Abstract
Abstract: The present study examined and compared the spasmolytic effects of 3 harmala alkaloids, harmine, harman, and harmaline, on carbachol‐, histamine‐, and KCl‐induced contractions of guinea‐pig isolated tracheal preparations. All 3 compounds relaxed the tracheal preparations contracted by these spasmogens with similar or different EC50 values, harmine being the most potent one. The cumulative concentration‐response curves of all 3 compounds for carbachol‐induced contraction were shifted to the right by propranolol (1 μM) pretreatment, indicating the involvement of the activation on the β‐adrenoceptors. All 3 compounds shifted the concentration‐response curves of carbachol to the right in a parallel manner with the pA2 values comparable with their relaxation EC50 values, indicating a competitive antagonism at the muscarinic receptors. Receptor binding assays indicated that all 3 compounds interacted with lung muscarinic receptors (Ki=11–13 μM), histamine H1 receptors (Ki=27–107 μM), and β2‐adrenoceptors (Ki=20–51 μM). Therefore, in addition to their actions on receptor‐linked and voltage‐dependent Ca2+ channels as reported in other types of smooth muscle, the present study suggests that the actions on muscarinic receptors, histamine H1 receptors, and β2‐adrenoceptors are also involved in their spasmolytic effects on airway smooth muscles.
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