Muscarinic Receptor Subtypes: Implications for Therapy

At least three subtypes of muscarinic receptor have been localised to human airways. M1-receptors facilitate ganglionic transmission and therefore enhance cholinergic reflexes, M2-receptors are localised to post-ganglionic cholinergic nerve terminals and inhibit the release of acetylcholine and M3-receptors on airway smooth muscle mediate constriction and on submucosal glands mediate increased mucus secretion. M3 or mixed M1/M3-receptor antagonists should be preferable to the non-selective anticholinergic drugs in current use, since they would not increase acetylcholine release from cholinergic nerves.