exists not only at the surface but within the brain parenchyma and may involve current flow through glial cells.
在麻醉大鼠中，当电流通过组织表面时，离子选择性微电极已被用于测量新皮层和小脑皮层表面下细胞外空间中钾离子（K⁺）和钙离子（Ca²⁺）活性的变化。内向电流使细胞外钾离子浓度（[K⁺]o）降低，外向电流使其升高，而细胞外钙离子浓度（[Ca²⁺]o）变化不显著。[K⁺]o的变化在组织表面正下方最大，但在约1毫米深度处仍可检测到。在小脑皮层中适当放置电极，22微安/平方毫米的电流持续400秒，内向电流产生的变化平均为-42%，外向电流产生的变化平均为+66%。表面附近的[K⁺]o变化在电流开始后立即最为迅速，几十秒后则较为缓慢。在组织更深层，变化速率更为均匀，刺激结束后恢复到基线的速度较慢。[K⁺]o变化的幅度、深度依赖性和时间进程与一个模型计算结果合理相符，该模型中K⁺部分通过细胞外空间移动，但主要通过组织内的膜和细胞质移动。[K⁺]o的变化并非归因于神经元活动的变化，尽管单位活动可被电流改变，因为交流电未能产生[K⁺]o变化，且0.ImM的河豚毒素或5mM的锰离子均未消除这些变化。局部应用哇巴因（4×10⁻⁴M）、2,4-二硝基苯酚（20mM）或碘乙酸盐（10mM），或窒息均未消除[K⁺]o的变化。因此，[K⁺]o的变化不依赖于代谢。数据表明，脑组织中存在一种在电化学梯度中被动转运K⁺的选择性机制，并导致比细胞外液中离子迁移所能支持的更高的K⁺通量。这种机制不仅存在于表面，也存在于脑实质内，可能涉及电流通过神经胶质细胞。
