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 Phenoxybenzamine
- 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
- Alpha-1 as well as beta receptor antagonists
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