Interactions of agonists with an allosteric antagonist at muscarinic acetylcholine M2 receptors.
The interaction of heptane-1,7-bis(dimethyl-3'-phthalimidopropylammonium bromide) (C7/3'-phth), with several agonists, was investigated at the muscarinic M2 receptor in guinea-pig left atria. C7/3'-phth shifted concentration-response curves for the agonists, carbachol, oxotremorine-M and (+)-cis-dioxolane, to the right in a parallel fashion. Arunlakshana-Schild regressions of the data yielded slopes significantly different to unity, suggesting non-competitive antagonism. Non-linear regression analysis, using an equation based on allosteric modulation, gave quantitative estimates of co-operativity (alpha values) and the dissociation constant of C7/3'-phth (KB). In all cases, the KB estimates for C7/3'-phth were not significantly different. Increasing the carbachol contact time 10-fold did not significantly influence the KB or the alpha value obtained with C7/3'-phth. Changing from Krebs to Tyrode solution did not significantly alter the KB for C7/3'-phth, although alpha values obtained were consistently lower in Tyrode solution, suggesting that the allosteric action may be sensitive to buffer composition. A 4-fold higher degree of negative, heterotropic co-operativity between C7/3'-phth and agonists than between C7/3'-phth and competitive antagonists was also found.
Lanzafame
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A
Department of Pharmaceutical Biology and Pharmacology, Victorian College of Pharmacy (Monash University), Parkville, Australia.
Christopoulos
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A
Mitchelson
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F
D000109:Acetylcholine / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001667:Binding, Competitive; D008725:Bis-Trimethylammonium Compounds / Q000378:metabolism / Q000494:pharmacology*; D002021:Buffers; D002217:Carbachol / Q000378:metabolism / Q000494:pharmacology; D004148:Dioxolanes / Q000378:metabolism / Q000494:pharmacology; D004347:Drug Interactions; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D054833:Isoindoles; D007700:Kinetics; D008297:Male; D018721:Muscarinic Agonists / Q000378:metabolism / Q000494:pharmacology*; D018727:Muscarinic Antagonists / Q000378:metabolism / Q000494:pharmacology*; D010095:Oxotremorine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic / Q000187:drug effects* / Q000378:metabolism
D008725:Bis-Trimethylammonium Compounds; D002021:Buffers; D004148:Dioxolanes; D054833:Isoindoles; D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D043585:Receptor, Muscarinic M2; D011976:Receptors, Muscarinic; C011268:heptane-1,7-bis(dimethyl-3'-phthalimidopropylammonium); D010095:Oxotremorine; C042743:oxotremorine M; D002217:Carbachol; D000109:Acetylcholine; C010962:formal glycol