In the present study, Desmodium velutinum (D. velutinum) leaves were used. These leaves were harvested from the village SOSSI, in the department of Niari in the month of November 2018. The samples of these leaves were identified at the Institut national de Recherche en Sciences Exactes et Naturelles (IRSEN) at the national herbarium service at number 636. These samples were then dried at the laboratory of the Institut National de Recherche en Sciences de la Santé (IRSSA) for 12 days in the shade.

The animal material was consisted of wistar rats (males and females) aged 16 to 20 weeks and weighing between 250 and 270 g. These animals were raised in the animal house of the Faculty of Sciences and Techniques of Marien Ngouabi University at an ambient temperature of 27˚C ± 1˚C with free access to food and tap water, under a cycle of 12 hours of light and 12 hours of darkness.

The hydroethanol extract of D. velutinum leaves was fractionated in a bioguided manner. For this purpose, 1 g of the hydroethanolic extract of D. velutinum leaves was chromatographed on an open column of Polyamide 6 (Fluka) of 1.5 mm diameter and 50 cm long. Elution was carried out with a water ethanol mixture of decreasing polarity, in the proportions: 100% H₂O, EtOH-H₂O (30:70 v/v), EtOH-H₂O (70:30 v/v), and 100% EtOH.

The blood pressure recording of the animals was done in this study following the invasive method after anesthetizing the rats with urethane 15% at 1 ml/100g. Blood pressure was assessed by the invasive method (bloody) or direct method. The technique consists in recording the blood pressure using a catheter introduced into the carotid artery of the anesthetized rat [4] . The catheter is linked to the transducer connected to the hemodynamic recorder Biopac Student Lab M35-type to visualize the different tracings recorded on the computer screen. After a period of stabilization of the cardiovascular parameters of approximately 30 minutes, all the substances were administered through the catheter attached to a syringe at the level of the femoral vein of the rat. These substances, dissolved in 0.9% NaCl solution, were administered at a rate of 0.1 ml/100g rat body weight [5] .

15 normotensive rats were divided into 5 lots of 3 rats each, treated as follows:

Ÿ Lot 1 received the aqueous extract of the leaves of D. velutinum at a rate of 2.5/mg/kg per i.v (intravenous);

Ÿ Lots 2, 3, 4 and 5 received the different fractions of D. velutinum (F1, F2, F3 and F4) at a dose of 2.5 mg/kg, per i.v respectively.

Systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR) were measured for one hour during the experiment.

The values expressed in the figures are the means of a series of values plus or minus Standard Error of the Mean (SEM). These values were calculated using Excel. The comparison of the means of the two series of values was done using the t-Student test. The significance threshold was set at p < 0.05.

Phenolic compounds can be characterized on the thin-layer chromatographic method (TLC). A small quantity of the mixture to be separated is deposited on the stationary phase and this phase is put in contact with the mobile phase (eluent). The mobile phase migrates upwards, by capillary action, along the fixed phase, carrying the components of the mixture. This is the phenomenon of elution, which allows the separation of the components of the mixture to be analyzed [6] . Each component migrates from a certain height (different components migrate at different speeds), characteristics of the substance, which is called frontal ratio or frontal retention (fr).

The determination of total polyphenols was performed using a spectrophotometer. We determined the optical densities of our extract and fractions and compared them to that obtained by a gallic acid standard of known concentration. The extract and fractions were assayed as follows: to 0.1 ml of extract and fractions introduced in different test tubes, were added 0.9 ml of distilled water; 0.9 ml of Folin Ciocalteu reagent (1N); then immediately 0.2 ml of a 2CO₃ solution (20%). The resulting mixture was incubated at room temperature for 40 minutes in the dark. The absorbance was then measured with a spectrophotometer at 725 nm against an ethanol solution used as the blank. Note that a calibration line was previously performed before the analysis with gallic acid under the same conditions as the samples to be analyzed [7] . The results obtained were expressed in mg gallic acid equivalent per 100 grams of dry matter (mgEa/100gMs).

Total flavonoids were also assayed using a spectrophotometer, as follows: 250 μl of the extract and fractions were placed in different test tubes, 1 ml of distilled water was successively introduced into each test tube. At the initial time (0 minute), 75 μl of a NaNO₂ (5%) was added, followed by 75 μl of AlCl₃ (10%) 5 minutes later. 6 minutes later, 500 μl l of NaOH (1N) and 2.5 ml of distilled water were successively added to the mixture. The absorbance of the resulting mixture was directly measured by UV-visible spectrophotometer at 510 nm and the results were expressed as mg quercetin equivalent per 100 grams of dry matter (mgEct/100g Ms). A calibration curve was developed with standard solutions of quercetin prepared at different concentrations.

Intravenous administration of the aqueous extract (2.5 mg/kg) and the different fractions (F1, F2, F3, and F4) at the dose of 2.5mg/kg caused an immediate decrease in diastolic blood pressure (DBP), about 9 seconds after their administration [8] (Figure 1), and the respective decrease in DBP of −47.60% ± 0.52%; −27.59% ± 5.86%; −23.72% ± 2.74%; − 49.99% ± 0.75% and −26.88% ± 3.44% (p < 0.001) (extract, F1, F2, F3, and F4). This decrease was followed by a slight rise in DBP at the 5th minute not exceeding the initial value, followed by a progressive decrease in DBP until the 60th minute. Regarding systolic blood pressure (SBP), the extract and fractions caused an abrupt decrease in SBP about 9 seconds after their administration at the same dose (Figure 2), the decrease in SBP is −23.28% ± 0.65%; −11.99% ± 0.16%; −12.38% ± 0.94%; −56.03% ± 0.23% et −24.37% ± 0.39% (p < 0.001) for extract, F1, F2, F3, and F4, respectively. This decrease was followed by a slight rise in DBP at the 5th minute not exceeding the initial value except for the F1 fraction, followed by a progressive decrease in SBP until the 60th minute.

Concerning heart rate (HR), at the same dose, the F2 and F4 fractions caused a significant decrease of −9.71% ± 0.09%; et −9.98% ± 0.16%; (p < 0.01) respectively. This decrease is followed by a constant rise in HR at the 5th minute (Figure 3) not exceeding the initial value, this rise was again followed by a considerable decrease until the 60th minute unlike the F1 fraction. Only the extract and the F3 fraction showed respectively a very significant decrease of −47.99% ± 0.06%; −50.25% ± 2.13% (p < 0.001), followed by a rise in the 5th minute and then a progressive relapse until the 60th minute.

The chromatogram in Figure 4 shows the chromatographic profiles of the aqueous extract and the different fractions (F1, F2, F3 and F4) of the leaves of D. velutinum. This profile shows, after the sputtering of the plate with Neu and visualization at 366 nm, different streaks that can materialize the presence of several chemical families.

The orange-yellow fluorescences at the frontal ratios (0.15; 0.28; 0.85 and 0.58) highlighted in the extract and fractions, could be attributed to the derivatives of an ortho-di-hydroxylated flavonol at the 3' and 4' position in the flavonol

ring, the blue to bluish-white fluorescent compounds at the ratio (0.72; 0.85; 0.28) on fractions F2, F4 and the extract could be attributable to phenolic acid on fractions F2, F4 and the extract could be attributable to phenolic acid derivatives, the light yellow compounds at the frontal ratios (0.87 and 0.84) their presence is characteristic to quercetin and myrtecin derivatives.

The results of the quantitative analysis by UV-visible spectrophotometer of the extract and the different fractions of D. velutinum studied are represented in Figure 5. In this composition we can see that the fractions are quantitatively richer in phenolic compounds (polyphenols and total flavonoids) than the extract. The quantitative analysis of total polyphenols and flavonoids also shows that fractions F4, F3, and F2 (Figure 5) are quantitatively richer in flavonoids and total polyphenols than fraction F1. The contents of total polyphenols and total flavonoids in the extract are respectively 354.55 mgEAG/100g/Ms and 114.22 mgEQt/100gMs and the contents of total flavonoids in the fractions are respectively: 266.38; 221.9; 355.54 and 376.67 mgEQt/100gMs and those of polyphenolic compounds are: 404.27; 530.55; 566.76 and 612.25 mgEAG/100gMs for fractions F1, F2, F3 and F4 respectively.