Unusual effects of histamine antagonists on canine nasal blood vessels.

This paper documents some of the lesser-known effects of antihistamines on nasal blood vessel smooth muscle. An in vitro muscle tension detecting technique was utilized. Three different H1 receptor antagonists (pyrilamine maleate, chlorpheniramine maleate, promethazine hydrochloride), and the H2 antagonist metiamide were used. Besides their basic antihistaminic effect, H1 antagonists (classical antihistamines) induced a contraction, enhanced norepinephrine- and epinephrine-induced contractions, and relaxed the sustained contraction induced by methoxamine hydrochloride or high concentrations of potassium. Although these effects were quite similar to those induced by histamine, the mechanism of each effect was apparently different. Metiamide did not produce these effects except for the relaxation of the sustained contraction. The actions of histamine antagonists seem to be much more complicated than expected.