Chain length dependence of the interactions of bisquaternary ligands with the Torpedo nicotinic acetylcholine receptor.
The interactions of a series of bisholine esters [(CH3)3N+CH(2)CH2OCO-(CH2)n-COOCH2CH2N+(CH3)3] with the Torpedo nicotinic acetylcholine receptor have been investigated. In equilibrium binding studies, [3H]-suberyldicholine (n=6) binds to an equivalent number of sites as [3H]-acetylcholine and with similar affinity (KD approximately 15 nM). In competition studies, all bischoline esters examined displaced both radioligands in an apparently simple competitive manner. Estimated dissociation constants (KI) showed clear chain length dependence. Short chain molecules (n<or=2) were of lower affinity (KI's of 150-300 nM), whereas longer ligands (n>6) had high affinity similar to suberyldicholine. Functional responses were measured by either rapid flux techniques using Torpedo membrane vesicles or voltage-clamp analyses of recombinant receptors expressed in Xenopus oocytes. Both approaches revealed that suberyldicholine (EC50 approximately 3.4 microM) is 14-25-fold more potent than acetylcholine. However, suberyldicholine elicited only about 45% of the maximum response of the natural ligand, i.e., it is a partial agonist. The potency of this bischoline series increased with chain length. Whereas the shorter ligands (n<or=3) displayed potencies similar to acetylcholine, longer ligands (n>or=4) had similar (or higher) potency to suberyldicholine. Ligand efficacy had an approximately bell-shaped dependence on chain length and compounds where n<or=3 and >or=8 were very poor partial agonists. Based on estimates of interonium distances, we suggest that bisquaternary ligands can interact with multiple binding sites on the nAChR and, depending on the conformational state of the receptor, these sites are 15-20A apart.
Carter
Chris R J
CR
Department of Pharmacology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7.
Cao
Liren
L
Kawai
Hideki
H
Smith
Peter A
PA
Dryden
William F
WF
Raftery
Michael A
MA
Dunn
Susan M J
SM
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002794:Choline / Q000031:analogs & derivatives / Q000378:metabolism; D008024:Ligands; D018722:Nicotinic Agonists / Q000737:chemistry / Q000494:pharmacology*; D011487:Protein Conformation; D011978:Receptors, Nicotinic / Q000737:chemistry / Q000378:metabolism*; D013329:Structure-Activity Relationship; D013804:Thenoyltrifluoroacetone / Q000494:pharmacology; D014101:Torpedo
D008024:Ligands; D018722:Nicotinic Agonists; D011978:Receptors, Nicotinic; D013804:Thenoyltrifluoroacetone; C009335:subecholine; D002794:Choline; D000109:Acetylcholine