Mechanism of relaxant effect mediated by an aqueous extract of Hibiscus sabdariffa petals in isolated rat aorta

Abstract The mechanism by which aqueous extracts of Hibiscus sabdariffa (HS) relaxes the vascular smooth muscle has been investigated. We used 2 mm ring segments of isolated aorta of rat mounted in organ baths containing physiological salt solution (PSS). Cumulative addition of HS (0.02-0.96 mg/ml) had a significant (p < 0.05) relaxation effect on noradrenaline precontracted rings but not on potassium chloride (KCl) precontracted rings. Similarly, HS significantly attenuated the contractile response to calcium chloride (CaCl2) in noradrenaline stimulated rings while it has no effect on KCl-stimulated rings. HS also significantly reduced the magnitude of noradrenaline phasic contraction. The results of this study suggest that HS relaxes vascular smooth muscle via a mechanism that is associated with an inhibition of Ca2+ influx through receptor operated channels and also on inhibition of Ca2+ release from intracellular stores.