Heterogeneity of sodium-dependent D-glucose transport sites along the proximal tubule: evidence from vesicle studies

The glucose transport properties of brush border membrane vesicles from the outer cortex (early proximal tubule) and outer medulla (late proximal tubule) of rabbit kidney were studied. In the outer cortical preparation the behavior of the sodium-dependent component of D-glucose flux indicated the presence of a low-affinity transport system with Km congruent to 6 mM and Vmax congruent to 10 nmol.min-1.mg protein-1 as measured under zero trans conditions at 40 mM NaCl and 17 degrees C. By contrast, in the outer medullary preparation this component of flux behaved as a high-affinity system with Km congruent to 0.35 mM and Vmax congruent to 4 nmol.min-1.mg protein-1. Differences in transport specificity between the two preparations were also indicated and glucose uptake by the outer cortical vesicles was significantly more sensitive to inhibition by phlorizin. These results suggest the existence of two distinct sodium-dependent D-glucose transport systems in the renal proximal tubule brush border membrane. The kinetic studies presented here were done under zero trans sodium and glucose conditions. The rationale and methodology for carrying out these measurements reliably are discussed in detail.