Cholinergic Muscarinic Antagonists

Muscarinic receptors are G protein-coupled receptors for acetylcholine at the parasympathetic neuroeffector junction, in autonomic ganglia and in the brain; the cardiac M2 subtype couples to Gi/Go and slows the heart by inhibiting adenylate cyclase. Drugs in this class occupy those receptors without activating them, so parasympathetic tone is withdrawn: heart rate rises, secretions and gut motility fall, and the pupil dilates. A drug belongs here because it blocks the muscarinic rather than the nicotinic acetylcholine receptor.