Mechanisms underlying the antihypertensive effect of Alstonia scholaris Idris Bello a , Nasiba Salisu Usman a , Roziahanim Mahmud b , Mohd. Zaini Asmawi a,n a Department of Pharmacology, School of Pharmaceutical Sciences, Universiti Sains Malaysia (USM), 11800 Pulau Pinang, Malaysia b Department of Pharmaceutical Chemistry, School of Pharmaceutical Sciences, University Sains Malaysia (USM),11800 Pulau Pinang, Malaysia article info Article history: Received 24 June 2015 Received in revised form 24 September 2015 Accepted 26 September 2015 Available online 30 September 2015 Keywords: Aorta rings Vasorelaxation Mechanisms Hypertension Alstonia scholaris abstract Ethnopharmacological relevance: Alstonia scholaris has a long history of use in the Ayurveda traditional treatment of various ailments including hypertension. We have reported the blood pressure lowering activity of the extract of A. scholaris. The following research aim to delineate the pharmacological me- chanism involve in the antihypertensive action. Materials and method: Vasorelaxant effect of the n-butanol fraction of A. scholaris (NBF-ASME) was evaluated on rat aorta pre-contracted with phenyelphrine (PE, 1 mM). Aortic rings preparation were pre- incubated with various antagonists like 1H-[1,2,4] oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ 10 μM), methylene blue (MB 10 μM), Nω-nitro-L-arginine methyl ester hydrochloride (L-NAME 10 μM), atropine (10 μM), indomethacin (1 μM), ML-9 and various K þ channel blockers such as glibenclamide (10 μM) and tetraethyl ammonium (TEA 10 μM) for mechanism study. Result: The results showed that pre-incubation of aortic rings with the extract (0.5, 1 and 2 mg/mL) significantly inhibit the contractile response of the rings to phenylephrine-induced contraction (p o0.05–0.001). Removal of endothelium, incubation with L-NAME, indomethacin, atropine and pro- pranolol did not significantly affect the relaxation effect of NBF-ASME. Furthermore, the K þ channel blockers, TEA and glibenclamide showed no inhibitory effect. However, aortic rings pretreated with ODQ and ML-9 showed a significant suppression of the relaxation curve of NBF-ASME (p o0.01–0.001). In Ca 2 þ -free solution, NBF-ASME inhibits the release of intracellular Ca 2 þ from the sarcoplasmic reticulum. NBF-ASME also inhibits calcium chloride (CaCl 2 )-induced contraction in endothelium-denuded aortic rings. Conclusion: The results from this study suggests that A. scholaris exerts vasodilation via calcium channels blockade, direct activation of soluble guanylate cyclase and possibly by also inhibiting the formation of inositol 1, 4, 5-triphosphate. & 2015 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Alstonia scholaris, a genus of the family apocynaceae, is a va- luable medicinal herb that is known for a number of its medicinal properties worldwide (Baliga, 2012; Bhanu et al., 2013; Dey, 2011). It is a native of the Indian subcontinent and Southeast Asia. The plant has been reported for its anti-inflammatory and analgesic effects (Shang et al., 2010); antidiabetic and antihyperlipidemic effects (Arulmozhi et al., 2010); antimalarial (Gandhi and Vinayak, 1990) and antimicrobial (Bonvicini et al., 2014; Mahapatra and Banerjee, 2010) among other diseases. The boiled decoction was reported to be used to treat several diseases such as asthma, hy- pertension, lung cancer and pneumonia, and also as a remedy for fever (Ashok Kumar et al., 2014; Manjeshwar Shrinath, 2010; Stocklin, 1986). Recent studies have also reported the potent broncho-dilatory effect of the ethanol extract of A. scholaris in the anaesthetized rats. The extract also produced its effects on cardi- ovascular system reflected by significant inhibition in carbachol- induced hypotension (Channa et al., 2005). A previous in vivo and in vitro studies from our laboratory re- vealed the blood pressure lowering effect in Spontaneous Hy- pertensive Rats (SHR) orally treated with methanol extract of A. scholaris (ASME) and the presence of potent vasorelaxant activity in the extract and its n-butanol fraction (NBF-ASME) through its ability to relax phenylephrine and KCl-induced contractions in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jep Journal of Ethnopharmacology http://dx.doi.org/10.1016/j.jep.2015.09.031 0378-8741/& 2015 Elsevier Ireland Ltd. All rights reserved. Abbreviations: PE, phenylephrine; KCl, potassium chloride; ASME, Alstonia scho- laris methanol extract; ASWE, Alstonia scholaris water extract; DCF-ASME, di- chloromethane fraction of methanol extract; EAF-ASME, ethyl acetate fraction of methanol extract; NBF-ASME, n-butanol fraction of methanol extract; AQF-ASME, water fraction of methanol extract. n Corresponding author. E-mail addresses: amzaini@usm.my, boy3hc2005@yahoo.com (Mohd.Z. Asmawi). Journal of Ethnopharmacology 175 (2015) 422–431