Phytochemical and pharmacological studies on Hibiscus sabdariffa and Polygonum salicifolium

  • Ahmed Mohammed Attyah Zheoat

Student thesis: Doctoral Thesis


Medicinal plants are an important resource for the discovery of new and potent therapeutic compounds. Hibiscus sabdariffa (Malvaceae) is a plant that is widely recognised for its antihypertensive properties; however the constituent(s) responsible for this biological activity are presently unknown. The aim of this study was to identify the potential compounds that are responsible for the vasorelaxant activity of Hibiscus sabdariffa. A second aim was to determine whether the aquatic plant, Polygonum salicifolium (Polygonaceae) has vasorelaxant activity. Thereafter, the mechanisms involved in producing the vasorelaxation were investigated. The plants were subjected to hot solvent extraction in a sequential manner (72 h each) with n-hexane, ethyl acetate and methanol using a Soxhlet apparatus. The methanolic extract was subjected to bioassay-guided fractionation in order to isolate pure compounds and determine their vasorelaxant activity. The vascular effects of the crude methanolic extracts, sub-fractions, and pure compounds were studied on the rat aorta in vitro using myography techniques. In this study, hibiscus acid has been isolated as one of the main organic acids of Hibiscus sabdariffa. Parallel studies were also carried out with garcinia acid, which is a diastereoisomer of hibiscus acid and is commercially available. Hibiscus acid produced a concentration-dependent relaxation of the aorta pre-contracted with either phenylephrine (3 μM) or KCl (60 mM), irrespective of the presence or absence of the endothelium. When the tissue was pre-contracted with phenylephrine, the concentration required to produce 50% relaxation (IC50), was 0.09 ± 0.01 mg/ml. Moreover, garcinia acid was found to have an almost identical vasorelaxant effect. When the aorta was pre-contracted with the Ca2+ channel activator Bay K8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl] phenyl) pyridine-3-carboxylic acid methyl ester or FPL 64176 (2,5-dimethyl-4-[2-(phenylmethyl) benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester), both hibiscus and garcinia acid were able to produce almost complete relaxation. Hibiscus acid had no effect on the phasic contraction induced by phenylephrine in Ca2+-free physiological solution; but it did affect the component of the contraction that is due to Ca2+ influx. Previous studies have showed that the crude extracts of H. sabdariffa have negative inotropic activity and relaxant effects on the tracheal tissues. Therefore, it was plausible to determine whether hibiscus acid has similar effects. Both hibiscus and garcinia acid, at a concentration of 0.6 mg/ml almost completely relaxed the electrically evoked contraction of the rat atria. Furthermore, both organic acids produced a concentration-dependent relaxation of the rat trachea, pre-contracted with either carbachol (1 μM) or KCl (60 mM). The vasorelaxant action of both compounds appears to be through inhibition of extracellular calcium influx, most likely by inhibition of voltage-dependent Ca2+ channels. There may be a slight modulatory role of the endothelium in the action of these substances; but the endothelium is not essential for the vasorelaxant response. With regard to the activity of Polygonum salicifolium, the crude methanolic extract of this plant produced a concentration-dependent relaxation of the aorta pre-contracted with phenylephrine. The IC50 was 0.04 ± 0.007 mg/ml and the extract caused complete relaxation of the aorta. The vasorelaxant activity of the crude extract was completely inhibited upon removal of the endothelium. Also, the crude extract was unable to relax the aorta when it was pre-contracted with KCl (60 mM). In conclusion, the diastereomers, hibiscus acid and garcinia acid were demonstrated to cause vasorelaxation in a similar manner. This thesis also reported for the first time the vasorelaxant activity of the medicinal plant Polygonum salicifolium, and the vasorelaxant action of this plant appears to be mediated through an endothelium-dependent mechanism.
Date of Award25 Jul 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorValerie Ferro (Supervisor) & Robert Drummond (Supervisor)

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