Local anaesthetics block noncompetitively the depolarisation of Electrophorus electroplax by cholinergic agonists applied in the bathing medium. The most potent local anaesthetics tested are dimethisoquin (apparent dissociation constant I50 = 2 µM) and dibucaine (I50 = 5 µM). In addition, local anaesthetics decrease the initial rate of the binding of a tritiated α-neurotoxin from Naja nigricollis to membrane fragments purified from Electrophorus and Torpedo electric tissues. They also inhibit competitively the binding of [3H]acetylcholine to the cholinergic receptor site from Torpedo. As in the case of cholinergic effectors, the concentrations of local anaesthetics which decrease by half the initial rate of [3H]α-toxin binding are very similar to their dissociation constants for the receptor site determined by inhibition of [3H]acetylcholine binding. However, these dissociation constants are one to two orders of magnitude larger than their apparent dissociation constants, I50, measured with the isolated electroplax. It is concluded that local anaesthetics bind to the cholinergic receptor site with a low affinity, but that they inhibit the depolarisation of the electroplax by binding at different sites situated on or near the cholinergic receptor protein.
ACKNOWLEDGMENTS We thank Professor P. Boquet for purification and a generous gift of pure α-toxin; Drs. A. Menez, J. L. Morgat and P. Fromageot for its tritiation; Professor P. G. Waser for the gift of muscarone; and the Laboratoire Roger Bellon for the gift of dimethisoquin and prilocaine. We thank Drs. R. L. Baldwin, H. Buc, J. B. Cohen, G. L. Hazelbauer, H. Lester, J. C. Meunier, R. W. Olsen, and R. Sealock for helpful criticism and suggestions and aid in the preparation of the manuscript. We thank Dr. J. Patrick for the privileged communication of a manuscript in publication.
- Copyright ©, 1974, by Academic Press, Inc.