Many illustrious personalities (physiologist /pharmacologists/ clinicians) have worked on histamine; some prominent names being Dale, Laidlaw, Best and Lewis.
Chemistry, biosynthesis and source –
i. Histamine is amine of the aminoacid histidine.
ii. Histamine is synthesized by enzymatic decarboxylation of histidine, the enzyme being histidine decaboxylase.
iii. In our body histamine can be obtained from many tissues but most notably from the mast cells of the tissues and the blood basophils.
iv. Other important sources are gastro intestinal mucosa and CNS.
v. Certain bee venom and stings of certain insects are rich in histamine.
In the mast cells, histamine is complexed with heparin and the heparin histamine complex is dissociated to cause release of pharmacologically active histamine. In allergic states following the contact of the antigen with a mast cell, the mast cell is degranulated (= mast cell granules are lysed) histamine is released from the mast cell.
The actions of histamine can be divided into two major categories :
(A) On smooth muscles (B) On exocrine glands of stomach.
(A) Actions on smooth muscles
1. It causes relaxation of the arterioles and probably also of the smooth muscles of precapillary sphincter, leading to engorgement of the capillaries.
2. Histamine causes contraction of bronchial (specially in susceptible persons) and intestinal smooth muscles leading to bronchospasm and diarrhea.
Mast cells occur plentifully in mucous membrane of tracheobronchial tree. An injection of histamine therefore can produce a sharp fall of blood pressure and violent attack of asthma; there may be urticaria. Fall of BP is due to acute dilatation of the arterioles (the‘resistance vessels’).
Urticaria is due to transudate of fluid from the capillary to the tissue which in turn is due to capillary engorgement. Ultimately, speaking, histamine injection in susceptible persons can produce, anaphylactic shock, characterized by severe fall of BP and asthma. In short, histamine stimulates the parietal cells of the stomach and thus increases the secretion of HCl.
1. In addition, histamine also is related to the pain and itching sensation.
2. Some rapidly growing tissues produce histamine in large amounts. Thus, embryos, regenerating liver and rapidly healing wounds produce histamine. It is possible, that for the smooth functioning ofthe rapidly growing tissues, histamine is necessary.
3. Some neurons of the brain are histaminergic, ie, their neurotransmitters is histamine. Such histaminergic neurons play a role in thermoregulation by hypothalamus.