It is now recognized that many toxic substances evolved by plants or ani￾mals act by interfering with the excitation process of nerve and muscle
cells. Specifically, these compounds alter the rapid and transient increase
in membrane permeability to Na ions (PNa), that initiates the action
potential. According to the general mode by which toxins modify the Na
permeability mechanism, or Na channels, two groups can be distinguished.
The first group of substances, including tetrodotoxin (TTX) and saxi￾toxin (STX), blocks the flow of Na ions through Na channels. The second
group of toxins maintains an open form of the Na channel, thereby pro￾longing the time during which Na ions flow into the cell. This group com￾prises alkaloids (ceveratrum alkaloids, batrachotoxin, aconitine), the diter￾penoid grayanotoxins, and polypeptides from scorpions, sea anemones
and coral and contains the most potent and most specific substances
known to increase the Na permeability of excitable membranes. Toxins Cardioactive Substances that Prolong the Open State of Sodium Channels 3
which prolong the open state of Na channels will be referred to as Na
channel-gate toxins to denote the specific function of the Na channel
affected by these agents, i.e. the gating mechanism which controls the
activation and inactivation of Na channels.