Antidiabetic Actions of Ethanol Extract of Camellia sinensis Leaf Ameliorates Insulin Secretion, Inhibits the DPP-IV Enzyme, Improves Glucose Tolerance, and Increases Active GLP-1 (7-36) Levels in High-Fat-Diet-Fed Rats

Abstract

Camellia sinensis (green tea) is used in traditional medicine to treat a wide range of ailments. In the present study, the insulin-releasing and glucose-lowering effects of the ethanol extract of Camellia sinensis (EECS), along with molecular mechanism/s of action, were investigated in vitro and in vivo. The insulin secretion was measured using clonal pancreatic BRIN BD11 β cells, and mouse islets. In vitro models examined the additional glucose-lowering properties of EECS, and 3T3L1 adipocytes were used to assess glucose uptake and insulin action. Non-toxic doses of EECS increased insulin secretion in a concentration-dependent manner, and this regulatory effect was similar to that of glucagon-like peptide 1 (GLP-1). The insulin release was further enhanced when combined with isobutylmethylxanthine (IBMX), tolbutamide or 30 mM KCl, but was decreased in the presence of verapamil, diazoxide and Ca²⁺ chelation. EECS also depolarized the β-cell membrane and elevated intracellular Ca²⁺, suggesting the involvement of a K_ATP-dependent pathway. Furthermore, EECS increased glucose uptake and insulin action in 3T3-L1 cells and inhibited dipeptidyl peptidase IV (DPP-IV) enzyme activity, starch digestion and protein glycation in vitro. Oral administration of EECS improved glucose tolerance and plasma insulin as well as inhibited plasma DPP-IV and increased active GLP-1 (7-36) levels in high-fat-diet-fed rats. Flavonoids and other phytochemicals present in EECS could be responsible for these effects. Further research on the mechanism of action of EECS compounds could lead to the development of cost-effective treatments for type 2 diabetes.

Keywords: Camellia sinensis; GLP-1; glucose; hyperglycaemia; insulin; phytoconstituents.

Figure 1

Effects of EECS on insulin secretion from (A,B) clonal pancreatic BRIN-BD11 β cells and (C) islets of Langerhans, (D) glycation of protein (E), secretion of insulin with known stimulators/inhibitors and (F) plus/minus extracellular calcium from BRIN-BD11 cells. Values n = 4–8 for insulin secretion and glycation of protein are mean ± SEM. *^, **^, *** p < 0.05–0.001 compared to control. ^{ϕ, ϕϕ, ϕϕϕ} p < 0.05–0.001 compared to 5.6 mM glucose with EECS. ^{Δ, ΔΔ, ΔΔΔ} p < 0.05–0.001 compared to respective incubation without EECS. EECS, ethanol extract of C. sinensis.

Figure 2

Effects of EECS on (A) membrane potential and (B) intracellular calcium in clonal pancreatic BRIN BD11 β cells and (C–G) glucose uptake, (H) starch digestion and (I) glucose diffusion in vitro. Changes in fluorescence intensity in differentiated 3T3L1 adipocyte incubated with EECS (E) minus or (F) plus 100 nM insulin. Magnification of 10x was used to capture the images. Values n = 6 for membrane potential and intracellular calcium, n = 4 for uptake of glucose, digestion of starch and diffusion of glucose are mean ± SEM. *, **, *** p < 0.05–0.001 compared to control.

Figure 3

Effects of EECS on (A) DPP-IV enzyme in vitro, (B) glucose tolerance, (C) plasma insulin, (D) DPP-IV and (E) active GLP-1 (7–36) in high-fat-diet-fed rats. In vivo parameters were evaluated before and after oral administration of glucose alone (18 mmol/kg body weight, control) or with EECS (250 mg/5 mL/kg body weight), sitagliptin and vidagliptin (both at 10 μmol/5 mL/kg, body weight). Plasma active GLP-1 (7–36) levels were measured 30 min following treatment. Values n = 4 for in vitro DPP-IV enzyme activity and n = 6 for in vivo parameters are mean ± SEM. *, **, *** p < 0.05–0.001 compared to control and ^{Δ, ΔΔ, ΔΔΔ} p < 0.05–0.001 compared to high-fat-diet-fed control rats.

Figure 4

Effects of EECS on (A) food intake during feeding test and (B) food intake, (C) fluid intake, (D) stool and (E) urine output after 24 h of metabolic study. (F) BaSO₄ traversed. Values n = 6 for feeding test and metabolic parameters are mean ± SEM. *, **, *** p < 0.05–0.001 compared to high-fat-diet-fed control rats.