Anti-hyperglycaemic and anti-oxidant activities of five Nigerian antidiabetic plants

Leaves of Olax subscorpioidea, Hoslundia opposita, Cleistopholis patens, Plumbago zeylanica and Dioscoreophyllum cumminsii that are used as anti-diabetics were evaluated for hyperglycaemic-lowering and antioxidant activities to justify their Nigerian ethnomedicinal usage. Leaf methanolic extracts (100, 200 and 400 mg/kg, p.o.) were assayed in normal, glucose- and alloxan-induced diabetic rats, while 1,1,-diphenyl-2-picrylhydrazyl, total antioxidant capacity, ferric reducing antioxidant power and hydroxyl radical scavenging tests were used for their antioxidant activity. Effects of n-hexane, dichloromethane, ethylacetate and aqueous partition fractions of the three most active anti-hyperglycaemic extracts were also tested in glucose-loaded rats. In normoglycaemic rats, all extracts generally lacked activity, while with glucose-loaded rats, only O. subscorpioidea (200mg/kg) and C. patens (400 mg/kg) at 1 h and O. subscorpioidea (200, 400 mg/kg) and C. patens (400 mg/kg) at 2-4 hours gave lowered (p < 0.05) blood glucose levels than glibenclamide (5 mg/kg), while activity of H. opposita and P. zeylanica (200 mg/kg) were only comparable to glibenclamide. When daily fed for fourteen days to alloxan-induced diabetic rats, all extracts, at their most active doses, gave significantly higher activity than glibenclamide. Olax subscorpioidea leaf extract had the highest hyperglycaemic-lowering and least antioxidant activities. Highest antioxidant activity of H. opposita may suggest some contribution of antioxidant property to its hyperglycaemic-lowering activity. The glucose-lowering and insulinotropic constituents of O. subscorpioidea, H. opposita and C. patens should be concentrated in their aqueous, aqueous and dichloromethane fractions, respectively. Anti-hyperglycaemic ethnomedicinal use of these plants was justified and H. opposita has additional antioxidant property. Keywords: Antioxidant activity; Diabetes mellitus; Nigerian anti-diabetic plants