Inhibits sodium channels (Nav) inactivation. Shows two types of inhibitory activities on channels. Inhibition of hNav1.6/SCN8A shows a large increase in the steady-state current component without any increase in the slow component, whereas inhibition of hNav1.1/SCN1A, hNav1.2/SCN2A, hNav1.3/SCN3A and hNav1.7/SCN9A shows a large increase in the slow component with only a small steady-state component (PubMed:20059541). Is 5-fold less potent than beta-PMTX for inducing repetitive action potentials in lobster neuromuscular junctions. Expressed by the venom gland. Does not or very slightly inactivate hNav1.4/SCN4A and hNav1.5/SCN5A sodium channels. Potency in mutagenesis experiments is measured by the concentration of these mutant toxins for inducing repetitive action potentials.