The equilibrium interactions of α-bungarotoxin, if-tubocurarine, and carbamylcholine with junctional and extrajunctional skeletal muscle acetylcholine receptors were examined. d-Tubocurarine is a competitive inhibitor of the bindings of α-bungarotoxin to the acetylcholine receptor. No substantive difference was observed in the association of d-tubocurarine with the junctional and extrajunctional receptors. In contrast, the carbamylcholine inhibition of toxin binding is not competitive. The data indicate that either the single set of α-bungarotoxin and d-tubocurarine binding sites contains two subsets of carbamylcholine sites or that the carbamylcholine binds in a cooperative manner to a single set of sites. In addition, the affinity of carbamylcholine for extrajunctional receptors may be higher than the affinity for junctional receptors.
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