α-ADRENOCEPTOR ANTAGONISTS (also known as α-adrenergic receptor antagonists, α-adrenoceptor blocking drugs or α-blockers) are drugs that inhibit certain actions of the sympathetic nervous system by preventing the action of adrenaline and noradrenaline (catecholamine mediators acting predominantly as hormone or neurotransmitter, respectively) by acting as antagonists at the α-adrenoceptors on which the catecholamines act. (Correspondingly, β-ADRENOCEPTOR ANTAGONISTS are drugs used to inhibit the remaining actions, by occupying the other class of adrenoceptor, β-adrenoceptors).
In disease states some sympathetic actions may be inappropriate, exaggerated and detrimental, so α-blockers may be used to restore a balance. One use of antagonists is in lowering blood pressure when it is raised in cardiovascular disease (see ANTIHYPERTENSIVE AGENTS), since they prevent the vasoconstrictor actions of noradrenaline and adrenaline (including in phaeochromocytoma), though a high incidence of side-effects means they are nowadays much less used. The apblockers are also used to treat urinary retention in benign prostatic hyperplasia (through an action on the blood circulation within the prostate).
Examples of α1-blockers include compounds of diverse structures, such as the synthetic heterocyclics prazosin, indoramin. phentolamine; the ergot alkaloids ergotamine and dihydroergotamine; and the haloalkylamine irreversible alkylators, e.g. phenoxybenzamine. Examples of antagonists relatively selective for α2-receptors over α1-receptors, are the natural indolealkylamine alkaloid yohimbine and its diastereoisomer rauwolscine (though they also have affinity for 5-HT receptors). However, many of the α1-blockers (especially prazosin) also have some affinity at the α2-adrenoceptor site.