Antihyperglycemic action of sinapic acid in diabetic rats.

Sinapic acid is a hydroxycinnamic acid contained in plants. In an attempt to know the hyperglycemic effect of sinapic acid, this study applied streptozotocin (STZ) to induce type 1-like diabetic rats and fed fructose-rich chow to induce type 2-like diabetic rats. Sinapic acid dose-dependently reduced the hyperglycemia of STZ-diabetic rats (9.8 +/- 1.8%, 11.6 +/- 0.7%, and 19.4 +/- 3.2% at 5 mg/kg, 10 mg/kg, and 25 mg/kg, respectively). Also, sinapic acid attenuated the postprandial plasma glucose without changing plasma insulin in rats. Repeated treatment of sinapic acid increased the gene expression of GLUT4 in soleus muscle of STZ-diabetic rats. Moreover, sinapic acid enhanced glucose uptake into isolated soleus muscle and L6 cells (337.0 +/- 29.6%). Inhibition of phospholipase C (PLC) using U73122 (1.00 +/- 0.02 mu g/mg protein) or protein kinase C (PKC) using chelerythrine (0.97 +/- 0.02 mu g/mg protein) attenuated the sinapic acid-stimulated glucose uptake (1.63 +/- 0.02 mu g/mg protein) in L6 cells. Otherwise, the reduced glucose infusion rate (GIR) in fructose-rich chow-fed rats was also raised by sinapic acid. Our results suggest that sinapic acid ameliorates hyperglycemia through PLC-PKC signals to enhance the glucose utilization in diabetic rats.