Saxitoxin binding in nerves from walking legs of the lobster Homarus americanus. Two classes of receptors.

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Abstract
The binding of exchange-labeled saxitoxin (STX) to sodium channels has been investigated in the nonmyelinated fibers of the walking leg nerves of the lobster. The properties of the STX binding site differed systematically among the nerves from different walking legs. The equilibrium dissociation constant for STX binding (KSTX) to the front legs is approximately twice that for the binding to the rear legs; the average ratio of KSTX (front): KSTX (rear) from five separate experiments was 1.80 +/- 0.21 (mean +/- SE). The actual KSTX values ranged from 124.0 to 22.7 nM for the front leg nerves and from 8.6 to 12.7 nM for the rear leg nerves. KSTX values for the middle two walking leg nerves fell between those for the front and rear legs. The inhibitory dissociation constant for tetrodotoxin (KTTX), calculated from tetrodotoxin's inhibition of labeled STX binding, was 3.02 +/- 0.27 nM for the front legs and 2.20 +/- 0.33 nM for the rear legs. The ratio KSTX: KTTX was different in the front and rear leg nerves, being 5.5 and 4.2, respectively. The apparent P pKa of the STX receptor also differed between the two legs, being 4.6 +/- 0.3 for the front legs and 5.1 +/- 0.1 for the rear legs. These results demonstrate that one tissue type in one organism can contain different toxin binding sites. The difference in the receptors can be qualitatively accounted for by the location of an additional negative charge near the receptor site of the rear walking leg.