[3H]batrachotoxinin-A 20-alpha-benzoate binding to sodium channels in rat brain: sensitivity to tetrodotoxin and divalent cations.

The sodium channel blocker tetrodotoxin partially inhibited [3H]batrachotoxinin-A 20-alpha-benzoate binding to rat brain synaptosomes. This inhibition results from a decrease in the apparent affinity of the radioligand, which indicates that sites 1 and 2 of the sodium channel are not independent as thought previously. Computer-assisted data analysis allowed two binding sites for BTX-B to be distinguished. These sites could be differentiated by means of the divalent cations Mg2+ and Ca2+, that inhibit BTX-B binding completely. Tetrodotoxin diminished the inhibition by Mg2+ and vice versa, suggesting a common mechanism of action for the inhibition.