虎螈视杆光感受器Ih通道的离子选择性

Ionic selectivity of Ih channels of rod photoreceptors in tiger salamanders.

作者信息

Wollmuth L P, Hille B

机构信息

Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle 98195.

出版信息

J Gen Physiol. 1992 Nov;100(5):749-65. doi: 10.1085/jgp.100.5.749.

DOI:10.1085/jgp.100.5.749

PMID:1282144

原文链接:https://pmc.ncbi.nlm.nih.gov/articles/PMC2229118/

Abstract

Ionic selectivity of Ih channels of tiger salamander rod photoreceptors was investigated using whole-cell voltage clamp. Measured reversal potentials and the Goldman-Hodgkin-Katz voltage equation were used to calculate permeability ratios with 20 mM K+ as a reference. In the absence of external K+, Ih is small and hard to discern. Hence, we defined Ih as the current blocked by 2 mM external Cs+. Some small amines permeate Ih channels, with the following permeability ratios (PX/PK):NH4+, 0.17; methylammonium, 0.06; and hydrazine, 0.04. Other amines are tially impermeant: dimethylammonium (< 0.02), ethylammonium (< 0.01), and tetramethylammonium (< 0.01). When K+ is the only external permeant ion and its concentration is varied, the reversal potential of Ih follows the Nernst potential for a K+ electrode. Ih channels are also permeable to other alkali metal cations (PX/PK): T1+, > 1.55; K+, 1; Rb+, > 0.55; Na+, 0.33; Li+, 0.02. Except for Na+, the relative slope conductance had a similar sequence (GX/GK): T1+, 1.07; K+, 1; Rb+, 0.37; NH4+, 0.07; Na+, 0.02. Based on permeabilities to organic cations, the narrowest part of the pore has a diameter between 4.0 and 4.6 A. Some permeant cations have large effects on the gating kinetics of Ih channels; however, permeant cations appear to have little effect on the steady-state activation curve of Ih channels. Lowering K+ or replacing K+ with Na+ reduces the maximal conductance of Ih but does not shift or change the steepness of its voltage dependence. With ammonium or methylammonium replacing K+ a similar pattern is seen, except that there is a small positive shift of approximately 10 mV in the voltage dependence.

摘要

利用全细胞膜片钳技术研究了虎螈视杆光感受器Ih通道的离子选择性。以20 mM K⁺为参比，通过测量反转电位并运用戈德曼-霍奇金- Katz电压方程来计算通透率比。在无细胞外K⁺时，Ih电流较小且难以分辨。因此，我们将Ih定义为被2 mM细胞外Cs⁺阻断的电流。一些小胺类可通透Ih通道，其通透率比（PX/PK）如下：NH₄⁺为0.17；甲铵为0.06；肼为0.04。其他胺类基本不通透：二甲铵（<0.02）、乙铵（<0.01）和四甲铵（<0.01）。当K⁺是唯一的细胞外通透离子且其浓度变化时，Ih的反转电位遵循K⁺电极的能斯特电位。Ih通道对其他碱金属阳离子也具有通透性（PX/PK）：Tl⁺，>1.55；K⁺，1；Rb⁺，>0.55；Na⁺，0.33；Li⁺，0.02。除Na⁺外，相对斜率电导具有相似顺序（GX/GK）：Tl⁺，1.07；K⁺，1；Rb⁺，0.37；NH₄⁺，0.07；Na⁺，0.02。基于对有机阳离子的通透性，通道孔最窄部分的直径在4.0至4.6埃之间。一些通透阳离子对Ih通道的门控动力学有较大影响；然而，通透阳离子似乎对Ih通道的稳态激活曲线影响较小。降低K⁺浓度或用Na⁺替代K⁺会降低Ih的最大电导，但不会使其电压依赖性发生偏移或改变其陡峭程度。用铵或甲铵替代K⁺时也观察到类似模式，只是电压依赖性有大约10 mV的小正移。