Sodium channels and block

The fast sodium current is responsible for the upstroke of the action potential in most cardiac cells. With the exception of the sinoatrial node and the atrio-ventricular node, activation of the sodium channel determines excitability and conduction of the impulse throughout the heart. Binding of toxins or drugs to the channel results either in an increase or a decrease of the current. Among the toxins, tetrodotoxin and saxitoxin are well known for their selective block of the Na+ channel; substances such as veratridine, aconitine, batrachotoxin, scorpion and sea anemone toxin exert stimulatory effects. Man-made drugs also may block or enhance the current through Na+ channels. In this contribution the discussion will be restricted to blocking substances.