Genetically obese Zucker rats exhibit mild hyperglycaemia and hyperinsulinaemia suggesting the existence of peripheral
insulin resistance. We have examined the effects of
YM268, an analogue of
thiazolidinedione, on the content and translocation of a
glucose transporter (GLUT4) in epididymal adipose tissue in 11-week-old obese and lean Zucker rats. The administration of
YM268 at a dose of 10 mg/kg for 2 weeks ameliorated hyperglycaemia, hyperinsulinaemia, and
impaired glucose tolerance after
glucose load in obese rats. The GLUT4 content per fat pad in obese rats was reduced to 36% of that in lean littermates. Obese rats treated with
YM268 increased GLUT4 concentrations in their fat pads from a basal value of 36% up to 191% of the level in lean rats. Furthermore, in adipocytes prepared from obese rats, an increase in the ratio of GLUT4 in plasma membrane to the total amount of GLUT4 (PM-GLUT4 ratio) induced by the submaximal concentration of
insulin (0.3 nmol/l) was significantly attenuated compared with that in lean rats. But the maximum effect of
insulin (3 nmol/l) was not attenuated. Meanwhile,
YM268 had no significant effect on the attenuated PM-GLUT4 ratio in response to
insulin in obese rats. These data suggested that one of the mechanisms by which
YM268 improved
insulin resistance in obese Zucker rats was to normalize the decreased GLUT4 content in the adipose tissue.