Interaction of the hexamethonium derivative W84, an allosteric effector at muscarinic cholinoreceptors, with rat brain nicotinic binding sites.
The hexamethonium derivative W84 (hexamethylene-bis-[dimethyl-(3-phthalimidopropyl)-ammonium bromide]) combined with atropine has an overadditive protective action against organophosphorus intoxications. It affects allosterically the binding of (-) [3H]N-methylscopolamine [(3H]NMS) to muscarinic cholinoceptors. Because nicotinic receptors are involved in organophosphorus intoxications, the interaction of W84 with nicotinic cholinoceptors was investigated. (-) [3H]nicotine (2.5 nM) was used to label nicotinic binding sites in rat brain membranes in 50 nM Tris, pH 7.3 at 23 degrees. Under control conditions, (-) [3H]nicotine-binding revealed a KD of 4 X 10(-9) M and a Bmax of 53 fmol/mg membrane protein. W84 inhibited (- ) [3H]nicotine-binding with an IC50 of 3 X 10(-5) M by reducing the binding affinity. The IC50 of hexamethonium was 20 X 10(-5) M. At 10(-4) M, W84 did not affect the dissociation rate of (-)[3H]nicotine, suggesting a lack of allosteric activity. For sake of comparison, the action of W84 was checked on [3H]NMS-binding (control: KD approximately 1 X 10(-9) M, Bmax approximately 500 fmol/mg prot). W84 inhibited the binding of [3H]NMS (0.5 nM) with an IC50 of 1.5 X 10(-9) M. At 10(-4) M, W84 prevented [3H]NMS-dissociation almost completely, thus displaying the allosteric action at muscarinic cholinoceptors. In conclusion, the results of the (-)[3H]nicotine-binding experiments point to a pure competitive action of W84 at nicotine cholinoceptors, lacking any allosteric effect. This competitive action may contribute to the protective effect of W84 in organophosphorus poisoning.
Leithäuser
F
F
Department of Pharmacology, University of Kiel, FRG.
Mohr
K
K
Staschen
C M
CM
D000818:Animals; D000931:Antidotes / Q000494:pharmacology*; D001665:Binding Sites; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism* / Q000648:ultrastructure; D054833:Isoindoles; D007700:Kinetics; D008297:Male; D019832:N-Methylscopolamine; D009538:Nicotine / Q000378:metabolism*; D010276:Parasympatholytics / Q000378:metabolism; D010797:Phthalimides / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011950:Receptors, Cholinergic / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism; D012602:Scopolamine Derivatives / Q000378:metabolism; D014316:Tritium
D000931:Antidotes; D054833:Isoindoles; D010276:Parasympatholytics; D010797:Phthalimides; D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D012602:Scopolamine Derivatives; D014316:Tritium; C013017:hexamethylenebis(dimethyl-(3-phthalimidopropyl)ammonium bromide); D009538:Nicotine; D019832:N-Methylscopolamine