Effect of antihistamines on antigen-induced increase of rat tracheal mucous gel layer thickness

The effect of orally administered H1 and H2 antihistamines on antigen-induced changes in tracheal mucous gel layer thickness of actively sensitized rats was determined. Cimetidine and ranitidine, H2 antagonists, produced dose-dependent inhibition of antigen-induced increase in gel thickness at doses ranging from 0.32 to 3.16 and from 0.10 to 1.0 mg/kg, respectively. H1 antagonists - like chlorpheniramine, diphenhydramine, and pyrilamine which possess anticholinergic properties - tended to enhance the antigen-induced gel thickening in a dose-dependent fashion, while astemizole and terfenadine had no effect. These data indicate that an antigen-induced increase of rat tracheal mucous gel layer thickness may result from stimulation of the H2 receptor, while histamine operating at the H1 receptor does not play a role. Therefore, an antihistamine that possesses both H1 and H2 antagonist properties while lacking anticholinergic properties appears to be the most appropriate for use in patients with asthma and associated bronchorrhea.