P 28: Some contributions to the evaluation of ethnomedical antidiabetic claims of African medicinal plants

Abstract

Plants with ethnomedicinal claims in Africa have continued to play a key role in the management of diabetes. Some of these plants were evaluated to justify these claims, using normal, glucose- and alloxan-induced hyperglycaemic rats, and in vitro insulin secreting cell line (INS-1) models. Glibenclamide was the standard drug. Activity of Clausena lansium stem bark was attributed to the insulinotropic chalepin and imperatorin while that of Stachytarpheta cayennensis leaf was suggested to be mainly due to isoverbascoside. This was confirmed by the relative proportions of these constituents in the active fractions, as indicated by HPLC. The insulinotropic activities of Jatropha tanjorensis and Eugenia uniflora leaves were limited to their ethylacetate and n-hexane fractions, respectively. Quercetin and its glycoside (rutin), with in vitro insulinotropic and insulin inhibitory activities, respectively, were the respective major and minor constituents of the anti-hyperglycaemic Bauhinia monandra leaf fraction. In vivo, the two compounds were insulin stimulating, as rutin has been reported to be metabolised in the gut to quercetin. A 1:1 mixture of α-amyrin and β-amyrin cinnamates (1a/1b) was identified as the main insulinotropic constituent of Gongronema latifolium stem and root while contributions of lupenyl cinnamate (2), lupenyl acetate (3) and an unidentified triterpenoid Z confirmed the synergistic nature of the constituents. Also, insulin stimulation was reported as a hitherto unreported mechanism of action of this plant. The identification of these plants’ insulinotropic active constituents, comparable (p > 0.05) to glibenclamide, conclusively justified their antidiabetic claims. Additional six medicinal plants with better antihyperglycaemic activities than glibenclamide gave the hope of discovery of new templates for drug development. Antidiabetic activity of Murraya koenigii leaf was slow acting and suggested best for type 2 diabetes of insulin resistant aetiology, due to the insulin inhibitory activity of its extract and constituents. Murrayaquinone-A was the most active constituent with significant contributions from girinimbine and koenimbine. This shows that plants may help better in the management of type 2 diabetes due to insulin resistant or its insufficiency. The co-occurrence of antihyperglycaemic and hyperglycaemic, insulinotropic and insulin inhibiting fractions and constituents in these plants may confirm the safety margins of herbal drugs and the need to identify the active constituents. Additional hepatoprotective, anti-microbial, -parasitic, -infective, -hypertensive, -inflammatory and -malarial, etc properties of these herbs may account for the reduced death of the African diabetic patients using herbal drugs singly or co-administered with orthodox drugs. Stem and root juices of Musa paradisiaca “prevented/delayed” onset of diabetes in glucose loaded rats. Implications of these results in the management of diabetes and drug development will be presented.