Selectively inhibits activation of voltage-gated sodium channels (VGSC) (Nav1.5/SCN5A) in rat cardiac myocytes, followed by shifting activated voltage in a depolarizing direction. The binding site on VGSC is suggested to be site 4 located at the extracellular S3-S4 loop of the channel. Also binds to voltage-gated potassium channels (Kv2.1/KCNB1). Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. Belongs to the neurotoxin 10 (Hwtx-1) family. 44 (Jztx-4) subfamily.