Effects of Syzygium aromaticum-derived oleanolic acid on glucose transport and glycogen synthesis in the rat small intestine.

BACKGROUND

In the present study, we investigated the effects of oleanolic acid (OA), which has hypoglycemic properties, on glucose transport and glycogen synthesis in the small intestine, an organ that secretes enzymes involved in carbohydrate metabolism.

METHODS

The OA was isolated from Syzygium aromaticum ethyl acetate-soluble fractions followed by recrystallization with ethanol. It was diluted to required concentrations in freshly prepared dimethyl sulfoxide (2 mL) and normal saline (19 mL) before being administered to rats (p.o.). Glycogen concentrations were determined in isolated small intestines from fasted and non-fasted non-diabetic and streptozotocin-diabetic rats after 18 h treatment with 80 mg/kg, p.o., OA or standard hypoglycemic drugs (i.e. 100 μg/kg, s.c., insulin; 500 mg/kg, p.o., metformin). In a separate series of experiments, the effects of 30-min incubation with graded concentrations of OA (0.82-6.56 mmol/L) on d-glucose were evaluated by monitoring changes in glucose concentrations inside and outside of intestinal sacs isolated from fasted, non-diabetic rats and mounted in an organ bath containing Krebs-Henseleit bicarbonate buffer.

RESULTS

All in vivo treatments increased the glycogen concentration in rat small intestine, although the effects of metformin treatment in non-fasted diabetic rats failed to reach statistical significance. In vitro, both OA (1.64-6.56 mmol/L) and phlorizin (10(-5) -10(-3) mol/L) decreased glucose transport from the mucosa to the serosa.

CONCLUSIONS

The data suggest that OA may be a potential alternative drug treatment for postprandial hyperglycemia because of its inhibition of glucose uptake across the small intestine and its concomitant conversion of glucose to glycogen in the intestinal wall.