Chapter 16 Cholinergic Agonists and Antagonists PDF

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Chapter 16: Cholinergic Agonists and Antagonists I. Cholinergic Agonists n - Drugs that stimulate the parasympathetic nervous system are called cholinergic agonists, or parasympathomimetics, because they mimic the parasympathetic neurotransmitter acetylcholine. - The two types of cholinergic receptors are muscarinic receptors, which stimulate smooth muscle and slow the heart rate, and nicotinic receptors (neuromuscular), which affect the skeletal muscles. A. Direct-Acting Cholinergic Agonists 1. Act on receptors to activate a tissue response; mimic acetylcholine and act on the receptor 2. primarily selective to the muscarinic receptors but are nonspecific because the muscarinic receptors are located in the smooth muscle of the GI and genitourinary tracts, glands, and heart. 3. SIDE EFFECTS: hypotension, tachycardia, blurred vision, excessive salivation, increased gastric acid secretion, abdominal cramps, diarrhea, bronchoconstriction, and, in some cases, cardiac dysrhythmias. 4. Muscarinic agonists are contraindicated for patients with intestinal or urinary tract obstruction, severe bradycardia, or active asthma. 5. *Pilocarpine is a direct-acting cholinergic agonist that constricts the pupils of the eyes, thus opening the Schlemm canal to promote drainage of aqueous humor (fluid) a) This drug is used to treat glaucoma by relieving (intraocular) fluid pressure in the eye and to promote miosis in eye surgery and examinations. B. Indirect-Acting Cholinergic Agonists 1. The indirect-acting cholinergic agonists do not act on receptors; instead, they inhibit or inactivate the enzyme cholinesterase, permitting acetylcholine to accumulate at the receptor sites 2. ChE may destroy acetylcholine before it reaches the receptor or after it attaches to the sire.

3. 4. Cholinesterase inhibitors can be separated into reversible and irreversible inhibitors. The reversible inhibitors bind the ChE for several minutes to hours, and the irreversible inhibitors bind the enzyme permanently. The resulting effects vary with how long the ChE is bound.

II.

a) Reversible: used to produce pupillary constriction in the treatment of glaucoma and to increase muscle strength in patients with myasthenia gravis. (1) Common side effects from reversible cholinesterase inhibitors are hypotension, bradycardia, sweating, hypersalivation, and GI distress, which includes anorexia, nausea, vomiting, abdominal pain, and diarrhea. (2) Caution for reversible cholinesterase inhibitors is required for patients who have bradycardia, asthma, peptic ulcers, or hyperthyroidism. (3) Cholinesterase inhibitors are contraindicated for patients with intestinal or urinary obstruction. b) Irreversible: The enzyme cholinesterase must be regenerated before the drug effect diminishes, a process that may take days or weeks. These drugs are used to produce pupillary constriction C. Bethanechol 1. a direct-acting cholinergic agonist, acts on the muscarinic (cholinergic) receptor and is used primarily to increase urination in the treatment of urinary retention and neurogenic bladder. 2. -> increases bladder tone Cholinergic Antagonists (Anticholinergics) - Drugs that inhibit the actions of acetylcholine by occupying the acetylcholine receptors - The major body tissues and organs affected by the anticholinergic group of drugs are the heart, respiratory tract, GI tract, urinary bladder, eyes, and exocrine glands. - The major responses to anticholinergics are a decrease in GI motility, a decrease in salivation, dilation of pupils—also called mydriasis—and an increase in pulse rate.

A. Atropine 1. first derived from the belladonna plant (Atropa belladonna) and purified in 1831, is a classic anticholinergic—or muscarinic antagonist—drug

2. Scopolamine was the second belladonna alkaloid produced. 3. Atropine is useful primarily as a preop medication to decrease salivary secretions and as an agent to increase heart rate when bradycardia is present. 4. Atropine can also be used as an antidote for muscarinic agonist poisoning caused by an overdose of a cholinesterase inhibitor or a muscarinic drug such as bethanechol 5. SIDE EFFECTS: dry mouth, decreased perspiration, blurred vision, tachycardia, constipation, and urinary retention B. Antiparkinson-Anticholinergic Drugs 1. Anticholinergics affect the CNS by suppressing the tremors and muscular rigidity of parkinsonism, but they have little effect on mobility and muscle weakness. a) To treat parkinsonism: trihexyphenidyl hydrochloride, biperiden, and benztropine C. Anticholinergics for Treating Motion Sickness 1. Scopolamine a) An anticholinergic, classified as an antihistamine for motion sickness; comes as a patch....