adrenoceptors

Specialized areas in the membranes of cells on which noradrenaline, and to a lesser extent adrenaline, act to convey messages from the nerves of the sympathetic nervous system to their target tissues. There are two main types of adrenoceptors: alpha receptors and beta receptors. Stimulation of alpha receptors by noradrenaline causes (among other effects) constriction of arteries (vasoconstriction); stimulation of beta receptors causes an increase in heart rate, widening of airways, relaxation of the muscle of the uterus, and dilatation of arteries (vasodilatation).

Drugs that act on adrenoreceptors either stimulate them and increase their normal activity or block them and prevent their normal actions. Drugs that mimic the effects of noradrenaline are called sympathomimetic drugs. There are two classes of drugs that block the effects of noradrenaline they are known as alpha blockers and beta blockers (they used to be called sympatholytic drugs). Some drugs (carvedilol and labetalol) block both alpha and beta receptors and are used to treat hypertension.

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