Objectives. Thioperamide is used as an antagonist to the histamine H3 receptor. During administration of the drug, the trachea may be affected via nasal or oral inhalation. This study was to determine the effects of thioperamide on the trachea of rats in vitro. Methods. We tested the effectiveness of thioperamide on isolated rat trachea submersed in Kreb's solution in a muscle bath. Changes in tracheal contractility in response to the application of parasympathetic mimetic agents were measured. The following assessments of thioperamide were performed: 1) effect on tracheal smooth muscle resting tension; 2) effect on contraction caused by 10-6 M methacholine as a parasympathetic mimetic; 3) effect of the drug on electrically-induced tracheal smooth muscle contractions. Results. Thioperamide induced a significant relaxation response at a preparation concentration up to 10-4 M. The drug also inhibited the electrical field stimulation induced spike contraction. However, thioperamide alone had a minimal effect on the basal tension of the trachea at increasing concentrations. Conclusion. The study indicated that high concentrations of thioperamide might actually antagonize cholinergic receptors and block parasympathetic function of the trachea.
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