Binds to the sodium channels Nav1.1/SCN1A (EC(50)=165 nM), Nav1.5/SCN5A (EC(50)=103 nM) and Nav1.6/SCN8A (EC(50)=133 nM), thereby delaying their inactivation (PubMed:18342901, PubMed:21802465). Also inhibits Nav1.2/SCN2A, Nav1.3/SCN3A, and Nav1.4/SCN4A, but to a lesser extent (PubMed:21802465). Inhibits Nav1.5 differently from isoforms Nav1.1 and Nav1.6. In Nav1.5 the effect consists in a right-shift of inactivation; whereas in both Nav1.1 and Nav1.6 the effect consists in an incomplete inactivation (PubMed:21802465). Does not show activity on Nav1.7/SCN9A. Belongs to the sea anemone sodium channel inhibitory toxin family. Type I subfamily.