adrenergic agonists lecture 1_clark

What is an adrenergic response?

The fight or flight response

Lecture outline

• Catecholamine- Synthesis- Storage/release- Termination

Reuptake/Metabolism/Drug interventions- Receptor classification

Locale/Physiological effects- Molecular Mechanisms- Factors determining receptor activity

Nerve Endings

Tyrosine Hydroxylase

• Tyrosine Hydroxylase is rate limiting

• An increase in sympathetic n.s. activity leads to: in NE release in NE Pool and release of feedback inhibition in Feedback inhibition (NE, PKA, PKC) in TH activity in NE synthesis

Dopamine β-hydroxylase

• Found exclusively in vesicles that store NE or EPI• Lack of substrate specificity• Converts a broad range of phenylethylamines

(PEA’s) to β-hydroxylated products that can be stored.

Storage and Release

Sympathetic Nervous System (SNS) activation generates nerve action potentials and influx of Ca++ at the nerve terminus that stimulates

-NE and EPI (100:1) release from the presynaptic postganglionic neuron into the synaptic compartment: mechanism is termed Stimulus Secretion-Coupling (or exocytosis).

-EPI and NE release (85:15) from the Adrenal Medulla Chromaffin Granules (remember that stimulation of preganglionic nerves cause release of Ach that activates nicotinic AChR on the adrenals). Mechanism of release is similar to stimulus secretion coupling and catecholamines are emptied into the blood stream.

Note: ATP is present in vesicles at high concentration and plays significant but ill understood roles following activation of purinergic receptors (there are many).

STORAGE

INTERACTION WITH EFFECTOR CELL

Termination of NE Action

• Reuptake into adrenergic neurons: Two step uptake; uptake I (PM) and vesicular uptake

• NE feedback inhibition on α2-adrenergic receptors; blocks release of NE

• Diffusion from site of action and uptake at non- neuronal sites (uptake II)

• Inactivation by catechol-o-methyl transferase (COMT) and monoamine oxidase (MAO)

• Desensitization of NE action on the effector cell

Inhibitory feedback loop blocksNE release through 2-adrenergic

receptors

TERMINATION OF ACTION

Uptake1

Vesicularuptake Uptake 2

diffusion

NE transporter (uptake 1)

• Requires Na+ and ATP• Active process

• Steriospecific (l-NE); Km about 1.0 M

• Works against a concentration gradient• Many compounds are good substrates (EPI, NE,

DA, tyramine (TA), PEA, guanethidine, amphetamine

REUPTAKE BLOCKERS

NE Uptake 1 Inhibitors potentiate sympathetic stimulation

• Cocaine: -competitive inhibitor of NE uptake-relatively non-specific; blocks serotonin (5HT) and dopamine reuptake

• Tricyclic antidepressantsdesipramine (NE selective) imipramine (tofranil)amitryptyline (elavil), relatively 5HT selectivefloxetine (Prozac), blocks DA, NE and

serotonin uptake

Vesicular uptake system and inhibitors

• Vesicular uptake an active process requiring Mg and ATP• Uptake of catechols is non-specific; works for NE, EPI,

DA as well; • Inhibitor: Reserpine, an alkaloid from the roots of

Rauwolfia serpentina originally used as an anti-psychotic– Blocks the ability of vesicles to take up and store

biogenic amines NE, dopamine, 5HT – Essentially irreversible in action– Once used to treat hypertensionGuanethidine: somewhat similar drug is taken up and

concentrated in nerve endings and interferes with stimulus secretion coupling-bad side effects

Monoamine Oxidase and Inhibitors

• Converts catechols to aldehydes non-selectively• Found in sympathetic nerve endings, liver and intestinal

mucosa• Protects body against phenylethylamines in diet like

tyramine (found in beer, cheese, wines, yeast, soy sauce)• Inhibitors include:

-clorgyline for MAO-A (5HT selective)-selegiline for MAO-B (dopamine selective

therapy for Parkinson’s)isocarboxazid-rarely used

Question: What happens if a patient on a MAO inhibitor eats a large quantity of PEA’s?

COMT Inhibitors

• Entacapone and Tolcapone– USE: As adjunct therapy with DOPA (l-dopa)

in Parkinson’s Disease– MOA: inhibits COMT and prolongs DOPA

action

Excretion of Catecholamines in HumansAmount in Urine/ 24 hours

3-Methoxy-4-hydroxymandelic acid(VMA) 3.8

3-Methoxy-4-hydroxyphenylethylene glycol (MOPEG) 1.5

Normetanephrine 0.2

Metanephrine 0.1

Norepinephrine 0.03

Epinephrine 0.005

mg

Drugs can Enhance Adrenergic Neurotransmission by Different Mechanisms

Mechanisms

1. Increase release of NE

2. Mimic Interaction with postjunctional receptors

3. Inhibit termination of NE

Example

• Tyramine : displacement

• Yohimbine : blocks 2 receptors

• Phenylephrine : 1 agonist

• Dobutamine : 1 agonist

• Cocaine, desipramine : blocks neuronal uptake

• Pargyline, clorgyline, selegiline : MAO inhibitors

Drugs Can Inhibit Adrenergic Neurotransmission by Different Mechanisms

Mechanisms Example

Inhibit synthesis of NE - Methyltyrosine : inhibits tyrosine hydroxylase

Inhibit storage of NE Reserpine : blocks vesicle transport systemGuanethidine : blocks Ca++ dependent exocytosis

Inhibit release of NE Clonidine : agonist at presynaptic 2 receptors

Inhibit NE interactions with postjunctional receptors

Prazosin : 1 antagonistMetoprolol : 1 antagonist

Classification of Adrenergic Receptors

• Discovery

• Subtypes of alphas and betas

• Functions

• Localization

Discovery/Classification of Adrenergic Receptors

• Originally based on complex and often paradoxical physiological responses to NE, EPI and ISO: led to concept of and receptors

• Synthesis of new adrenergic agonists and antagonists allowed distinction of subtypes first, and then subtypes

• Cloning further clarified the subtypes and allowed their independent study after individual expression in cells grown in vitro

Comparison of agonist potency on - and - adrenergic receptors

: EPI>NE>>ISO: ISO>EPI>>NE

1: ISO>EPI = NE2: ISO>EPI>>NE

Adrenergic Receptor Classification

-Adrenergic

1

2

3

-adrenergic

1A

1B

1D

2A

2B

2C

Adrenergic Receptors

Alpha () receptors are generally excitatory (constriction/contraction) except in gut

Mechanism 1 : Ca++

Beta () receptors are inhibitory (e.g. 2 lung relaxation;) or stimulatory (heart, liver and fat cells)

Mechanism: cAMP

“Fight or Flight”

Epi

NE

FAT

Factors determining the actions of catecholamines on ARs

• Reuptake• Catabolism • Density of receptors• Relative proportion of

and locale of adrenergic receptors

• Potency of agonist• Efficiency of agonist

• Desensitization• Down regulation• Removal of 2nd

messenger• Pharmacogenetics• Reflexes

-Adrenergic Receptors1

Heart (stimulation)

Fat ( lipolysis; 3 also)Kidney (↑ renin release)

Amylase secretion (thick)

Eye: Increase in aqueous humor; relaxes ciliary muscle

2

Arterioles of liver and skeletal muscle (relax)

Coronary and cerebral vessels (relax)

Bronchioles (relax) Uterine smooth muscle

(relax)Metabolic (liver, muscles)Skeletal muscle (↑contraction)Mast Cell granule release (inhibition)

-Adrenergic Receptors

1-receptors– Constriction of smooth

muscle

• Arterioles of skin, mucosa, abdominal viscera

• arterioles of liver and

skeletal muscle

• Coronary/cerebral vessels

• Veins

• bladder sphincter, seminal tract, iris radial SM

Relaxation intestinal wall SM

2-receptors– Presynaptic inhibition of

NE release– Constriction in arterioles of

kidney, abdominal viscera and veins

– Platelet aggregations– Decreased secretion of

insulin

Different blood vessels contain different densities of / receptors

Renal, skin vessels, mucosa mainly 1

Abdominal viscera, /

Skeletal muscle vessels, mainly 2

(*Think about blood flow in Fight or Flight)

REDISTRIBUTION OF BLOOD FLOW

ANS AMAZING FACT

Cardiovascular reflexes may OVERRIDE the direct effects of a drug

Example: slowing of heart after injection of Norepinephrine

Homeostatic reflexes complicate drug effects

Example: what are the consequences of slowly infusing NE into a patient?

NE directly produces TACHYCARDIA and vasoconstriction.However, the net effect of NE infusion is often a large increase in TPR, a moderate increase in MAP, and BRADYCARDIA. Why?

Pharmacogenetics

2-adrenergic receptor is highly polymorphic (altered amino acid sequence)

• Altered structure may affect agonist activation or desensitization

– E.g.; Thr Ile 164 impairs agonist activation

– Patients with this variant of the 2-adrenergic receptor

appear to have increased risk of dying from congestive heart failure and go to the front of the line for transplant

Mechanisms of Desensitization of the β2-Adrenergic Receptor

Homologous PKA-mediated

Heterologous GRK/Arrestin- mediated

1.Agonist binding (NE or EPI) rapidly activates adenylyl cyclase and PKA and PKA phosphorylates the β2AR

2.Agonist binding to the β2AR also rapidly activates GRKs

3.GRK phosphorylation of the β2AR leads to Arrestin binding and uncoupling

3. Arrestin binding leads to clathrin coated pit internalization of the β2AR

4. Internalized receptor is recycled to the plasma membrane (dephosphorylation occurs both internally and at the plasma membrane resenstizing the β2AR )

5. A small fraction of the β2AR is downregulated (degraded) slowly (t1/2 = 3-4 hrs)

Billington and Penn, Respir. Res 2003,4:2