Bronchodilatory effects of S-isopetasin, an antimuscarinic sesquiterpene of Petasites formosanus, on obstructive airway hyperresponsiveness

In the presence of neostigmine (0.1 μM), S-isopetasin competitively antagonized cumulative acetylcholine-induced contractions in guinea pig trachealis, because the slope [1.18 ± 0.15 (n = 6)] of Schild's plot did not significantly differ from unity. The pA₂ value of S-isopetasin was calculated to be 4.62 ± 0.05 (n = 18). The receptor binding assay for muscarinic receptors of cultured human tracheal smooth muscle cells (HTSMCs) was performed using [³H]-N-methylscopolamine ([³H]-NMS). Saturation binding assays were carried out with [³H]-NMS in the presence (non-specific binding) and absence (total binding) of atropine (1 μM). Analysis of the Scatchard plot (y = 0.247-1.306x, r² = 0.95) revealed that the muscarinic receptor binding sites in cultured HTSMCs constituted a single population (n_H = 1.00). The equilibrium dissociation constant (K_d) and the maximal receptor density (B_max) for [³H]-NMS binding were 766 pM and 0.189 pmol/mg of protein, respectively. The - logIC₅₀ values of S-isopetasin, methoctramine, and 1,1-Dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP) for displacing 0.4 nM [³H]-NMS-specific binding were 5.05, 6.25, and 8.56, respectively, which suggests that [³H]-NMS binding is predominantly on muscarinic M₃ receptors of cultured HTSMCs. The inhibitory effects of S-isopetasin on enhanced pause (P_enh) value were similar to that of ipratropium bromide, a reference drug. The duration of action of S-isopetasin (20 μM), also similar to that of ipratropium bromide (20 μM), was 3 h. In contrast to ipratropium bromide, which non-selectively acts on muscarinic receptors, S-isopetasin preferentially acts on muscarinic M₃ receptors. In conclusion, S-isopetasin may be beneficial as a bronchodilator in the treatment of chronic obstructive pulmonary disease and asthma exacerbations.