Effects of local anesthetics and histrionicotoxin on the binding of carbamoylcholine to membrane-bound acetylcholine receptor.

The effects of local anesthetics and perhydrohistrionicotoxin on the kinetic mechanism of carbamoylcholine binding to the membrane-bound acetylcholine receptor have been studied by stopped-flow methods. Receptor-enriched membrane fragments from Torpedo californica were reduced and then alkylated by 5-(iodoacetamido)salicylic acid, and the agonist binding kinetics were monitored by the fluorescence changes of this bound probe. The alkylation procedures did not alter the ability of the receptor to mediate agonist-induced cation flux. Preincubation of such modified receptor preparations with saturating concentrations of lidocaine, prilocaine, or dimethisoquin did not significantly affect the equilibrium dissociation constant for carbamoylcholine binding. The multiphasic kinetic signal which accompanies the binding of the agonist was, however, much simplified in the presence of local anesthetics, and the observed kinetics could be described by a mechanism in which a single conformational change follows the formation of the initial complex. Perhydrohistrionicotoxin did not act in the same way as the local anesthetics examined since saturating concentrations did not significantly perturb the agonist binding kinetics.