(adrenoreceptor, adrenergic receptor) any cell *receptor that binds with the catecholamines adrenaline or noradrenaline, the neurotransmitters of the *sympathetic nervous system. There are two principal types of adrenoceptor, alpha (?) and beta (?), with various subtypes of each. Alpha adrenoceptors have a slightly higher affinity for adrenaline than for noradrenaline; they can be divided into subtypes ?1 and ?2. ?1-adrenoceptors mediate contraction of smooth muscle; in the walls of arteries, for example, their stimulation causes constriction of arteries and a rise in blood pressure. ?2-adrenoceptors occur in the presynaptic membranes of neurons in the sympathetic nervous system, where they restrict the release of catecholamines from these neurons. Beta adrenoceptors also have two subtypes. ?1-adrenoceptors have an equal affinity for adrenaline and noradrenaline and are found mainly in cardiac muscle; their stimulation causes an increase in heart rate. ?2-adrenoceptors have a slightly higher affinity for adrenaline. They mediate relaxation of smooth muscle in the blood vessels, bronchi, bladder, uterus, and other organs and thus cause widening of the airways and *vasodilatation.
Drugs that stimulate these receptors (alpha agonists and beta agonists) are described as *sympathomimetic. Drugs that block their effects are the *alpha blockers and *beta blockers.
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