Alpha blockers: Selectivity

Alpha receptors
Alpha-1 Predominantly expressed on vascular smooth muscle. Agonists cause increase in tone resulting in hypertension (and reflex bradycardia). Think phenylephrine. Antagonists have the opposite effect.
Alpha-2 Expressed on presynaptic receptors. Agonist activity results in feedback inhibition of sympathetic outflow. Thus, a decrease in BP and bradycardia (dexmedetomidine or clonidine).

Alpha Blockers

  • Irreversible antagonist of a1>a2 receptors
  • Primarily used in the treatment of pheochromocytoma, as it helps to block the effects of circulating catechols


  • Competitive (reversible) antagonist of a1 = a2
  • Alpha-2 antagonism can result in positive inotropic and chronotropic effect on the heart
  • Can be injected subcutaneously to prevent dermal necrosis after extravasation of drugs with a1 agonist activity (pressors)

Prazosin (and other -zosins)

  • Alpha-1 selective antagonist
  • Useful in treatment of HTN, as well as BPH (alpha-1 also expressed on prostate)
  • Major side effect is orthostatic hypotension


  • Alpha-2 selective antagonist
  • Formerly used in the treatment of orthostatic hypotension
  • Unavailable in US

Labetalol, Carvedilol

  • Alpha-1 as well as beta receptor antagonists


Keyword history

Defined by: Pablo Kollmar, MD