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28 June 2014 1
Adrenergic agonists
Sympathomimetics drugs
Pharmacology I/ Lecture 5
Dr. Hiwa K. Saaed, HD, M.Sc, Ph.D
28 June 2014 2
agents that act on pathways mediated by
the endogenous catecholamines (CAO);
1. norepinephrine
2. epinephrine.
NEP & EP are modulate the
 Rate and force of contraction of heart.
 Resistance (constriction and dilation) of blood
vessels and bronchioles.
 Release of insulin,
 breakdown of fat (lipolysis).
Adrenergic agonists
28 June 2014 3
synthesis,
storage,
release,
binding
removal (reuptake) of
the neurotransmitter
They are frontline
therapies for
hypertension,
depression, shock,
asthma, angina & etc.
drugs target:
28 June 2014 4
 Tyrosine hydroxylase can be inhibited by methyl-p-
tyrosine.
 MAO: inhibitors of MAO (e.g., phenelzine,
tranylcypromine)
 The mobile pool; many indirect-acting
sympathomimetics (e.g., amphetamine, ephedrine,
tyramine) can displace NE from the mobile pool
 Uptake: some indirect-acting sympathomimetics
(cocaine, TCA).
Drug Targets
28 June 2014 5
 Prejunctional α-receptors: (e.g., clonidine, alpha
methyldopa) cause inhibition of NE release.
 Granular uptake of NE: blocker of granular uptake
of NE (e.g., reserpine) .
 NE release from granules: blockers (e.g.,
guanethidine).
 Postjunctional receptors: postjunctional receptors
can be activated or blocked.
Drug Targets
Classification
28 June 2014 6
divided into subgroups
on the basis of their
 Spectrum of action:
 α, β, or dopamine
receptor affinity
 Mode of action:
 direct, indirect or
both
28 June 2014 7
Adrenergic agonists
 Direct acting:
I. α agonists:
• Non selective,
• α1-selective,
• α2-selective
II. β agonists:
• Non selective,
• β1-selective,
• Β2-selective
28 June 2014 8
 Indirect acting ↑ CAO in the synapse:
1. Releaser: Amphetamine, tyramine
Potentiate by MAOI, COMT blocker. Why?
2. Reuptake inhibitor: Cocaine, TCA
 Mixed: Ephedrine, metaraminol
Adrenergic agonists
28 June 2014 9
Removal of NE may:
 Diffuse out and enter the general circulation.
 Be metabolized by COMT in the synaptic
 Be recaptured by an uptake systems into the
neuron
28 June 2014 10
Adrenoceptors
 Selective for NE & EP.
 dopamine can also activate some adrenoceptors at
very high ‘supraphysiologic’ concentrations.
 Divided into two main classes:
 α & β adrenoceptors
 All are members of GPCR superfamily.
28 June 2014 11
α-receptors:
EP≥NE>>Isoproterenol
β-receptors:
Isop>EP>NE
28 June 2014 12
28 June 2014 13
 based on their affinities for a agonists and blocking
drugs, α-receptors are subdivided into two subgroups
α1 & α2,
e.g., α1 receptors have a higher affinity for
phenylephrine than do α2 receptors.
 Conversely, clonidine selectively binds to α2
receptors and has less effect on α1 receptors.
α-adrenoceptors (α1 & α2)
28 June 2014 14
α-receptors:
 α1 Are present on the postsynaptic membrane
 α2 Located primarily on presynaptic nerve endings.
The stimulation of α2 receptors causes feedback
inhibition of the ongoing release of NE;
 α2 Located on other cells such as the β-cell of the
pancreas control insulin output.
28 June 2014 15
28 June 2014 16
β-receptors:
 Subdivided to β1, β2 and β3-receptors
 β1-receptors have ~equal affinities for both EP &
NE.,
 β2-receptors have higher affinity for EP than for
NE.
 thus tissue with a predominance of β2-receptors
(vasculature of skeletal muscle) are particularly
responsive to hormonal effects of circulating EP
released by adrenal medulla.
28 June 2014 17
β-receptors
Mechanism of action:
 binding of neurotransmitter at the β1 or β2-receptor→
result in activation of AC→↑cAMP concentrations
within the cell.
28 June 2014 18
Mechanisms of action of adrenergic receptors :
28 June 2014 19
28 June 2014 20
Desensitization of receptors:
 Prolonged exposure to the CAO reduces the
responsiveness of the receptors due to:
1. Sequestration of the receptors
2. Downregulation (destruction, or decreased
synthesis)
3. An inability to couple to G-protein
28 June 2014 21
A. Catecholamine properties:
 High potency in activating α & β receptors
 Rapid inactivation by:
1. COMT postsynaptically, gut wall,
2. MAO intraneuronally, liver or gut
Thus,
CAO have only a brief duration of action when given
parenterally, and are ineffective when administered
orally because of inactivation.
 Poor penetration into the CNS (polar)
28 June 2014 22
B. Non Catecholamine properties :
 phenylephrine, ephedrine, amphetamine
 Have longer t1/2 because they are not inactivated by
COMT, and they are poor substrate for MAO
 Increased lipid solubility permits the greater access to
the CNS
28 June 2014 23
Major effects mediated by adrenoceptors
28 June 2014 24
SITE OF ACTION
28 June 2014 25
SITE OF ACTION

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L5: Adrenergic agonists; sympathomimetics

  • 1. 28 June 2014 1 Adrenergic agonists Sympathomimetics drugs Pharmacology I/ Lecture 5 Dr. Hiwa K. Saaed, HD, M.Sc, Ph.D
  • 2. 28 June 2014 2 agents that act on pathways mediated by the endogenous catecholamines (CAO); 1. norepinephrine 2. epinephrine. NEP & EP are modulate the  Rate and force of contraction of heart.  Resistance (constriction and dilation) of blood vessels and bronchioles.  Release of insulin,  breakdown of fat (lipolysis). Adrenergic agonists
  • 3. 28 June 2014 3 synthesis, storage, release, binding removal (reuptake) of the neurotransmitter They are frontline therapies for hypertension, depression, shock, asthma, angina & etc. drugs target:
  • 4. 28 June 2014 4  Tyrosine hydroxylase can be inhibited by methyl-p- tyrosine.  MAO: inhibitors of MAO (e.g., phenelzine, tranylcypromine)  The mobile pool; many indirect-acting sympathomimetics (e.g., amphetamine, ephedrine, tyramine) can displace NE from the mobile pool  Uptake: some indirect-acting sympathomimetics (cocaine, TCA). Drug Targets
  • 5. 28 June 2014 5  Prejunctional α-receptors: (e.g., clonidine, alpha methyldopa) cause inhibition of NE release.  Granular uptake of NE: blocker of granular uptake of NE (e.g., reserpine) .  NE release from granules: blockers (e.g., guanethidine).  Postjunctional receptors: postjunctional receptors can be activated or blocked. Drug Targets
  • 6. Classification 28 June 2014 6 divided into subgroups on the basis of their  Spectrum of action:  α, β, or dopamine receptor affinity  Mode of action:  direct, indirect or both
  • 7. 28 June 2014 7 Adrenergic agonists  Direct acting: I. α agonists: • Non selective, • α1-selective, • α2-selective II. β agonists: • Non selective, • β1-selective, • Β2-selective
  • 8. 28 June 2014 8  Indirect acting ↑ CAO in the synapse: 1. Releaser: Amphetamine, tyramine Potentiate by MAOI, COMT blocker. Why? 2. Reuptake inhibitor: Cocaine, TCA  Mixed: Ephedrine, metaraminol Adrenergic agonists
  • 9. 28 June 2014 9 Removal of NE may:  Diffuse out and enter the general circulation.  Be metabolized by COMT in the synaptic  Be recaptured by an uptake systems into the neuron
  • 10. 28 June 2014 10 Adrenoceptors  Selective for NE & EP.  dopamine can also activate some adrenoceptors at very high ‘supraphysiologic’ concentrations.  Divided into two main classes:  α & β adrenoceptors  All are members of GPCR superfamily.
  • 11. 28 June 2014 11 α-receptors: EP≥NE>>Isoproterenol
  • 13. 28 June 2014 13  based on their affinities for a agonists and blocking drugs, α-receptors are subdivided into two subgroups α1 & α2, e.g., α1 receptors have a higher affinity for phenylephrine than do α2 receptors.  Conversely, clonidine selectively binds to α2 receptors and has less effect on α1 receptors. α-adrenoceptors (α1 & α2)
  • 14. 28 June 2014 14 α-receptors:  α1 Are present on the postsynaptic membrane  α2 Located primarily on presynaptic nerve endings. The stimulation of α2 receptors causes feedback inhibition of the ongoing release of NE;  α2 Located on other cells such as the β-cell of the pancreas control insulin output.
  • 16. 28 June 2014 16 β-receptors:  Subdivided to β1, β2 and β3-receptors  β1-receptors have ~equal affinities for both EP & NE.,  β2-receptors have higher affinity for EP than for NE.  thus tissue with a predominance of β2-receptors (vasculature of skeletal muscle) are particularly responsive to hormonal effects of circulating EP released by adrenal medulla.
  • 17. 28 June 2014 17 β-receptors Mechanism of action:  binding of neurotransmitter at the β1 or β2-receptor→ result in activation of AC→↑cAMP concentrations within the cell.
  • 18. 28 June 2014 18 Mechanisms of action of adrenergic receptors :
  • 20. 28 June 2014 20 Desensitization of receptors:  Prolonged exposure to the CAO reduces the responsiveness of the receptors due to: 1. Sequestration of the receptors 2. Downregulation (destruction, or decreased synthesis) 3. An inability to couple to G-protein
  • 21. 28 June 2014 21 A. Catecholamine properties:  High potency in activating α & β receptors  Rapid inactivation by: 1. COMT postsynaptically, gut wall, 2. MAO intraneuronally, liver or gut Thus, CAO have only a brief duration of action when given parenterally, and are ineffective when administered orally because of inactivation.  Poor penetration into the CNS (polar)
  • 22. 28 June 2014 22 B. Non Catecholamine properties :  phenylephrine, ephedrine, amphetamine  Have longer t1/2 because they are not inactivated by COMT, and they are poor substrate for MAO  Increased lipid solubility permits the greater access to the CNS
  • 23. 28 June 2014 23 Major effects mediated by adrenoceptors
  • 24. 28 June 2014 24 SITE OF ACTION
  • 25. 28 June 2014 25 SITE OF ACTION