Veenstra R D, Wang H Z, Beblo D A, Chilton M G, Harris A L, Beyer E C, Brink P R
Department of Pharmacology, State University of New York Health Science Center at Syracuse 13210, USA.
Circ Res. 1995 Dec;77(6):1156-65. doi: 10.1161/01.res.77.6.1156.
Connexins form a variety of gap junction channels that vary in their developmental and tissue-specific levels of expression, modulation of gating by transjunctional voltage and posttranslational modification, and unitary channel conductance (gamma j). Despite a 10-fold variation in gamma j, whether connexin-specific channels possess distinct ionic and molecular permeabilities is presently unknown. A major assumption of the conventional model for a gap junction channel pore is that gamma j is determined primarily by pore diameter. Hence, molecular size permeability limits should increase and ionic selectivity should decrease with increasing channel gamma j (and pore diameter). Equimolar ion substitution of 120 mmol/L KCl for potassium glutamate was used to determine the unitary conductance ratios for rat connexin40 and connexin43, chicken connexin43 and connexin45, and human connexin37 channels functionally expressed in communication-deficient mouse neuroblastoma (N2A) cells. Comparison of experimental and predicted conductance ratios based on the aqueous mobilities of all ions according to the Goldman-Hodgkin-Katz current equation was used to determine relative anion-to-cation permeability ratios. Direct correlation of junctional conductance with dye transfer of two fluorescein-derivatives (2 mmol/L 6-carboxyfluorescein or 2',7'-dichlorofluorescein) was also performed. Both approaches revealed a range of selectivities and permeabilities for all five different connexins that was independent of channel conductance. These results are not consistent with the conventional simple aqueous pore model of a gap junction channel and suggest a new model for connexin channel conductance and permselectivity based on electrostatic interactions. Divergent conductance and permeability properties are features of other classes of ion channels (eg, Na+ and K+ channels), implying similar mechanisms for selectivity.
连接蛋白形成多种间隙连接通道，这些通道在发育和组织特异性表达水平、跨连接电压对门控的调节以及翻译后修饰，以及单通道电导（γj）方面存在差异。尽管γj存在10倍的差异，但目前尚不清楚连接蛋白特异性通道是否具有独特的离子和分子通透性。间隙连接通道孔的传统模型的一个主要假设是，γj主要由孔径决定。因此，随着通道γj（和孔径）的增加，分子大小的通透性限制应该增加，离子选择性应该降低。用120 mmol/L氯化钾等摩尔离子替代谷氨酸钾，以确定在缺乏通讯的小鼠神经母细胞瘤（N2A）细胞中功能性表达的大鼠连接蛋白40和连接蛋白43、鸡连接蛋白43和连接蛋白45以及人连接蛋白37通道的单通道电导比。根据戈德曼-霍奇金-卡茨电流方程，基于所有离子的水相迁移率，比较实验和预测的电导比，以确定相对阴离子与阳离子的通透性比。还进行了连接电导与两种荧光素衍生物（2 mmol/L 6-羧基荧光素或2',7'-二氯荧光素）染料转移的直接相关性研究。两种方法都揭示了所有五种不同连接蛋白的一系列选择性和通透性，这与通道电导无关。这些结果与间隙连接通道的传统简单水相孔模型不一致，并提出了一种基于静电相互作用的连接蛋白通道电导和通透选择性的新模型。不同的电导和通透性特性是其他类离子通道（如Na+和K+通道）的特征，这意味着存在相似的选择性机制。
