9-6. TYPES OF CHOLINERGIC AGENTS
a. Direct Acting Agents. Direct acting drugs have molecules that resemble
acetylcholine molecules; thus, they have a direct action on the acetylcholine receptor
sites of the postganglionic synapse. These drugs are usually specific in their site of
action. An example of a direct acting agent is pilocarpine hydrochloride (Isopto-
(1) Pilocarpine hydrochloride (Isopto-Carpine). Pilocarpine hydrochloride
is a direct acting parasympathomimetic. It is used in the treatment of glaucoma. It
causes the contraction of the iris sphincter muscle; this results in miosis (pupil
constriction). Pilocarpine can produce the following side effects: muscle tremors,
unusual increase in perspiration, unusual watering of the mouth, blurred vision, and eye
pain. The patient instilling this medication into the eye should be informed that the drug
could cause a change in his near or distant vision. Therefore, he should ensure that his
vision is clear before he drives or does any jobs that require him to see well.
(2) Bethanecol chloride (Urecholine). Bethanecol chloride is a direct acting
parasympathomimetic. It is used in the treatment of non-obstructive urinary retention.
Bethanecol can produce side effects such as shortness of breath, blurred vision, and
dizziness. This drug should not be administered to patients who have bronchial
asthma. Patients should be instructed to take the drug on an empty stomach (one or
two hours before meals) in order to decrease the probability of having stomach upset.
b. Indirect Acting Agents. Indirect acting agents alter or inhibit the activity of
acetylcholinesterase. Since the activity of acetylcholinesterase is inhibited or altered,
the acetylcholine levels will increase causing cholinergic activity. The indirect acting
agents form a complex with acetylcholinesterase. Based upon the type of complex they
form, the agents are placed into two groups:
(1) Reversible cholinesterase inhibitors. These agents form a temporary
complex with acetylcholinesterase.
(a) Neostigmine (Prostigmin). Neostigmine is a reversible indirect
acting acetylcholinesterase inhibitor. This drug is used in the treatment of myasthenia
gravis, a condition characterized by muscle weakness and fatigue. The drug is also
used to treat urinary bladder atony. Side effects associated with this agent are diarrhea,
abdominal cramps, increased salivation, and increased bronchial secretions.
(b) Physostigmine (Eserine). Physostigmine is a reversible indirect
acting acetylcholinesterase inhibitor. It is used in the treatment of glaucoma. Side
effects associated with the use of physostigmine include loss of bladder control, muscle
weakness, unusual increase in perspiration, blurred vision or change in distant vision,
and headache. The patient using this medication should be warned that it can cause a
change in near or distant vision; therefore, the patient should ensure that his vision is
clear before he drives or performs any job which requires that he see well.