Cardiovascular system
Histamine produces dilatation of capillaries and venules accompanied by a fall in blood pressure. The mechanism is direct relaxation of the smooth muscles of blood vessels. This effect cannot be adequately reversed by antihistaminic agents but by adrenaline.
It also has positive inotropic and chronotropic actions on the heart, impairs AV conduction, and increases coronary blood flow.
Smooth Muscles:
Histamine directly stimulates the smooth muscles of various tissues including the bronchi and uterus. Histamine-induced bronchospasm is effectively antagonized by adrenaline.
Exocrine Glands:
It is a powerful stimulant of HCl secretion by the gastric mucosa.
CNS:
Histamine is formed locally in the brain and is believed to be a “waking amine”, acting by “increasing the sensitivity of large cerebral areas to excitation inputs”
Miscellaneous actions include induction of itching and pain.
Histamine has no valid therapeutic use currently. But it plays very important role in anaphylaxis and other forms of allergic reactions. Its release may be induced by various agents including certain venoms, drugs, trauma (thermal, chemical, radiation), and antigen-antibody reactions.
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