title: A role for M(2) and M(3) muscarinic receptors in the contraction of rat and human small airways.
abstract: Large airway bronchoconstriction acts mainly through cholinergic pathways via muscarinic M3 receptors with some contribution from M2 receptors. By contrast, the mechanisms of small airway contraction are largely unknown. This study used precision cut lung slices to examine the role of muscarinic M2 and M3 receptors in the contractile response of rat and human small airways. In rat small airways, the M3 preferential antagonist, 4-DAMP, inhibited carbachol-mediated contraction (1×10(-6) M) more than that of the M2 selective antagonist, AF-DX116 (pIC50 values: 8.85±0.18 and 6.31±0.19, n=6-8 respectively). Tiotropium, inhibited the contractile response to carbachol with (pIC50: 9.86±0.07, n=6), but could not distinguish between M2 and M3 mediated effects. Similar experiments using human small airways with tiotropium and AF-DX116, gave a pIC50 of 10.35±0.05 and a pKB of 6.37±0.13, n=5 respectively. Therefore, M3 receptors play a key role in muscarinic contraction of small airways in both rats and humans but the effect of M2 receptors cannot be excluded. To investigate the role of M2 receptors, carbachol-induced contraction of small airways was performed in the presence and absence of a β2-agonist in order to elevate intracellular cAMP levels prior to contraction. Isoproterenol-induced relaxation was significantly increased by AF-DX116 (P<0.001) in rat small airways and by AF-DX116 (P<0.01), gallamine (P<0.05) and pertussis toxin (P<0.05) in human small airways. Taken together, these data suggest that cholinergic antagonism of muscarinic receptors in human and rat small airways inhibits airway contraction via direct inhibition of contraction through M3 receptors, and by M2 receptor mediated inhibition of relaxation.
authors: Airways Disease, National Heart and Lung Institute, Imperial College London, Dovehouse Street, London SW3 6LY, UK.
authors: Stephanie M
authors: SM
authors: Brown
authors: Akira
authors: A
authors: Koarai
authors: Richard G
authors: RG
authors: Sturton
authors: Andrew G
authors: AG
authors: Nicholson
authors: Peter J
authors: PJ
authors: Barnes
authors: Louise E
authors: LE
authors: Donnelly
mesh_terms: D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008168:Lung / Q000187:drug effects* / Q000502:physiology; D018727:Muscarinic Antagonists / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects; D010880:Piperidines / Q000494:pharmacology*; D010890:Pirenzepine / Q000031:analogs & derivatives* / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D043585:Receptor, Muscarinic M2 / Q000037:antagonists & inhibitors / Q000502:physiology*; D043587:Receptor, Muscarinic M3 / Q000037:antagonists & inhibitors / Q000502:physiology*; D012602:Scopolamine Derivatives / Q000494:pharmacology; D000069447:Tiotropium Bromide
chemical_list: D018679:Cholinergic Agonists; D018727:Muscarinic Antagonists; D010880:Piperidines; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D012602:Scopolamine Derivatives; D010890:Pirenzepine; C042375:4-diphenylacetoxy-1,1-dimethylpiperidinium; D002217:Carbachol; C048593:otenzepad; D000069447:Tiotropium Bromide