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Receptors on Cells
On their surface, most cells have many different types of receptors. A receptor is a molecule with a specific 3-dimensional structure, which allows only substances that fit precisely to attach to it—as a key fits in its lock. Receptors enable natural (originating in the body) substances outside the cell to influence the activity of the cell. Examples of such substances include neurotransmitters (chemicals that conduct messages between cells in the nervous system) and hormones (chemicals released into the bloodstream by one organ to affect another organ). That influence may be to stimulate or inhibit a process inside the cell. Drugs tend to mimic these natural substances and thus use receptors in the same way. For example,morphineand related pain-relieving drugs act on or affect the same receptors in the brain used by endorphins, which are substances produced by the body to help control pain. Some drugs attach to only one type of receptor. Other drugs, like a master key, can attach to several types of receptors throughout the body. A drug’s selectivity can often be explained by how selectively it attaches to receptors. A receptor on the cell’s surface has a 3-dimensional structure that allows a specific substance, such as a drug, hormone, or neurotransmitter, to bind to it because the substance has a 3-dimensional structure that perfectly fits the receptor, as a key fits a lock. Drugs that target receptors are classified as agonists or antagonists.Agonist drugsactivate, or stimulate, their receptors, triggering a response that increases or decreases the cell’s activity.Antagonist drugsblock the access or attachment of the body’s natural agonists, usually neurotransmitters, to their receptors and thereby prevent or reduce cell responses to natural agonists. Agonist and antagonist drugs can be used together in people with asthma. For example,albuterolcan be used withipratropium.Albuterol, an agonist, attaches to specific (adrenergic) receptors on cells in the respiratory tract, causing relaxation of smooth muscle cells and thus widening of the airways (bronchodilation).Ipratropium, an antagonist, attaches to other (cholinergic) receptors, blocking the attachment of acetylcholine, a neurotransmitter that causes contraction of smooth muscle cells and thus narrowing of the airways (bronchoconstriction). Both drugs widen the airways (and make breathing easier) but in different ways. Beta-blockers, such aspropranolol, are a widely used group of antagonists. Beta-blockers are used to treat high blood pressure, angina (chest pain caused by an inadequate blood supply to the heart muscle), and certain abnormal heart rhythms and to prevent migraines. They block or reduce stimulation of the heart by the agonist neurotransmittersepinephrine(adrenaline) andnorepinephrine(noradrenaline), which are released during stress. Antagonists such as beta-blockers are most effective when the concentration of the agonist is high in a specific part of the body. Similar to the way a roadblock stops more vehicles during the 5:00 PM rush hour than at 3:00 AM, beta-blockers, given in doses that have little effect on normal heart function, may have a greater effect during sudden surges of hormones released during stress and thereby protect the heart from excess stimulation. 