Glucose transporters of rat proximal tubule: differential expression and subcellular distribution

In the late proximal tubule, glucose reabsorption progressively lowers the concentration of luminal glucose, and concentrative glucose influx increases to ensure complete glucose reabsorption. The change in glucose influx is effected by luminal Na(+)-dependent glucose transporters (Na(+)-GLUT), which exhibit higher Na(+)-to-glucose stoichiometric ratios in the late proximal tubule. In this work, the corresponding changes in the axial distribution of basolateral glucose efflux transporters (GLUTs) were examined. mRNAs encoding high-affinity facilitative basolateral transporter GLUT1, low-affinity GLUT2, and apical Na(+)-GLUT were identified in mixed populations of proximal convoluted and straight tubules. The organization of the cognate proteins was also appraised on Western blots. GLUT1 was present in glomeruli, proximal convoluted, and straight tubules, GLUT2 was only expressed in the proximal convoluted tubule, and Na(+)-GLUT was present in both proximal convoluted and straight segments. GLUT1 and GLUT2 were confined to the basolateral membrane, whereas Na(+)-GLUT was preferentially localized to the brush-border membrane. These data are consistent with the idea that glucose influx in early and late proximal tubule is achieved through Na(+)-GLUT, that GLUT1 and GLUT2 are responsible for glucose efflux in the early proximal tubule, and that in the late proximal tubule, where transcellular glucose flux is lower, only GLUT1 mediates glucose efflux.