Isoleucine, a potent plasma glucose-lowering amino acid, stimulates glucose uptake in C2C12 myotubes

To examine which branched-chain amino acids affect the plasma glucose levels, we investigated the effects of leucine, isoleucine, and valine (0.3 g/kg body weight p.o.) in normal rats using the oral glucose tolerance test (OGTT, 2 g/kg). A single oral administration of isoleucine significantly reduced plasma glucose levels 30 and 60 min after the glucose bolus, whereas administration of leucine and valine did not produce a significant decrease. Oral administration of valine significantly enhanced the plasma glucose level at 30 min after the glucose administration and leucine had a similar effect at 120 min. At each measurement timepoint, the insulin levels of the treated groups were lower than that of the control group. We then investigated the effects of leucine, isoleucine or valine at the same concentration (1 mM) on glucose metabolism in C2C12 myotubes in the absence of insulin. Glucose consumption was elevated by 16.8% in the presence of I mM isoleucine compared with the control. Conversely, I mM leucine or valine caused no significant changes in glucose consumption in the C2C12 myotubes. The 2-deoxyglucose uptake Of C2C12 myotubes significantly increased upon exposure to 1-10mM isoleucine and 5-10mM leucine. However, isoleucine caused no significant difference in glycogen synthesis in C2C12 myotubes, although leucine and valine caused a significant increase in intracellular glycogen compared with the control. The isoleucine effect on glucose uptake was mediated by phosphatidylinositol 3-kinase (PI3K), but was independent of mammalian target of rapamycin (mTOR). These results suggest that isoleucine stimulates the insulin-independent glucose uptake in skeletal muscle cells, which may contribute to the plasma glucose-lowering effect of isoleucine in normal rats. (C) 2003 Elsevier Inc. All rights reserved.