Vascular effects and antioxidant activity of two Combretum species from Democratic Republic of Congo

ETHNOPHARMACOLOGICAL RELEVANCE: Combretum racemosum P. Beauv (Combretaceae) leaves (CrLv) and root bark (CrRB) and Combretum celastroides subsp. laxiflorum Welw (Combretaceae) leaves (ClLv) are used in Congolese traditional medicine for several therapeutic purposes, notably for the treatment of conditions consistent with hypertension. The present study aims to investigate the vasorelaxant and in vitro antioxidant activities of these plants polar extracts and to examine the in vivo antihypertensive effect of the extract which displays the most potent vasorelaxant effect. MATERIAL AND METHODS: The vasorelaxant effect of CrLv, CrRB and ClLv methanolic extracts was studied on rat aorta rings pre-contracted with phenylephrine (PE, 1μM) in the presence or absence of the endothelium. In some experiments, prior to the addition of the extract, rings were incubated for 30min with either L-Nᴳ-nitroarginine methyl ester (L-NAME; 100μM), a nitric oxide synthase (NOS) inhibitor, indomethacin (10μM), a cyclooxygenase inhibitor, or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10μM), a guanylate cyclase inhibitor. The antioxidant activity was determined by the measurement of the scavenging ability of extracts towards the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Blood pressure was measured on normotensive Wistar rats and spontaneously hypertensive rats (SHR) treated orally with a daily dose (40mg/kg) of the CILv extract for 5 weeks. Tested extracts have been characterised by TLC profiles targeted at flavonoids. RESULTS: All tested extracts showed an important DPPH scavenging activity, ranging from 0.6 to 1.1 quercetin-equivalents. They caused a concentration-dependent vasorelaxation on intact aortic rings pre-contracted with PE (1μM). The responses to CrRB and CrLv methanolic extracts reached 74.0±5.1% and 62.2±8.6% at a cumulative concentration of 50μg/ml, respectively. The ClLv (10μg/ml) extract was more active and, in the same conditions, relaxed aortic rings by 90.3±5.8%. The vasorelaxant activity of all extracts disappeared or was significantly attenuated by removal of the endothelium or after pretreatment with L-NAME or ODQ. Indomethacin only inhibited the activity of CrLv and CrRB extracts. The ClLv extract was able to lower the systolic blood pressure in SHR rats by 7% after a 5-week treatment. CONCLUSIONS: The present study shows that methanolic extracts from ClLv, CrRB and CrLv have an antioxidant activity and an endothelium-dependent vasorelaxant effect. ClLv induces the vasorelaxant effect through the NO-cGMP pathway while CrLv and CrRB extracts also act via a prostanoid pathway. ClLv extract demonstrated a modest but significant antihypertensive activity in SHR rats.