Phrixotoxin-2. A Potent Blocker of Kv4 Channels . (k)-theraphotoxin-Ps1b, (k)-TRTX-Ps1b, PaTX2. Phrixotoxin-2 is a natural peptide isolated from the Phrixotrichus auratus Tarantula venom and purified to homogeneity. Phrixotoxin-2 blocks specifically and reversibly Kv4.2 and Kv4.3 channels. These channels are probably the molecular counterparts of the A-type current (Ito1) in the heart. Cloned Kv4.2 and Kv4.3 channels heterologously expressed in COS cell were blocked with IC50 of 34 and 71 nM of Phrixotoxin-2 respectively. Ion Channel Modulators; K+ Channel Blockers.