Effect of various cations and anions on the action of tetrodotoxin and saxitoxin on frog myelinated nerve fibers.

The influence of Mg2+, La3+, NO-3, and SCN- on the equilibrium effect of tetrodotoxin (TTX) and saxitoxin (STX) on single myelinated nerve fibres of the frog Rana esculenta was studied under voltage clamp conditions. Mg2+ and La3+ reduce the sodium permeability, shift the voltage dependence of the Na permeability PNa towards more positive potentials and reduce the effectiveness of TTX and STX. NO-3 and SCN- reduce the sodium permeability too, but shift the voltage dependence of PNa towards more negative potentials and increase the action of TTX and STX. In all experiments the change in effectiveness is larger for the divalent STX than for the monovalent TTX. It is concluded that changes of the monovalent TTX. It is concluded that changes of the external surface potential induced by Mg2+, La3+, NO-3 and SCN- affect the TTX and STX binding to toxin receptors. The apparent potential change at the toxin receptor is only a fraction of the change "seen' by the Na channel gates.