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What is the primary mechanism by which norepinephrine (NE) is removed from the synaptic cleft?
What is the result of the uptake 2 process in the removal of norepinephrine (NE)?
What is the function of catechol-O-methyltransferase (COMT) in the metabolism of norepinephrine (NE)?
What is the classification of adrenergic receptors that respond more strongly to norepinephrine (NE) than to isoproterenol?
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What is the secondary messenger involved in the signaling pathway of adrenergic receptors?
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What is the characteristic of alpha-adrenergic receptor agonists?
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What is the result of the metabolism of norepinephrine (NE) by monoamine oxidase (MAO)?
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What is the mechanism by which norepinephrine (NE) is reabsorbed into neurons?
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What is the effect of stimulating α1 receptors in blood vessels?
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Which neurotransmitter is NOT a member of the catecholamine family?
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What is the function of β2 receptors in the cardiac muscles?
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What is the effect of NE administration on the uterus?
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What is the primary function of α2 receptors?
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Which of the following is NOT a physiological function regulated by adrenergic neurotransmitters?
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What is the effect of adrenergic drugs on carbohydrate metabolism?
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What is the primary function of β2 receptors in skeletal muscles?
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What is the essential feature of direct acting sympathomimetics?
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What is the effect of replacing H with an alkyl group in direct acting sympathomimetics?
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Why do direct acting sympathomimetics exhibit stereoselectivity?
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Which of the following is NOT inactivated orally?
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What is the effect of β-OH group on receptor binding?
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What is the characteristic of R(-) enantiomer of direct acting sympathomimetics?
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What is the consequence of metabolic inactivation of direct acting sympathomimetics?
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What is the pharmacological effect of direct acting sympathomimetics?
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Study Notes
Termination of Action
- Norepinephrine (NE) may be removed from the action site through two processes: uptake and metabolism
- Uptake process: the greatest amount of NE is rapidly reabsorbed into neurons and then into its storage sites through an active transport mechanism (carrier protein)
- Two types of uptake: uptake 1 (highly stereospecific, occurs at nerve terminal on presynaptic cell membrane) and uptake 2 (less stereospecific, occurs at the receptor site on the postsynaptic cell membrane)
Metabolism
- NE is metabolized to inactive metabolites excreted in the urine
- Two enzymes involved in metabolism: Monoamine Oxidase (MAO) and Catechol-O-Methyltransferase (COMT)
- MAO converts NE into an acid or aldehyde
- COMT converts NE into 3-methoxy-4-hydroxymandelic acid
Adrenergic Receptors
- Adrenoreceptors are G protein-coupled receptors that are targets of catecholamines (NE and E)
- Classified into two types: α and β receptors
- α-adrenergic receptor primary binding involves a small N-substituent and at least one OH
- β-adrenergic receptor primary binding involves a bulky N-substituent and two catecholic OHs
- α1 or α2 response is stronger to NE or E than isoproterenol
- β1, β2, or β3 response is stronger to isoproterenol than E or NE
Sympathomimetic Drugs
- Direct-acting sympathomimetics are phenylethylamine derivatives
- Structure-activity relationship (SAR) requirements:
- β-OH is essential
- N atom should bear at least one H (1ry or 2ry)
- N atom should be separated from the ring by 2 C
- 3-OH and 4-OH are important for activity
- Vasoconstrictor activity is increased by replacing H with alkyl groups (increases lipophilicity)
Adrenergic Drugs
- Adrenergic drugs stimulate or inhibit adrenergic receptors (α and β receptors)
- Adrenergic neurotransmitters regulate many physiological functions, including:
- Blood pressure
- Heart rate
- GIT motility
- Bronchial tone
- Carbohydrate metabolism
- Transport of lipid from fat depots to the liver
- NE is a neurotransmitter in the autonomic nervous system and in the CNS
Adrenergic Receptors and Physiological Functions
- α1 receptors: stimulation causes vasoconstriction
- α2 receptors: mostly postsynaptic
- β2 receptors: stimulation causes:
- Increased heart rate and various arrhythmias
- Contraction of skeletal muscles
- Increased GIT motility and pancreatic secretion
Catecholamines
- Include NE, E, and dopamine
- Chemical structures:
- NE: OH, NH2, HO
- E: OH, N, HO, NH2
- Dopamine: OH, NH2, HO
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Description
This quiz covers the termination of action of neurotransmitters, including uptake process and metabolism, in adrenergic and dopaminergic neurons in the CNS and ANS.
