Furosemide inhibits glucose transport in isolated rat adipocytes via direct inactivation of carrier proteins.

Furosemide inhibits 3-O-methyl-D-glucose equilibrium flux in isolated adipocytes. The inhibition is saturable with an increasing concentration of furosemide and shows a noncompetitive type of kinetics. Both basal and insulin-stimulated fluxes are equally affected by the inhibition. Hydrochlorothiazide and piretanide also inhibit the flux with a similar potency, whereas bumetanide, a more potent diuretic, is much less potent. To understand the molecular basis of this inhibition, effects of furosemide on the glucose-sensitive cytochaslasin B binding activities of adipocytes were studied. Furosemide inhibits the glucose-sensitive cytochalasin B binding of both microsomal and plasma membrane preparations. For both preparations, the inhibition is time dependent and only slowly reversible, is saturable with an increasing concentration of furosemide, shows a noncompetitive type of kinetics with apparent Ki (the inhibitor concentration that gives the half-maximum effect) of 3.5 and 0.7 mM after 2 and 18 h incubation, respectively, and is essentially identical between the basal and insulin-stimulated adipocytes. The inhibition develops with a first-order rate constant of approximately 0.12/h at 4 degrees C. These results indicate that furosemide inhibits glucose transport in adipocytes by directly inactivating transport carriers of both plasma membranes and microsomal reserve pool. This inactivation of glucose carrier may play a part in the diuretic-induced glucose intolerance frequently observed during diuretic therapy.