Na+ Channels with Binding Sites of High and Low Affinity for Tetrodotoxin in Different Excitable and Non‐excitable Cells

Abstract

The properties of interaction of tetrodotoxin with its receptor site on the voltage-sensitive Na+ channel were analyzed by titrating Na+ channels with a tetrodotoxin derivative, [3H]ethylenediamine-tetrodotoxin, and by studying the physiological properties of interaction of the toxin with its receptor from 22Na flux measurements. Cells like mouse neuroblastoma N1E 115 cells, Chinese hamster lung fibroblast CCl 39 cells, embryonic chick cardiomyocytes and chick skeletal myotubes only had 1 family of Na+ channels with high-affinity binding sites (in the nM range) for tetrodotoxin. These Na+ channels were the same ones as those that were activated by the alkaloid and polypeptide toxins that accelerated 22Na+ influx. C9 cells had Na+ channels with low-affinity binding sites for tetrodotoxin. These Na+ channels were also activated by alkaloid and polypeptide toxins (the median inhibitory concentration for tetrodotoxin inhibition of 22Na+ influx through these Na+ channels was 300 nM). Rat myotubes that had differentiated in culture in the absence of neuronal influence had high-affinity binding sites (in the nM range) detected with a 3H-tetrodotoxin derivative and low-affinity binding sites (in the .mu.M range) detected by 22Na+ flux experiments. Only low-affinity binding sites corresponded to Na+ channels that could be activated with alkaloid and polypeptide toxins.