Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C139
Synergistic hypoglycaemic effects between Syzygium aromaticum derived oleanolic acid and insulin in streptozotocin-induced diabetic rats
C. T. Musabayane1, R. F. Mapanga1
1. Discipline of Human Physiology, University of KwaZulu-Natal, Durban, South Africa.
Syzygium cordatum leaf ethyl acetate extract solubles (EAS) contain triterpene mixtures (oleanolic acid (OA) and ursolic acid (UA) with hypoglycaemic properties (1). We speculated that Syzygium aromaticum extracts contain similar compounds. Accordingly, this study investigated the anti-hyperglycaemic effects of OA isolated from S. aromaticum EAS in streptozotocin (STZ)-induced diabetic rats (diabetes mellitus was induced in male Sprague-Dawley rats with a single intraperitoneal injection of STZ, 60 mg/kg). We also investigated possible synergistic anti-hyperglycaemic effects between OA and insulin since evidence suggests that OA is an insulin secretagogue. OA was studied because the triterpene exists in food products and is the major constituent of many African plant species used in traditional medicine. Extraction and fractionation of S. aromaticum flower bud EAS yielded mixtures of OA/UA and methyl maslinate/methyl corosolate which on recrystallisation using ethanol afforded OA whose structure was confirmed by NMR spectroscopy (1H & 13C). Oral glucose tolerance (OGT) responses to various doses of OA and/or standard anti-diabetic drugs were monitored in separate groups of non-diabetic and STZ-induced diabetic rats given a glucose load after an 18-h fast. Rats treated with deionized water (3 ml kg-1, po) or standard anti-diabetic drugs acted as untreated and treated positive controls, respectively. Blood glucose concentrations were measured at 15-min intervals for the first hour, and hourly thereafter for 3 h. Short-term effects were assessed in animals treated twice daily with OA and/or standard anti-diabetic drugs at the end of 5-weeks. Statistical comparison of the differences between the control means and experimental groups was performed using one-way analysis of variance (ANOVA) and Tukey-Kramer test; p<0.05 was considered significant. Spectroscopic analysis of the white powder obtained after recrystallization of EAS with ethanol and the structure elucidated by 1H- and 13C-NMR (1D and 2D) spectroscopy compared with previously reported data of oleanolic acid (2). The mean fasting blood glucose concentration of control STZ-induced diabetic rats was significantly higher as compared to control non-diabetic animals (21.25±0.25 vs 3.26±0.10 mmol l-1, n=6 in both groups). OA improved OGT responses of both non-diabetic and diabetic rats, as did some standard drugs except glibenclamide which did not exhibit any effects in STZ-induced diabetic animals. Combined OA and insulin administration had even greater anti-hyperglycaemic response, suggestive of a synergistic effect of the two. OA however, did not have any apparent effects on plasma insulin concentrations. Short-term OA administration to rats increased hepatic glycogen concentrations implying that the hypoglycaemic effects could in part be mediated via increased hepatic glycogen synthesis. The results suggest that OA has a potential role in diabetes mellitus treatment.
Where applicable, experiments conform with Society ethical requirements