Effect of histamine on tracheal mucosal perfusion, water content and airway smooth muscle in sheep.

We examined the response of tracheal mucosal blood flow normalized for systemic arterial pressure (Qtrn), water content (VH20) and luminal dead space (Vtr) to nebulized histamine in intact, lightly anesthetized sheep. Nebulized histamine produced rapid increases in mean Qtrn (+84%) and VH2O (+85%), and a decrease in mean Vtr (-17%) (P less than 0.05) within 5 min post completion of challenge. Mean Vtr rapidly returned to baseline, while mean Qtrn and VH2O remained elevated for 60 and 90 min after challenge, respectively. Pretreatment with chlorpheniramine (H1-antagonist) blocked the changes in Vtr and VH2O, and attenuated the increase in Qtrn. Metiamide (H2-antagonist) pretreatment abolished the increase in Qtrn and blunted the increase in VH2O, but had no effect on the decrease in VTR. 2-methylhistamine (H1-agonist) decreased mean Qtrn and Vtr (P less than 0.05) and dimaprit (H2-agonist) increased mean Qtrn (P less than 0.05) without changing Vtr. Neither 2-methylhistamine nor dimaprit significantly altered VH2O. Atropine blocked histamine induced decreases in Vtr and slightly attenuated the increases in Qtrn and VH2O. Thus, histamine increased airway smooth muscle tone and mucosal water content principally via H1 receptors, and mucosal perfusion via H2 receptors. The airway smooth muscle contraction involved muscarinic pathways.