NA-D-GLUCOSE COTRANSPORT IN RENAL BRUSH-BORDER MEMBRANE-VESICLES OF AN EARLY TELEOST (ONCORHYNCHUS-MYKISS)

Brush-border membrane vesicles (BBMW) enriched with alkaline phosphata se (8.1-fold) and gamma-glutamyl transpeptidase (11.5-fold) were prepa red from the rainbow trout kidney. D-[H-3]glucose uptake was stimulate d by inward Na gradients but not by K, choline, Li, N-methyl-D-glucami ne, or mannitol gradients. Na-dependent glucose uptake displayed overs hoot in voltage-polarized vesicles (VPV; negative inside) but not in s hort-circuited vesicles (SCV). Recognition of carbons 2 and 3 of the g lucopyranose ring was essential for glucose uptake. Phlorizin inhibite d Na-dependent D-glucose uptake with an inhibition constant of 11.4 mu M. The Michaelis-Menten constant of glucose was 0.58 mM in VPV and in creased to 1.49 mM in SCV, whereas that for sodium was 193 mM in VPV a nd similar in SCV. Maximum velocity of Na was reduced in SCV. The Hill coefficient was 1 for both Na and glucose in VPV and SCV. Our studies indicate a single Na-D-glucose cotransporter that transports Na and g lucose with a 1:1 stoichiometry and voltage-dependent kinetics. The tr ansporter shares functional properties with both mammalian transporter s SGLT1 and SGLT2.