Muscarinic receptors in lung.

Muscarinic receptors are present on several different cells in lung and are predominantly involved in bronchoconstriction and mucus secretion. Muscarinic receptor activation leads to breakdown of phosphoinositides in airway smooth muscle cell membranes, which leads to intracellular calcium release and contraction. Muscarinic receptor subtypes are now recognised, and muscarinic receptors on cholinergic nerves which inhibit acetylcholine release (auto-receptors) may differ from those on airway smooth muscle. There is evidence that muscarinic receptor density is less in peripheral airways, so that cholinergic drugs may be less effective against small airway obstruction. The possibility that muscarinic receptors function abnormally in airway disease has been considered but is unlikely since the airways are hyperresponsive to several different stimuli, in addition to cholinergic agonists.