Krueger B K, Worley J F, French R J
Ann N Y Acad Sci. 1986;479:257-68. doi: 10.1111/j.1749-6632.1986.tb15574.x.
The block of single, batrachotoxin-activated sodium channels by saxitoxin (STX), tetrodotoxin (TTX), and Ca2+ has been investigated in planar bilayers. All three substances block in a voltage-dependent manner with hyperpolarizing potentials favoring block. Extracellular Ca2+ competitively inhibits binding of STX and relieves STX block. Trimethyloxonium, a carboxyl-methylating agent, eliminates block by STX and TTX and dramatically reduces block by Ca2+. These results suggest that STX, TTX, and Ca2+ compete for a negative site on the outside of the channel. The voltage dependence of block by STX (divalent cation) and TTX (monovalent) was similar (40 mV/e-fold), suggesting that voltage dependence is due to a conformational change in the channel rather than to the toxins entering the membrane electric field to block. A physical model, with an external binding site for toxins and Ca2+ and another site deeper within the electric field (associated with the "selectivity filter") that is accessible to Ca2+ but not toxins, predicts voltage-dependence Ca2+ block without invoking the conformational change needed to explain the voltage dependence of block by TTX and STX.
在平面双层膜中研究了石房蛤毒素（STX）、河豚毒素（TTX）和Ca2+对单个经蛙毒素激活的钠通道的阻断作用。这三种物质均以电压依赖性方式阻断，超极化电位有利于阻断。细胞外Ca2+竞争性抑制STX的结合并解除STX阻断。三甲氧鎓，一种羧基甲基化剂，可消除STX和TTX的阻断，并显著降低Ca2+的阻断作用。这些结果表明，STX、TTX和Ca2+在通道外部竞争一个负性位点。STX（二价阳离子）和TTX（一价阳离子）阻断的电压依赖性相似（40 mV/十倍变化），这表明电压依赖性是由于通道的构象变化，而不是毒素进入膜电场进行阻断。一个物理模型，具有毒素和Ca2+的外部结合位点以及电场中更深的另一个位点（与“选择性过滤器”相关），Ca2+可进入该位点但毒素不能进入，该模型预测了电压依赖性Ca2+阻断作用，而无需调用解释TTX和STX阻断的电压依赖性所需的构象变化。
