Evaluation of Ethnomedical Claim III: Anti-hyperglycemic activities of Gongronema latifolium root and stem

Background: The insulinotropic activity of the combined root and stem of Gongronema latifolium (Asclepiadaceae) was evaluated to justify its African ethnomedicinal use in the management of diabetes. Methods: A methanolic extract and its chromatographic fractions (A(1)-A(6)) were tested for glucose-reducing and in vitro insulin-stimulating abilities using glucose-loaded rats and INS-1 cells, respectively. In vivo insulin-releasing activities for the significantly (P < 0.05) active antihyperglycemic A(5) and A(6) and in vitro insulinotropic activity of the C-1 fraction and its isolated constituents were also similarly determined. Results: The extract (100 mg/kg) had higher in vivo antihyperglycemic activity than the individual A(1)-A(6), indicating a synergistic effect of the plant constituents. Higher in vivo insulin release in response to A(5) (100 mg/kg) than A(6), agreed with their in vivo antihyperglycemic activities and confirmed insulin release as a mechanism of action of the plant. Compared with 5.6 mmol/L glucose (negative control), the extract and the A(3), A(6), and C-1 (all at 100.0 mg/mL) elicited significantly higher in vitro insulin release that was similar to that of glibenclamide (1.0 mu g/mL; P > 0.05). Fraction C-1 yielded a 1: 1 mixture of alpha-amyrin and beta-amyrin cinnamates (1a: 1b), lupenyl cinnamate (2), lupenyl acetate (3), and two unidentified triterpenoids, Y and Z. The 1a: 1b mixture (100.0 mg/mL) demonstrated the highest insulinotropic activity that was comparable (P > 0.05) to that of glibenclamide (1.0 mu g/mL). Conclusions: The results confirm pancreatic activity as a mechanism underlying the antidiabetic action of G. latifolium and justify its ethnomedical use.