The Petal Extract of Hibiscus sabdariffa Produces Relaxation of Isolated Rat Aorta

Abstract

A water extract of the petals of Hibiscus sabdariffa (HS) has been tested for vascular smooth muscle contractile activity by applying cumulative concentrations on isolated rat aortic rings under baseline tension. Relaxation responses to HS were also tested on tissues precontracted with 100−7M noradrenaline (NA). Relaxation experiments were conducted on endothelium-intact, methylene blue-treated, and endothelium-denuded tissues. In the concentration range of 0.02-7.68 mg/ml, HS did not show a measurable contractile effect on the rat aorta. In contrast, in tissues with NA-induced tone, vasorelaxation was observed with cumulative concentrations of HS and reached a mean of 91 ± 4% at a concentration of 1.70 mg/ml, with an EC50 of 0.53 ± 0.06 mg/ml. This relaxation response was significantly attenuated by removal of the endothelium, since endothelium-denuded rings showed a mean relaxation of 28 ± 1% at 1.70 mg/ml, with an EC50 of 2.53 ± 0.09 mg/ml (p<0.05). Moreover, endothelium-intact rings which were pretreated with 106M methylene blue showed significantly attenuated relaxation to threshold concentrations of HS, with EC10 values of 0.25 ± 0.01 mg/ml and 0.15 ± 0.01 mg/ml (p<0.05) with and without methylene blue treatment, respectively. The effect of methylene blue was not significant, however, at higher concentrations of HS. Thus, HS-induced relaxation of rat aortic smooth muscle occurs by both endothelium-dependent and -independent mechanisms. Since the endothelium-dependent response was largely unaffected by methylene blue treatment, this component of r's activity does not appear to involve EDRF.