The mechanism underlying the spasmolytic and bronchodilatory activities of the flavonoid-rich red onion "Allium cepa L." peel extract

The flavonoid-rich hydro-acetone extract of red onion (Allium cepa peel, ACPE) was studied for its spasmolytic and bronchodilator activities using ex-vitro and in-vivo assays. In isolated rabbit jejunum preparations, ACPE produced a concentration-dependent (0.03-1 mg L-1) relaxation of spontaneous and high K+ (80 mM)-induced contractions equipotently, nearly similar to that caused by papaverine, whereas, verapamil was relatively more potent against K+-induced contractions. ACPE also caused the right ward shift in the Ca++ concentration-response curves (CRCs), similar to that of verapamil and papaverine. In normotensive anesthetized rats, ACPE dose-dependently (3-30 mg kg-1) suppressed the carbachol (CCh, 1 mg kg-1) induced bronchoconstriction similar to the effect observed with aminophylline. In guinea-pig tracheal preparation, ACPEexhibited concentration-dependent relaxation of both CCh (1 μM) and high K+-induced contraction at similar concentrations (0.3-3 mg mL-1) and also shifted the isoprenaline-induced inhibitory CRCs to the left, similar to that caused by papaverine. Theresults of this study indicated that the spasmolytic and bronchodilatory activities of ACPE are mediated through the dual inhibition of Ca++ channels and phosphodiesterase enzyme like-mechanisms, which might add an evidence-based medicinal value to the red onion peel in the treatment of gastrointestinal and respiratory disorders, e.g. diarrhea and asthma, respectively.