title: Divergence of allosteric effects of rapacuronium on binding and function of muscarinic receptors.
abstract: BACKGROUND
  Many neuromuscular blockers act as negative allosteric modulators of muscarinic acetylcholine receptors by decreasing affinity and potency of acetylcholine. The neuromuscular blocker rapacuronium has been shown to have facilitatory effects at muscarinic receptors leading to bronchospasm. We examined the influence of rapacuronium on acetylcholine (ACh) binding to and activation of individual subtypes of muscarinic receptors expressed in Chinese hamster ovary cells to determine its receptor selectivity.
  
  
  RESULTS
  At equilibrium rapacuronium bound to all subtypes of muscarinic receptors with micromolar affinity (2.7-17 microM) and displayed negative cooperativity with both high- and low-affinity ACh binding states. Rapacuronium accelerated [3H]ACh association with and dissociation from odd-numbered receptor subtypes. With respect to [35S]GTPgammaS binding rapacuronium alone behaved as an inverse agonist at all subtypes. Rapacuronium concentration-dependently decreased the potency of ACh-induced [35S]GTPgammaS binding at M2 and M4 receptors. In contrast, 0.1 microM rapacuronium significantly increased ACh potency at M1, M3, and M5 receptors. Kinetic measurements at M3 receptors showed acceleration of the rate of ACh-induced [35S]GTPgammaS binding by rapacuronium.
  
  
  CONCLUSIONS
  Our data demonstrate a novel dichotomy in rapacuronium effects at odd-numbered muscarinic receptors. Rapacuronium accelerates the rate of ACh binding but decreases its affinity under equilibrium conditions. This results in potentiation of receptor activation at low concentrations of rapacuronium (1 microM) but not at high concentrations (10 microM). These observations highlight the relevance and necessity of performing physiological tests under non-equilibrium conditions in evaluating the functional effects of allosteric modulators at muscarinic receptors. They also provide molecular basis for potentiating M3 receptor-mediated bronchoconstriction.
authors.affiliation: Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic. jakubik@biomed.cas.cz
authors.forename: Jan
authors.initials: J
authors.lastname: Jakubík
authors.forename: Alena
authors.initials: A
authors.lastname: Randáková
authors.forename: Esam E
authors.initials: EE
authors.lastname: El-Fakahany
authors.forename: Vladimír
authors.initials: V
authors.lastname: Dolezal
mesh_terms: D000109:Acetylcholine / Q000378:metabolism*; D000494:Allosteric Regulation / Q000187:drug effects; D000495:Allosteric Site / Q000187:drug effects; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D005260:Female; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D018721:Muscarinic Agonists / Q000494:pharmacology; D019832:N-Methylscopolamine / Q000378:metabolism; D003473:Neuromuscular Nondepolarizing Agents / Q000494:pharmacology*; D011869:Radioligand Assay / Q000379:methods; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D014673:Vecuronium Bromide / Q000031:analogs & derivatives* / Q000494:pharmacology
chemical_list: D018721:Muscarinic Agonists; D003473:Neuromuscular Nondepolarizing Agents; D011976:Receptors, Muscarinic; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D014673:Vecuronium Bromide; C082938:rapacuronium; D000109:Acetylcholine; D019832:N-Methylscopolamine