鸡胚晶状体的离子浓度、通量及电学特性

Bassnett S, Becker T M, Beebe D C

Department of Anatomy and Cell Biology, Uniformed Services University of the Health Sciences, Bethesda, MD 20814-4799.

Exp Eye Res. 1992 Aug;55(2):215-24. doi: 10.1016/0014-4835(92)90185-u.

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12.5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mM K+ and 26 mM Cl-. A value for intracellular Na+ of 14 mM was obtained using Na(+)-sensitive microelectrodes. A double-exponential model was used to fit the efflux of 86Rb+, 22Na+, 36Cl- and [3H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K+ over Na+. To test this further, lenses were perfused with AAH containing varying concentrations of K+. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K+, Na+ and Cl- of 1.0, 0.36, 0.51 respectively. Replacing most or all of the Na+ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K+ equilibrium potential predicted by the Goldman model. Including the K+ ionophore valinomycin in the low Na(+)-AAH solutions caused a large increase in 86Rb+ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na+ was not due to voltage or pH inactivation of lens K+ channels.

利用同位素和电学技术对鸡胚胎晶状体的膜特性进行了表征。晶状体含水量相对较高（80%），细胞外空间较大（12.5%）。同位素摄取测量表明，晶状体细胞质中含有118 mM的K⁺和26 mM的Cl⁻。使用对Na⁺敏感的微电极测得细胞内Na⁺值为14 mM。采用双指数模型拟合了⁸⁶Rb⁺、²²Na⁺、³⁶Cl⁻和[³H]甘露醇（一种细胞外空间标记物）从晶状体的流出。用人工房水（AAH）溶液灌注时，胚胎晶状体的膜电位在-20至-40 mV之间。在较老胚胎的晶状体中通常观察到更负的值。还鉴定出一个对哇巴因敏感的成分，它对膜电位的贡献为-7 mV。相对去极化的膜电位表明，晶状体膜对K⁺相对于Na⁺的选择性较弱。为了进一步验证这一点，用含有不同浓度K⁺的AAH灌注晶状体。根据戈德曼模型解释了由此产生的电位变化。戈德曼电位方程的最佳拟合表明，在存在哇巴因的情况下，鸡晶状体膜对K⁺、Na⁺和Cl⁻的相对渗透率分别为1.0、0.36、0.51。在AAH中替换大部分或全部Na⁺只会使膜电位发生微小变化，而不是像戈德曼模型预测的那样朝着K⁺平衡电位大幅超极化。在低Na⁺-AAH溶液中加入K⁺离子载体缬氨霉素会导致⁸⁶Rb⁺流出量大幅增加，但不会导致额外的超极化。这表明膜电位对细胞外Na⁺降低的不敏感性不是由于晶状体K⁺通道的电压或pH失活。