𝝰-ADRENOCEPTOR ANTAGONISTS (𝝰 blockers)...
EXAMPLES: Doxazosin, prazosin, tamsulosin, alfuzosin.
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.
Tags:
Pharmacology