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Cholinergic Receptors Antagonists. 59-291 Section 2, lecture 3. Muscarinic Receptor Antagonists Belladonna alkaloids Derived from plants; Atropa belladonna (the deadly night shade) Atropine, scopolamine Well absorbed from the gut and distributed to CNS

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cholinergic receptors antagonists

Cholinergic Receptors Antagonists

59-291 Section 2, lecture 3

Muscarinic Receptor Antagonists
    • Belladonna alkaloids
      • Derived from plants; Atropa belladonna (the deadly night shade)
      • Atropine, scopolamine
      • Well absorbed from the gut and distributed to CNS
      • Systemic administration, short half life of ~ 2h
      • Topical ocular admin, longer half life.
        • Bind to iris pigments and are released over days ,darker irises bind more

Pharmacologic effects- muscarinic receptor antagonists inhibit parasympathetic nerve stimulation

  • relax smooth muscle
  • increase heart rate
  • Increase conductivity in the heart
  • Inhibit exocrine gland secretion

These effects are dose dependent

cardiac effect
Cardiac effect
  • Standard dose
    • Blocks the effect of vagus nerve
      • Increases heart rate
      • Increases AV conduction velocity
  • Low dose
    • When delivered by IV, at low dose slows heart rate
      • by stimulating vagal motor nucleus in the brain
  • Used to treat sinus bradycardia which can lead to hypotension, ischemia if left untreated
central nervous system
Central Nervous System
  • Atropine, scopolamine;
    • Block muscarinic receptors
      • Sedation
      • Excitement
  • Scopolamine
    • More sedating than atropine
    • Used as an adjunct to anesthesia
  • Atropine
    • Mild stimulation followed by a slower and longer-lasting sedative effect
    • Higher dose>> deliruim, hallucination
nicotinic receptor antagonists
Nicotinic Receptor Antagonists
  • Ganglionic blocking agents (e.g. Trimethaphan)
    • Block NN receptors at sympathetic and parasy.
    • Effect on a tissue: depends on sympathetic or parasy. system is dominant
    • No longer are used to treat chronic hypertension
    • Trimethaphan is occasionally used in cases of hypertensive emergency, when extremely high blood pressure must be lowered rapidly
nicotinic receptor antagonists7
Nicotinic Receptor Antagonists
  • Neuromuscular blocking agents
    • Inhibit neurotransmission at skeletal muscle
    • Causing muscle weakness and paralysis
    • Non-depolarizing blockers
    • Depolarizing blockers
nondeporolarizing neuromuscular blocking agents curariform drugs
Nondeporolarizing Neuromuscular blocking agents (Curariform drugs)
  • Positively charged, e.g. Pancuronium, Tubocurarine
  • Are not well absorbed from the gut, hence they do not cause poisoning when ingested with contaminated meat
  • Do not cross blood-brain barrier (BBB)
  • Competitive antagonists of Ach at NM receptors
nondeporolarizing neuromuscular blocking agents
Nondeporolarizing Neuromuscular blocking agents
  • Paralyze small fast moving muscles of eyes and face> larger muscles of the limbs and trunk > finally the intercostal muscles and the diaphragm
  • Is used for relaxation of abdominal muscles for surgical procedures without producing apnea
  • Side effects: stimulate mast cells to release histamine> tachycardia, hypotension and bronchospasm
depolarizing neuromuscular blocking agents
Depolarizing Neuromuscular Blocking Agents
  • Succinylcholine; two molecules of Ach
  • Binds to NM receptors and depolarizes the motor end plate
  • First transient muscle contraction (fasciculation) followed by a sustained muscle paralysis
  • Ultra-short duration action (5-10 min) due to the effect of plasma cholinesterase
  • Is used to produce muscle relaxation before and during surgery
adrenergic receptor agonists
Adrenergic Receptor Agonists
  • Diverse pharmacological effect: treatment of a wide spectrum of clinical conditions
  • Cardiovascular emergencies to common cold
  • Sympathetic Stimulation> release of NE, E > Adrenergic receptors> physiological effects

Relaxes bronchial, uterine, and vascular smooth muscle

Contract vascular smooth muscle, iris, bladder sphincter muscle

Inhibits NE release

b adrenergic receptors
b-Adrenergic Receptors
  • Activation of b1 adrenergic receptors
    • Positive chronotropic effect ( heart rate)
    • Positive inotropic effect ( contractility)
    • Positive dromotropic effect ( impulse conduction velocity)
  • Activation of b2adrenergic receptors
    • Relaxation of bronchial, uterine, vascular smooth muscle cells
    • Potassuim uptake in skeletal muscles
    • Glycogenolysis
  • Activation of b3adrenergic receptors
    • lipolysis
dopamine receptors
Dopamine Receptors
  • Dopamine receptors only activated by dopamine and not by any other adrenergic receptor agonist
    • D1: Muscle relaxation in vascular smooth muscles
    • D2: modulate neurotransmitter release
imidazoline receptors
Imidazoline Receptors
  • Activated by adrenergic receptor agonists and other substances that contain imidazoline structure
  • Found in CNS and PNS
signal transduction
Signal transduction
  • Adrenergic, dopamine and imidazoline receptors are G-protien binding receptors
  • a1 Activate phospholipase C, which catalyzes the release of IP3 and DAG from membrane phospholipid
  • IP3 releases Ca2+ from sarcoplasmic reticulum in SM cells and muscle contraction> vasoconstriction> increase BP
  • a2 activation> inhibition of adenylate cyclase> cAMP
  • b and D1 receptor activation > stimulation of adenylate cyclase > cAMP


practice questions
Practice Questions
  • Determine the location and function of the following adrenergic receptors
  • a1 receptors
    • Postjunctional smooth muscles;
    • Contraction of vascular SM, iris dilator muscle, bladder sphincter muscle
  • a2 receptors
    • Presynaptic neurons postganglionic neurons
    • Feedback inhibition of NE release
  • b1 receptors
    • Cardiac cells
    • Positive chronotropic, inotropic, dromotropic effects
What is succinylcholine? What is the effect and structure of this drug?
  • Nicotinic receptor antagonist
  • Depolarizing blocking agent
  • Two molecules of Ach
  • Binds to nicotinic receptors and causes persistent depolarization of motor end plate
  • Fasciculation followed by sustained paralysis