| Summary: | <i>Orthosiphon stamineus</i> is a popular folk herb used to treat diabetes and some other disorders. Previous studies have shown that <i>O. stamineus</i> extracts were able to balance blood glucose levels in diabetic rat animal models. However, the antidiabetic mechanism of <i>O. stamineus</i> is not fully known. This study was carried out to test the chemical composition, cytotoxicity, and antidiabetic activity of <i>O. stamineus</i> (aerial) methanol and water extracts. GC/MS phytochemical analysis of <i>O. stamineus</i> methanol and water extracts revealed 52 and 41 compounds, respectively. Ten active compounds are strong antidiabetic candidates. Oral treatment of diabetic mice with <i>O. stamineus</i> extracts for 3 weeks resulted significant reductions in blood glucose levels from 359 ± 7 mg/dL in diabetic non-treated mice to 164 ± 2 mg/dL and 174 ± 3 mg/dL in water- and methanol-based-extract-treated mice, respectively. The efficacy of <i>O. stamineus</i> extracts in augmenting glucose transporter-4 (GLUT4) translocation to the plasma membrane (PM) was tested in a rat muscle cell line stably expressing myc-tagged GLUT4 (L6-GLUT4myc) using enzyme-linked immunosorbent assay. The methanol extract was more efficient in enhancing GLUT4 translocation to the PM. It increased GLUT4 translocation at 250 µg/mL to 279 ± 15% and 351 ± 20% in the absence and presence of insulin, respectively. The same concentration of water extract enhanced GLUT4 translocation to 142 ± 2.5% and 165 ± 5% in the absence and presence of insulin, respectively. The methanol and water extracts were safe up to 250 µg/mL as measured with a Methylthiazol Tetrazolium (MTT) cytotoxic assay. The extracts exhibited antioxidant activity as measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. <i>O. stamineus</i> methanol extract reached the maximal inhibition of 77 ± 10% at 500 µg/mL, and <i>O. stamineus</i> water extract led to 59 ± 3% inhibition at the same concentration. These findings indicate that <i>O. stamineus</i> possesses antidiabetic activity in part by scavenging the oxidants and enhancing GLUT4 translocation to the PM in skeletal muscle.
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