Tuesday 14 June 2011

Adrenergic Neuroeffector Junctions

CATECHOLAMINE SYNTHESIS, ACTION, AND DEGRADATION

The important aspects of  the adrenergic neuroeffector junction are summarized in Figure



 Adrenergic Neuro-Effector Junction

Tyrosine  is actively transported into nerve endings and is converted to dihydroxyphenylalanine
(DOPA)  via  tyrosine hydroxylase (1).  This step is rate limiting in the synthesis of NE. DOPA is
converted to dopamine (DA) via L-aromatic amino acid decarboxylase  (DOPA decarboxylase).
DA  in turn is metabolized to NE via DA  beta hydroxylase and is  taken up and stored in gran-
ules (6).  Inactivation of NE via monoamine oxidase (MAO) (2) may regulate prejunctional  lev-
els of transmitter in the mobile pool (3) but not the NE stored in granules.

Presynaptic membrane depolarization opens voltage-dependent  ca2+  channels. Influx of  this
ion causes fusion of  the synaptic granular membranes, with the presynaptic membrane leading
to WE  exocytosis  into the neuroeffector junction (7). NE then activates postjunctional receptors
(8),  leading  to tissue-specific responses depending on the adrenoceptor subtype activated.

Termination of NE actions is mainly due  to removal from the neuroeffector junction back into the
sympathetic nerve ending via a NE transporter system (uptake 1) (4). At  some sympathetic nerve
endings,  the NE released may activate prejunctional alpha adrenoceptors (5)  involved in  feedback
regulation,  which results  in decreased release of the neurotransmitter. Diffusion away from the neu-
roeffector junction may also contribute to termination of actions. NE accumulated  into target cells
(e.g.,  via uptake 2) is rapidly inactivated by catechol-0-methyltransferase (COMT).

DRUG "TARGETS"

1.  Tyrosine Hydroxylase

Tyrosine hydroxylase can be  inhibited by methyl-p-tyrosine and  is  subject  to feedback inhibi-
tion by high levels of NE in  the mobile pool.

2.   MAO

Inhibitors of MA0 (e.g.,  phenelzine, tranylcypromine) may increase prejunctional  levels of NE.
Note that MA0 type A,  the enzyme form that metabolizes NE, also metabolizes tyramine and
serotonin  (5HT).

3. The Mobile Pool

Many  indirect-acting sympathomimetics  (e.g., amphetamine, ephedrine, tyramine)  can dis-
place NE from the mobile pool.

4. Uptake 1

Some indirect-acting sympathomimetics  (eg,  cocaine, tricyclics) inhibit uptake into the nerve
cell, increasing the postjunctional actions of NE.

5. Prejunctional Alpha Receptors

Activators of prejunctional alpha receptors (e.g., clonidine, alpha methyldopa) cause inhibition
of NE release from synaptic vesicles.

6. Granular Uptake of NE

Blockers of  granular uptake of  NE  (e.g.,  reserpine) decrease prejunctional levels available for
release.

7.  NE Release From Granules

Blockers of NE  release  from granules (e.g.,  guanethidine) decrease postjunctional actions of NE.

8.  Postjunctional Receptors

Postjunctional receptors can be activated  by  many  direct-acting  sympathomimetics. These
receptors are also  "targets"  for many antagonist drugs.















 Fig:   Effect of Alpha Activators on Heart Rate and Blood Pressure

Drugs
Phenylephrine (al):  decongestant-mydriasis  without cycloplegia.
Methoxamine  (al):  use in paroxysmal atrial  tachycardia-elicits vagal reflex.
 
a,  Agonists
Stimulate prejunctional receptors in the CNS  to decrease vasomotor outflow and decrease mean
BP. Primary use is in mild-to-moderate HTN.
 
Drugs
 
Clonidine:  initial increase in BP (some al  activity) followed by decrease-abrupt  discontinua-
tion (DC) causes rebound HTN. 
a-Methyldopa: a pro-drug  forming a-methyl NE.
 
Beta-Agonists

Beta1: increase HR, SV, and CO. 
Beta2:decrease PVR



Fig: Effect of Beta  Activators on Heart Rate and Blood Pressure.


Fig: Effect of Norepinephrine on Heart Rate and Blood Pressure

Norepinephrine  (NE) has little effect on p2  receptors. It increases PVR and both diastolic and
systolic BP.  Positive inotropic action of NE causes a small to moderate increase in pulse pres-
sure  (PP). Compensatory  vagal  reflexes  tend  to  overcome the direct positive chronotropic
effects of NE (reflex bradycardia may ensue), but the positive inotropic effects are maintained.


Fig: Effect of Epinephrine on Heart Rate and Blood Pressure

Other actions-lungs  (bronchodilation), liver (glycogenolysis stimulated-increase  serum glucose),
pancreas  (J  insulin release because  a2  predominates), muscle (glycogenolysis  and glycolysis),
fat (activates lipase-?  free fatty acids  [FFAs]).

Clinical uses-anaphylaxis,  cardiac arrest, adjunct to local anesthetics, glaucoma.

INDIRECT-ACTING ADRENOCEPTOR AGONISTS

Drugs
 
Tyramine: Present in certain foods and beverages-rapidly metabolized by MA0 type A  in GI
tract  and  liver. At  high  levels  displaces NE  from  the mobile  pool,  thus  increasing pressor
response-potential problem when MA0  inhibitors are present, leading to hypertensive crisis.
No CNS entry. Amphetamine. Releases NE from mobile pool in nerve ending, causing increased
NE activity-peripheral effects are those of  sympathetic stimulation with reflex bradycardia.
CNS  actions  include  release of  both NE  and DA. Clinical  uses  include ADHD,  short-term
weight loss, and narcolepsy (see CNS section).
 
Ephedrine: Releases NE from sympathetic neurons and can cause amphetamine-like CNS  effects.
Clinical use: decongestant (note  similar use of phenylpropanolamine and pseudoephedrine).
 
Cocaine: Blocks reuptake of NE into sympathetic nerve endings, causing vasoconstriction (this
plus block of axonal membrane Na+ channels forms the basis for clinical use). CNS effects are
probably via block of reuptake of both NE and DA  (see CNS section).
 
Dopamine: DA  is probably not a "natural" transmitter in periphery but can be  released from
some sympathetic fibers. When infused, it has dose-dependent actions on DA and adrenocep-
tors. Low dose  increases  renallmesenteric blood  flow via Dl activation  + ? RBF  and GFR.
Medium dose ? CO (positive inotropy) via P1  activation. Useful in management of shock states.
Very high dose causes vasoconstriction-? systolic and diastolic BP via al  activation.






Clinical Uses andlor Characteristics  of Beta Blockers

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