How does histamine affect smooth muscle?
Histamine affects several receptors found in different tissues. Sometimes blocking the receptor and sometimes adding to it.
Smooth muscle around bronchi in the lungs and within the intestinal tract respond to histamine stimulation by contraction, although the magnitude of response varies considerably among species.
These effects also depend on which receptor is being bound by histamine; for example, the H2 receptor mediates bronchodilation.
One of the first bioassays for histamine involved measuring contraction of guinea pig intestinal muscle. These effects on smooth muscle are manifest in a number of allergic reactions, for example, bronchoconstriction in response to inhaled allergens.
Four histamine receptors have been identified, all of which are G protein-coupled receptors. These different receptors are expressed on different cell types and work through different intracellular signaling mechanisms, which explains, at least at a simple level, the diverse effects of histamine in different cells and tissues.