𝝰-adrenoceptor antagonists (𝝰 blockers)

𝝰-adrenoceptor antagonists (𝝰 blockers)...

1. EXAMPLES: Doxazosin, prazosin, tamsulosin, alfuzosin.
2. MECHANISM OF ACTION: Inhibit 𝝰1-adrenoceptors in arterioles, thereby reducing tone of vascular smooth muscle and reducing total peripheral resistance.
• Inhibition of 𝝰1-adrenoceptors in periurethral prostatic stroma results in relaxation of internal urethral sphincter and some relief of obstructive urinary symptoms in males.

• Hypertension (i.e. doxazosin, particularly in resistant cases as part of polytherapy).
• Benign prostatic hyperplasia.

• Caution in patients with a susceptibility to heart failure.

• Postural hypotension.
• Dizziness.
• Weakness and fatigue.
• Reflex tachycardia.
• Headache.
• Dry mouth.
• Ejaculatory failure.

• Variable – e.g. doxazosin (t½ ～22 h) extensively metabolised by liver; alfuzosin (t½ ～3–5 h) partially metabolised.

• May cause severe first-dose hypotension therefore need to start at low dose and warn patient of side-effects.

• Enhanced hypotensive effect with antihypertensives and alcohol.

• Centrally acting 𝝰2-adrenoceptor agonists (e.g. clonidine, 𝝰 methyldopa) also have an antihypertensive effect (mediated via suppression of the vasomotor centre in the brain).
• These agents are rarely used due to infrequent but potentially severe adverse effects (𝝰 methyldopa may cause hepatitis). 𝝰 methyldopa continues to be used for hypertension in pregnancy.