Adrenoceptor agonists (eg, phenylephrine) elicit vasoconstriction and mydriasis and are used as nasal decongestants and in eye examinations. α2-Adrenoceptor agonists (eg, clonidine) bind to presynaptic receptors and activate a negative feedback loop that inhibits further release of norepinephrine; they serve as anti-hypertensive agents.
α1-Adrenoceptor antagonists (eg, doxazosin) are also used to treat hypertension. β1-Adrenoceptor agonists (eg, dobutamine) augment sympathetic innervation of the heart and are used as cardiac stimulants. β1-Adrenoceptor antagonists (eg, atenolol) attenuate sympathetic innervation of the heart and function as antihypertensive agents. β2-Adrenoceptor agonists (eg, albuterol) stimulate bronchodilation and are used to treat asthma. Certain drugs (eg, isoproterenol and labetalol) affect multiple receptor types. Adverse effects include vasoconstriction, vasodilation, and tachycardia.