Binding properties of sea anemone toxins to sodium channels in the crayfish giant axon.

1. Effects of four different sea anemone toxins from Anthopleura (AP-A and AP-C), Anemonia (ATX II) and Parasicyonis (PaTX), and a scorpion toxin from Leiurus (LqTX) on crayfish giant axons were studied. 2. These toxins slowed the Na channel inactivation process, inducing a maintained Na current during a depolarizing pulse. 3. The binding rates for these toxins markedly decreased under depolarization. The decrease in AP-A binding was mainly derived from an increased dissociation rate under depolarization whereas that in PaTX binding from a reduced association rate. 4. The potential-dependent toxin binding kinetics seemed to be related to the gating mechanism of the Na channel. 5. Competitive bindings between these toxins were demonstrated.