Antioxidant Activity of a Cissus verticillata Fraction and Tyramine, its Bioactive Constituent, on Alloxan-Induced Diabetic Rats

In the present study, the antioxidant activity of a methanol soluble fraction (MSF) from Cissus verticillata, used in Brazil and elsewhere as a hypoglycemic and antidiabetic medicinal plant, and tyramine (TYR), one of its main bioac- tive constituents, was assessed. For this, male Wistar rats were submitted to alloxan injection (40 mg/kg, i.v.) in order to induce a diabetic state and, 48 h later, glycemia was determined. Animals were distributed into groups: normal controls (NC); diabetic controls (DC); DC plus MSF; and DC plus TYR. Another group was treated with glibenclamide (GLI), used as a positive control. After 5-day treatments, animals were sacrificed for liver dissection, and determination of anti- oxidant markers such as thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH), catalase and nitrite concentrations. The antioxidant effect was also evaluated on the pentobarbital-induced sleeping time, before and after CCl4 treatment. Under our experimental conditions, diabetic rats did not present any alteration in liver lipid peroxidation, before (DC) or after treatments with the MSF; TYR or GLI, as compared to normal controls (NC). Levels of GSH were significantly increased in 79% in DC, as related to NC, and the effects were partially reversed in diabetic rats, after MSF treatments at the higher dose. However, while similar effects were observed after TYR and GLI, both drugs brought val- ues of GSH to normality. The DC group had increased liver catalase activity, as compared to NC, and these effects were partially reversed by MSF and almost completely by TYR and GLI. Significant increases were also detected in nitrite concentrations in livers of DC, as an index of free radical formation, and a large reduction was observed after MSF, TYR and GLI treatments of diabetic rats, as compared to NC. MSF and TYR also prevented prolongation of the pentobarbital- induced sleeping time by CCl4, suggesting hepatoprotective and anti-oxidative effects. In conclusion, we showed that the antioxidant activity probably plays an important role in the antidiabetic effect of C. verticillata, and TYR is at least in part responsible for this property.