Peripheral neural transmission: Adrenoceptor agonists & antagonists Flashcards
α
Noradrenaline
Agonist
α1 > α2, β1»_space; β2
Adrenaline
Uses?
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.
Isoprenaline
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
Phenylephrine
Uses?
Agonist
α1 selective (some v minimal action at β1)
Used to raise bloop pressure in acute hypotension.
Not taken up by uptake 1
MethylNA
α adrenoceptor agonist
α2»_space;> α1
Clonidine
Uses?
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.
Xylazine
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
Salbutamol
Uses?
β2 selective agonist
(some v minimal action at β1)
Used in asthma to relieve bronchoconstriction.
Dobutamine
Uses?
β1 selective agonist
(some minimal action at β2)
Used in acute cardiogenic shock
Phentolamine
Antagonist
α1, α2
Non selective- largely obsolete as antihypertensive as causes reflex tachycardia, postural hypotension.
Phenoxybenzamine
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.
Prazosin
Uses?
Antagonist
α1»_space; α2 (selective)
Used in hypertension
(But does give rise to postural hypotension)
Yohimbine
Antagonist
α2 > α1
Idazoxan
α2 selective antagonist
α2»_space; α1
Propranolol
Uses?
Antagonist
β1 , β2
Once widely used in hypertension, now replaced by more selective agents less likely to give rise to bronchoconstriction (β2 blockade)
Atenolol
Uses?
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.
Butaxamine
β2 selective antagonist
some minimal action at β1
Labetalol
Uses?
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
Xylometazoline
Uses?
α agonist (relatively non-selective)
Used to relieve nasal congestion
Salmeterol
Long acting β2 agonist (selective)
Asthma: used for prevention of a bronchospasm but not relief of acute attack (acts too slowly)
Mirabegron
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.
Tamsulosin
Uses?
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
Dutasteride
Uses?
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
Ergotamine
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.
