adrenoceptor

= 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 (α) and beta (β), with various subtypes of each. Alpha adrenoceptors have a slightly higher affinity for adrenaline than for noradrenaline; they can be divided into subtypes α₁ and α₂. α₁-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. α₂-adrenoceptors occur in the presynaptic membranes of neurones in the sympathetic nervous system, where they restrict the release of catecholamines from these neurones. Beta adrenoceptors also have two subtypes. β₁-adrenoceptors have an equal affinity for adrenaline and noradrenaline and are found mainly in cardiac muscle; their stimulation causes an increase in heart rate. β₂-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.