Campbell D T
Hatfield Marine Science Center, Oregon State University, Newport 97365-5296.
Pflugers Arch. 1993 Jun;423(5-6):492-6. doi: 10.1007/BF00374946.
The electrical signals of nerve and muscle are fundamentally dependent on the voltage-gated Na+ channel, which is responsible for the rising phase of the action potential. At least two kinds of Na+ channel are expressed in the membrane of frog dorsal root ganglion (DRG) cells: Na+ channels with fast kinetics that are blocked by tetrodotoxin (TTX) at high affinity, and Na+ channels with slower kinetics that are insensitive to TTX. Recordings of single-channel currents from frog DRG cells, under conditions favoring Na+ as the charge carrier, reveal two distinct amplitudes of single-channel events. With 300 mM external Na+, single-channel events that can be measured in the presence of 1 microM TTX have a slope conductance 7.5 pS. In the absence of TTX, events with a slope conductance of 14.9 pS dominate. Ensemble averages of the smaller single-channel events display the slower kinetics characteristic of the macroscopic TTX-insensitive Na+ currents, and ensemble averages of the larger events display the faster kinetics characteristic of the TTX-sensitive currents. The results are consistent with the idea that the toxin-binding site is sufficiently close to the pore to influence ion permeation.
神经和肌肉的电信号从根本上依赖于电压门控钠通道，该通道负责动作电位的上升阶段。青蛙背根神经节（DRG）细胞的膜上至少表达两种钠通道：具有快速动力学的钠通道，在高亲和力下被河豚毒素（TTX）阻断；以及动力学较慢、对TTX不敏感的钠通道。在有利于钠作为电荷载体的条件下，对青蛙DRG细胞的单通道电流进行记录，揭示了单通道事件的两种不同幅度。在外部钠浓度为300 mM时，在存在1 microM TTX的情况下可测量的单通道事件的斜率电导为7.5 pS。在没有TTX的情况下，斜率电导为14.9 pS的事件占主导。较小单通道事件的总体平均值显示出宏观TTX不敏感钠电流较慢的动力学特征，而较大事件的总体平均值显示出TTX敏感电流较快的动力学特征。这些结果与毒素结合位点足够靠近孔以影响离子渗透的观点一致。
