Histamine stimulates bronchiolar smooth muscle contraction through activation of Hj-receptors. A much smaller bronchodilatory response is evoked by stimulation of H2-receptors. Asthmatics are generally more sensitive to the bronchoconstrictor actions of histamine than are nonasthmatics.
Histamine is able to cause uterine contraction. Although the magnitude of this effect in humans is normally small, the large amounts of histamine released during anaphylactic reactions can initiate abortion in pregnant women. Histamine can also stimulate contraction of gastrointestinal smooth muscle, with large doses able to produce diarrhea.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.