10.01.2 Actions of Histamine
Histamine has a major role in Type 1 allergic reaction, which is the most common type of allergic reaction. Type 1 allergy can lead to anaphylactic reactions. Type I allergy is also known as immediate hypersensitivity reaction, and is often just called allergy. Allergy can be caused by a number of things including grass pollen, house dust mites, certain foods, animal fur…. Medicines that can cause allergy include aspirin, angiotensin converting enzyme inhibitors, and penicillin. With allergy, there is the release of contents of the mast cells and basophils. The contents of mast cells include histamine, eicosanoids, and cytokines. There local chemical mediators cause vasodilation, oedema, and inflammation. Sometimes allergy is limited to certain sites e.g. food allergies affect gastrointestinal tract. Whereas contact allergies may be limited to skin, where allergy is manifest as urticaria (hives) and atopic dermatitis (itching + raised red rash). Allergy to inhalants e.g. grass pollen, may be limited to the respiratory system and manifest as rhinitis and asthma. Mild allergic conditions may be treated with antihistamines. Antihistamines are not effective in severe allergic reactions e.g. anaphylaxis, as mediators other than histamine are also involved.
On the lung the hypersensitivity reaction of histamine is a H1-receptor-mediated bronchoconstriction. On the nasal mucous membranes/skin, the hypersensitivity reaction is a H1-receptor-mediated dilation of terminal arterioles, which leads to the accumulation of red blood cells and produces the redness associated with allergy. In the severe hypersensitivity reaction anaphylaxis, histamine H1-receptor mediated dilation causes a major decrease in blood pressure (hypotension). The H1-receptor-mediated contraction of the endothelial cells of capillaries increases the space between the cells, allowing the contents of the capillaries (fluids, proteins) to flow into extracellular space to cause oedema (hives) (Figure 10.1).
Figure 10.1 Oedema with histamine (Copyright QUT, Sheila Doggrell)
The other major effect of histamine in the periphery is a H2-receptor mediated increase in gastric secretion. Histamine is released, possibly from the enterochromaffin cells in the gut, and acts at H2-receptors on the parietal cells. Stimulation of the H2-receptors leads to activation of adenylate cyclase, which in turn activated the H+/K+ ATP-ase, the proton pump. The proton pump, pumps H+ into the stomach to cause the acid environment.