Heterocycles in Medicine 171Acetylcholine (ACh)
Acetylcholine mediates two different types of activity at corresponding receptors – muscarinic and nicotinic, which
are related to the pharmacological activities of two natural products, muscarine and nicotine. The fi rst type of action
occurs in nerve synapses and the latter at neuromuscular junctions and peripheral ganglia. The term cholinergic is used
for the general effects of acetylcholine.
Some clinically useful muscarinic agents are simple choline derivatives although the alkaloid pilocarpine is a mus-
carinic agonist used in the treatment of glaucoma. The muscarinic antagonist pirenzepine is an alternative to the H 2
antagonists for the treatment of peptic ulcers and some cholinergic side-effects caused by radiotherapy. The best-
known nicotinic agonist is nicotine! Nicotinic antagonists include the natural quaternary bis-tetrahydroisoquinoline
alkaloid tubocurarine, the paralytic agent from curare (South American Indian arrow poison), which is used in surgery
as a muscle-relaxant. Other muscarinic antagonists include the saturated heterocyclic alkaloid atropine, which has a
number of medical applications, including dilation of the pupil of the eye, but the shorter acting and less toxic analogue
tropicamide is preferred for this purpose in routine ophthalmological examinations.