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Histamine
Histamine is a tissue amine abundant in animal tissues and plays critical roles in hypersensitivity and physiological processes. It is synthesized from histidine in mast cells and has notable pharmacological actions such as vasodilation and increased capillary permeability, mediated through H1 and H2 receptors. Adverse effects of histamine release can include itching, hypotension, and bronchospasm.
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Histamine
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- 2. Introduction Histamine Means: Tissueamine Chemical formula: C5H9N3
- 3. 3 Structure of Histamine- C5H9N3 H H H H H H H H H
- 4. 4 Histamine • Meaning "tissueamine" (histos — tissue) — abundantly present in animal tissues — also in plants like "stinging nettle" • Mediator of hypersensitivity and tissue potential tissue injury — Physiological role • The primary site the mast cell granules (or basophils) — skin, intestinal and gastric mucosa, lungs, liver and placenta • Other sites central nervous system: neurotransmitter the fundus of the stomach: major acid secretagogues, epidermis, gastric mucosa and growing regions also blood, body secretions, venoms & pathological fluids
- 5. 5 Histamine – Synthesis Synthesizedlocally from amino acid histidine Histidine Histamine L-histidine decarboxylase
- 6. 6 Histamine – Storageand Release • Metabolized by P450 system, 2 pathways: Methylation to N-me histamine (N-me transferase), & to N-me imidazole acetic acid (MAO) - eliminated in urine Oxidative deamination to imidazole acetic acid (DAO), and to imidazole acetic acid riboside - eliminated in urine • In mast cells— held by acidic protein and heparin (-ve charged) — histamine is +ve charged • Ineffective orally — liver destroys all absorbed from intestine
- 7. 7 Histamine – Pharmacologicalactions •Blood vessels: Dilatation of small vessels — arterioles, capillaries and venules • SC administration — flushing, heat, increased HR and CO — little fall in • Rapid IV injection: Fall in BP early (H1) and persistent (H2) — only H1 effect with low dose • Dilatation of cranial vessels • H1 component vasodilatation — mediated indirectly by EDRF But H2 component - mediation is directly on smooth muscle Of blood vessels • Larger arteries and veins — constriction mediated by H1 receptor • Increased capillary permeability — exudation of plasma
- 8. 8 Mechanism of Actionof Histamine Histamine H1 Receptors Ca2+ Smooth muscle contraction Increased capillary permeability Vasodilation Sensory nerve endings pain & itching H1 Receptors cAMP Gastric acid secretion Blood vessels: vasodilation Increased capillary permeability H1 Receptors cAMP histamine release Secretion Vasodilation
- 9. 9 Adverse effects ofhistamine release • Itching, Urticaria • Flushing • Hypotension • Tachycardia • Bronchospasm • Angioedema • Wakefullness • Increased acidity (Gastric acid secretion)
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