Evidence for a high-affinity sodium-dependent D-glucose transport system in the kidney

Renal brush-border membrane D-glucose transport was studied in vesicles prepared from early proximal tubules (outer cortical tissue) and late proximal tubules (outer medullary tissue). Kinetics of sodium-dependent D-glucose uptake, determined with zero trans sodium gradient of 200 mM at 21 degrees C demonstrated a curvilinear Woolf-Augustinsson-Hofstee plot in both cortical and medullary vesicles. Three sodium-dependent systems were discerned in the cortex, possessing Km values of 12.35 +/- 2.2 mM, 520 +/- 39 microM, and 12 +/- 1 microM, respectively. In contrast, only the high-affinity, low-capacity systems were present in the medullary tissue; with respective Km values of 484 +/- 95 microM and 11 +/- 1 microM and mean Vmax values of 2.4 +/- 0.1 and 0.44 +/- 0.08 nmol X mg protein-1 X min-1. The high-affinity systems were more abundant in the medulla than in the cortex. These studies support the notion of multiple transport processes for sodium-dependent D-glucose transport in the pig kidney. Further, they provide evidence for a third, high-affinity system located principally in the medullary tissue, perhaps the late proximal tubule.