title: Electrostatic interactions regulate desensitization of the nicotinic acetylcholine receptor.
abstract: To determine the importance of electrostatic interactions for agonist binding to the nicotinic acetylcholine receptor (AChR), we examined the affinity of the fluorescent agonist dansyl-C6-choline for the AChR. Increasing ionic strength decreased the binding affinity in a noncompetitive manner and increased the Hill coefficient of binding. Small cations did not compete directly for dansyl-C6-choline binding. The sensitivity to ionic strength was reduced in the presence of proadifen, a noncompetitive antagonist that desensitizes the receptor. Moreover, at low ionic strength, the dansyl-C6-choline affinities were similar in the absence or presence of proadifen, a result consistent with the receptor being desensitized at low ionic strength. Similar ionic strength effects were observed for the binding of the noncompetitive antagonist [(3)H]ethidium when examined in the presence and absence of agonist to desensitize the AChR. Therefore, ionic strength modulates binding affinity through at least two mechanisms: by influencing the conformation of the AChR and by electrostatic effects at the binding sites. The results show that charge-charge interactions regulate the desensitization of the receptor. Analysis of dansyl-C6-choline binding to the desensitized conformation using the Debye-Hückel equation was consistent with the presence of five to nine negative charges within 20 A of the acetylcholine binding sites.
authors.affiliation: Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: X Z
authors.initials: XZ
authors.lastname: Song
authors.forename: S E
authors.initials: SE
authors.lastname: Pedersen
mesh_terms: D000818:Animals; D001665:Binding Sites; D002217:Carbachol / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D003619:Dansyl Compounds / Q000493:pharmacokinetics; D004557:Electric Organ / Q000502:physiology; D004996:Ethidium / Q000493:pharmacokinetics; D005456:Fluorescent Dyes; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D018722:Nicotinic Agonists / Q000493:pharmacokinetics; D018733:Nicotinic Antagonists / Q000493:pharmacokinetics; D009994:Osmolar Concentration; D011487:Protein Conformation; D000644:Quaternary Ammonium Compounds / Q000493:pharmacokinetics; D011869:Radioligand Assay; D011978:Receptors, Nicotinic / Q000737:chemistry* / Q000378:metabolism*; D055672:Static Electricity; D014101:Torpedo
chemical_list: D003619:Dansyl Compounds; D005456:Fluorescent Dyes; D018722:Nicotinic Agonists; D018733:Nicotinic Antagonists; D000644:Quaternary Ammonium Compounds; D011978:Receptors, Nicotinic; C027933:(1-(5-dimethylaminonaphthalene)sulfonamido)-n-hexanoic acid-beta-N-trimethylammonium ethyl ester; D002217:Carbachol; D004996:Ethidium