alpha-adrenoceptor
Alpha-adrenoceptors are one of the two main families of receptors that listen for norepinephrine and adrenaline. Their signature job is to tighten things, above all the smooth muscle wrapped around blood vessels, which is why activating them raises blood pressure.
They are G-protein-coupled receptors with two subtypes. Alpha-1 receptors sit on smooth muscle (vessels, bladder neck, prostate, pupil), couple to Gq, and cause contraction. Alpha-2 receptors are largely presynaptic, couple to Gi, and act as a brake: when stimulated on the nerve terminal they reduce further norepinephrine release, while alpha-2 receptors in the brainstem lower sympathetic outflow overall.
This split makes them rich drug targets. Alpha-1 blockers relax the prostate and lower blood pressure; alpha-1 agonists constrict vessels and act as decongestants. Alpha-2 agonists such as clonidine exploit the central brake to treat hypertension and ease withdrawal, a neat example of an agonist that ultimately calms the sympathetic system.
Tamsulosin, an alpha-1 blocker, relaxes prostate and bladder-neck smooth muscle to ease urine flow in benign prostatic enlargement.
Alpha-1 blockade to relieve urinary symptoms.
The counterintuitive point worth memorizing: an alpha-2 agonist lowers blood pressure even though it activates a receptor, because the receptor it activates is itself an inhibitory brake on sympathetic activity.