Acetylcholine and Muscarinic Receptors
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Acetylcholine and Muscarinic Receptors

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What is the primary mode of action for M1, M3, and M5 muscarinic receptors?

  • Inhibit adenylyl cyclase
  • Regulate ion channels via Gi and Go
  • Directly activate nicotinic receptors
  • Activate the Gq-PLC pathway (correct)

    • Which muscarinic receptor subtype is primarily involved in the modulation of nicotinic cholinergic transmission in the ganglia?

  • M4
  • M2
  • M1 (correct)
  • M5

    • What effect do M2 and M4 muscarinic receptors have on cellular activity?

  • They inhibit adenylyl cyclase (correct)
  • They stimulate adenylyl cyclase
  • They enhance polyphosphoinositides hydrolysis
  • They activate the Gq-PLC pathway

    • Which muscarinic receptor subtype has effects on the hydrolysis of polyphosphoinositides?

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

    What is a common therapeutic use of muscarinic receptor agonists?

    <p>Promote salivation</p> Signup and view all the answers

    Which statement correctly describes the structure of muscarinic receptors?

    <p>They are G protein-coupled receptors</p> Signup and view all the answers

    What is a significant toxicological consideration with muscarinic receptor antagonists?

    <p>Possible stimulation of peripheral nicotinic receptors</p> Signup and view all the answers

    Which of the following is a characteristic of the pharmacological effects of acetylcholine?

    <p>It can activate both muscarinic and nicotinic receptors</p> Signup and view all the answers

    Which muscarinic receptor agonist effect is primarily associated with the M2 receptor subtype?

    <p>Decreased heart rate</p> Signup and view all the answers

    What is the dominant cardiovascular response when administering atropine?

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

    Why is atropine not the preferred choice for treating patients with COPD?

    <p>It affects mucociliary clearance negatively.</p> Signup and view all the answers

    What receptor primarily mediates the increase in secretory activity of the stomach and intestines?

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

    How does atropine administration affect the heart rate at maximal exertion?

    <p>It has no effect on heart rate.</p> Signup and view all the answers

    What happens to heart rate at average clinical doses of atropine?

    <p>Transitional bradycardia may occur.</p> Signup and view all the answers

    What effect does the activation of M2 and M3 receptors have on norepinephrine (NE) release?

    <p>Inhibits NE release</p> Signup and view all the answers

    In the SA node, what is the primary outcome of increased intracellular calcium affecting heart rate?

    <p>Decreased heart rate</p> Signup and view all the answers

    What limits the systemic administration of acetylcholine (ACh)?

    <p>The rapid hydrolysis by plasma acetylcholinesterase</p> Signup and view all the answers

    Which of the following is true regarding the effect of atropine on presynaptic M1 receptors?

    <p>It inhibits M1 receptors which influences heart rate response.</p> Signup and view all the answers

    Which inhaled medications are preferred over atropine for COPD patients?

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

    How does the activation of M3 receptors affect salivary secretion?

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

    Which of the following is NOT a site where cholinergic synapses occur?

    <p>Neuromuscular junctions in smooth muscle</p> Signup and view all the answers

    What is the impact of cholinergic agonists on the atrioventricular (AV) node?

    <p>Slows conduction and increases refractory period</p> Signup and view all the answers

    What is one of the pharmacological effects of acetylcholine on the cardiovascular system?

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

    Which muscarinic receptor is primarily responsible for bronchoconstriction?

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

    The term 'miosis' refers to which action in the eye?

    <p>Contraction of the pupillary sphincter muscle</p> Signup and view all the answers

    What characteristic of acetylcholine's chemical structure affects its ability to penetrate the blood-brain barrier?

    <p>Presence of quaternary ammonium ions</p> Signup and view all the answers

    What effect does hyperpolarization have in the atria of the heart?

    <p>Decreases action potential duration</p> Signup and view all the answers

    Which type of receptor contributes significantly to cholinergic stimulation of salivary secretion?

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

    Which of the following is a negative chronotropic effect of acetylcholine?

    <p>Decreased heart rate</p> Signup and view all the answers

    What happens to the ventricular rate during atrial flutter/fibrillation with increased parasympathetic tone?

    <p>Decreases ventricular rate</p> Signup and view all the answers

    What occurs when acetylcholine stimulates the cardiovascular system?

    <p>Reduced myocardial oxygen consumption</p> Signup and view all the answers

    What role does B1 adrenergic receptor have in the presence of M2 and M3 receptors regarding NE?

    <p>Counteracts increased cAMP levels</p> Signup and view all the answers

    What is one of the effects of acetylcholine on the respiratory tract?

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

    Which of the following statements about muscarinic receptors is correct?

    <p>M3 receptors are found in smooth muscle and glands.</p> Signup and view all the answers

    In which of the following scenarios would acetylcholine prove to be less significant?

    <p>Negative inotropic effects in the ventricles</p> Signup and view all the answers

    What is the primary effect of muscarinic receptor antagonists like atropine?

    <p>Dry mucous membranes of the respiratory tract</p> Signup and view all the answers

    Which drug is specifically noted for undergoing rapid hydrolysis in plasma?

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

    What is a side effect commonly associated with muscarinic antagonists?

    <p>Dry mouth</p> Signup and view all the answers

    Which of the following drugs is primarily used for its effects on the respiratory tract?

    <p>Both B and C</p> Signup and view all the answers

    What is the role of the allosteric site on the muscarinic receptor?

    <p>It allows binding of ligands other than agonists or antagonists.</p> Signup and view all the answers

    What pharmacological effect does atropine have on rhinorrhea?

    <p>It decreases runny nose symptoms.</p> Signup and view all the answers

    Which of the following statements is true regarding ipratropium and tiotropium?

    <p>They have minimal effect on mucociliary clearance relative to atropine.</p> Signup and view all the answers

    Which compound is not classified as a quaternary ammonium compound?

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

    What is a common indication for using muscarinic receptor antagonists?

    <p>Inhibition of bronchial secretion</p> Signup and view all the answers

    Which of the following would likely not be a clinical reason for using atropine?

    <p>To induce bronchoconstriction</p> Signup and view all the answers

    Study Notes

    Acetylcholine & Muscarinic Receptor Target

    • Acetylcholine (ACh) is a neurotransmitter that interacts with muscarinic receptors.
    • Muscarinic receptors are G protein-coupled receptors (GPCRs) involved in various physiological functions.

    Muscarinic Receptor Subtypes

    • Muscarinic receptors are divided into five subtypes: M1, M2, M3, M4, and M5.
    • M1, M3, and M5 activate Gq-PLC pathway, leading to hydrolysis of polyphosphoinositides and mobilization of intracellular Ca2+.
    • M2 and M4 inhibit adenylyl cyclase and regulate specific ion channels via coupling to pertussis toxin-sensitive Gi and Go.
    • M1 plays a crucial role in modulating nicotinic cholinergic transmission in the ganglia.

    Pharmacological Effects of Acetylcholine

    • ACh is a quaternary ammonium compound with limited penetration into the CNS due to its chemical structure.
    • ACh's systemic administration is limited due to rapid hydrolysis by plasma butyrylcholinesterase (bAChE).

    Cardiovascular System

    • ACh exerts four primary effects on the cardiovascular system: vasodilation, decreased heart rate (negative chronotropic), decreased AV conduction velocity (negative dromotropic), and decreased force of cardiac contraction (negative inotropic).
    • ACh's effects on the heart are primarily mediated by parasympathetic (vagal) stimulation.
    • In the SA node, ACh slows down spontaneous depolarization, delaying the attainment of threshold potential, resulting in decreased heart rate.
    • ACh's effects on contractility are mediated by inhibiting cAMP production and the release of norepinephrine (NE).
    • In the atria, ACh induces hyperpolarization and decreases action potential duration by increasing IK-ACh.
    • In the AV node, ACh slows down conduction and increases refractory period by inhibiting calcium channel opening (ICa-L), potentially leading to complete heart block with high doses of cholinergic agonists.

    Respiratory Tract

    • ACh causes bronchoconstriction, increased tracheobronchial secretions, and stimulation of chemoreceptors of the aortic and carotid bodies, primarily through M3 receptors.

    Gastrointestinal Tract

    • M3 receptors are primarily responsible for increased amplitude of contractions and secretory activity in the stomach and intestine, while M2 receptors also contribute.

    Secretory Effects

    • ACh increases secretions in lacrimal, nasopharyngeal, salivary, and sweat glands through M3 receptors.
    • M1 receptors significantly contribute to the cholinergic stimulation of salivary secretion.

    Eye

    • ACh causes miosis (contracting pupillary sphincter muscle) and accommodation (contracting ciliary muscle for near vision), mainly mediated by M3 receptors.

    Muscarinic Receptor Antagonists

    • Muscarinic receptor antagonists competitively inhibit the actions of ACh and its congeners.

    Atropine

    • Atropine is a prototypical muscarinic antagonist.
    • Atropine's pharmacological effects differ from other muscarinic antagonists.
    • Atropine is used for its effects on the cardiovascular, respiratory, and gastrointestinal systems.

    Cardiovascular System

    • Atropine mainly alters heart rate, primarily causing tachycardia.
    • Smaller doses (0.4-0.6 mg) can temporarily induce bradycardia, but this is usually followed by tachycardia.
    • Atropine blocks presynaptic M1 receptors in the SA node, reducing the release of ACh and contributing to its tachycardia effect.
    • Atropine blocks M2 receptors on the nodal pacemaker cells, further increasing heart rate.
    • Large doses of atropine can progressively increase heart rate by blocking M2 receptors on the pacemaker cells.
    • Atropine can prevent or abruptly abolish bradycardia.
    • Important note:* In infants, elderly patients, and individuals with heart failure, even large doses of atropine may fail to accelerate the heart rate.

    Respiratory System

    • Atropine dries the mucous membranes of the respiratory tract by inhibiting secretions from the nose, mouth, pharynx, and bronchi.
    • Atropine is used to reduce rhinorrhea (runny nose) and prevent irritating inhalational anesthetics (e.g., diethyl ether) from increasing bronchial secretions.
    • Quaternary ammonium compounds like ipratropium, tiotropium, aclidinium, and umeclidinium are used primarily for their effects on the respiratory tract.
    • Aclidinium undergoes rapid hydrolysis in plasma to inactive metabolites, reducing systemic exposure.
    • Ipratropium and tiotropium have minimal inhibitory effects on mucociliary clearance compared to atropine, minimizing the accumulation of lower airway secretions.
    • Key takeaway:* For COPD patients, ipratropium and tiotropium are preferred over atropine because atropine also inhibits mucociliary clearance, exacerbating their condition.

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    Description

    Delve into the complexities of acetylcholine and its interaction with muscarinic receptors. This quiz covers the various subtypes of these receptors, their physiological roles, and the pharmacological effects of acetylcholine in the body. Test your knowledge on this vital neurotransmitter and its significance in the cardiovascular system and beyond.

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