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Case: Muscarinic cholinomimetic agents

A 61-year-old man is noted to have increased intraocular pressure on a routine eye examination. The visual acuity is normal in both eyes. The dilated eye examination reveals no evidence of optic nerve damage. Visual field testing shows mild loss of peripheral vision. He is diagnosed with primary open-angle glaucoma and is started on pilocarpine ophthalmic drops.

What is the action of pilocarpine on the muscles of the iris and cilia?

What receptor mediates this action?

Is pilocarpine the appropriate first-line drug for treatment of primary open-angle glaucoma?

Answers to case: Muscarinic cholinomimetic agents

Summary: A 61-year-old man with open-angle glaucoma is prescribed pilo-carpine ophthalmic drops.

Action of pilocarpine on muscles of the iris and cilia: Constriction of the muscles

Receptor that mediates this action: Muscarinic cholinoreceptor

First-line drugs to treat primary open-angle glaucoma: Prostaglandin analogs

Clinical correlation

Open-angle glaucoma is a disease caused by obstruction of the outflow of aqueous humor into the canal of Schlemm, causing an increase in intraocular pressure. The use of a direct-acting muscarinic agonist, such as pilocarpine, causes contraction of the muscles of the cilia and iris. Because these are circular muscles, the pupil is constricted, which helps to relieve the outflow obstruction and lower the intraocular pressure. Although not common with the use of topical ophthalmic drops, bronchospasm and pulmonary edema has been noted with the use of pilocarpine drops. More commonly, blurred vision and myopia (nearsightedness) occur as a result of the impairment of accommodation caused by the contraction of the iris and ciliary muscles.

The use of a direct-acting muscarinic agonist such as pilocarpine to treat open-angle glaucoma is now not common due to its numerous side effects, the need to administer it up to four times per day, and the availability of other agents. Prostaglandin analogs such as latanoprost are now considered first-line therapy for this condition followed by β-adrenoceptor agonists.

Approach to muscarinic cholinomimetic agents

Objectives

1. Be able to list the receptors of the parasympathetic nervous system.

2. Contrast the actions and effects of direct and indirect stimulation of muscarinic cholinoreceptors.

3. List the therapeutic uses of parasympathomimetic agents.

4. List the adverse effects of parasympathomimetic agents.

Definitions

Parasympathetic nervous system: An anatomic division of the autonomic nervous system (the other is the sympathetic nervous system) that originates in nuclei of the CNS. Preganglionic fibers exit through cranial and sacral spinal nerves to synapse via short postganglionic nerve fibers on ganglia, many of which are in the organs they innervate.

Cholinomimetic agents: Agents that mimic the action of ACh. These act directly or indirectly to activate cholinoreceptors. Some directly acting agents (pilocarpine, bethanechol, carbachol) are designed to act selectively on either muscarinic or nicotinic cholinoreceptors, whereas indirectly acting agents (such as neostigmine, physostigmine, edrophonium, demecarium), which inhibit the enzyme acetylcholinesterase (AChE) that is responsible for the inactivation of ACh, can activate both. Pilocarpine is a directly acting cholinomimetic agent that acts chiefly at muscarinic cholinoreceptors. Additional selectivity of pilocarpine and other cholinomimetics in the treatment of glaucoma is achieved by the use of an ophthalmic (topical) preparation.

Continuation: Case: Muscarinic cholinomimetic agents. Class

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