an amine, C
5H
9N
3, produced by decarboxylation of
histidine, found in all body tissues. It induces capillary dilation (which increases capillary permeability and lowers blood pressure); contraction of most smooth muscle tissue; increased gastric acid secretion; and acceleration of the heart rate. It is also a mediator of
immediate hypersensitivity. adj.,
adj histamin´ic.
There are three types of cellular
histamine receptors :
H1 receptors mediate contraction of smooth muscle and capillary dilation, and
H2 receptors mediate acceleration of heart rate and promotion of gastric acid secretion. Both types mediate contraction of vascular smooth muscle. H
3 receptors occur in a number of systems including the central nervous system and peripheral nerves, and are believed to play a role in regulation of the release of histamine and other neurotransmitters from neurons. Histamine is used as a diagnostic aid in testing gastric secretion and in the diagnosis of pheochromocytoma. An excess of histamine apparently is released when the body comes in contact with certain substances to which it is sensitive. This excess histamine is believed to be the final cause of
hay fever,
urticaria (hives), and most other allergies, as well as certain stomach upsets and some headaches.
There are two types of histamine antagonists in clinical use that act at either the H
1 or the H
2 receptors. Drugs such as
diphenhydramine (Benadryl) and
chlorpheniramine(
Chlor-Trimeton) are referred to as
antihistamines and act on the H
1 receptors; they block the effects of histamine on vascular, bronchial, and gastrointestinal smooth muscle and on capillary permeability. They are used for relief of allergic and gastrointestinal disorders and in over-the-counter cold medicines. Drugs such as
cimetidine(
Tagamet) act at the H
2 receptors and block stimulation of gastric acid secretion.
histamine phosphate the phosphate salt of histamine, having the same actions as the base; administered by inhalation to test airway hyperresponsiveness in diagnosis of
asthma, subcutaneously as a positive control in skin testing for allergy, and as a diagnostic aid to assess production of
hydrochloric acid in the stomach.
histamine test 1. a formerly used test in which histamine was injected to stimulate gastric secretion and measure output of gastric acid.
2. a formerly used test for presence of a
pheochromocytoma; persons with such a tumor would show first a fall and then a marked rise in
blood pressure.
3. a skin prick test used in evaluation of patients with
allergies; skin responses to
allergens are compared to the response to a histamine wheal.