REGULATION OF GLUCOSE-TRANSPORT IN CULTURED MUSCLE-CELLS BY NOVEL HYPOGLYCEMIC AGENTS

The antidiabetic agent troglitazone (CS-045) and a metabolite designated M3 have potent blood glucose-lowering actions. The mechanism of the hypoglycemic effects of troglitazone and MB was investigated in cultured L6 muscle cells. Short-term (a-hour) exposure of fully differentiated myotubes to troglitazone had no effect on glucose transport activity; M3 exposure caused a modest (50% to 60%) increase in basal and insulin-stimulated transport. Long-term (72-hour) treatment of myotubes with troglitazone resulted in a doubling of glucose transport in the absence of insulin, whereas MB treatment resulted in a fivefold increase in basal glucose transport. Transport activity in MB-treated myotubes was greater than that seen after short-term insulin treatment. Insulin did not stimulate transport further in long-term MB-treated cells. A similar effect of prolonged exposure to M3 was observed in nondifferentiated myocytes. The agent had no influence on cell growth or the extent of differentiation. Augmentation of basal glucose transport by M3 was slow in onset, requiring 18 to 24 hours before significant effects were observed and 72 hours for full stimulation. M3 action on glucose transport was also dose-dependent, with half-maximal stimulation at 5 mu g/mL of the agent and full effects at 10 to 20 mu g/mL. Total membranes were prepared from control and MB-treated L6 myocytes and myotubes, and glucose transporter (GLUT1 and GLUT4) protein levels were measured by Western blotting. GLUT1 content was increased 2.9- +/- 1.3- and 2.8- +/- .2-fold by M3 treatment in myocytes and myotubes, respectively. GLUT4 content in myotubes was also increased, but to a lesser extent (1.9- +/- .3-fold). In conclusion, the novel antidiabetic agent M3 acts to increase glucose transport into muscle cells in the absence of insulin. This effect appears to occur mainly through an increase in the level of glucose transporter proteins, which could explain, at least in part, its hypoglycemic actions. Copyright (C) 1995 by W.B. Saunders Company