Binds specifically to voltage-gated sodium channels (Nav) (site 3), thereby delaying their inactivation. This toxin retains the greatest capacity to discriminate between the cardiac (Nav1.5/SCN5A) and neuronal sodium channels (2.5 nM versus 120 nM, when electrophysiologically tested and 14 nM versus 400 nM, when tested by ion flux), whereas its paralog Anthopleurin-B has the highest affinity of all anemone toxins for the mammalian sodium channel (PubMed:13806, PubMed:7612595, PubMed:17092528). Its ability to differentiate between cardiac and skeletal channels appears to be associated with domain 4 of the channel (PubMed:9306007). This toxin does not slow or inhibit closed-state inactivation of cardiac sodium channels, but selectively modifies inactivation from the open-state (PubMed:8576699). It does not display phospholipid-binding activities, suggesting that the domain IV S3-S4 linker is located at the extracellular surface and not buried in the phospholipid bilayer (By similarity). Belongs to the sea anemone sodium channel inhibitory toxin family. Type I subfamily.