Butt A M, Hargittai P T, Lieberman E M
Department of Physiology, School of Medicine, East Carolina University, Greenville, NC 27858.
Neuroscience. 1990;38(1):175-85. doi: 10.1016/0306-4522(90)90383-f.
The physiological basis of the high selective potassium permeability of the crayfish glial perineurium was studied. The transient spike-like perineurial potential generated in high external [K+] was used as a measure of barrier K+ permeability. The medial giant axon membrane potential was used to monitor interstitial [K+]. Perineurial current-voltage relations of the perineurium were used to measure electrical resistance and to determine changes in K+ conductance of the perineurial barrier. Of a range of cations studied only Rb+, in addition to K+ generated a large transient sheath potential. In some experiments "regenerative" multiple spikes were observed during the continued exposure of the perineurium to high [Rb+]0. This degree of ion selectivity is typical of glial cell membranes and K channels. Barrier conductance increased only very briefly in Rb+; the potential falling rapidly to a steady 5-10 mV. The PCl/PRb and the PCl/PK ratios at the peak transient potential were similar suggesting the permeability site for these cations was the same. The permeability of Rb+ in the plateau phase was significantly lower than K+ suggesting that high [Rb+]0 may act to block K+ channels. The K(+)-selective permeability was reversibly blocked by extracellular Ba2+ at both the peak and the plateau phase, in a concentration-dependent manner. Other K-channel blocking agents, tetraethylammonium ions (10 mM), caesium ions (20 mM), and 3,4-diaminopyridine (0.5 mM) were ineffective. The effect of Ba2+ on the peak potential was similar to the removal of external Ca2+ or exposure to the Ca2(+)-channel blockers, verapamil (10(-4) M) or La3+ (5 mM). The time- and concentration-dependent reversible block of the K+ permeability of the perineurium was consistent with the known action of these agents on voltage-gated Ca2+ channels in nerve and glia. La3+ caused an irreversible decrease in perineurial conductance and K+ influx. Lanthanum titration of the negative charges of glial membranes and mucopolysaccharide matrix of the intercellular space suggest they may be important factors in determining the magnitude of the perineurial leak and paracellular K+ permeability. Electron microscopic examination of La3+ distribution demonstrated a diffusion barrier at the outer layer of perineurial glia. The binding of La3+ at the basolateral membranes of the glial barrier suggested this was the site at which La3+ had its physiological actions. The results suggest that the increase in glial membrane K+ conductance in high [K+]0 was most likely due to voltage-gated Ca2+ and K+ channels and Ca2(+)-activated K+ channels of the membranes of perineurial glia.
研究了小龙虾神经胶质周膜高选择性钾通透性的生理基础。在高细胞外[K⁺]中产生的瞬态尖峰状周膜电位被用作屏障K⁺通透性的指标。内侧巨轴突膜电位用于监测细胞间[K⁺]。周膜的周膜电流-电压关系用于测量电阻并确定周膜屏障K⁺电导的变化。在所研究的一系列阳离子中，除了K⁺外，只有Rb⁺能产生大的瞬态鞘电位。在一些实验中，在周膜持续暴露于高[Rb⁺]₀期间观察到“再生”多个尖峰。这种离子选择性程度是神经胶质细胞膜和K⁺通道的典型特征。Rb⁺中屏障电导仅短暂增加；电位迅速下降至稳定的5 - 10 mV。瞬态电位峰值时的PCl/PRb和PCl/PK比值相似，表明这些阳离子的通透位点相同。平台期Rb⁺的通透性显著低于K⁺，表明高[Rb⁺]₀可能起到阻断K⁺通道的作用。细胞外Ba²⁺在峰值和平台期均以浓度依赖性方式可逆地阻断K⁺选择性通透性。其他K⁺通道阻断剂，四乙铵离子（10 mM）、铯离子（20 mM）和3,4 - 二氨基吡啶（0.5 mM）无效。Ba²⁺对峰值电位的影响类似于去除细胞外Ca²⁺或暴露于Ca²⁺通道阻滞剂维拉帕米（10⁻⁴ M）或La³⁺（5 mM）。周膜K⁺通透性的时间和浓度依赖性可逆阻断与这些药物对神经和神经胶质中电压门控Ca²⁺通道的已知作用一致。La³⁺导致周膜电导和K⁺内流不可逆降低。镧对神经胶质细胞膜和细胞间空间粘多糖基质负电荷的滴定表明，它们可能是决定周膜渗漏和细胞旁K⁺通透性大小的重要因素。对La³⁺分布的电子显微镜检查显示在周膜神经胶质外层有扩散屏障。La³⁺在神经胶质屏障基底外侧膜的结合表明这是La³⁺发挥其生理作用的位点。结果表明，在高[K⁺]₀中神经胶质细胞膜K⁺电导的增加最可能是由于周膜神经胶质细胞膜的电压门控Ca²⁺和K⁺通道以及Ca²⁺激活的K⁺通道。
