antagonist /an·tag·o·nist/ (an-tag´o-nist)
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.antagonis´tic
2. antagonistic muscle. 
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Antagonist. The triceps brachii extends the forearm at the elbow while the biceps brachii, its antagonist, flexes the elbow.
3. a tooth in one jaw that articulates with one in the other jaw.
α-adrenergic antagonist alpha-adrenergic blocking agent; see adrenergic blocking agent.
β-adrenergic antagonist beta-adrenergic blocking agent; see adrenergic blocking agent.
folic acid antagonist an antimetabolite, e.g., methotrexate, that interferes with DNA replication and cell division by inhibiting the enzyme dihydrofolate reductase; used in cancer chemotherapy.
H1 receptor antagonist any of a large number of agents that block the action of histamine by competitive binding to the H1 receptor; they also have sedative, anticholinergic, and antiemetic effects and are used for the relief of allergic symptoms, as antiemetics, as antivertigo agents, and as antidyskinetics in parkinsonism.
H2 receptor antagonist an agent that blocks the action of histamine by competitive binding to the H2 receptor; used to inhibit acid secretion in the treatment of peptic ulcer.
alpha
the first letter of the Greek alphabet, A or α; used to denote the first position in a classification system; as, in names of chemical compounds, to distinguish the first in a series of isomers, or to indicate the position of substituent atoms or groups; also used to distinguish types of radioactive decay, brain waves or rhythms, adrenergic receptors, and secretory cells that stain with acid dyes, such as the alpha cells of the pancreas.
alpha-adrenergic antagonist
see alpha-blocking agents (below).
alpha-blocking agents
a group of drugs that selectively inhibit the activities of alpha receptors in the sympathetic nervous system. As with beta-blocking agents, alpha-adrenergic blocking agents compete with the catecholamines at peripheral autonomic receptor sites. This group includes ergot and its derivatives, and phenotolamine.
alpha brain waves
human brain-wave currents during electroencephalography having a frequency of approximately 8 to 13 hertz (pulsations per second), best seen when patient's eyes are closed and the patient is physically relaxed. See also electroencephalography.
alpha cells
glucagon-producing cells of the pancreas.
alpha-hemolysin
see alpha hemolysis.
alpha hemolysis
see alpha hemolysis.
alpha particles
a type of emission produced by the disintegration of a radioactive substance. The atoms of radioactive elements such as uranium and radium are very unstable; they are continuously breaking apart with explosive violence and emitting particulate and nonparticulate types of radiation. The alpha particles, consisting of two protons and two neutrons, have an electrical charge and form streams of tremendous energy when they are released from the disintegrating atoms. These streams of energy (alpha rays) are used to advantage in the treatment of various malignancies. See also radiation and radiotherapy.
alpha-responsive sympathomimetic drugs
drugs which cause vasoconstriction and maintain correct vascular permeability.
alpha-sheet
α-sheet a common structural feature of many proteins in which a single polypeptide chain turns regularly about itself to make a rigid cylinder in which each peptide bond is regularly hydrogen-bonded to other peptide bonds elsewhere in the chain.
alpha toxin
the toxins of many bacteria are classified as alpha, beta, etc.
alpha-adrenergic antagonist
An adrenergic blocking agent which produces miosis and a slight reduction in intraocular pressure. It is used mainly to reverse the mydriatic effect of sympathomimetic drugs (e.g. phenylephrine hydrochloride), or even some antimuscarinic drugs (e.g. tropicamide). Common agents include dapiprazole and moxisylyte (thymoxamine). Syn. alpha-blocker. See sympatholytic drugs.
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