Insulin secretory actions of ethanolic extract of Acacia arabica bark in high fat-fed diet-induced obese Type 2 diabetic rats

Abstract

Acacia arabica commonly known as 'babul' has been widely used for the treatment of numerous diseases, including diabetes due to their potential pharmacological actions. The aim of the present study was to investigate the insulinotropic and antidiabetic properties of ethanol extract of Acacia arabica (EEAA) bark through in vitro and in vivo studies in high fat-fed (HFF) rats. EEAA at 40-5000 µg/ml significantly increased (P<0.05-0.001) insulin secretion with 5.6 and 16.7 mM glucose, respectively, from clonal pancreatic BRIN BD11 β-cells. Similarly, EEAA at 10-40 µg/ml demonstrated a substantial (P<0.05-0.001) insulin secretory effect with 16.7 mM glucose from isolated mouse islets, with a magnitude comparable to 1 µM glucagon-like peptide-1 (GLP-1). Diazoxide, verapamil, and calcium-free conditions decreased insulin secretion by 25-26%. The insulin secretory effect was further potentiated (P<0.05-0.01) with 200 µM isobutylmethylxanthine (IBMX; 1.5-fold), 200 µM tolbutamide (1.4-fold), and 30 mM KCl (1.4-fold). EEAA at 40 µg/ml, induced membrane depolarization and elevated intracellular Ca2+ as well as increased (P<0.05-0.001) glucose uptake in 3T3L1 cells and inhibited starch digestion, glucose diffusion, dipeptidyl peptidase-IV (DPP-IV) enzyme activity, and protein glycation by 15-38%, 11-29%, 15-64%, and 21-38% (P<0.05, 0.001), respectively. In HFF rats, EEAA (250 mg/5 ml/kg) improved glucose tolerance, plasma insulin, and GLP-1 levels, and lowered DPP-IV enzyme activity. Phytochemical screening of EEAA revealed the presence of flavonoids, tannins and anthraquinone. These naturally occurring phytoconstituents may contribute to the potential antidiabetic actions of EEAA. Thus, our finding suggests that EEAA, as a good source of antidiabetic constituents, would be beneficial for Type 2 diabetes patients.

Keywords: GLP-1; Insulin; glucose; obesity; phytoconstituents; type 2 diabetes.

Figure 1. Effects of ethanol extract of Acacia arabica (EEAA) bark on insulin secretion from (A,B) clonal pancreatic BRIN BD11 β cells, as well as (C) pancreatic islets of Langerhans, (D) protein glycation, (E) insulin secretion with known stimulators or inhibitors and (F) with or without extracellular calcium

Values are mean ± SEM; n = 4–8 for insulin secretion and glycation of protein. ^{*, **, ***}P<0.05–0.001 compared with control. ^ϕP<0.05 and ^ϕϕϕP<0.001 compared with 5.6 mM glucose with EEAA. ^{Δ, ΔΔ, ΔΔΔ}P<0.05–0.001 compared with respective incubation without EEAA. EEAA, ethanol extract of Acacia arabica (bark).

Figure 2. Effects of ethanol extract of Acacia arabica (EEAA) bark on (A) membrane potential and (B) intracellular calcium in clonal pancreatic BRIN-BD11 β-cells and, (C–G) glucose uptake by differentiated 3T3L1 adipocytes, (H) starch digestion and (I) glucose diffusion in vitro

The intensity of fluorescence was measured for cells incubated with EEAA (E) minus or (F) plus 100 nM insulin. The images were captured at 10x magnification. (G) Glucose uptake in 3T3L1 cells and percentage of glucose liberation from (H) starch digestion and (I) glucose diffusion in vitro were represented in the scatter dot plot. The values are mean ± SEM; n = 6 for membrane potential and intracellular calcium, n = 4 for glucose uptake, starch digestion and glucose diffusion. ^{*, **, ***}P<0.05–0.001 compared with control.

Figure 3. Effects of ethanol extract of Acacia arabica (EEAA) bark on (A) in vitro dipeptidyl peptidase-4 (DPP-IV) enzyme, (B) glucose tolerance, (C) plasma insulin, (D) plasma DPP-IV and (E) active GLP-1 (7-36) in HFF rats

Parameters were measured before and after oral administration of glucose alone (18 mmol/kg body weight, control) or with EEAA (250 mg/5 ml/kg body weight), sitagliptin and vidagliptin (both at 10 μmol/5 ml/kg, body weight) in vivo. Plasma active GLP-1 (7-36) levels were evaluated at 60 min following treatment. Values are mean ± SEM; n=4 for in vitro DPP-IV enzyme activity and n=6, for in vivo parameters. ^{*, **, ***}P<0.05–0.001 compared with control and ^{Δ, ΔΔ, ΔΔΔ}P<0.05–0.001 compared with HFF diet control rats.