Alpha Blockers: Relaxing Vessels and the Prostate
Block the α1 receptor and two very different things happen: blood vessels relax and blood pressure falls, and the muscle around the prostate loosens so an older man can finally urinate freely. The same drug family that eases a blocked bladder also defuses one of medicine's most dramatic emergencies — a tumour flooding the body with adrenaline. But these drugs open with a notorious first move that can drop a patient to the floor.
An older man is delighted: after months of a weak urine stream and endless nighttime trips to the bathroom from an enlarged prostate, his new tablet has finally let him urinate normally. But the first night he took it, he stood up from bed and nearly fainted — the room spun, his vision greyed, and he had to grab the wall. Same drug, two faces: it relaxed the muscle strangling his urine flow, but it also relaxed his blood vessels so suddenly that his blood pressure dropped when he stood. Both effects come from blocking one receptor — α1.
What blocking α1 does
α1 receptors sit on vessels and internal sphincters. Remember the receptor map: α1 stimulation constricts blood vessels and contracts the smooth muscle of the bladder neck and prostate. So BLOCKING α1 does the reverse: it dilates blood vessels (lowering blood pressure) and relaxes the bladder-neck and prostatic muscle (letting urine flow). That's why one α1-blocker can be used for two seemingly unrelated problems — high blood pressure and the urinary symptoms of an enlarged prostate (benign prostatic hyperplasia, BPH).
The two groups of alpha blockers
The selective α1 blockers all end in '-osin': prazosin, doxazosin, terazosin, tamsulosin, and alfuzosin. They're used mainly for BPH — tamsulosin is especially 'uroselective,' targeting the prostate with less blood-pressure effect — and sometimes as an add-on for hypertension (though not first-line). Prazosin has a niche in treating the nightmares of post-traumatic stress disorder. The non-selective alpha blockers — phenoxybenzamine (irreversible) and phentolamine (reversible) — block both α1 and α2 and are reserved for a special emergency: a catecholamine-secreting tumour called pheochromocytoma.
A pheochromocytoma pumps out adrenaline and noradrenaline, causing episodes of pounding headache, sweating, palpitations, and severe spikes of blood pressure. Before surgery to remove it, doctors must blockade the α receptors to control the dangerous vasoconstriction — using phenoxybenzamine. Only AFTER good alpha blockade may a beta blocker be added. Give a beta blocker first and you invite disaster (see the pearl).
In pheochromocytoma: alpha blockade FIRST, then beta. If you give a beta blocker first, you block the β2 vasodilation that was partly relaxing the vessels, leaving adrenaline's α1 vasoconstriction completely 'unopposed' — and the blood pressure can rocket into a hypertensive crisis. 'A before B' is one of pharmacology's most important ordering rules, and the same 'unopposed alpha' danger is why beta blockers are avoided in cocaine toxicity.
- Blocking α1 dilates vessels (↓BP) and relaxes the bladder neck/prostate (↑urine flow).
- '-osin' drugs (tamsulosin, doxazosin…) → mainly BPH, sometimes hypertension.
- Phenoxybenzamine/phentolamine (non-selective) → pheochromocytoma.
- In pheochromocytoma: alpha block BEFORE beta block (avoid unopposed α crisis).
The first-dose phenomenon and other effects
The dizzy spell in our patient has a name: the first-dose phenomenon. The very first dose of an α1 blocker can cause a sharp drop in blood pressure on standing (orthostatic hypotension), sometimes with fainting — which is why these drugs are started at a low dose, at bedtime. Related effects include general dizziness, a reflex fast heartbeat as the body tries to compensate for the lower pressure, and nasal stuffiness (from dilated nasal vessels). One more to know: tamsulosin can cause 'floppy iris syndrome,' complicating cataract surgery, so eye surgeons need to know a patient is taking it.
- Giving a beta blocker before an alpha blocker in pheochromocytoma. Risks a hypertensive crisis.
- Starting an α1 blocker at full dose. The first-dose drop can cause fainting — start low, at night.
- Forgetting to tell the eye surgeon about tamsulosin (floppy iris syndrome).
- Using α1 blockers as first-line for hypertension. They're add-ons, not first choice.
Why must alpha blockade precede beta blockade when preparing a pheochromocytoma patient?
- α1 blockers dilate vessels (↓BP) and relax bladder neck/prostate (↑urine flow).
- '-osin' drugs → BPH (tamsulosin most uroselective); phenoxybenzamine/phentolamine → pheochromocytoma.
- Watch the first-dose orthostatic hypotension; start low, at bedtime.
- Pheochromocytoma rule: alpha block before beta block (avoid unopposed α).
- Katzung BG. Basic & Clinical Pharmacology — Adrenoceptor antagonists: alpha blockers.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — α-Adrenergic receptor antagonists.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Alpha-adrenoceptor antagonists.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Alpha-adrenergic blockers.
- AUA / clinical guidelines — Alpha blockers in BPH; pheochromocytoma perioperative alpha blockade.
