Nymphaea pubescens extract as an herbal ingredient for erectile dysfunction and vascular diseases

Abstract

Background: Due to the increase in the elderly population, the disease caused by degeneration (degenerative diseases) is becoming an important health problem. Cardiovascular disease (CVD) has many common risk factors, including age, hypertension, diabetes, insulin resistance, smoking, increased body mass index (BMI), cholesterol, and lower high-density lipoprotein (HDL). These factors are also linked to erectile dysfunction (ED). ED and CVD are pathophysiological consequences of atherosclerosis and endothelial dysfunction. Endothelial dysfunction also plays a role in the progression of vascular disease in pulmonary arterial hypertension (PAH). From the previous study, we found that the petal of Nymphaea pubescens Willd., the member of Nymphaeaceae family, showed high inhibition of phosphodiesterase 5, the enzyme that is involved in the cardiovascular system and erection process. This plant or the plants in the related family might be sources of bioactive compounds for ED and CVD.

The aim of the study: The main aim of this project was exploration of the possibility to apply water lilies and lotuses in families of Nelumbonaceae and Nymphaeaceae for vascular health benefit. The PDE5 and arginase inhibitors from the active extract would be isolated and identified. The underlying mechanisms on vasodilator action in various blood vessels as well as the cytotoxicity of the extract and its bioactive constituents would be studied. Finally, the methods for quality control and standardization of the water lily extract would be established.

Materials and methods: Thirty-seven ethanolic extracts from different parts of plants in the genus Nymphaea and Victoria of Nymphaeaceae and genus Nelumbo of Nelumbonaceae were screened for PDE5 and arginase inhibitory activities. The bioactive compounds in the active extract were isolated and identified by chromatographic and spectroscopic methods, respectively. The vasorelaxant effects of the extract and its chemical constituents were evaluated on rat PA, aorta and mesenteric arteries. The cytotoxicity of the extract was also tested on the vascular smooth muscle cells (VSMCs) isolated from the rats. The characterization and quantitative analysis of constituents in the active extracts were performed by HPLC.

Results: The ethanolic extracts of the receptacles and pollens of Nelumbo nucifera Gaertn., petals of Nymphaea cyanea Roxb. ex G.Don, Nymphaea stellata Willd., and Victoria amazonica (Poepp.) Sowerby and the petals and receptacles of N. pubescens showed IC50 values on PDE5 of less than 25 µg/mL while none of the extracts showed effects on arginase. The most active extract, N. pubescens petal extract (WLE), was fractionated to isolate and identify the PDE5 inhibitors. The six flavonoid constituents including quercetin 3'-O-β-xylopyranoside (1), quercetin 3-methyl ether 3'-O-β-xylopyranoside (2), quercetin (3), 3-O-methylquercetin (4), kaempferol (5) and 3-O-methylkaempferol (6) inhibited PDE5 with IC50 values at the micromolar level. Among these compounds, 2 was the major compound in WLE (17.31±0.05 mg/g) and showed the strongest activity (IC50=5.63±0.20 mM).

The WLE relaxed PA (EC50 = 4.96±0.81 µg/ml) more than the aorta (EC50 = 27.50±7.61 µg/ml, p<0.001), suggesting its selectivity on the PA vs the aorta. PA vasorelaxation was reduced by endothelial removal or NG-nitro-L-arginine methyl ester (L-NAME), but was unaffected by indomethacin, apamin plus charybdotoxin, 4-aminopyridine (4-AP), glibenclamide, iberiotoxin, and BaCl2. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin -1- one (ODQ) slightly reduced the relaxation induced by the WLE. Sodium nitroprusside (SNP)-induced relaxation was enhanced by the WLE. WLE cannot inhibit Ca2+ channels (extracellular Ca2+ influx through ROCCs/VOCCs and intracellular Ca2+ release from the SR), and PE-induced contraction via α1-receptor. Compounds 2, 3 and 5 non-selectively relaxed the PA and aorta rings with and without endothelium (EC50 = 26 - >100 µM). VSMCs incubated in the WLE for 1 hr showed no acute cytotoxicity. The results of mesenteric arteries showed that both WLE and compound 2 induced vasorelaxant effects with EC50 of 0.08±0.01 mg/mL and 42.8±6.3 µM, respectively. A significant decrease in these relaxations was observed with L-NAME, but not with apamin-charybdotoxin or indomethacin. In endothelium-denuded condition, WLE-induced relaxation was enhanced by 4-aminopyridine and glibenclamide while iberiotoxin and ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one) had no effect. By contrast, compound 2-induced relaxation was not changed by any of these inhibitors. Both WLE and compound 2 enhanced sodium nitroprusside-induced relaxation and inhibited receptor-operated Ca2+ channels. Only WLE was able to reduce PE-induced contraction (p<0.001).

Conclusion: Among 37 samples from Nymphaeaceae and Nelumbonaceae families, our research found that N. pubescens petals had the strongest PDE5 inhibitory effect. Six flavonoids (1-6) with PDE5 inhibitory activity have been characterized as the constituents in the extract by the HPLC method. The major compound, 2 (17.31±0.05 mg/g of WLE), together with 1 and 6 were found to inhibit PDE5 for the first time in this study. The WLE induced PA relaxation via the endothelial nitric oxide (NO) pathway and mild stimulation of sGC. Importantly, the WLE showed no cytotoxicity on the VSMCs. The vasorelaxant effect of WLE and compound 2 on mesenteric arteries, relying on the potentiation of the NO-cGMP pathway and on calcium inhibitory effects.

-