Peripheral neural transmission: Adrenoceptor agonists & antagonists

Question 1

Noradrenaline

Answer 1

Agonist

α1 > α2, β1&raquo_space; β2

Question 2

Adrenaline

Uses?

Answer 2

Agonist
α2 > α1 > β1, β2

Treatment of acute cardiac failure.
Treatment of anaphylactic shock
(Anaphylactic reactions involve swelling of skin and mucous membranes, bronchospasm- obstructs breathing, & cardiac collapse due to vasodilatation. Histamine is an imp mediator)
Adr given i.v or i.m gives powerful vasopressor effect & relieves bronchospasm.

Question 3

Isoprenaline

Answer 3

Agonist
Selective: β1, β2
About 10x more potent than Adr

Used in asthma but increase in HR due to β1 effects was a major problem- replaced by more selective agents

Question 4

Phenylephrine

Uses?

Answer 4

Agonist
α1 selective (some v minimal action at β1)
Used to raise bloop pressure in acute hypotension.
Not taken up by uptake 1

Question 5

MethylNA

Answer 5

α adrenoceptor agonist

α2&raquo_space;> α1

Question 6

Clonidine

Uses?

Answer 6

α adrenoceptor agonist
α2 > α1 (slightly selective)

Anti-hypertensive: stimulates receptors in the hind brain. But omitting a single dose can lead to rebound hypertension- not used widely in UK.

Question 7

Xylazine

Answer 7

α2 selective agonist
(some v minimal action at α1- more selective than clonidine)

Used in vetmed for its sedative effects through an action on α2 receptors in CNS

Question 8

Salbutamol

Uses?

Answer 8

β2 selective agonist
(some v minimal action at β1)

Used in asthma to relieve bronchoconstriction.

Question 9

Dobutamine

Uses?

Answer 9

β1 selective agonist
(some minimal action at β2)

Used in acute cardiogenic shock

Question 10

Phentolamine

Answer 10

Antagonist
α1, α2

Non selective- largely obsolete as antihypertensive as causes reflex tachycardia, postural hypotension.

Question 11

Phenoxybenzamine

Answer 11

Irreversible antagonist (a 2-chloroethylamine like BCM) 
α2 > α1

May be used with atenolol to prevent effects of large release of catecholamines that takes place during surgery to remove a phaeochromocytoma when the tumour is displaced.

Question 12

Prazosin

Uses?

Answer 12

Antagonist
α1&raquo_space; α2 (selective)

Used in hypertension
(But does give rise to postural hypotension)

Question 13

Yohimbine

Answer 13

Antagonist

α2 > α1

Question 14

Idazoxan

Answer 14

α2 selective antagonist

α2&raquo_space; α1

Question 15

Propranolol

Uses?

Answer 15

Antagonist
β1 , β2

Once widely used in hypertension, now replaced by more selective agents less likely to give rise to bronchoconstriction (β2 blockade)

Question 16

Atenolol

Uses?

Answer 16

β1 selective antagonist
(some minimal action at β2)

Antihypertensive, cardiac dysrhthmias, angina
May be used with phenoxybenxamine to prevent effects of large release of catecholamines that takes place during surgery to remove a phaeochromocytoma when the tumour is displaced.

Question 17

Butaxamine

Answer 17

β2 selective antagonist

some minimal action at β1

Question 18

Labetalol

Uses?

Answer 18

Antagonist
α1, β1, β2

Four isomers, each of which has different actions: 
R,R: β blocker, weak α1 blocker
R,S: no activity
S,R: α1 blocker, v weak β blocker
S,S: α1 blocker

Used to treat hypertension in pregnancy

Question 19

Xylometazoline

Uses?

Answer 19

α agonist (relatively non-selective)

Used to relieve nasal congestion

Question 20

Salmeterol

Answer 20

Long acting β2 agonist (selective)

Asthma: used for prevention of a bronchospasm but not relief of acute attack (acts too slowly)

Question 21

Mirabegron

Answer 21

β3 agonist
Relexes bladder detrusor muscle- use in treatment of overactive bladder (increases bladder capacity).

Recommended by NICE in July 2013 only for patients in whom anti-muscarinic drugs ate ineffective/ contra-indicated/ not tolerated.

Question 22

Tamsulosin

Uses?

Answer 22

Selective α1A receptor antagonist

Used in benign prostatic hyperplasia: relaxes smooth muscle in prostate & neck of bladder, allows better bladder emptying.

May be used as co-therapy with dutasteride

Question 23

Dutasteride

Uses?

Answer 23

Antiandrogenic effects
Inhibits all 3 isoforms of 5-α reductase (rate limiting step in synthetic pathway for testosterone)

May be used as co-therapy with Tamsulosin

Question 24

Ergotamine

Answer 24

Ergot alkaloid
Partial agonist at α- receptors
Also acts at dopamine, 5-HT receptors
Used in migrane- probably involves 5-HT receptors

Ingested by eating ergot-infected cereals -> causes St Anthony’s fire, an intense peripheral vasoconstriction (mediated by α1) that can lead to gangrene.