Polyphenols-rich Vernonia amygdalina shows anti-diabetic effects in streptozotocin-induced diabetic rats

This study aims to investigate the hypoglycemic properties of Vernonia amygdalina Del. (VA) and its possible mechanisms of action in a single-dose STZ induced diabetic rat model. A dose–response study was conducted to determine optimum dose for the hypoglycemic effect of VA in STZ-induced diabetic rats. The optimum dose (400 mg/kg) was used throughout the 28-day chronic study. Body weight, food and water intakes of the rats were monitored daily. Fasting blood serum, pancreas, liver and soleus muscle were collected for biochemical analyses. Chemical composition of VA was analysed using HPLC and LC–ESI-MS. The study reveals that ethanolic extract of VA contains high level of polyphenols mainly 1,5-dicaffeoyl-quinic acid, dicaffeoyl-quinic acid, chlorogenic acid and luteolin-7-O-glucoside. In an oral glucose tolerance test, 400 mg/kg VA exhibited a significant improvement in glucose tolerance of the STZ-induced diabetic rats. 28-day treatment with 400 mg/kg VA resulted in 32.1% decrease in fasting blood glucose compared to diabetic control. VA also caused significant decrease (18.2% and 41%) in triglyceride and total cholesterol level. Besides, VA showed protective effect over pancreatic β-cells against STZ-induced damage, causing a slight increase in insulin level compared to diabetic control. VA administration also showed positive regulation of the antioxidant system, both enzymatic and non-enzymatic. Furthermore, VA was found to increase expression of GLUT 4 (24%) in rat skeletal muscle. Further tissue fractionation revealed that it can increase the GLUT 4 translocation (35.7%) to plasma membrane as well, suggesting that VA may stimulate skeletal muscle's glucose uptake. This observation is in line with the restoration in skeletal muscle glycogenesis of VA-treated group. However, no alteration was observed in GLUT 1 expression. In addition, VA also suppressed (40% inhibition) one of the key hepatic gluconeogenic enzymes, glucose-6-phosphatase (G6Pase). VA possesses antihyperglycemic effect, most probably through increasing GLUT 4 translocation and inhibiting hepatic G6Pase. The polyphenols in the extract may be the candidates that are responsible for the above-mentioned biological activities.