Histamine is formed by decarboxylation of histidine. It is stored preformed in cytoplasmatic granules of mast cells and basophils. Mast cells in man are found in the loose connective tissue of all organs. They occur particularly around blood and lymph vessels around nerves, and are more abundant in the skin, upper and lower respiratory tract, gastrointestinal tract and reproductive mucosa. Mast cells comprise up to 2% of the alveolar tissue cells. They are found especially in the connective tissue beneath the airway basement membrane, the bronchial lumen, near submucosal blood vessels and glands throughout muscle bundles, and in the alveolar septa.
Histamine release can be induced by very many substances:
Two histamine receptors have been identified: H1 and H2. Their actions are as follows:
H1
H2
Briefly histamine causes airway narrowing as follows:
Histamine is metabolized within minutes and therefore by itself does not accumulate. However, an effect of inhaled histamine on airway diameter in usual dosages may be detectable for up to 70 minutes. Therefore, when subsequent doses are inhaled a small cumulative effect is to be expected.