Novel mechanism of blocking axonal Na+ channels by three macrocyclic polyamine analogues and two spider toxins

1 The mechanism of Na+ channel block by three macrocyclic polyamine derivat ives and two spider toxins was studied with voltage clamp and internal perf usion method in squid axons. 2 All these chemicals specifically block Na+ channels in the open state onl y from the internal surface, and do not affect K+ channels. 3 The blocking effect is enhanced as the depolarizing pulse becomes larger. Blocked channels are unable to shift to the inactivated state. 4 In the case of cyclam and guanidyl-side armed cyclam (G-cyclam), quick re lease of these chemicals from the binding sites is proven by the increase i n the tail current and prolongation of the time course of the off gating cu rrent. On the other hand, in the presence of N-4 and the spider toxins, the ir detachment was delayed significantly. 5 Molecular requirements for the block of Na+ channels by these molecules a re the presence of positive charge and hydrophobicity.