Alpha receptor blocker

1. Dr. Veena Nayak
Associate . Professor
Dept of pharmacology
KMC, Manipal

2. Receptor Location Effect of agonist
α1 Smooth muscle
Blood vessels, anal and urinary
sphincter,
Radial muscle of iris, trigone of
bladder, vas deferens and
prostate
Contraction of all except intestinal
smooth muscle
α2 Presynaptic on nerve endings Inhibits negative feedback on release of
nor adrenaline
α2 Postsynaptic
Beta cells of pancreas, ciliary
epithelium
decrease secretion of insulin and
aqueous humor

3. Alpha blockers

4. Classification
Non-selective
 Reversible - Phentolamine
 Irreversible- Phenoxybenzamine
Selective
α1- prazosin, terazosin, doxazosin, tamsulosin, alfuzosin, silodosin
α2 – yohimbine

5. Other drugs with alpha blocking property
Chlorpromazine
Ergotamine
Ergotoxine

6. Alpha agonist Alpha antagonist
Eye Mydriasis Miosis
BP Increases BP Hypotension (decreased venous return , CO)
Reflex tachycardia ( presynaptic action )
GIT Relaxation of smooth
muscle
Increased peristalsis
Bladder Contraction of sphincter Decreased tone of bladder trigone, sphincter
and prostate
Urine flow increases
Male
reproductive
organs
Ejaculation Inhibits ejaculation and impotence

7.  Nasal stuffiness
 Hypotension – decrease in GFR – reflex increase in Na and
water reabsorption
Sodium water retention
Expansion of blood
volume
Increased Renin
production

8. Phenoxybenzamine
 Non selective (α1 and α2 ) and irreversible blocker of alpha
receptors
 Cyclises to produce reactive metabolite which bind covalently to
the receptors
 Has a long duration of action
 Side effects – postural hypotension, tachycardia, diarrhoea, nasal
stuffiness, giddiness, impotence
 Can be used orally, parenterally
 Used in pheochromocytoma

9. Phentolamine
 Competitively blocks α1, α2 receptors
 Rapid onset and short duration of action
Uses
 Diagnosis of pheochromocytoma
 intraoperatively during surgery of pheochromocytoma
 hypertensive emergencies
 Local infiltration to overcome extravasation
 Side effects – tachycardia, palpitation, arrythmias, precipitation of
angina

10.  C/F- sudden and paroxysmal rise in BP, headache,
palpitation, sweating
 Urinary VMA elevated
 Confirmation – CT scan, MRI
 Definitive treatment – surgery
 Preoperative for 1-2 weeks– Tab Phenoxybenzamine and Propranolol
 Intraoperatively – inj phenoxybenzamine / phentolamine given
1) Pheochromocytoma

11.  Propranolol - beta blocker.
 To control tachycardia and arrythmias due to excess release of
catecholemines
 Should be used only after an alpha blocker

12. Selective alpha blockers

13. Prazosin
 Selectively blocks α1
Hypotension but no tachycardia
"First dose effect"
Other side effects – less
Route: orally
Uses
Hypertension
Raynaud's disease and benign hypertrophy of prostate (BHP)

14.  Terazosin Doxazosin – longer acting (24h)- HTN, BHP
 Alfuzosin- short acting (3-5h) - BHP
 Tamsulosin – uroselective blocker α1A/1D
No antihypertensive effect
Used as SR tablets for patients with BHP
 Sildosin-selective α1A blocker, used in BHP

16. Other drugs
 Yohimbine
 Chlorpromazine
 Ergot derivatives

17. Uses of alpha blockers
 Pheochromocytoma – phenoxybenzamine, phentolamine
 Essential hypertension - Prazosin, terazosin, doxazosin
 Benign hypertrophy of the prostate – tamsulosin, silodosin
 Hypertensive emergencies ( cheese reaction, clonidine withdrawal) –
phentolamine IV
 To prevent tissue necrosis- phentolamine local infiltration
 Peripheral vascular diseases – Raynauds –prazosin

18. Adverse effects of alpha blockers
 First dose effect
 Nasal stuffiness
 Miosis
 Reflex tachycardia
 Impotence
 diarrhoea

19. Take home points – alpha blockers
 Phenoxybenzamine , phentolamine – non selective
 Prazosin – HTN, first dose phenomena
 Tamsulosin , silodosin- BHP