α-blockers

Also known as

• Alpha blockers

Key examples

• Doxazosin
• Tamsulosin
• Alfuzosin

Common indications

• As a first-line medical option to improve symptoms in benign prostatic hyperplasia.
• As an add-on treatment in resistant hypertension, when other medicines (e.g. calcium channel blockers, ACE inhibitors, thiazide diuretics) are insufficient.

 

Mechanisms of action

• Although often described using the broad term ‘α-blocker,’ most drugs in this class (including doxazosin, tamsulosin and alfuzosin) are highly selective for the α₁-adrenoceptor. 
• Alpha₁-adrenoceptors are found mainly in smooth muscle, including in blood vessels and the urinary tract (the bladder neck and prostate in particular). 
• Stimulation induces contraction; blockade induces relaxation. 
• Alpha₁-blockers therefore cause vasodilatation and a fall in blood pressure, and reduced resistance to bladder outflow.

 

Important adverse effects

• Predictably from their effects on vascular tone, α-blockers can cause postural hypotension, dizziness and syncope. This is particularly prominent after the first dose (rather like with ACE inhibitors and angiotensin receptor blockers).

 

Warnings

• Alpha-blockers should not be used in patients with existing postural hypotension.

 

Important interactions

• In general, combining antihypertensive drugs results in additive blood pressure lowering effects (this may well be the therapeutic aim). To avoid pronounced first-dose hypotension, it may be prudent to omit doses of one or more existing antihypertensive drugs on the day the α-blocker is started. This is particularly the case for β-blockers, which inhibit the reflex tachycardia that forms part of the compensatory response to vasodilatation.