Antagonist

In biochemistry, an antagonist acts against and blocks an action. For example, insulin lowers the level of glucose (sugar) in the blood, whereas another hormone called glucagon raises it; therefore, insulin and glucagon are antagonists. An antagonist is the opposite of an agonist which stimulates an action. Antagonists and agonists are key players in pharmacology and in the chemistry of the human body.

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Something opposing or resisting the action of another; certain structures, agents, diseases, or physiologic processes that tend to neutralize or impede the action or effect of others. Cf.:synergist.

- α-adrenoceptor a. SYN: α-adrenergic blocking agent.

- β-adrenoreceptor a. SYN: β-adrenergic blocking agent.

- aldosterone a. an agent that opposes the action of the adrenal hormone aldosterone on renal tubular mineralocorticoid retention; these agents, e.g., spironolactone, are useful in treating the hypertension of primary hyperaldosteronism, or the sodium retention of secondary hyperaldosteronism.

- associated a. one of two muscles or groups of muscles which pull in nearly opposite directions, but which, when acting together, move the part in a path between their diverging lines of action.

- calcium a. SYN: calcium channel-blocking agent.

- competitive a. an antimetabolite.

- enzyme a. an antimetabolite or inhibitor of enzyme action.

- folic acid antagonists modified pterins, such as aminopterin and methotrexate, that interfere with the action of folic acid and thus produce the symptoms of folic acid deficiency; have been used in cancer chemotherapy and inflammatory disorders.

- 5-hydroxy tryptamine antagonists agents that block serotonin receptors and hence interfere with the biological actions of serotonin (5-HT).

- insulin a. substances in the β- and γ-globulin or β₁-lipoprotein fractions of serum that may induce a functional insulin deficiency; may include nonprecipitating antibodies against nonhuman insulin.

- leukotriene receptor a. a class of agents, of which zileuton, montelukast, zafirlukast are the best known, used in the prophylactic and chronic treatment of asthma in older children and adults; these drugs are not bronchodilators in themselves, but act by interfering with the leukotriene-mediated inflammatory process present in asthma.

- muscarinic a. drugs that bind with muscarinic cholinergic receptors but do not activate them, thus preventing access to acetylcholine; examples include atropine, scopolamine, propantheline, and pirenzepine.

- opioid antagonists agents such as naloxone and naltrexone that have high affinity for opiate receptors but do not activate these receptors. These drugs block the effects of exogenously administered opioids such as morphine, heroin, meperidine, and methadone, or of endogenously released endorphins and enkephalins.

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an·tag·o·nist -nəst n an agent that acts in physiological opposition <contact between a tooth and its \antagonist in the opposing jaw>: as

a) a muscle that contracts with and limits the action of an agonist with which it is paired called also antagonistic muscle compare AGONIST (1), SYNERGIST (2)

b) a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate) esp one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor compare AGONIST (2)

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n.

1. a muscle whose action (contraction) opposes that of another muscle (called the agonist or prime mover). Antagonists relax to allow the agonists to effect movement.

2. a drug or other substance with opposite action to that of another drug or natural body chemical, which it inhibits. Examples are the antimetabolite.

• antagonism n.

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an·tag·o·nist (an-tagґə-nist) [Gr. antagōnistēs an opponent] 1. a substance that tends to nullify the action of another, as a drug that binds to a cell receptor without eliciting a biological response, blocking binding of substances that could elicit such responses. 2. antagonistic muscle. 3. a tooth in one jaw that articulates with a tooth in the other jaw.

Antagonist. The triceps brachii extends the forearm at the elbow while the biceps brachii, its antagonist, flexes the elbow.