Questions and Answers
What is the mechanism of action of indirect acting parasympathomimetics?
Inhibition of cholinesterase enzyme
Which of the following is a reversible cholinesterase inhibitor mentioned in the text?
Physostigmine
What effect does ACh have on the heart?
Increase effective refractory period at the atrium
What effect does ACh have on smooth muscles in blood vessels?
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Which compound is mentioned as an irreversible cholinesterase inhibitor in the text?
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What is the main pharmacological action of ACh on smooth muscles in blood vessels?
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What is the main reason oximes must be given very early after exposure?
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Which oxime is mentioned as more potent than pralidoxime?
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Which of the following is NOT a natural occurring alkaloid mentioned in the text?
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Which class of drugs is known for blocking muscarinic receptors without affecting nicotinic receptors?
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Which drug is mentioned specifically for eye applications in the text?
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Which drug class has little central nervous system effects and is used in bronchial asthma?
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Which type of receptors in the parasympathetic nervous system are blocked by atropine?
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Which type of cholinergic receptor is found in the autonomic ganglia and is blocked by d-tubocurarine?
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Which type of cholinergic receptor causes inhibition of adenyl cyclase leading to a decrease in cAMP?
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Which classification of parasympathomimetics includes drugs that act by directly binding to the receptors?
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Which cholinergic agonist is a Choline ester?
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Which type of cholinergic receptor is G-protein coupled and causes stimulation of phospholipase C?
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Study Notes
Autonomic Nervous System
- The parasympathetic nervous system is affected by parasympathomimetics, which are drugs that mimic the action of acetylcholine (ACh) on cholinergic receptors.
Cholinergic Receptors
- There are two types of cholinergic receptors: muscarinic and nicotinic receptors.
- Muscarinic receptors:
- Blocked by atropine
- G-protein coupled receptors
- Subtypes: M1 (in autonomic ganglia), M2 (in the heart), M3 (in smooth muscles and secretory glands)
- Nicotinic receptors:
- Not blocked by atropine
- Ligand-gated ion channels
- Subtypes: Nm (in neuromuscular junction), Nn (in autonomic ganglia, adrenal medulla, and CNS)
Molecular Mechanisms and Signal Transduction of Cholinergic Receptors
- Nicotinic receptors: ligand-gated ion channels that increase permeability to Na+ and K+.
- Muscarinic receptors:
- M1, M3, and M5: Gq/11, stimulating phospholipase C, increasing [Ca++] and IP3
- M2 and M4: Gi, opening K+ channels, decreasing cAMP
Classification of Parasympathomimetics (Cholinergic Agonists)
- Direct-acting parasympathomimetics:
- Bind directly to receptors
- Examples: acetylcholine, methacholine, carbachol, bethanechol, pilocarpine, and muscarine
- Indirect-acting parasympathomimetics:
- Inhibit cholinesterase enzyme, increasing ACh levels
- Examples: physostigmine, neostigmine, and organophosphorus compounds
Pharmacology of Cholinergic Agonists
- Choline Esters (quaternary compounds):
- Pharmacological actions of ACh:
- Muscarinic actions
- Effect on the heart: bradycardia, decreased strength of contraction, and decreased A-V conduction
- Effect on smooth muscles: vasodilation and hypotension
- Pharmacological actions of ACh:
Parasympatholytics (Cholinergic Blocking Drugs)
- Antimuscarinic drugs that block muscarinic receptors, but not nicotinic receptors
- Natural occurring alkaloids: atropine and scopolamine
- Synthetic and semisynthetic:
- Tertiary ammonium drugs (cross BBB, affect CNS): benztropine, procyclidine, homatropine, cyclopentolate, and tropicamide
- Quaternary ammonium drugs (little CNS effects): ipratropium, tiotropium, propantheline, and glycopyrrolate
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