Local anesthetics and histrionicotoxin are allosteric inhibitors of the acetylcholine receptor. Studies of clonal muscle cells.

Measurements in BC3H-1 clonal muscle cells allow the simultaneous estimation of agonist occupation and the coupled permeability response of the nicotinic acetylcholine receptor in the intact cell. The present experiments combine binding and permeability measurements to examine the action of several noncompetitive inhibitors of the receptor. Several local anesthetics and histrionicotoxin (HTX) appear to block activation of the receptor of intact cells by enhancing desensitization in a heterotropic allosteric fashion. Apparent dissociation constants KR and KRv are measured for carbamylcholine binding to low affinity, activatable, and high affinity desensitized receptor states using the assay of competition between carbamylcholine and ‘2SI-labeled a-toxin for surface receptors. KR equals 6 X M and KK 2 X M. Using these measured values, K R and KK, a cooperative two-state model for desensitization is fit to the binding function measured following prolonged exposure of receptor to carbamylcholine and reveals the ratio of high affinity to low affinity states in the absence of agonist. This ratio, or allosteric constant, M is for the receptor in BC3H-1 cells. HTX and dibucaine increase the affinity of the receptor for carbamylcholine more than 30-fold, apparently by increasing the allosteric constant up to 2000-fold. Moreover, these heterotropic inhibitors increase M in a cooperative fashion consistent with their association with at least 2 sites/receptor. These heterotropic inhibitors also accelerate the onset of desensitizaton and block agonistinduced 22Na+ influx at concentrations which increase the allosteric constant. Antagonism of the permeability response is compared with changes in agonist affinity induced by HTX, dibucaine, and QX314 and the parallel changes in both parameters indicate that the three inhibitors have similar mechanisms of action on the acetylcholine receptor in BC3H-1 cells.