Histamine relaxes constricted trachea and bronchi of horse

Horse trachea and bronchi contracted to slow-reacting substance of anaphylaxis, carbachol, 5-hydroxytryptamine, histamine, 2-methyl-histamine, bradykinin and prostaglandin F₂. Prostaglandins E₁ and E₂, isoprenaline and 4-methylhistamine caused relaxation of carbachol-contracted bronchi. Mepyramine (H₁-blocker) specifically inhibited histamine bronchoconstriction. In the presence of mepyramine, histamine caused relaxation of carbachol-contracted bronchi. Metiamide (H₂-blocker) inhibited histamine bronchorelaxation but not relaxation of the trachea. This suggests (1) the presence of both H₁- and H₂-receptors mediating bronchoconstriction and relaxation respectively (2) the existence of an atypical histamine receptor in the trachea. The study suggests that in equine respiratory hypersensitivity under therapy with classical (H₁) antihistaminics, further release of histamine may exert a beneficial broncholytic effect on airways contracted by other chemical mediators of immediate-type inflammation.