Blood pressure lowering effect and vascular activity of Phyllanthus niruri extract: The role of NO/cGMP signaling pathway and β-adrenoceptor mediated relaxation of isolated aortic rings

Ethnopharmacological relevance: Phyllanthus niruri have a long history of use in the traditional treatment of various ailments including hypertension. Literature reports have indicated that it is a potent antihypertensive herbal medication used traditionally. Aim of the study: This study was carried out to investigate the antihypertensive and vasodilatory activity of four solvents extracts of P. niruri namely; petroleum ether (PEPN), chloroform (CLPN), methanol (MEPN) and water (WEPN), with the aim of elucidating the mechanism of action and identifying the phytochemical constituents. Materials and methods: Male Spontaneous Hypertensive Rats (SHRs) were given oral gavage of P. niruri extract daily for two weeks and the blood pressure was recorded in vivo. We also determine the vasodilation effect of the extracts on rings of isolated thoracic aorta pre-contracted with phenylephrine (PE, 1 mu M). Endothelium-intact or endothelium-denuded aorta rings were pre-incubated with various antagonists like 1H-(1,2,4] oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 mu M) and Methylene blue (MB 10 mu M), sGC inhibitors; Nw-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 10 mu M) a nitric oxide synthase (NOS) inhibitor; atropine (10 mu M), a cholinergic receptor blocker; indomethacin (10 mu M), a cyclooxygenase inhibitor and various K+ channel blockers such as glibenclamide (10 mu M) and tetraethyl ammonium (TEA 10 mu M) for mechanism study. Results: SHRs receiving P. niruri extracts showed a significant decrease in their blood pressure (BP) when compared to the baseline value, with PEPN being more potent. The extracts (0.125-4 mg/mL) also induced vasorelaxation on endothelium-intact aorta rings. PEPN elicited the most potent maximum relaxation effect (R-max). Mechanism assessment of PEPN showed that its relaxation effect is significantly suppressed in endothelium-denuded aorta rings. Pre-incubation of aorta rings with atropine, L-NAME, ODQ, indomethacin, and propranolol also significantly attenuated its relaxation effect. Conversely, incubation with TEA and glibenclamide did not show a significant effect on PEPN-induced relaxation. Conclusion: This study indicates that the antihypertensive activity of P. niruri extract is mediated by vasoactive phytoconstituents that dilate the arterial wall via endothelium-dependent pathways and beta-adrenoceptor activity which, in turn, cause vasorelaxation and reduce blood pressure.