Alpha Blockers – Study Notes

Mechanism of Action

• Block α-receptors (mainly α1, sometimes α2).

• α1 on vascular smooth muscle → normally: NE/Epi → vasoconstriction → ↑TPR, ↑afterload, ↑BP.

  • Blocked → vasodilation → ↓TPR → ↓afterload + ↓BP.

• α1 on prostate + internal urethral sphincter → normally contraction → urine retention, aids ejaculation.

  • Blocked → relaxation → ↑urine flow, helps BPH symptoms.

Drug Classes

• Selective α1 blockers (act only on α1):

  • Terazosin, Doxazosin, Prazosin, Tamsulosin.

• Non-selective (α1 + α2 blockers):

  • Phenoxybenzamine → allosteric, irreversible.

  • Phentolamine → active site, reversible.

Indications

• Hypertension (2nd-line, esp. with BPH).

• BPH → relax sphincter + prostatic smooth muscle → ↑urine outflow.

• Tamsulosin (Flomax): also for ureteral calculi → relaxes ureter smooth muscle → stone passage.

• Pheochromocytoma:

  • Phenoxybenzamine (preferred, irreversible).

  • Phentolamine (reversible, less effective at high NE/Epi).

• Hypertensive crisis (MAOI + tyramine).

• Cocaine/meth-induced HTN: avoid β-blocker alone (risk unopposed α); use α-blocker (phentolamine/phenoxybenzamine).

Side Effects / Adverse Drug Reactions

• Vasodilation effects:

  • Hypotension.

  • Reflex tachycardia (↑SNS, RAAS activation).

  • Flushing.

  • Peripheral edema.

• Urinary system:

  • Urinary frequency, possible incontinence.

  • Retrograde ejaculation (semen → bladder).

• Sexual effects: ↓ejaculatory response (vas deferens relaxation).

• Ophthalmic:

  • Tamsulosin → intraoperative floppy iris syndrome (IFIS) in cataract surgery.

• α2 blockade (non-selective):

  • Prevents NE feedback inhibition → ↑NE release → pronounced reflex tachycardia.

Contraindications / Cautions

• Cataract surgery patients → avoid tamsulosin (risk of IFIS).

• Urinary incontinence → worsens symptoms.