OTHER ALPHA-ADRENOCEPTOR ANTAGONISTS
Alfuzosin is anα1-selective quinazoline derivative that is approvedfor use in BPH. It has a bioavailability of about 60%, is extensively metabolized, and has an elimination half-life of about 5 hours. It may increase risk of QT prolongation in susceptible individuals. Silodosin resembles tamsulosin in blocking theα1Areceptor andis used in treatment of BPH. Indoramin is another α1-selective antagonist that also has efficacy as an antihypertensive. It is not available in the USA. Urapidil is anα1 antagonist (its primary effect) that also has weak α2-agonist and 5-HT1A-agonist actions and weak antagonist action at β1 receptors. It is used in Europe as an antihypertensive agent and for benign prostatic hyperplasia. Labetalol has bothα1-selective andβ-antagonistic effects; it isdiscussed below. Neuroleptic drugs such as chlorpromazine and haloperidol are potent dopamine receptor antagonists but are alsoantagonists at α receptors. Their antagonism of α receptors prob-ably contributes to some of their adverse effects, particularly hypotension. Similarly, the antidepressant trazodone has the capacity to block α1 receptors. Ergot derivatives, eg, ergotamine and dihydroergotamine, cause reversible α-receptor blockade, probably via a partial agonist action .
Yohimbine, an indole alkaloid, is anα2-selective antagonist.It is sometimes used in the treatment of orthostatic hypotension because it promotes norepinephrine release through blockade of α2receptors in both the central nervous system and the periph-ery. This increases central sympathetic activation and also pro-motes increased norepinephrine release in the periphery. It was once widely used to treat male erectile dysfunction but has been superseded by phosphodiesterase-5 inhibitors like sildenafil . Yohimbine can greatly elevate blood pressure if administered to patients receiving norepinephrine transport blocking drugs. Yohimbine reverses the antihypertensive effects of α2-adrenoceptor agonists such as clonidine. It is used in veterinarymedicine to reverse anesthesia produced by xylazine, anα2 agonist used to calm animals. Although yohimbine has been taken off the market in the USA solely for financial reasons, it is available as a “nutritional” supplement.