Merck Manual

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Targets in The Body: Cell Receptors

Targets in The Body: Cell Receptors

Certain natural substances in the body, such as neurotransmitters and hormones, target specific receptors on the surface of cells. When these substances bind with the receptor on a cell, they stimulate that receptor to perform its function, which is to produce or to inhibit a specific action in the cell. Drugs can also target and bind with these receptors.

Some drugs act as agonists, stimulating the receptor in the same way that the body’s natural substances do. Others act as antagonists, blocking the action of the natural substance on the receptor. Each type of receptor has many subtypes, and drugs may act on one or several subtypes of receptors.

Type of Receptor

Body’s Natural Agonist

Resulting Action

Drugs That Target the Receptor


Alpha 1

Epinephrine and norepinephrine

“Fight-or-flight” reactions: Constriction of the blood vessels in the skin, digestive tract, and urinary tract

Breakdown of glucose in the liver (releasing energy)

Decrease in activity of the stomach and intestines

Contraction of smooth muscle in the genital and urinary organs

Agonist: Methoxamine and phenylephrine

Antagonist: Doxazosin, prazosin, tamsulosin, and terazosin

Alpha 2

Epinephrine and norepinephrine

A decrease in insulin secretion, in the clumping of platelets, in the constriction of blood vessels in the skin and intestines, and in the release of norepinephrine from nerves

Agonist: Clonidine

Antagonist: Yohimbine

Beta 1

Epinephrine and norepinephrine

An increase in heart rate, in the force of heart contraction, and in secretion of renin (a hormone involved in controlling blood pressure)

Agonist: Dobutamine and isoproterenol

Beta 2

Epinephrine and norepinephrine

Dilation of smooth muscle in the blood vessels, airways, digestive tract, and urinary tract

Breakdown of glycogen in skeletal muscles (releasing glucose for energy)

Agonist: Albuterol, isoetharine, and terbutaline

Antagonist: Propranolol




A decrease in heart rate and the force of the heart’s contraction

Constriction of airways

Dilation of blood vessels throughout the body

Increase in activity of the stomach, intestines, bladder, and salivary, lacrimal, and sweat glands

Agonist: Bethanechol and carbachol

Antagonist: Atropine, ipratropium, and scopolamine



Contraction of skeletal muscles

Agonist: None commonly used

Antagonist: Atracurium, pancuronium, and tubocurarine




Production of an allergic response

Contraction of muscles in the airways and digestive tract

Dilation of small blood vessels

Drowsiness (sedation)

Agonist: None commonly used

Antagonist: Cetirizine, chlorpheniramine, clemastine, diphenhydramine, fexofenadine, and loratadine



Stimulation of stomach secretions

Agonist: None commonly used

Antagonist: Cimetidine, famotidine, and nizatidine



Constriction of blood vessels within the brain

Stimulation of activity (motility) in the digestive tract

Contraction of blood vessels

Effects on sleep, memory, sensory perception, temperature regulation, mood, appetite, and hormone secretion

Partial agonist: Buspirone

Agonist*: Sumitriptan and zolmitriptan

Antagonist: Methysergide and ondansetron



Involvement in movement, mood, thinking, learning, and reward-seeking

Also increases blood flow to the kidneys, which allows for increased urine excretion

Agonist: Pramipexole and ropinirole

Antagonist: Olanzapine and risperidone

* Antidepressants called selective serotonin reuptake inhibitors (SSRIs) act by enhancing the effects of serotonin but are not agonists (they do not act on the serotonin receptor).