Anti-diabetic Activity of Convallatoxin Isolated from the Root Bark of Parquetina nigrescens (Afzel.) Bullock (Asclepiadaceae)

This study investigated the anti-diabetic activity of the root bark extract of Parquetina nigrescens and the isolated compound, convallatoxin, from the root bark. A powdered sample of the plant was extracted with methanol, and the extract (A) was tested in glucose-loaded normal rats at 100, 200 and 400 mg/kg for the determination of the most active dose. The anti-diabetic activity of A at 200 mg/kg was carried out on streptozotocin-induced diabetic rats. A was further partitioned to obtain its n-hexane (B1), dichloromethane (B2), ethyl acetate (B3) and mother liquor (B4) fractions that were tested for blood glucose lowering activity using glucose-loaded normal rats model. The anti-diabetic activity of the isolated compound from B3 was carried out on streptozotocin-induced diabetic rats. The results were subjected to one way ANOVA followed by Bonferroni post hoc tests and p< 0.05 was considered significant. A showed dose-independent and time dependent blood glucose level reduction activity at 200 mg/kg with the highest percentage of 30% at 4 h that was comparable to the standard, glibenclamide at 5 mg/kg. A at 200 mg/kg showed a blood glucose level reduction of 49, 68 and 70% by Day 4, 7 and 10 respectively as against glibenclamide at 5 mg/kg of 18, 33 and 39% by Day 4, 7 and 10 respectively on streptozotocin-induced diabetic rats. B3 showed a time-dependent blood glucose level reduction activity up to the fourth hour similar to glibenclamide with 35% as against 38% of glibenclamide. B1, B2 and B4 were devoid of blood glucose level reduction activity. Convallatoxin isolated from B3 gave 83 % blood glucose levels reduction at day 10 as against 38% of glibenclamide. The anti-diabetic activity of convallatoxin was significantly (p<0.05) more than glibenclamide at all-time point. The structure of convallatoxin was determined using IR, 1D and 2D NMR spectroscopy and the spectroscopic data compared well with published data in the literature.