Abstract
The present study examined the relaxing effect of a water extract of Baibu (Stemonae radix, the root tuber of Semona sessilifolia (Miq.) Franch. et Sav.) on carbachol-, histamine- and KCl-induced contractions of the guinea-pig isolated tracheal preparations. The results showed that Baibu (1–50 mg/ml) concentration-dependently relaxed the tracheal preparations contracted by these spasmogens with an IC50 value (mg/ml) of 2.0±0.1 for carbachol, 41.2±0.8 for histamine and 18.6±0.9 for KCl. The effect of Baibu was not affected by the pretreatment with a β-adrenoceptor antagonist propranolol (10−6 M), indicating that Baibu's effect was not due to an activation on β-adrenoceptors. Baibu shifted the concentration-response curve of carbachol to the right in a parallel manner without changing the maximal response, having a pA2 value of 0.16±0.07 mg/ml (equivalent to a KB=0.70±0.11 mg/ml). This indicates a competitive antagonism at the muscarinic receptors. Receptor binding assay indicated that Baibu interacted with the muscarinic receptors (Ki=0.51±0.12 mg/ml) and the dihydropyridine (DHP) binding site of l-type Ca2+ channels (Ki=8.0±1.9 mg/ml), but not with the histamine H1 receptors. Therefore, the present study demonstrates that Baibu contains the principle(s) acting on the muscarinic receptors and DHP binding sites, which contribute its relaxation effect on the airway smooth muscles.
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