Conformational effects of volatile anesthetics on the membrane-bound acetylcholine receptor protein: facilitation of the agonist-induced affinity conversion.

The rate of the carbamylcholine-induced affinity conversion of the membrane-bound acetylcholine receptor protein from Torpedo californica is enhanced by pretreatment of the membranes under an atmosphere of 3% halothane or 1% chloroform. The enhancement is much more pronounced in the presence of low rather than high concentrations of carbamylcholine since the volatile anesthetics alter the apparent dissociation constant for carbamylcholine from 17 to 3 microM without affecting the first-order rate constant for the ligand-induced conversion (0.07 s-1). These results indicate that the acetylcholine receptor is assuming a conformational form with intermediate affinity for carbamylcholine in addition to the previously described low- and high-affinity forms. The dissociation constants for carbamylcholine obtained from kinetic studies of the carbamylcholine-induced transition are 3-15-fold lower than those obtained as inhibition constants from the rate of 125I-labeled alpha-bungarotoxin binding to the low-affinity conformer of the acetylcholine receptor protein. This pattern, observed in both the presence and absence of anesthetic, provides further evidence that the acetylcholine receptor has nonequivalent ligand binding sites for carbamylcholine.