A role for M2 and M3 muscarinic receptors in the contraction of rat and human small airways

Large airway bronchoconstriction acts mainly through cholinergic pathways via muscarinic M-3 receptors with some contribution from M-2 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 M-2 and M-3 receptors in the contractile response of rat and human small airways. In rat small airways, the M-3 preferential antagonist, 4-DAMP, inhibited carbachol-mediated contraction (1 x 10(-6) M) more than that of the M-2 selective antagonist, AF-DX116 (pIC(50) values: 8.85 +/- 0.18 and 6.31 +/- 0.19, n=6-8 respectively). Tiotropium, inhibited the contractile response to carbachol with (pIC(50): 9.86 +/- 0.07, n=6), but could not distinguish between M-2 and M-3 mediated effects. Similar experiments using human small airways with tiotropium and AF-DX116, gave a pIC(50) of 10.35 +/- 0.05 and a pK(B) of 6.37 +/- 0.13, n=5 respectively. Therefore, M-3 receptors play a key role in muscarinic contraction of small airways in both rats and humans but the effect of M-2 receptors cannot be excluded. To investigate the role of M-2 receptors, carbachol-induced contraction of small airways was performed in the presence and absence of a beta(2)-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 M-3 receptors, and by M-2 receptor mediated inhibition of relaxation. (C) 2013 Elsevier B.V. All rights reserved.