The American Journal of Chinese Medicine, Vol. 37, No. 5, 991–1008 © 2009 World Scientific Publishing Company Institute for Advanced Research in Asian Science and Medicine Characterization of the Possible Mechanisms Underlying the Hypotensive and Spasmogenic Effects of Loranthus ferrugineus Methanolic Extract Omar Z. Ameer, ∗ Ibrahim M. Salman, ∗ Mohammad Jamshed A. Siddiqui, † Mun F. Yam, ∗ Raghava N. Sriramaneni, ∗ Amirin Sadikun, † Zhari Ismail, † Amin M. Shah ∗ and Mohamed Z. Asmawi ∗ ∗ Department of Physiology and Pharmacology † Department of Pharmaceutical Chemistry School of Pharmaceutical Sciences Universiti Sains Malaysia Minden 11800 Penang, Malaysia Abstract: In the present study, L. ferrugineus methanol extract (LFME) was evaluated for its blood pressure lowering effect in anesthetized normotensive Sprague Dawley (SD) rats and its spasmogenic effect in isolated guinea pig ileum. The possible mechanism(s) of action were also investigated. LFME was obtained by Soxhlet extraction. The rats were fasted overnight and anesthetized with sodium pentobarbitone (60 mg/kg i.p.). LFME was administered in i.v. boluses in the concentrations of 25, 50, 100 and 200 mg/kg respectively, with concomitant mon- itoring of mean arterial pressure (MAP). It was found that LFME dose-dependently reduced MAP. An i.v. bolus injection of atropine significantly decreased the blood pressure lowering effect of LFME. Similarly, L-NAME (Nω-nitro-L-arginine methyl ester) significantly lowered both the MAP and the action duration. Conversely, no significant change in MAP was seen fol- lowing i.v. injections of neostigmine, hexamethonium, prazosin and propranolol. LFME also produced a dose-dependent contractile effect in guinea pig ileum. This contraction was sig- nificantly reduced in atropine pre-incubated tissue segments, yet it was significantly enhanced in the presence of neostigmine. No appreciable change in the ability of LFME to contract guinea pig ileum was seen in the presence of hexamethonium. Accordingly, it can be postu- lated that LFME possesses a marked hypotensive effect that can be attributed to stimulation of muscarinic receptors and/or stimulation of nitric oxide (NO) release. Moreover, LFME Correspondence to: Dr. Omar Z. Ameer, Department of Physiology and Pharmacology, School of Pharmaceutical Sciences, Universiti Sains Malaysia, Minden 11800 Penang, Malaysia. E-mail: omar_3m@yahoo.com 991