Alphaagonists

Alpha agonists, or alpha-adrenergic agonists, are drugs that activate alpha-adrenergic receptors, primarily the alpha-1 and alpha-2 subtypes. They differ in tissue distribution and effects, but collectively they modulate vascular tone, reflexes, and secretions.

Activation of alpha-1 receptors on vascular smooth muscle causes vasoconstriction, increasing peripheral vascular resistance and blood pressure. Alpha-2 receptors are mainly presynaptic and in the central nervous system; their stimulation inhibits norepinephrine release and, in the CNS, reduces sympathetic outflow, which can lower blood pressure and produce sedative effects in certain contexts.

Common alpha-1 agonists include phenylephrine and oxymetazoline. Phenylephrine is used as a nasal decongestant, a mydriatic agent for eye exams, and a vasopressor to raise blood pressure in hypotensive states. Midodrine is an oral alpha-1 agonist used to treat orthostatic hypotension. Topical alpha-1 agonists such as oxymetazoline provide temporary nasal decongestion but can cause rebound congestion with prolonged use.

Alpha-2 agonists include clonidine, guanfacine, and dexmedetomidine. Brimonidine and apraclonidine are used in glaucoma to reduce intraocular pressure. Dexmedetomidine serves as a sedative and analgesic adjunct in anesthesia and critical care, reflecting a central sympatholytic action.

Adverse effects vary by agent and site of action. They include hypertension or reflex bradycardia with alpha-1 drugs, local irritation and rebound congestion with topical formulations, and sedation, dry mouth, or bradycardia with central alpha-2 agonists. Caution is needed in patients with cardiovascular disease, vascular compromise, or glaucoma.