histamine(redirected from H substance (inflammation))
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A biologically active amine that is formed by the decarboxylation of the amino acid histidine. It is widely distributed in nature and is found in plant and animal tissues as well as in insect venoms. In humans, histamine is a mediator of inflammatory reactions, and it functions as a stimulant of hydrochloric acid secretion in the stomach.
Most tissue histamine is found stored in mast cells, where it can be released by a variety of stimuli. Once released, it can cause many effects, including constriction of bronchiolar, gastrointestinal, uterine smooth muscle, and lowering of blood pressure. If histamine is released in the skin, itching, a flare (area of redness) due to vasodilation, and a wheal due to leaking of fluid into the tissue are observed. The increase in vascular permeability that permits this leakage is due to an action on the endothelial cells of postcapillary venules.
All of these actions of histamine are mediated by the activation of histamine receptors, designated either H-1 or H-2. Antihistamine drugs exert their effects by blocking the combination of histamine with these receptors. See Antihistamine
Histamine release can be caused by tissue injury, by physical stimuli such as cold or pressure, by drugs such as heroin, and most importantly by immunologic mechanisms. Mast cells in the skin, the lung, the nasal passages, or other sites may become sensitized to antigens such as ragweed or other pollens, and then release histamine and other biologically active substances upon exposure to them. The released histamine may then cause the effects commonly associated with allergic responses. If the allergic reaction becomes generalized and severe, life-threatening anaphylactic shock may ensue. The prompt administration of epinephrine, which exerts effects opposite to those of histamine, can be life-saving in such cases. See Allergy, Antigen, Epinephrine, Hypersensitivity, Immunology
[β-imidazole-4(5)-ethylamine], a tissue hormone that is strongly biologically active; one of the biogenic amines. It is formed as a result of the decarboxylation of the amino acid, histidine:
Histamine is contained in large quantities in inactive, bound form in various organs and tissues of animals and humans (lungs, liver, and skin), and also in thrombocytes and leucocytes. It is liberated in cases of anaphylactic shock and inflammatory and allergic reactions. It produces dilation of the capillaries and an increase in their permeability, constriction of the large vessels, contraction of the smooth musculature, and sharp increase of hydrochloric-acid secretion in the stomach. The release of histamine from its bound state during allergic reactions leads to reddening of the skin, itching, burning, and formation of blisters. Histamine decomposes under the action of the enzyme histaminase, mainly in the intestinal tract and the kidneys. Histaminase (diaminoxidase) catalyzes the oxidative deamination of histamine, as a result of which a nontoxic product (imidazole-acetaldehyde) is formed. The enzyme is active only in the presence of oxygen; it may deaminate diamines other than histamine.
E. V. PETUSHKOVA