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Questions and Answers
What is the effect of stimulating β1 adrenergic receptors?
What is the effect of stimulating β1 adrenergic receptors?
Which statement correctly describes the action of α2 adrenergic receptors?
Which statement correctly describes the action of α2 adrenergic receptors?
What role does choline acetyl-transferase play in the synthesis of acetylcholine?
What role does choline acetyl-transferase play in the synthesis of acetylcholine?
Which of the following neurotransmitters stimulates both α and β adrenergic receptors, but varies in potency?
Which of the following neurotransmitters stimulates both α and β adrenergic receptors, but varies in potency?
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How is acetylcholine released into the synaptic space?
How is acetylcholine released into the synaptic space?
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What substance blocks the transport of acetylcholine into synaptic vesicles?
What substance blocks the transport of acetylcholine into synaptic vesicles?
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What happens to acetylcholine immediately after its release in the synaptic cleft?
What happens to acetylcholine immediately after its release in the synaptic cleft?
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Which receptor is associated with bronchodilation when stimulated?
Which receptor is associated with bronchodilation when stimulated?
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What is the first step in the synthesis of noradrenaline?
What is the first step in the synthesis of noradrenaline?
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Which mechanism primarily regulates the release of noradrenaline in the synaptic cleft?
Which mechanism primarily regulates the release of noradrenaline in the synaptic cleft?
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How is noradrenaline predominantly released from presynaptic neurons?
How is noradrenaline predominantly released from presynaptic neurons?
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What role does Monoamine oxidase (MAO) play in the metabolism of catecholamines?
What role does Monoamine oxidase (MAO) play in the metabolism of catecholamines?
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What type of receptor is primarily involved in the contraction of blood vessels?
What type of receptor is primarily involved in the contraction of blood vessels?
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What is the primary function of the catecholamine reuptake mechanisms?
What is the primary function of the catecholamine reuptake mechanisms?
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Which enzyme converts DOPA to dopamine in the synthesis of noradrenaline?
Which enzyme converts DOPA to dopamine in the synthesis of noradrenaline?
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Which uptake mechanism is more selective to noradrenaline?
Which uptake mechanism is more selective to noradrenaline?
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Study Notes
Adrenergic Transmission
- Synthesis of Noradrenaline (NA): Begins with tyrosine, converted through several steps involving enzymes like tyrosine hydroxylase, DOPA decarboxylase, and dopamine hydroxylase.
- Adrenaline Synthesis: NA is further converted to adrenaline by phenylethanolamine N-methyl transferase enzyme, primarily in the adrenal medulla.
Storage and Release of Noradrenaline
- NA is stored with ATP in presynaptic vesicles.
- Release: Triggered by calcium influx via voltage-gated calcium channels, leading to exocytosis.
- Non-exocytotic Release: Certain drugs like amphetamine can displace and release NE from vesicles.
Regulation of Noradrenaline Release
- Auto-inhibitory Feedback Mechanism: Release of NA stimulates α2 receptors in the presynaptic neuron, inhibiting further release.
Elimination of Catecholamines
-
Reuptake:
- Uptake 1 (presynaptic): More selective for NA.
- Uptake 2 (extraneuronal): Occurs in effector organs like the heart and smooth muscle; more selective for adrenaline.
-
Metabolism:
- Monoamine Oxidase (MAO): Found in mitochondria, primarily in neurons.
- Catechol-O-Methyl Transferase (COMT): Found in various neuronal and non-neuronal tissues.
Classification of Adrenoceptors
- Divided into two main classes:
- α receptors: (α1 and α2)
- β receptors: (β1, β2, and β3)
Actions of Adrenergic Receptors
-
α1 Receptors:
- Found in smooth muscle of various organs, causing contraction of blood vessels.
- Relaxation of the gastrointestinal tract (GIT).
- Contraction of all sphincters (GIT, urinary).
- Mydriasis (dilation of pupil) due to contraction of the dilator pupillae muscle.
-
α2 Receptors:
- Found in presynaptic neurons, CNS, and blood vessels.
- Inhibit neurotransmitter release.
- Promote platelet aggregation.
- When postsynaptically, they can function similar to α1 receptors.
-
β1 Receptors:
- Primarily found in the heart, increasing contractility and heart rate upon stimulation.
-
β2 Receptors:
- Present in smooth muscle, leading to bronchodilation, peripheral vasodilation, relaxation of visceral smooth muscle, and skeletal muscle tremor.
- Increase aqueous humor secretion.
- Increase liver glycogenolysis.
-
β3 Receptors:
- Role in lipolysis and thermogenesis.
Adrenergic Receptor Agonist Potency
- Adrenaline, noradrenaline, and isoprenaline act on both α and β receptors with varying potencies.
- Noradrenaline more strongly stimulates α receptors.
- Isoprenaline more strongly stimulates β receptors.
Acetylcholine (ACh) Synthesis
- Choline Uptake: Transported from extracellular fluid into the cytoplasm of cholinergic neurons by an energy-dependent carrier system.
- Acetylcholine Synthesis: Choline acetyltransferase catalyzes the reaction of choline with acetyl CoA to form acetylcholine.
Storage of Acetylcholine
- Packaged into presynaptic vesicles via active transport.
- Mature vesicles contain ACh, ATP, and proteoglycans.
- Vesamicol blocks the uptake of ACh into synaptic vesicles.
Release of Acetylcholine
- Action potential arrival at the nerve terminal opens voltage-gated calcium channels, increasing intracellular calcium concentration.
- This triggers fusion of vesicles with the cell membrane, releasing their contents into the synaptic space.
- Botulinum toxin: Blocks the release of ACh.
Degradation and Recycling of Acetylcholine
- Degradation: Acetylcholinesterase (AChE) cleaves ACh into choline and acetate in the synaptic cleft.
- Recycling: Choline is recaptured by a high-affinity uptake system, transported back into the neuron, and used for the synthesis of new acetylcholine.
Acetylcholine Receptors
- Two major classes of receptors in the parasympathetic system:
- Nicotinic receptors:
- Muscarinic receptors:
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Description
This quiz covers the synthesis, storage, release, and regulation of noradrenaline and adrenaline in adrenergic transmission. Key processes including enzyme involvement, auto-inhibitory feedback mechanisms, and catecholamine elimination are explored. Test your understanding of this vital physiological process.
