Muscarinic Receptors Overview
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Muscarinic Receptors Overview

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Questions and Answers

What is the predominant subtype of muscarinic receptors involved in cholinergic control of the heart?

  • M1 (correct)
  • M3
  • M5
  • M2

    • Which pathway do the M1, M3, and M5 muscarinic receptors activate?

  • Gi pathway
  • Gq-PLC pathway (correct)
  • Adenylyl cyclase pathway
  • cAMP pathway

    • What effect does acetylcholine have on heart rate?

  • Decrease in heart rate (correct)
  • Fluctuation in heart rate
  • Increase in heart rate
  • Stabilization of heart rate

    • Which muscarinic receptor subtype is primarily responsible for bronchoconstriction?

    <p>M3</p> Signup and view all the answers

    What is one of the primary effects of acetylcholine on the vascular system?

    <p>Vasodilation</p> Signup and view all the answers

    How does acetylcholine affect conduction velocity in the AV node?

    <p>Decreases conduction velocity</p> Signup and view all the answers

    Which muscarinic receptor subtype primarily inhibits adenylyl cyclase?

    <p>M2</p> Signup and view all the answers

    What is the effect of acetylcholine on the force of cardiac contraction?

    <p>Decrease in contraction force</p> Signup and view all the answers

    What role do M2 muscarinic receptors play in smooth muscle secretory glands?

    <p>They inhibit secretory activity.</p> Signup and view all the answers

    What is methacholine primarily used for?

    <p>Diagnosis of bronchial airway hyperreactivity</p> Signup and view all the answers

    Which drug is indicated for the treatment of glaucoma?

    <p>Pilocarpine</p> Signup and view all the answers

    What is a contraindication for using muscarinic receptor agonists?

    <p>Chronic obstructive pulmonary disease</p> Signup and view all the answers

    What is the primary action of bethanechol?

    <p>Stimulates peristalsis</p> Signup and view all the answers

    Which muscarinic agonist is preferred for treating xerostomia with fewer side effects?

    <p>Cevimeline</p> Signup and view all the answers

    What characterizes naturally occurring muscarinic antagonists?

    <p>Atropine and scopolamine</p> Signup and view all the answers

    Which option is NOT a function of pilocarpine?

    <p>Stimulation of peristalsis</p> Signup and view all the answers

    Which muscarinic agonist can be used to ease swallowing in patients after head and neck radiation?

    <p>Cevimeline</p> Signup and view all the answers

    What is a common side effect of muscarinic receptor agonists?

    <p>Hypotension</p> Signup and view all the answers

    Which drug is specifically indicated for the treatment of urinary retention?

    <p>Bethanechol</p> Signup and view all the answers

    What is a prominent effect of Scopolamine at therapeutic doses?

    <p>Amnesia</p> Signup and view all the answers

    Which muscarinic antagonist is used exclusively for respiratory problems?

    <p>Ipratropium</p> Signup and view all the answers

    Which of the following side effects is associated with the use of muscarinic antagonists?

    <p>Xerostomia</p> Signup and view all the answers

    What is a therapeutic use of atropine in the cardiovascular system?

    <p>Increasing heart rate in sinus bradycardia</p> Signup and view all the answers

    Which statement about quaternary antagonists is true?

    <p>Systemic absorption is limited when inhaled.</p> Signup and view all the answers

    What is the dominant cardiovascular response to atropine administration?

    <p>Tachycardia</p> Signup and view all the answers

    What condition is not treated using muscarinic antagonists?

    <p>Asthma attacks</p> Signup and view all the answers

    Which muscarinic receptor does atropine primarily block to slow heart rate at clinical doses?

    <p>M1 receptors</p> Signup and view all the answers

    Which muscarinic receptor subtype primarily mediates detrusor muscle contraction in the bladder?

    <p>M3 receptors</p> Signup and view all the answers

    How does atropine affect bronchoconstriction?

    <p>It inhibits bronchoconstriction.</p> Signup and view all the answers

    What effect do muscarinic receptor agonists have on the gastrointestinal tract?

    <p>Increased tone and amplitude of contractions</p> Signup and view all the answers

    What is one of the effects of atropine on the eyes?

    <p>Photophobia</p> Signup and view all the answers

    Which choline ester is known to be resistant to hydrolysis by cholinesterases?

    <p>Methacholine</p> Signup and view all the answers

    What happens to gastric secretions when atropine is administered?

    <p>They are incompletely inhibited.</p> Signup and view all the answers

    Which statement about the absorption of muscarinic receptor agonists is correct?

    <p>Tertiary amines can cross the blood-brain barrier and are readily absorbed.</p> Signup and view all the answers

    In the eye, what response is caused by muscarinic receptor activation?

    <p>Miosis by contracting the pupillary sphincter muscle</p> Signup and view all the answers

    What effect does atropine have on salivary secretion?

    <p>Completely abolishes watery secretion.</p> Signup and view all the answers

    What is the therapeutic use of muscarinic agonists in treating xerostomia?

    <p>Increasing salivary secretion</p> Signup and view all the answers

    What is one consequence of atropine-induced cycloplegia?

    <p>Blurred vision for near objects.</p> Signup and view all the answers

    Which hormone is NOT known to mediate gastric secretions in response to atropine?

    <p>Norepinephrine</p> Signup and view all the answers

    What is the primary pharmacological action of muscarine?

    <p>Acts primarily at muscarinic receptors</p> Signup and view all the answers

    Which of the following is a function of M2 receptors in the urinary tract?

    <p>Reverses receptor-cyclic AMP-mediated relaxation of the detrusor muscle</p> Signup and view all the answers

    How does larger doses of atropine affect the heart rate?

    <p>Inhibits parasympathetic activity causing tachycardia.</p> Signup and view all the answers

    What is the main therapeutic consideration when using atropine with respiratory disease patients?

    <p>Inhibits bronchoconstriction.</p> Signup and view all the answers

    Which of the following cholinergic agonists is a partial agonist at muscarinic receptors?

    <p>Pilocarpine</p> Signup and view all the answers

    Muscarinic agonists crossing the blood-brain barrier can lead to which of the following effects?

    <p>Activation response</p> Signup and view all the answers

    Study Notes

    Muscarinic Receptors

    • Found in the peripheral nervous system (PNS) on autonomic effector cells innervated by postganglionic parasympathetic nerves
    • Found in the central nervous system (CNS) in the hippocampus, cortex, and thalamus
    • Muscarinic agonists mimic the effects of acetylcholine (ACh) at these sites and are typically longer-acting
    • All are G protein-coupled receptors (GPCRs)

    Muscarinic Receptor Subtypes

    • M1, M3, and M5: Activate the Gq-PLC pathway, causing hydrolysis of polyphosphoinositides and mobilization of intracellular Ca2+
    • M2 and M4: Inhibit adenylyl cyclase and regulate specific ion channels via coupling to the pertussis toxin-sensitive G proteins (Gi and Go)

    Pharmacological Effects of Acetylcholine

    Cardiovascular System

    • Vasodilation: Stimulates muscarinic receptors (primarily M3) on vascular endothelial cells, producing nitric oxide (NO) which relaxes vascular smooth muscle
    • Decrease in heart rate: Negative chronotropic effect by decreasing the rate of spontaneous depolarization, delaying the attainment of threshold potential
    • Decrease in conduction velocity in the AV node: Negative dromotropic effect by increasing the refractory period (inhibits Ca2+ channel opening)
    • Decrease in the force of cardiac contraction: Negative inotropic effect by hyperpolarizing the atria and decreasing action potential (increasing K+ channel opening), and inhibiting cAMP formation and release of norepinephrine (NE)

    Respiratory Tract

    • Bronchoconstriction
    • Increased tracheobronchial secretion
    • Stimulation of the chemoreceptors of the carotid and aortic bodies
    • Mediated by M3 muscarinic receptors on bronchial and tracheal smooth muscle

    Urinary Tract

    • Detrusor muscle (bladder) contraction: Primarily by M3 receptors, with some contribution from M2 receptors
    • Increased voiding pressure
    • Ureteral peristalsis

    Gastrointestinal Tract

    • Increased tone and amplitude of contractions
    • Secretory activity of stomach and intestine
    • Primarily mediated by M3 receptors, with some contribution from M2 receptors

    Other Effects

    • Lacrimal, nasopharyngeal, salivary, and sweat glands: Increased secretions via M3 receptors
    • Eye: Miosis (pupillary sphincter contraction) and accommodation for near vision (ciliary muscle contraction)
    • Central nervous system: Muscarinic agonists crossing the blood-brain barrier (BBB) can cause arousal or activation responses

    Muscarinic Receptor Agonists

    • Choline Esters: Methacholine, carbachol, and betanechol
    • Cholinomimetic Alkaloids: Muscarine, arecoline, and pilocarpine

    ADME of Receptor Agonists:

    • Quaternary Amines: Poorly absorbed, do not cross the BBB, resistant to hydrolysis, short half-life (renal elimination)
    • Tertiary Amines: Readily absorbed, cross the BBB, examples include arecoline and pilocarpine

    Therapeutic Uses of Muscarinic Agonists

    • Treatment of urinary bladder disorders
    • Treatment of xerostomia (dry mouth)
    • Diagnosis of bronchial hyperreactivity

    Muscarinic Receptor Antagonists

    • Naturally occurring alkaloids: Atropine and scopolamine
    • Semisynthetic derivatives: Differ in disposition and duration of action
    • Synthetic derivatives: Limited selectivity for specific receptor subtypes

    Pharmacological Effects of Atropine

    Cardiovascular System

    • Tachycardia: Blocks M1 receptors on the SA node, inhibiting ACh release and leading to tachycardia.
    • Larger doses: Can block M2 receptors on the SA node, further inhibiting parasympathetic activity

    Respiratory System

    • Inhibits bronchoconstriction: Can block bronchoconstriction caused by histamine, bradykinin, and eicosanoids
    • Stops secretions: Dries mucus membranes of the respiratory tract

    Eyes

    • Mydriasis (pupil dilation) and Cycloplegia (paralysis of accommodation)
    • Photophobia: Sensitivity to light
    • Blurred vision: Lens is fixed for far vision, near objects are blurred
    • Smaller appearance of objects: Normal pupillary reflex and convergence is abolished

    Gastrointestinal Tract

    • Incompletely inhibits GI responses to vagal stimulation: GI tract has other non-cholinergic synapses and plexuses
    • Reduced gastric secretions: Gastric secretions are also mediated by gastrin-releasing peptides and other hormones (gastrin, histamine)
    • Reduced salivary secretions: Can completely abolish watery secretion, making swallowing hard

    Sweat Glands and Temperature

    • Inhibits sweat gland activity: Leads to hot and dry skin

    Central Nervous System

    • Atropine: Minimal effects at therapeutic doses, toxic doses lead to restlessness, disorientation, and in larger doses, depression, circulatory collapse, respiratory failure, paralysis, and coma
    • Scopolamine: Prominent effects including drowsiness, amnesia, fatigue, dreamless sleep, decreased REM sleep
    • Anti-motion sickness: Effective by blocking neural pathways in the vestibular apparatus of the inner ear

    ADME of Muscarinic Antagonists

    • Tertiary antagonists: Belladonna alkaloids, synthetic and semisynthetic derivatives, rapid absorption from GIT, limited skin absorption
    • Quaternary antagonists: Limited systemic absorption, less readily penetrate the eye, lack central effects

    Therapeutic Effects of Muscarinic Antagonists

    • Respiratory Tract: Ipratropium, tiotropium, aclidinium, and umeclidinium are used for respiratory problems like COPD
    • Genitourinary Tract: Treat overactive bladder, lower intravesicular pressure, reduce frequency of contractions
    • Gastrointestinal Tract: Treat diarrhea, once used for peptic ulcers
    • Eye: Topical administration for mydriasis (examining retina and optic disc)
    • Cardiovascular System: Atropine for excessive vagal tone (sinus bradycardia)
    • Central Nervous System: Scopolamine for motion sickness
    • Anesthesia: Atropine or glycopyrrolate block parasympathomimetic effects of neostigmine

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    Pharmacology - Muscarinic Receptor Agonists and Antagonists PDF

    Description

    Explore the functions and subtypes of muscarinic receptors, including their roles in the peripheral and central nervous systems. This quiz covers the pharmacological effects of acetylcholine, particularly in the cardiovascular system and how different subtypes activate specific pathways.

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