The use of this toxin as a biochemical probe has elucidated two distinct types of voltage-gated sodium channels present in mammals: tetrodotoxin-sensitive voltage-gated sodium channels (TTX-s Na+ channels) and tetrodotoxin-resistant voltage-gated sodium channels (TTX-r Na+ channels). Tetrodotoxin inhibits TTX-s Na+ channels at concentrations of around 1-10 nM, whereas micromolar concentrations of tetrodotoxin are required to inhibit TTX-r Na+ channels. Nerve cells containing TTX-r Na+ channels are located primarily in cardiac tissue, while nerve cells containing TTX-s Na+ channels dominate the rest of the body.