ANTI-HYPERTENSIVE PROPERTIES AND MECHANISMS OF ACTION OF THE EXTRACT AND FRACTIONS FROM Persea americana MILL. LEAF IN RATS

Abstract

Hypertension is a chronic cardiovascular disease characterised by persistent, elevated arterial blood pressure. High cost and adverse effect burden of current antihypertensive therapies necessitate the need for alternative remedies. Persea americana leaf is used in ethnomedicine to treat hypertension, with limited scientific justification. This study therefore evaluated the antihypertensive properties and mechanisms of action of fractions from P. americana leaf in rats.

Fresh leaves of P. americana collected from the University of Ibadan was authenticated at the University Herbarium (UIH No. 22381), air-dried and extracted in 95% methanol. Persea americana methanol leaf extract (PAM) was partitioned with n-hexane, dichloromethane, chloroform, ethyl acetate and n-butanol. An initial study was carried out using 25, 50 and 100 mg/kg PAM to determine optimal effective dose. Thirty noradrenaline-induced hypertensive Wistar rats, distributed into six groups (n=5) were treated intravenously as follows: control (normal saline; 0.6 mL/kg), PAM (100 mg/kg), PAM fractions (100 mg/kg); n-hexane, dichloromethane, ethyl acetate and n-butanol. The PAM ethyl acetate fraction (PEtOAc) was fractionated on silica gel using column chromatography. Four pooled fractions (CFEt1, CFEt2, CFEt3 and CFEt4) were evaluated intravenously at 25 mg/kg for antihypertensive activity in noradrenaline- or NG-nitro-L-arginine methyl-ester (L-NAME)-induced hypertension using twenty-five rats. Mean Arterial Pressure (MAP) was used as index for antihypertensive activity. Antihyperlipidemic effect of PAM and CFEt3 were also evaluated in thirty-five rats distributed into 7 groups (n=5): Triton X100-induced hyperlipidemic rats (100 mg/kg; 72h; intraperitoneally) were treated orally with normal saline (10 mL/kg), atorvastatin (10 mg/kg), PAM (50 and 100 mg/kg), CFEt3 (25 mg/kg); non-hyperlipidemic rats treated with normal saline and PAM (100 mg/kg). The effect of PAM (100 mg/mL) and CFEt3 (25 mg/mL) were also tested on noradrenaline-induced contraction of thoracic aortic rings (n=6). The Angiotensin Converting Enzyme (ACE) inhibition by PAM, CFEt3 and captopril (100 µg/mL) was also investigated. The constituents of CFEt3 were identified by GC-MS. Data were analysed using descriptive statistics and ANOVA at α 0.05

The most active column fraction of PAM was the PEtOAc. The percentage reductions in noradrenaline-induced raised MAP were 31.6%, 32.0% and 32.3% observed for PAM (100 mg/kg), PEtOAc (100 mg/kg), and CFEt3 (25 mg/kg) respectively. Similarly, only PAM and CFEt3 caused 21.2% and 23.3% reductions respectively in L-NAME-induced raised MAP. Also, PAM (100 mg/kg), CFEt3 (25 mg/kg) and Atorvastatin significantly lowered Atherogenic Index of plasma from 1.8 ± 0.5 to 0.03 ± 0.001 when compared with Triton X 100. Additionally, PAM (100 mg/kg) and CFEt3 significantly reduced noradrenaline contraction of endothelium-intact (74.4% and 65.0%) and endothelium-denuded (36.6% and 27.8%) aortic rings respectively. The percentage in vitro inhibition of ACE by PAM, CFEt3 and Captopril were 68.1 ± 1.9, 78.7 ± 2.9 and 59.6 ± 5.0 respectively. The GC-MS analysis revealed 11-Tetradecyn-1-ol acetate (16.6%) and 14-methyl-(Z)-8-Hexadecenal (16.5%) as major components of CFEt3.

Persea americana leaf methanol extract and its ethyl acetate column fraction 3 possess in vitro relaxant and in vivo antihypertensive activities in rats which may be mediated through inhibition of vascular alpha adrenoceptors, angiotensin converting enzyme and amelioration of dyslipidemia.