Quantity of Na/K-ATPase and glucose transporters in the plasma membrane of rat adipocytes is reduced by in vivo triiodothyronine

The expression of sodium-potassium pumps and glucose transporters in pure adipocyte plasma membranes from a hyperthyroid animal model was studied. Hyperthyroidism was induced by enteral administration of five doses of 90 μg of triiodothyronine every second day to 8-week-old rats. Following isolation of epididymal adipocytes, 3-O-methylglucose transport was measured and the number of Na/K-ATPase-(α 1 - and α 2 -isoforms) and glucose transporter (GLUT1 and GLUT4) molecules in sheets of adipocyte plasma membrane were determined by quantitative immunoelectron microscopy, using gold labelling. Maximal in vitro insulin stimulation of adipocytes increased the glucose transport rate and the amount of GLUT4 in the plasma membrane 15-fold, whereas the amount of α 2 was unaffected. In adipocytes from hyperthyroid rats, mean adipocyte volume was decreased by 18% and the quantities of GLUT4 per unit area of plasma membrane (maximal insulin stimulation) and of α 2 were decreased by 19% and 15%, respectively. Thus, hypotrophia of fat tissue in the hyperthyroid state is associated with a decreased expression in the plasma membrane of the glucose transporter GLUT4 and the α 2 -isoform of Na/K-ATPase.