The β3 -Adrenergic Agonist BRL37344 Increases Glucose Transport into L6 Myocytes through a Mechanism Different from That of Insulin

In the present study, we examined the effects of BRL37344, a selective β3-adrenergic agonist, on glucose transport into L6 myocytes and the results were compared with the effects of insulin. Insulin increased 2-deoxyglucose (2-DG) uptake in a dose-dependent manner, with maximal stimulation at 10−7 M. BRL37344 ranging from 10−7 to 10−5 M also enhanced 2-DG uptake in the absence of insulin. The effects of insulin and BRL37344 were completely additive, suggesting that these two agents enhance glucose uptake by L6 myocytes through different mechanisms. In fact, BRL37344 apparently did not increase tyrosine phosphorylation of cellular proteins in L6 myocytes, whereas insulin stimulated tyrosine phosphorylation of 180–190 and 95 kDa proteins. Furthermore, BRL37344-induced increase in glucose transport was not blocked by wortmannin, an inhibitor of phos-phatidylinositol 3-kinase, whereas the insulin-induced effect was completely abolished. When L6 myocytes were incubated with insulin, the content of GLUT4 in the plasma membrane was increased. However, BRL37344 did not affect the GLUT4 content in the plasma membrane. BRL37344. did not increase the Vmax value for glucose uptake but decreased the Km, value, although insulin increased the Vmax value. These results suggest that BRL 37344 enhances glucose transport into L6 myocytes through a signaling pathway different from that of insulin and that the mechanism does not involve the translocation of GLUT4, but may be due to an increase in the intrinsic activity of GLUT present in the plasma membrane.