Comparative Antidiabetic Activity of Aqueous, Ethanol, and Methanol Leaf Extracts of Persea americana and Their Effectiveness in Type 2 Diabetic Rats

Native to Mexico, Persea americana Mill. (avocado) is a fruit tree whose different parts (leaf, bark, roots, and stone) are used in traditional medicine especially against diabetes mellitus. The aim of this study was to investigate the beneficial effects of 28-day treatment with aqueous, ethanolic, and methanolic leaf extracts on glucose homeostasis in type 2 diabetic mellitus using Wistar rats. Type 2 diabetes was induced with nicotinamide (120 mg/kg, i.p.) and streptozotocin (65 mg/kg, i.p.). After 28 days of treatment, histopathological examination of the pancreas, kidneys, liver, and muscle (tibialis anterior) were realized. Biochemical markers were determined and an intestinal absorption test of D-glucose was performed. All extracts (100 mg/kg/day, p.o.) significantly (p < 0.001) reduced blood glucose level at the 28th day of treatment with a more pronounced effect for methanolic extract. The treatments were well tolerated and induced a restoration of T-CHOL and HDL-C levels compared to the control group. Methanolic extract reduced the AIP (atherogenic index of plasma) by 45%. Histopathological analyzes of the pancreas showed regeneration of islets of Langerhans. Methanolic extract was the most effective in preventing intestinal glucose uptake up to 60.90% in relation to metformin. These results justify the use of this plant in traditional medicine against type 2 diabetes. However, other complementary studies should be done to identify the molecules responsible for this activity and their signaling voice.


Introduction
Diabetes mellitus is a metabolic disease caused by an insufficiency of insulin secretion associated or not with a bad use of this hormone by the body [1]. Once installed, this dysfunction causes a disruption of carbohydrate homeostasis (hyperglycemia), proteins, lipids, and electrolytes [2]. Unfortunately, 47.5% of people with diabetes are not screened. In the long run, this untreated metabolic pathology can lead to serious cardiovascular, neurological, and nephrological complications that are often fatal. In 2017, diabetes mellitus was the 7th leading cause of death and it killed 3.2 to 5 million humans in the world [3,4].
In Africa, for economic and cultural reasons, 80% of the population uses medicinal plants from traditional medicine to treat themselves [5]. In the case of diabetes mellitus, approximately 800 plants have been identified and used alone or in combination in ethnomedicine as an antidiabetic treatment worldwide [6]. In African countries such as Côte d'Ivoire, Nigeria, Kenya, and Egypt, several ethnobotanical surveys confirmed that many of these medicinal plants are used against diabetes mellitus [7][8][9]. ey include Abizia harveyi, Ximenia americana, Eremophila maculata, Cola nitida, Punica granatum, and Persea americana Mill. which showed potent activity against type 2 diabetes [10][11][12][13][14][15]. Persea americana, which is the subject of our study, is a tree native to the southeast of Mexico from where it has spread to all the tropical and subtropical regions of the world. It belongs to the class of Magnoliopsida and the Lauraceae family, which has more than 50 genera and about 3000 species [16,17]. It is registered under No. 8845 in the national herbarium of Côte d'Ivoire. Ethnobotanical surveys have revealed that different parts of this plant (leaves, bark, fruit, and stone) are used alone or in combination with other plants against other diseases such as headache, rheumatism, dental pain, and skin diseases [18,19].
Moreover, studies have shown antibacterial, anti-inflammatory, cardiovascular, antifungal, and antidiabetic activity of P. americana. With regard to diabetes, the work of Anitia et al. [20] and Lima et al. [21] revealed that aqueous and hydroethanolic extracts of Persea americana leaves have antihypoglycaemic activity on a streptozotocin-and alloxaninduced type 1 diabetic Wistar rat model. ese studies demonstrated that this activity is due to the activation of the protein kinase B (PKB/Akt) pathway intracellularly [2,20,22].
However, there are little data on the antidiabetic activity of P. americana on type 2 diabetes although 90% of diabetes cases are type 2 according to the International Diabetes Federation [3]. In addition, the pathophysiology and risk factors of type 2 diabetes are different from those of type 1 diabetes. Obesity appears as one of the major risk factors. It would cause inflammatory reactions under the effect of hyperlipidemia leading to a decrease in the sensitivity of peripheral tissues to insulin, a phenomenon responsible for hyperglycemia [23,24]. e first part of our study showed that the leaves of this plant seem quite tolerated [12]. e aim of this study work was to study the antidiabetic activity of aqueous, ethanolic, and methanolic extracts of P. americana leaves on a type 2 diabetes rat. Specifically, it was about evaluating and comparing the hypoglycaemic and hypolipidemic activity of these three extracts on the one hand. On the other hand, it was about to determine the therapeutic effects of these extracts on the islets of Langerhans and to evaluate their biotolerance at the hepatic, renal, and muscular level after a 28-day treatment. Finally, this work carries out a preliminary study of the mechanism of action of these extracts by measuring their capacity to prevent the intestinal absorption of glucose. . Hyperglycemia was confirmed by polyuria, polydipsia, polyphagia, and elevation of blood glucose levels 72 h after the STZ-nicotinamide administration. After one week, rats that showed a fasting blood glucose level range of 200-300 mg/dl were considered T2DM and included in the study.

Selection of Doses.
e selected dose (100 mg/kg/day, bw) of extracts (AE, EE, and ME) of P. americana was based on the preliminary study of their acute oral toxicity and hypoglycaemic activity of AE and ME [12]. Extracts were prepared in 2% aqueous Tween 80, considered as vehicle. Treatment was administered orally on a daily basis in a single dose for 28 consecutive days.

Histopathological Studies of Pancreas, Liver, Kidney, and
Muscle. After euthanasia of rat and fasting blood collection, pancreas tissue, liver, muscle (tibialis anterior), and both kidneys were carefully removed, weighted, and then fixed in 10% buffered formalin. Histological preparations of these organs were performed in the pathological anatomy laboratory of the teaching hospital, Cocody (Côte d'Ivoire). Paraffin sections of 2-4 μm were cut with microtome and stained with hematoxylin-eosin (HE), gomori trichrome, and Perls for microscopic examination (Motic ® 1820, Hong Kong, China).

Intestinal Glucose Absorption
Test. e intestinal glucose absorption test was performed according to Lima et al.'s protocol [21]. Normoglycaemic rats were sampled into 5 groups (n � 5). e fasting blood glucose level of each rat was determined at t � 0 h after overnight fasting (for 16 h) with free access to water. Groups 1 to 5 were treated orally with vehicle (10 ml/kg), metformin (15 mg/kg), AE (100 mg/kg), EE (100 mg/kg), and ME (100 mg/kg), respectively. After 30 min, D (+) glucose monohydrate (500 mg/kg, bw) was orally administered. Sixty minutes (60 min) later, all rats were euthanized in the similar condition described earlier on. en, small intestines were carefully removed, and their contents were collected by perfusion of 50 ml distilled water. e content was centrifuged at 1500 g for 5 min, and the supernatant was used to determine glucose level based on the glucose dehydrogenase method using spectrophotometer.

Statistical Analysis.
e results were presented as mean ± SEM. en, statistical analysis of all the data obtained was evaluated using one-way ANOVA followed by the Newman-Keuls test (GraphPad Prism, version 5.01). e differences were considered as significant at p ≤ 0.05. Biochemical parameters variation (4) percentages were calculated as follows: where DCG: diabetic control glycaemia and TGG: test group glycaemia.
Evidence-Based Complementary and Alternative Medicine 3

Treatment Incidence on Glycaemia.
e glycaemia regulation capacity of P. americana extracts on type 2 diabetes induced in Wistar rats treated during a 28-day period is shown in Figure 1. Before treatment, all diabetic rats had a hyperglycemia estimated at a mean value of 223 mg/dl. One week after starting the treatment, the average serum glucose of daily treated groups with P. americana AE, EE, and ME at a dose of 100 mg/kg was reduced at respective rates of 13.5, 14.3, and 31.8% in comparison with initial values, respectively. On the contrary, the untreated control group kept a constant and high glycaemia value of approximatively 210.8 mg/dl. At the end of the 28-day treatment period, the extracts brought about significant reduction of the glycaemia in diabetic rats with respective rates of 16.3%, 20.8%, and 37.4% for AE, EE, and ME. ese results proved all three extracts to have regulated type 2 diabetes in rats as compared to the untreated control group whose glycaemia remained elevated. However, extract ME seemed to be the more active than glibenclamide.
is extract reduced average glycaemia of diabetic rats at 145 mg/dl, while glibenclamide at 133.8 mg/dl.

Incidence of Treatment on T2DM Rats' Nutritional
State.
e influence of the administration of P. americana extracts on body weight, water uptake, and food uptake is present in Figure 2. After the 28-day treatment period, all diabetic rats treated with AE, EE, and ME at a daily dose of 100 mg/kg showed an increase in body weight, whereas the weights of DC and GLIB groups remained the same (see Figure 2(a)). e respective increase rate in body weight of AE, EE, and ME reached 7.61, 10.52, and 15.22%. us, the weight gain has been significant for rats fed with ME since the first week of treatment. Although the rise of body weight of treated rats was significantly superior to that of the DC group (not treated), it was significantly inferior to the average of normoglycaemic rats (NDC group). e increase in average was 16.64% (39.8 g) consecutive to the 28 days treatment period.
As for the daily feeding during experiment, data collected (Figure 2(b)) showed that T2DM rats, which received each extract dose of 100 mg/kg/day, took lesser food than the untreated control group (DC).
is polyphagia was slighter for groups EE and ME than that in groups DC and GLIB. Figure 2(c) shows variation of daily consumption of water per rat. From the first week up to the 14 th treatment day, rat groups AE, EE, and ME had significantly reduced polydipsia at 12.04, 25.64, and 16.74%, respectively, which was maintained between 31.9 and 44 ml/rat/day, until the 28 th day. In contrast, water uptake by DC and GLIB groups increased progressively to reach respective values of 53 and 63.4 ml/rat/day. As for the NDC group, its average water consumption was 20.66 ml/rat/day.

Organ to Body Weight Ratio.
After the 28-day treatment, kidney to body weight ratios of T2DM rats from DC and GLIB groups were significantly (p < 0.01) higher than that of normoglycaemic animals (NDC).
Additionally, the liver to body weight ratio significantly increased for the group treated with glibenclamide. For groups treated with the different extracts, the ratios were within the range of normal values (Table 1).

Lipidic Profile.
e lipidic profile of the animals set on day 28 of the experiment is presented in Figure 3. Glibenclamide (10 mg/kg/day) reduced serum lipidic charges.
is decreased rate in T-CHOL, T-LIP, VLDL-C, and TG was significant with respective values of 38.7, 26.2, 43.9, and 43.9% in relation to the DC group. In addition, AE, EE, and ME at a dose of 100 mg/kg/day were able to restore proportion of T-CHOL and HDL-C at a level comparable to that of the NDC group (0.70 g/L and 0.28 g/L). Yet, rates of TG, VLDL-C, and T-LIP were slightly below NDC group's normal values. e LDL-C and the AIP were lower in groups treated with the extracts than in untreated diabetic groups (DC). ese decreases reached respective rates of 67.5% and 45.0% for groups EE and ME in the case of LDL-C. Table 2 presents the hepatic, renal function markers, and ionogram of T2DM rats after the 28 days of treatment.

Histopathological Analyses.
Histopathological preparations of pancreatic tissue of rats are presented in Figure 4. Observation of histopathological sections of NDC rats group showed several islets of Langerhans with normal architecture (Figure 4(a)). However, a severe atrophy of pancreatic islets and a reduction in the number of cells due to type 2 diabetes appeared in control diabetic rats (Figure 4(b)). In rats of the GLIB group, this atrophy was slight (Figure 4(c)). But, islets cells in diabetic rats treated with AE, EE, and ME during the 28 days were recovered partially (Figures 4(d)-4(f )). e kidney tissue showed a normal appearance with glomeruli located in the renal cortex zone surrounded by a clear space (Bowman space) and apparent proximal and distal convoluted tubules ( Figure 5). e liver tissues showed hepatic lobules with regular and normal hepatic cells. ere was no lymphocytal infiltration. e paraffin sections stained with Masson trichrome and Perls did not reveal neither fibrosis nor hemosiderinic deposits ( Figure 6). Transversal histological sections of skeletal muscle tibialis anterior observed by lower (×250) and higher (×400) increments showed that muscular fibers with cells were made up of nuclei without cytological atypia (Figure 7).

Intestinal Absorption of Glucose in Normoglycaemic Rats.
e results of the intestinal absorption of glucose in normoglycaemic rats are shown in Figure 8.
ese results showed an intestinal glucose depletion in normoglycaemic rats with prior AE, EE, and ME administration (30 min) at a dose of 100 mg/kg. e respective rates obtained were 30.45, 36.83, and 60.90% compared with positive control (metformin). Precisely, only the ME inhibited significantly (p < 0.01) glucose absorption in contrast with the negative control. However, this hypoglycaemic activity of ME is weaker than that of the reference compounds (metformin).

Discussion
e study of the antidiabetic activity of the aqueous, ethanolic, and methanolic extracts of P. americana leaves administered at a dose of 100 mg/kg/day for 28 days confirmed the ability of these extracts to help the organs of diabetic animals to regulate glucose metabolism in type 2 diabetes. e fasting hyperglycemia initially between 200 and 300 mg/ dL has fallen to 195.1 mg/dL for the AE group, 163.1 mg/dL for the EE group, and 145 mg/dL for the ME group after 28 days of treatment, whereas the diabetic control (DC) group maintained high fasting hyperglycemia at 245.3 mg/ dL. is reduction in blood glucose was significant as early as the second week of treatment in the case of ethanolic and methanolic extracts. is early ability of P. americana extracts to regulate fasting glucose levels in diabetic animals was highlighted by the works of Brai and Lima [21,32]. In effect, this efficacy is reflected clinically by the significant improvement of hyperglycemia-related symptoms of polyuria, polyphagia, and polydipsia in treated rats compared to NDC rats.
us, this glycaemic reduction explains the limitation of the weight loss of treated diabetic rats in relation to the NDC rats and unlike glibenclamide, which would lead to a severe weight loss [21]. is improvement in clinical symptoms that characterized type 2 diabetes had already been reported in the work of Oliveira and Lima et al. [21,33].
is ability of organic extracts, in particular the methanolic extract, to regulate blood glucose levels has already been demonstrated with several African pharmacopoeia plants, such as Albizia harveyi, Ximenia americana, Eremophila maculata, Cola nitida, and Punica granatum [10,11,[13][14][15]. is potential could be explained by the composition of these extracts in bioactive phytochemical molecules, especially in polyphenols. Indeed, a phytochemical study carried out previously had highlighted the presence of polyphenols, flavonoids, saponins, alkaloids, tannins, sterols, terpenes, and coumarins in these extracts. e concentration of these polyphenol extracts was 2707.3 ± 155.4, 2952.7 ± 166.0, and 1873.1 ± 63.5 (GAE) μg/g of extract, respectively, for AE, EE, and ME [12]. ese secondary metabolites have the ability to regulate blood glucose through several signaling pathways. Saponins, polyphenols, and, especially, flavonoids have a hypoglycemic activity by inhibition of intestinal absorption of glucose and glycogenolysis, restoring beta-cells' integrity and enhancing insulin release. Inhibition of the activity of several enzymes such as α-amylase, α-glucosidase, and glucose-6-phosphatase (G6Pase) would lead to reduction of glucose bioavailability [34][35][36][37]. us, these molecules would increase the peripheral use of glucose by stimulating translocation of GLUT4 in skeletal muscle [38]. e extracts have an effect comparable to that of metformin at the intestinal level. e extracts of P. americana were able to inhibit the intestinal absorption of glucose, in particular the methanolic extract whose inhibitory activity represented 60.9% of that of metformin. e half-maximal inhibitory concentration (IC 50 ) of the methanolic extract of P. americana leaves was 0.219 ± 0.012 mg/mL for α-amylase and 0.067 ± 0.001 mg/mL for α-glycosidase [39]. e work of Kim et al. [37] has confirmed this hypothesis by showing its ability to inhibit the activity of the intestinal glucose transporter SGLT1 too. e present study shows that extracts of P. americana would act via insulin. In fact, the islets of Langerhans, which had been destroyed by streptozotocin and nicotinamide, were regenerated under the extracts (see Figure 4). is regeneration or islet protection was observed with the 28-day treatment with a hydroethanolic extract of P. americana leaves at 300 mg/kg/day in type 1 diabetic rats [21]. is confirms its usefulness not only in type 1 diabetes but also in type 2 diabetes. e study of the lipid profile of rats at the end of the treatment period showed that extracts of P. americana were generally lipid lowering. Indeed, the effects of glibenclamide on T-LIP, TG, T-CHOL, and VLDL-C are comparable to those of the extracts. Moreover, the extracts lead to a fall in the LDL-C level and the AIP (LDL/HDL) and an increase in HDL-C. erefore, these extracts would be antiatherogenic and would help reduce the phenomenon of insulin resistance by regulating lipid homeostasis [40].
is hypocholesterolemic effect was also observed in Brai's work in 2007, on normoglycemic rats on a hypercholesterolemic diet [32]. Postprandial hyperglycemia and elevated LDL/HDL ratio in type 2 diabetes are risk factors for the occurrence of cardiovascular complications (microangiopathies and macroangiopathies) such as arteriosclerosis and retinopathy. ese results confirm that the extracts have comparable effects to metformin by inhibiting postprandial hyperglycemia, on the one hand, and antihyperglycemic and hypolipidemic effects similar to glibenclamide. On the other hand, these extracts would reduce the risk of cardiovascular complications by significantly reducing the atherogenic index of plasma. Another advantage is that the first part of this study had shown that these extracts would be nonhypoglycemic in contrast to glibenclamide. In addition, it seems well tolerated (LD50 ≥ 5000 mg/kg) [12]. e ratio of kidney or liver mass of rats to body mass of treated diabetic animals was comparable to that of healthy rats (NDC). It is different from that of the DC group and GLIB group, which had a significantly high ratio (see Table 1). e results suggest that the extracts of this plant would have protected the kidneys and liver of diabetic rats from complications related to type 2 diabetes. is nephroprotective and hepatoprotective activity of P. americana extracts is attributable to alkaloids, flavonoids, tannins and saponins that would have an antioxidant, vaxorelaxant, bradycardic, and hypotensive property. In addition, these extracts would improve glomerular filtration [22,[41][42][43][44]. is ability of P. americana extracts to protect the kidneys and liver is confirmed by the enhancement of tissue, liver, and kidney markers except AE that seems toxic to the liver and kidneys. is biotolerance of P. americana has been reported by the work of Kamagaté et al. [12,45,46], which showed that the ethanolic and methanolic extracts of this plant had a cytoprotective effect against the toxicity induced by the high dose of paracetamol (2000 mg/kg, p.o.) and streptozotocin (50 mg/kg, i.p.). Histological analyzes of the liver, kidneys, and muscle (tibialis anterior) would confirm this biotolerance by revealing no abnormalities visible under the light microscope.

Conclusion
e present study showed that P. americana is able to help organs restore glucose and lipid homeostasis, particularly in the case of type 2 diabetes while being well tolerated. is justifies the use of this plant in traditional medicine to treat diabetes mellitus and avoid the complications generated by this disease.
However, it would be important to conduct further studies to elucidate its mechanism of action and evaluate its long-term toxicity in order to popularize its use.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper. group treated with ethanol extract, 100 mg/kg; ME: group treated with methanol extract, 100 mg/kg; metformin: group treated with metformin, 15 mg/kg. e results are expressed as mean ± SEM. (n � 5). Statistically significant vs control: * * p < 0.01; * * * p < 0.001.