Cholinergic Drugs

• Muscarinic agonists bind to muscarinic receptors and activate the receptors; response to drug is similar to a parasympathetic response
• Muscarinic antagonists block acetylcholine from binding to muscarinic receptors; response to drug is the opposite of a parasympathetic response
• Cholinesterase inhibitors prevent the breakdown of acetylcholine; they are indirect cholinergic agonist
• Bethanechol is a muscarinic agonist (cholinergic agonist) that is used for treatment of (nonobstructive) urinary retention
• Bethanechol- mechanism of action: binds to muscarinic receptors and activates them
• Bethanechol contraindications: urinary tract obstructions, intestinal obstructions, peptic ulcer disease, coronary insufficiency/bradycardia, hypotension, and asthma
• Bethanechol adverse effects: hypotension, bradycardia, abdominal cramps, nausea, vomiting, diarrhea, salivation, bronchoconstriction (exacerbation of asthma)
• To help reduce gastric upset, patients can take bethanechol before or after eating
• Signs of muscarinic agonist toxicity include excessive salivation, lacrimation (tears), urinary incontinence, diarrhea, GI (abdominal) cramps, vomiting (emesis) , miosis (pupil constriction)
• Treatment for muscarinic agonist toxicity is administration of Atropine (muscarinic antagonist)
• Atropine is a muscarinic antagonist (anticholinergic) that is used as a pre-anesthetic and to treat bradycardia, biliary colic, hyper mobility of the intestines, and muscarinic agonist overdose
• Muscarinic agonists will cause pupil constriction, increased salivation, decreased heart rate, vasodilation, bronchoconstriction, increased GI motility, increased gastric secretions, and increased urine output
• Muscarinic antagonists (anticholinergics) will cause pupil dilation, decreased saliva, increased heart rate, vasoconstriction, bronchodilator, decreased GI motility, increased glucose levels, and decreased urine output
• Atropine- mechanism of action: binds to muscarinic receptors and blocks acetylcholine or muscarinic agonists from activating them
• Atropine contraindications: glaucoma, intestinal atony, urinary tract obstructions, and tachycardia; caution for patients with asthma
• Atropine adverse effects: dry mouth, blurred vision, light sensitivity, increase ocular pressure, urinary retention, constipation, decreased sweating, tachycardia, and thickened lung mucus
• When taking Atropine, patients should take pills with water, stay hydrated, void before taking it, and increase fiber to prevent constipation
• Treatment for muscarinic antagonist toxicity is administration of a cholinesterase inhibitor
• Pyridostigmine is a cholinesterase inhibitor that is used to increase muscle strength in patients with myasthenia gravis
• Pyridostigmine- mechanism of action: binds to cholinesterase enzyme and causes the enzyme to break down the drug rather than break down acetylcholine; increases acetylcholine levels
• Pyridostigmine contraindications: obstruction of urinary tract or intestines; use with caution for patients with peptic ulcer disease, bradycardia, asthma, and hyperthyroidism
• Pyridostigmine adverse effects: excess salivation, diarrhea, abdominal cramps, nausea, vomiting, bradycardia, miosis, and urine incontinence
• For MG patients taking pyridostigmine, encourage them to take it before any activities to increase energy levels and educate them on the difference between muscarinic toxicity (too much ACh) and a myasthenia crisis
• Cholinesterase inhibitors like donepezil can be used to treat Alzheimer's disease