Waltheria indica L. (Malvaceae) has been recognized in traditional medicine for its various therapeutic effects, including anti-inflammatory, antioxidant, anticancer, and bronchorelaxant activities. This study aimed to investigate the relaxant effects of W. indica extracts on rat tracheal smooth muscle and explore the involvement of β2-adrenergic and muscarinic receptors, calcium and potassium channels, nitric oxide (NO), and the cyclooxygenase pathway in this response. Tracheal rings were contracted using KCl (80 mM) or acetylcholine (10 μM), and cumulative concentrations of W. indica extract (10, 30, 100, 300, 1000, 2000, and 3000 μg/ml) were added to the organ bath at 15-min intervals. Isoprenaline (10, 30, 100, 300, 1000, 2000, and 3000 μg/ml) was used as a positive control in unincubated tissue. The relaxant effects of W. indica extracts were assessed in unincubated tissues and tissues incubated with propranolol, atropine, glibenclamide, indomethacin, tetraethylammonium (TEA), methylene blue, and N(gamma)-nitro-L-arginine methyl ester (L-NAME). W. indica demonstrated a concentration-dependent relaxant effect on unincubated isolated tracheal rings contracted by KCl (80 mM) or acetylcholine (10 μM) (P < 0.01 to P < 0.001). There was no significant difference in the relaxant effects of W. indica between unincubated tissues and those incubated with glibenclamide, TEA, indomethacin, or L-NAME. However, the relaxant effects were significantly reduced in tissues incubated with methylene blue and atropine compared to unincubated tissues. The relaxant effects of different concentrations of W. indica extract were not significantly different from those of isoprenaline. W. indica extracts exhibited potent relaxant effects on isolated rat tracheal rings. These results suggest that the stimulation of soluble guanylate cyclase and/or blockade of muscarinic receptors may be plausible mechanisms underlying the plant's relaxant effects.

GMPc release, muscarinic receptor, Waltheria indica, smooth muscle, soluble guanylate cyclase.