Effects of cetirizine on isolated rat's tracheal smooth muscle

Cetirizine (Zytec) is often used as a histamine receptor-1 (H₁) antagonist in rhinitis patients who are suffering from sneezing and rhinorrhea. This H₁ antagonist is used as an oral tablet or nasal spray solution. The effect of H₁ antagonist on nasal mucosa in vivo is well known; however, the effect of the drug on tracheal smooth muscle has been rarely explored. Therefore, during administration of the H₁ antagonist for nasal symptoms, it might also affect the trachea via oral intake or inhalation. We used our preparation to test the effectiveness of Zytec on isolated rat's tracheal smooth muscle. The following assessments of Zytec were performed: (1) effect on tracheal smooth muscle resting tension; (2) effect on contraction caused by 5 × 10^-6 M methacholine as a parasympathetic mimetic; (3) effect of the drug on electrically induced tracheal smooth muscle contractions. Results indicated that addition of a parasympathetic mimetic to the incubation medium caused the trachea to contract in a dose-dependent manner. Addition of Zytec at doses of 10^-5 M or above elicited a relaxation response to 5 × 10^-6 M methacholine-induced contraction. Zytec could inhibit electrical field stimulation induced spike contraction, and basal tension was increased at the same time. However, it alone had a minimal effect on the basal tension of trachea as the concentration increased. This study indicated that high concentrations of Zytec might actually inhibit parasympathetic function of the trachea.