Binding of tetrodotoxin to squid nerve fibers. Two kinds of receptors?

The effect of tetrodotoxin on the sodium currents of the squid (Doryteuthis plei and Sepioteuthis sepiodea) giant axons was studied under potential control conditions. The axons were immersed in artificial seawater at 21 degrees C and pH 7.5. When the effect of the toxin is studied in concentrations ranging from 0.1 to 50 nM the Eadie-Haldane plot is not a straight line and indicates that there are two populations of sodium channels open during activity. 19.0 +/- 4.7% of the channels are accociated to receptors with an apparent dissociation constant of 0.11 +/- 0.05 nM and 84.0 +/- 4.1% of the channels are related to receptors having an affinity constant of 4.90 +/- 0.49 nM (nine nerves).