Pharmacological analysis of histamine receptors in musculature and vasculature of the dog trachea in situ.

1 The role of histamine H1- and H2-receptors in the musculature and vasculature of the dog trachea was investigated in the blood-perfused trachea in situ. 2 Histamine and acetylcholine caused increases in blood flow (tracheal, vasodilatation) and in intraluminal pressure (tracheal constriction) in a dose-dependent manner. Histamine was almost equipotent to acetylcholine in causing tracheal vasodilatation but was about 30 times less potent in causing tracheal constriction. 3 The histamine H2-receptor agonist, dimaprit, caused a dose-dependent increase in tracheal blood flow but failed to cause tracheal constriction. 4 The tracheal constriction produced by histamine was inhibited strongly by diphenhydramine but not modifed by metiamide. The tracheal vasodilatation produced by histamine was antagonized by both diphenhydramine and metiamide; diphenhydramine was more effective than metiamide. 5 It is concluded that in the tracheal musculature, histamine receptors are exclusively of the H1-type and mediate constriction, whereas in the tracheal vasculature, both histamine H1- and H2-receptors mediate vasodilatation but histamine H1-receptors are predominant.