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Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.



Vung Nguyen Tien, Loi Vu Duc and Tung Bui Thanh
 
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ABSTRACT

Nerium oleander L. is an evergreen shrub in the dogbane family Apocynaceae and planted throughout the tropical region. It has cardiotonic, antioxidant, anti-inflammatory and anticancer activities. Nerium oleander L. has high amount of cardiac glycoside and flavonoids compounds. From methanol extract of flowers of Nerium oleander L. grown in vietnam, it has been isolated four compounds. Their structures were identified as (1) D16-Dehydroadynerigenin, (2) D16-Digitoxigenin, (3) Quercetin and (4) Kaempferol on the basis of spectroscopic data and by comparing their physicochemical and spectral data with those published studies. The effect of fraction HF2, which containing compound 1 and 2 on heart function was examined, revealing potent positive inotropic effect on isolated rabbit heart by increasing the contractility and coronary flow heart but does not alter heart rate.

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Vung Nguyen Tien, Loi Vu Duc and Tung Bui Thanh, 2016. Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.. Research Journal of Phytochemistry, 10: 21-29.

URL: https://scialert.net/abstract/?doi=rjphyto.2016.21.29
 
Received: September 06, 2015; Accepted: October 19, 2015; Published: December 03, 2015



INTRODUCTION

Vietnam has the tropical monsoon climate with plant species diversity. Vietnam has more than 12000 plant species of which nearly 4000 species can be used in traditional medicine belong to 300 families. Nerium oleander L. is a small tree perennial in the dogbane family Apocynaceae, widely distributed in temperate regions throughout the world. It is so widely cultivated as ornamental. The leaf and flower of Nerium oleander L. contain some cardiac glycosides that are extremely toxic (Bhuvaneshwari et al., 2007; Siddiqui et al., 2009; Khan et al., 2010; Kumar et al., 2013). Flowers of Nerium oleander L. has size from 2.5-5 cm, with funnel shape. Traditional medicine has been used in different treatment such as heart failure, asthma, corns, cancer, diabetes and epilepsy (Benson et al., 2015). The cardiac glycosides of Nerium oleander L. are mainly oleandrin and neriine compounds (Akhtar et al., 2014). Some cardenolides in Nerium oleander L. are capable of exerting positive inotropic effects on the hearts of animals and humans. The cardiotonic effect of oleanders have been used in therapeutic and also as an instrument of suicide since antiquity (Langford and Boor, 1996). The mechanism of Nerium oleander L. cause poisoning by inhibiting plasmalemmal Na+, K+-ATPase (Barbosa et al., 2008). Some studies have been showed the lethal dose of Nerium oleander L. is very small. For mice, the lethal dose of Nerium oleander L. leaves ethanolic extract were 520 mg kg–1 b.wt. (Saliem, 2010). Vietnam also has many case of poisoning from misuse, envenom, suicide of some preparations from Nerium oleander L. in every year. Identified compound from Nerium oleander L. is extremely important to determine the cause of poisoning that help to detoxify patients accurately and quickly. Therefore a study about the chemical composition of Nerium oleander L. is very needed. In this study, extracted, isolated and identified four compounds: quercetin, kaempferol, D16-dehydroadynerigenin and D16-digitoxigenin from flower of Nerium oleander L. grown in Vietnam.

MATERIALS AND METHODS

Materials: The flowers of Nerium oleander L. were collected in August 2013 from Ha Noi, Vietnam. Plant samples were authenticated and stored at the Institute of Medicine, Vietnam.

Chemicals and equipment: Melting points were measured on Mikroskopheiztisch PHMK-50 (VEB Waegetechnik Rapido, Germany). The FT-IR spectra were recorded on an IMPACT-410FT-IR spectrometer (CARL ZEISS JENA). The NMR [1H (500 MHz), 13C (125 MHz) and DEPT-90 and 135 MHz)] spectra were recorded on an AVANCE spectrometer AV 500 (Brucker, Germany) in the Institute of Chemistry, Vietnam Academy of Science and Technology (VAST). Chemical shifts were reported in ppm downfield from TMS with J in Hz. Electrospray Ionization Mass Spectra (ESI-MS) were recorded on a Varian Agilent 1100 LC-MSD mass spectrometer. Analytical TLC was performed on Kieselgel 60 F254 (Merck) plates (silica gel, 0.25 mm layer thickness) and RP-18 F254 (Merck) plates (0.25 mm layer thickness). Spots were visualized using ultraviolet radiation (at 254 and 365 nm) and by spraying with 10% H2SO4 followed by heating with a heat gun. Column chromatography was performed on silica gel (70-230 and 230-400 mesh, Merck). Organic solvents were of analytical grade.

Extraction and isolation: Nerium oleander L. flowers of 1.2 kg was dried at 60°C and extracted with methanol (5 L×3 times) at room temperature. The methanol extracts were combined and then evaporated to dryness in vacuo at 40°C. This crude extract (34 g) was then suspended in H2O and partitioned successively with n-hexane, chloroform and ethyl acetate. The EtOAc fraction (10,6 g) was chromatographed over a Sephadex LH-20 column using MeOH as the eluting solvent to yield six fractions (HF1 to HF6). Fraction HF2 (4.9 g) was further separated on Sephadex LH-20 column and eluted with MeOH/CH2Cl2 (95/5) to yield two fractions HF2.1 (1.2 g) and HF2.2 (200 mg). Fraction HF2.2 was crystallized in solvent system (n-hexane/acetone 4:1) to obtain the compound 1 (30 mg). Fraction Hf2.1 was chromatographed over a silica gel column and eluted with n-hexane/acetone (9:1) to yield compound 2 (10 mg). Fraction HF5 was applied to a silica gel column eluting with CH2Cl2/MeOH (95:5) to afford five subfractions (HF5.1-HF5.5). Fraction HF5.1 (420 mg) was further separated on Sephadex LH-20 column and eluted with MeOH/CH2Cl2 (4/1) to yield compound 3 (50 mg). Fraction HF5.2 (70 mg) was also separated on Sephadex LH-20 column and eluted with MeOH/CH2Cl2 (4/1) to yield compound 4 (50 mg).

Evaluate effect of fraction HF2 (contains compound 1 and 2) on isolated heart rabbit: We used common rabbits (Oryctolagus cuniculus) (1.5-2.5 kg) obtained from a local rabbit breeder. The animals were maintained at ambient temperature (22±1°C) with 12:12 h light-dark cycles and free access to water and food. All procedures were approved by the ethical committee of our institute and the experiments were performed according to international accepted guidelines for the use of animals. The rabbits were injected intravenously with heparin and anesthetized with ketamine and xylazine. After the heart was quickly removed, the ascending aorta was immediately cannulated and perfused with a Ringer-Locke solution at a constant pressure (60 cm H2O) at 37°C and continuously bubbled with a mixture of 95% O2/5% CO2 according to the Langendorff technique. The Ringer-Locke solution consisted of 154 mM NaCl, 5.63 mM KCl, 2.16 mM CaCl2, 2.10 mM MgCl2, 5.95 mM NaHCO3 and 5.55 mM glucose. Langendorff hearts were allowed to equilibrate for 15 min, hearts presenting any irregularities in function were discarded. The rabbits were randomly divided into three groups (n = 9 for each group):

HF2 (0.1%): HF2 (0.1%) was added to the Ringer-Locke solution
HF2 (0.5%): HF2 (0.5%) was added to the Ringer-Locke solution
HF2 (1%): HF2 (1%) was added to the Ringer-Locke solution

During the experiments each heart served as its own control before injection of each solution. The Heart Rate (HR), contractility and Coronary Flow (CF) were monitored with a physiograph (Ugo-Basile, Italy) by using fluid volume flow through the heart in each 5 min before and after perfused with the sample solution during 30 min.

RESULTS

In order to isolate the compounds from flower of Nerium oleander L., the EtOAc-soluble extract of Nerium oleander L. was subjected to a succession of chromatographic procedures including Sephadex LH-20, silica gel chromatography, RPC18 and HPLC to afford four compounds (Fig. 1).

Compound 1 (D16-Dehydroadynerigenin): It was obtained as a yellow, solid powder, melting point 209-210°C; IR (KBr) nmax 3362 (OH), 2930 (CH, aliphatic), 1742 (g-lactone α,β-unsaturated), 1628, 1443 (C = C) and 1160 (C-O-C) cm–1; ESI-MS: 371 [M+H]+; 1H NMR (500 MHz, CDCl3) δH ppm: 1.04 (3H, s, H-19); 1.21 (3H, s, H-18); 2.57 (1H, dd, J = 20.0; 2.0 Hz, H-15β); 2.61 (1H, dd, J = 20.0; 2.0 Hz, H-15α); 6.07 (1H, t, J = 3,0 Hz, H-16); 5.94 (1H, br s, H-22); 4.97 (1H, dd, J = 16.5; 1.5 Hz, H-21α); 4.91 (1H, dd, J = 16.5; 1.5 Hz; H-21β); 4.12 (1H, br s, H-3); 13C NMR (500 MHz, CDCl3) δH ppm: 15.63 (C-11); 19.91 (C-18); 24.51 (C-6); 24.58 (C-19); 26.78 (C-7); 28.08 (C-2); 29.60 (C-1); 33.04 (C-15); 33.16 (C-4); 33.32 (C-12); 35.94 (C-9); 36.00 (C-5); 36.94 (C-10), 44.74 (C-13); 65.12 (C-8); 66.49 (C-3); 70.10 (C-14); 71.40 (C-21); 112.82 (C-22), 132.24 (C-16); 143.01 (C-17); 157.68 (C-20); 174.28 (C-23).

Compound 2 (16-dehydrogitoxigenin or D16-Digitoxigenin): It was obtained as a white, solid powder, melting point 240-241°C; IR (KBr) nmax 3484 (OH), 2930 (CH, aliphatic), 1727 (γ-lactone α, β-unsaturated), 1619, 1456 (C = C) and 1170 (C-O-C) cm–1; ESI-MS: 373 [M+H]+, 355 [M-H2O+H]+; 1H NMR (500 MHz, CDCl3) δH ppm: 0.99 (3H, s, H-19); 1.27 (3H, s, H-18); 2.34 (1H, dd, J = 18.5; 3.5 Hz, H-15β); 2.72 (1H, br d, J = 18.5 Hz, H-15α); 4.07 (1H, br s, H-3), 5.00 (1H, dd, J = 16.5; 1.5 Hz; H-21β); 5.08 (1H, dd, J = 16.5; 1.5 Hz, H-21α); 5.94 (1H, br s, H-22); 6.20 (1H, br s, H-16). 13C NMR (500 MHz, CDCl3) δH ppm: 16.85 (C-18); 20.46 (C- 7); 21.71 (C-11); 24.29 (C-19); 27.10 (C-6); 28.20 (C-2); 30.32 (C-1); 33.72 (C-4); 35.88 (C-10); 36.71 (C-9); 36.99 (C-8); 39.13 (C-15); 40.78 (C-12); 41.54 (C-5); 52.93 (C-13); 67.05 (C-3); 73.01 (C-21); 86.13 (C-14); 111.74 (C-22), 134.20 (C-16); 144.49 (C-17); 161.13 (C-20); 176.76 (C-23).

Image for - Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.
Fig. 1(a-d): Chemical structure of compounds 1-4 isolated from the flower of Nerium oleander L.

Table 1:Effect of fraction HF2 on rate of isolated rabbits heart
Image for - Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.
We observed that three concentration of fraction HF2 did not influenced significantly (p>0.05) to heart rate isolated rabbit, HR: Heart rate

Compound 3 (Quercetin): It was obtained as a yellow, solid powder, melting point 304-305°C; IR (KBr) nmax cm–1: broad absorbance band 3426 (-OH), 1664 (C = O), 1614 (C = C), 1523, 1386 (C-H), 1020 (C-O-C) cm–1; ESI-MS: 301 [M-H]+; 1H NMR (500 MHz, CDCl3+ MeOH-d4) δH ppm: 6.13 (1H, d, J = 2.5 Hz, H-2’); 6.28 (1H, d, J = 2.0 Hz, H-8); 6.80 (1H, d, J = 8.5 Hz, H-5’); 7.52 (1H, dd, J = 8.5; 2.5 Hz, H- 6’); 7.61 (1H, J = 2.0 Hz, H-6).13C NMR (500 MHz, CDCl3+ MeOH-d4) δH ppm: 93.40 (C-8); 98.09 (C-6); 102.98 (C-4a); 114.42 (C-2’); 114.83 (C-5’); 120.32 (C-6’); 122.44 (C-1’); 135.29 (C-3); 144.25 (C-3’); 146.07 (C-2); 146.78 (C-4’); 156.46 (C-8a); 160.32 (C-5); 163.55 (C-7); 175.22 (C-4).

Compound 4 (Kaempferol): It was obtained as a yellow powder, melting point 275-276°C; IR (KBr) nmax cm–1: 3421 (OH phenol), 2992 (-CH-), 1661 (C = O), 1612, 1507, 1385 (C = C), 1178, 1007 (C-O-C), 821 (C-H); ESI-MS: 285 [M-H]+; 1H NMR (500 MHz, methanol-d4) δH ppm: 6.19 (1H, d, J = 2,0 Hz, H-6); 6.41 (1H, d, J = 2.0 Hz, H-8); 6.91 (2H, dd, J = 7.0; 2.0 Hz, H-5’ va H-3’); 8.01 (2H, dd, J = 7.0; 1.5 Hz, H- 6’ va H-2’).13C NMR (500 MHz, metanol-d4) δH ppm: 148.05 (C-2); 137.13 (C-3); 177.37 (C-4); 162.52 (C-5); 99.2 (C-6); 165.58 (C-7); 94.46 (C-8); 158.26 (C-8a); 104.54 (C-4a); 123.73 (C-1’); 130.68 (C-2’); 116.30 (C-3’); 160.55 (C-4’); 116.30 (C-5’); 130.68 (C-6’).

Effect of fraction HF2 on isolated rabbit heart: The results of fraction HF2 to heart rate, contractility heart (mm) and coronary flow on isolated rabbit heart were showed in Table 1-3.

Table 2:Effect of fraction HF2 on contractility of isolated rabbits heart
Image for - Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.

Table 3:Effect of fraction HF2 on coronary flow of isolated rabbits heart
Image for - Isolated Compounds and Cardiotonic Effect on the Isolated Rabbit Heart of Methanolic Flower Extract of Nerium oleander L.

We observed that three concentration of fraction HF2 did not influenced significantly (p>0.05) to heart rate isolated rabbit.

In Table 2 we observed that the three concentrations of HF2 significantly increased the contractile of isolated rabbits heart (p<0.005-p<0.001). When the concentration increased from 0.1-1%, the contractility of isolated rabbit’s heart was increased from 21.2-41.9%. As the concentration of HF2 increases, the contractility also increases.

Table 3 showed that the three concentrations of HF2 also significantly increased the coronary flow of isolated rabbits heart (p<0.05-p<0.001). When the concentration of HF2 increased from 0.1-1%, the coronary flow of isolated rabbit’s heart was increased from 4.5-13.0%. The effect of HF2 on coronary flow is concentration-dependent.

DISCUSSION

Compound 1 was isolated as a yellow solid, melting point at 209-210°C. The molecular formula was established as C23H30O4 based on a molecular ion peak at m/z 371 [M+H]+. The IR spectrum showed typical absorption bands arising from hydroxyl (nmax 3372 cm–1), γ-lactone α, β-unsaturated group (nmax 1789, 1742 cm–1) and double bond C = C (nmax 1628 cm–1). The 13C-NMR spectrum indicated the presence of 23 carbon atoms in the molecule. They are distinguished by DEPT 90 and DEPT 135 spectrum. There are a carbonyl carbon (δC 174.28), 4 carbons of two double bond (δC 143.01 and 132.24, 157.68 and 112.82), two methyl carbons (δC 19.91 and 24.58), 9 methylene carbons in which one oxymethylene carbon (δC 71.40), three sp3 methine carbon including a hydroxymethine carbon (δC 66.49) and four quaternary carbon sp3 including two carbon bond to oxygen. The 1H-NMR spectrum showed the presence of two methyl singlet group relatively in up-field at δH 1.04 (3H-19) and δH 1.21 (3H-18), two signals double doublet appear at dH 2.57 (J = 20; 2.0 Hz, H-15β) and dH 2.61 (J = 20.0; 2.0 Hz, H-15α) belong to two gemminal proton of methylene group. Additionally, the spectrum indicated at down-field a singlet broad signal of a proton hydroxymethine at dH 4.12, this signal has a configuration. The signal of two protons in oxymethylene group appears as two double doublet at δH 4.97 (J = 16.5, 1.5 Hz, H-21 α) and 4.91 (J = 16.5; 1. 5 Hz; H-21β). Signal of proton of unsaturated hydrocarbon appears as a triplet at δH 6.07 (J = 3.0 Hz, H-16) and a broad singlet at δH 5.94 (H-22). In addition, the 1H- NMR spectrum had signals as multiplets at up-field of methine and methylene group at dH 1.10-2.20 ppm region. Furthermore, from molecular formula C23H30O4 of compound 1, it can inferred the total rings and double bonds in the molecule is 9, which identify the molecule containing 3 double bonds and 6 rings and then this compound containing steroid ring with γ-lactone α, β-unsaturated ring. The steroid ring with γ-lactone α, β-unsaturated ring and a double bond contain 5 rings and 3 double bonds. Thus there is only one oxygen atom corresponding to a loop in the molecule, which indicates that the molecule must contain an epoxy ring. This is totally consistent with the presence of two carbons sp3 bond with oxygen at δC 65,12 (C-8) and δC 70,10 (C-14) on 13C NMR spectrum. Based on the above evidence and the literature data (Yamauchi et al., 1973; Siddiqui et al., 1986, 1997), compound 1 was identified as 3β-hydroxy-8, 14β-epoxy-5β-carda-16, 20(22)-dienolide or D16-Dehydroadynerigenin.

Compound 2 was isolated as a white solid with melting point at 240-241°C. Spectrum of compound 2 is very similar to compound 1. The 13C-NMR spectrum also indicated the presence of 23 carbon atoms in the molecule and distinguished by DEPT 90 and DEPT 135 spectrum. There are two methyls carbons (dC 16.85 va 24.29), nine methylene carbons including one oxymethylene carbons (dC 73.01), six methine carbons including two methine carbons unsaturated (dC 111.74 va 134.20), one hydroxymethine carbon (dC 67.05) and two sp3 methine and six quaternary carbon including a carbonyl carbon (dC 176.76), two quaternary carbon unsaturated (dC 144.49 va 161.13), one sp3 carbon bonds to oxygen (dC 86.13). The 1H-NMR spectrum of compound 2 has many similarities with the 1H-NMR spectrum of compound 1 as showed the signals of two singlets of two methyl groups at dH 0.99 and dH 1.27, two gemminal proton of methylene group as one signal of double doublet at dH 2.34 (J = 18.5; 3.5 Hz, H-15β) and one broad doublet at dH 2.72 (br d, J = 18.5 Hz, H-15α). In addition, at the down-field of spectrum there was a signal of hydroxymethine as a broad singlet at dH 4.07, two protons of a oxymethylene group as two double doublet at dH 5.00 (J = 16.5; 1.5 Hz; H-21β) and dH 5.08 (J = 16.5; 1.5 Hz, H-21α). Signals of two protons of double bonds appear as two broads singlet at dH 5.94 (br s, H-22) and dH 6.20 (br s, H-16). Additionally, there were also signals as multiplets of the methylene group and methine group at up-field dH 0.88-2.03. The molecular formula was established as C23H32O4 based on a molecular ion peak at m/z 373 [M+H]+. From molecular formula C23H30O4 of compound 2, we can inferred the total rings and double bonds in the molecule is 8. Based on spectrum data on, the compound has three double bonds, then it has five rings in the molecule. Compound 2 has spectrum data is very similar of compound 1, differing only in that compound 2 has more two hydrogen but less a ring and a carbon quaternary bonds to oxygen as compared to compound 1. Combining all the data, it can be concluded that the compound 2 has structure of compound 1 with epoxy ring opening. Based on the above evidence and the previous studies (Yamauchi et al., 1975; Siddiqui et al., 1986), compound 2 was identified as 16-anhydrogitoxigenin or D16-Digitoxigenin.

Compound 3 was isolated as a yellow powder melting point at 304-305°C. The 1H NMR spectrum showed only signal of five protons of 2 spin-spin interactions at down-field of proton in aromatic ring. The first interaction is an AX system of two protons in meta position in a benzene ring with four substituted [dH 6.28 (1H, d, J = 2.0 Hz, H-6) va 7.61 (1H, d, J = 2.0 Hz, H-8)]. The second interaction is an ABX system of three protons in a benzene ring with three substituted [dH 6.13 (1H, d, J = 2.5 Hz, H-2’); 6.80 (1H, J = 8.5 Hz, H-5’) va 7.52 (1H, dd, J = 8.5, 2.0 Hz)]. The 13C-NMR spectrum showed the presence of 15 carbon atoms at down-field (dC 93.40-175.24) in the molecule, in which 5 carbons belong methine group of double bond (= CH) [dC 93.40 (C-8), 98.09 (C-6), 114.42 (C-2’), 114.83 (C-5’) and 120.32 (C-6’)] seven quaternary carbons bond to oxygen (dC 135.29-175.22) and 2 other quaternary carbons sp2. The signal at dC 175.22 indicated that the molecule containing a carbonyl group.

The presence of carbonyl group in the molecule was confirmed by the presence of the absorption band at nmax 1664 cm–1 in IR spectrum. Also the IR spectrum showed typical absorption bands arising from some hydroxyl groups (nmax 3200-3500 cm–1) and benzen ring through absorption band for C = C bonds in aromatic rings (nmax 1614, 1523 cm–1). In addition, there is also a typical absorption band for C-O-C bond at nmax 1020 cm–1. The molecular formula was established as C15H10O7 based on a molecular ion peak at m/z 301 [M+H]+ and combining all the data, it can be concluded this compound is a flavonoid. Based on the above evidence and the study which reported by Ahmedova et al. (2012), compound 3 was identified as quercetin. Quercetin is a flavonol with has various biological activities and it is very common in some plants. This compound has strong antioxidant, anti-inflammatory and anti-cancer activities (Lee et al., 2007).

Compound 4 was isolated as a yellow crystal, melting point at 275-276°C. The 1H NMR spectrum showed signal of six protons at down-field of proton in aromatic ring. There is an interaction AX system of two protons in a benzene ring with four substituted [dH 6.19 (1H, d, J = 2.0 Hz, H-6); 6.41 (1H, d, J = 2.0 Hz, H-8)]. Other signal belonging to four protons in para position in a benzene ring with two substituted [dH 6.91 (2H, dd, J = 7.0; 2.0 Hz, H-5’ and H-3’); 8.01 (2H, dd, J = 7.0; 1.5 Hz, H- 6’ and H-2’)]. The 13C-NMR spectrum showed the presence of 15 carbon atoms at down-field (dC 94.46-177.39) in the molecule, in which 6 carbons belong methine group of double bond, nine quaternary carbons including one carbonyl carbon (dC 177.39), six quaternary carbons bond to oxygen and 2 other quaternary carbons sp2. The IR spectrum showed typical absorption bands arising from some hydroxyl groups (nmax 3300-3500 cm–1). The presence of methine double bond (= CH) group in the molecule was confirmed by the presence of the typical absorption band of C-H bond at nmax 2929 cm–1 in IR spectrum. The carbonyl group in the molecule gives an absorption band at nmax 1661 cm–1. Also the IR spectrum showed typical absorption bands arising from benzen ring through absorption band for C = C bonds in aromatic rings (nmax 1612, 1507 and 1385 cm–1). Additionally, there is also a typical absorption band for C-O-C bond at nmax 1178 cm–1. Combining all the data, we can conclude this compound is a flavonoid. The molecular formula was established as C15H10O6 based on a molecular ion peak at m/z 285 [M+H]+. Based on the above evidence and the study which reported by Furusawa et al. (2005), compound 4 was identified as kaempferol. Kaempferol is a flavonol and presents in some plants. This compound has strong anti-inflammatory, diuretic, antioxidant activity and inhibition of topoisomerase-II, adenosine deaminase, tyrosine kinase, xanthine oxidase enzyme (Lee et al., 2007).

In this study, methanolic extract of Nerium oleander elicited to increase the force of contraction, coronary flow heart but does not influence to heart rate. Present data can be used to explain the possible benefit of this herb in cardiovascular diseases. The increased force of contraction of the heart will cause the heart to expel more blood into the arterial system with each beat. The patient with failing heart will be beneficial by this properties of the herb (Adome et al., 2003). Mechanism of action of cardiac glycosides can explain by capacity of inhibit Na+-K+ pumps in the cardiac myocytes, then increased intracellular sodium concentration. The raises intracellular sodium levels lead to inhibit Na+/Ca2+ exchanger, then calcium ions are not extruded and begins to increase inside the cell. Increased cytoplasmic calcium concentrations allow for greater calcium release on stimulation so that myocytes could achieve faster and more powerful contraction by cross-bridge cycling (Radzyukevich et al., 2009). Our data was agreed with previous study of Adome et al. (2003). These authors have shown that the crude ethanolic extracts of the dried leaves of Nerium oleander increased the force of contraction and cardiac flow on the isolated guinea pig hearts in manner dose-dependent. When the force of cardiac contraction is increased, the failure is partially relieved, also reducing the size heart rate. In our present work the heart rate was not influenced, it is contradiction with Adome et al. (2003), which they showed the heart rate was increased. In other study, Gayathri et al. (2010) have shown the cardioprotective effect of hydroalcoholic extract of Nerium oleander flowers against isoproterenol-induced myocardial toxicity by improving the antioxidant defense system in rats model. The positive inotropic effect of fraction HF2 may due to present of two major compounds 1 and 2, which we have isolated from this fraction. In fact, dehydrogitoxigenin and digitoxigenin glucoside have been showed the inotropic activity in previous study (Altman et al., 1988; Packer et al., 1991). The pretreatment with extract of Nerium oleander prevented the elevation of marker enzymes such as lactate dehydrogenase, γ-glutamyl transferase, creatine kinase, aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase in plasma and also attenuated the lipid peroxidation and increased the levels of enzymatic such as superoxide dismutase, glutathione peroxidase and nonenzymatic antioxidants including reduced glutathione and nitrite (Gayathri et al., 2010).

CONCLUSION

From the flowers of Nerium oleander L. grown in Hanoi, Vietnam, we have isolated and identified successfully four compounds: (1) D16-Dehydroadynerigenin or 16-dehydrogitoxigenin, (2) D16-Digitoxigenin, (3) Quercetin and (4) Kaempferol based on IR, MS, NMR spectrum. This is the first time that four compounds were isolated from flower of Nerium oleander L. Fraction HF2 of flowers of Nerium oleander L. showed potent inotropic effect on isolated rabbit heart by increasing the contractility and coronary flow heart but does not alter heart rate.

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