The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.

DEFINITION:

histamine

Chemical name: 1H-imidazole-4-ethanamine. A decarboxylation product of histidine, C₅H₉N₃, found in all body tissues, particularly in the mast cells and their related blood basophils, the highest concentration being in the lungs. It is also present in ergot and other plants and may be synthesized outside the body from histidine or citric acid. It has several functions, including (1) dilation of capillaries, which increases capillary permeability and results in a drop of blood pressure, (2) contraction of most smooth muscle tissue, including bronchial smooth muscle of the lung, (3) induction of increased gastric secretion, and (4) acceleration of the heart rate. It is also responsible for the triple response, and is implicated as a mediator of immediate hypersensitivity. On the basis of the antagonistic effects of antihistamines, it is postulated that cellular receptors of histamine are of two types: The H₁ receptors mediate the contraction of smooth muscle and the effects on capillaries; the H₂ receptors mediate the acceleration of heart rate and the promotion of gastric acid secretion. Both H₁ and H₂ receptors mediate the contraction of vascular smooth muscle. Histamine has also been postulated to be a neurotransmitter in the central nervous system.