After being swallowed, injected, inhaled, or absorbed through the skin, mucosa under the tongue, or mucosa inside the cheek, most drugs enter the bloodstream and circulate throughout the body. (See also Definition of Drug Dynamics.) Some drugs are given directly to the area where they are wanted. For example, eye drops are put directly into the eyes. The drugs then interact with cells or tissues where they produce their intended effects (target sites). This interaction is called selectivity.
Selectivity is the degree to which a drug acts on a given site relative to other sites.
Relatively nonselective drugs
Relatively selective drugsNonopioid Pain Relievers), target any area where inflammation is present.
Highly selective drugs
Where drugs exert their effects has to do with how they interact with specific cells or substances such as enzymes.
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.
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 Perfect Fit
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. |
Agonists and antagonists
Drugs that target receptors are classified as agonists or antagonists. Agonist drugs activate, or stimulate, their receptors, triggering a response that increases or decreases the cell’s activity. Antagonist drugs block 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.
epinephrine (adrenaline) and norepinephrine (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.
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 |
Adrenergic | |||
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 | |
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 | 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) | Antagonist: Beta-blockers |
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) | |
Cholinergic | |||
Muscarinic | Acetylcholine | 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 | |
Nicotinic | Acetylcholine | Contraction of skeletal muscles | Agonist: None commonly used |
Histaminergic | |||
H1 | Histamine | 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 |
H2 | Histamine | Stimulation of stomach secretions | Agonist: None commonly used |
Serotoninergic | |||
Serotonin | 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 | ||
Dopaminergic | |||
Involvement in movement, mood, thinking, learning, and reward-seeking Also increases blood flow to the kidneys, which allows for increased urine excretion | |||
* 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). | |||
Enzymes
Some drugs target enzymes, which regulate the rate of chemical reactions, instead of attaching to receptors on cellsrifampin, the contraceptive is metabolized (that is, broken down into inactive components) and removed from the body more quickly than usual and may therefore be ineffective.
Chemical Interactions
Some drugs produce effects without changing the function of a cell and without attaching to a receptor. For example, most antacids decrease stomach acid through simple chemical reactions. Antacids are bases that chemically interact with acids to neutralize stomach acid.
