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Histamine and Antihistamine

 HISTAMINNE

Histamine, meaning 'tissue amine' (histos-tis-sue) is almost ubiquitously present in animal

tissues and in certain plants, e.g. stinging nettle.Its pharmacology was studied in detail by Dalein the beginning of the 20th century when close parallelism was noted between its actions and the manifestations of certain allergic reactions. Histamine was implicated as a mediator of hypersensitivity phenomena and tissue injury reactions. It is now known to play important physiological roles.Histamine is present mostly within storage granules of mast cells. Tissues rich in histamine are skin, gastric and intestinal mucosa, lungs, liver and placenta. Nonmast cell histamine Occurs in brain, epidermis, gastric mucosa and growing regions. Turnover of mast cell histamine is slow, while that of nonmast cell histamine is fast. Histamine is also present in blood, most body secretions, venoms and pathological fluids.

           Histamine is B imidazolylethylamine. It issynthesized locally from the amino acid his-tidine and degraded rapidly by oxidation and methylation . In mast cells, histamine(positively charged) is held by an acidic protein and heparin (negatively charged) within intracel-lular granules. When the granules are extruded by exocytosis, Na ions in e.c.f. exchange with histamine to release it free . Increase in intracellular cAMP (caused by B adrenergic agonists and methylxanthines) inhibits histamine release. Histamine is inactive orally because liver degrades all histaminethat is absorbedfrom theintestine.

 


Synthesis and degradation of Histamine MAO



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