muscarinic receptor
Muscarinic receptors are the slow, deliberate listeners for acetylcholine. Named after muscarine, a toxin from certain mushrooms that selectively activates them, they sit on the organs the parasympathetic system controls and translate the cholinergic signal into a measured response rather than a lightning-fast one.
They are G-protein-coupled receptors, not ion channels, so they work through second messengers and act over hundreds of milliseconds to seconds. There are five subtypes (M1 to M5). M1, M3, and M5 couple to Gq and tend to be excitatory (for example, glandular secretion and smooth muscle contraction), while M2 and M3 dominate in the heart and many smooth muscles; cardiac M2 receptors couple to Gi and slow the heart.
These receptors are the target of two big drug groups. Muscarinic agonists and cholinesterase inhibitors turn their signaling up, while antimuscarinics (anticholinergics) such as atropine turn it off. Subtype selectivity is the holy grail of design, so that a bladder drug, for instance, can spare the heart and brain.
Atropine blocks cardiac M2 muscarinic receptors, releasing the heart from vagal slowing and speeding it up in symptomatic bradycardia.
Blocking M2 to speed a slow heart.
A handy clinical shorthand: muscarinic blockade produces dry mouth, blurred near vision, fast heart, flushed skin, urinary retention, and confusion. Memorized as dry as a bone, blind as a bat, red as a beet, mad as a hatter.