Role of the electrochemical gradient for Na+ in D-glucose transport by mullet kidney.

Brush border membrane vesicles were prepared from striped mullet (Mugil cephalus) kidney. As judged by the marker enzyme activities, these membranes were enriched 10- to 15-fold compared with the initial homogenate. Uptake of D-glucose was phlorizin sensitive and Na+ dependent. L-glucose uptake was unaffected by either phlorizin or Na+. D-Glucose uptake reflected entry into the osmotically active intravesicular space, not binding to the membranes. When Na+ was increased from 25 to 100 mM, the maximum velocity of glucose uptake was increased from 92 to 423 pmol . s-1 . mg protein-1, whereas the apparent Km (1.27 +/- 0.23) was not altered as Na+ increased. 22Na+ uptake by these membrane vesicles was stimulated by D-glucose and inhibited by phlorizin. These results indicated that Na+ and glucose entered the vesicles via a cotransport process. Consistent with this interpretation, it was possible to show that glucose uptake could be driven by either the chemical or electrical component of the Na+ electrochemical gradient and that the contributions of these two components were additive when both were present. Finally, it was shown that the coupling ratio between Na+ and glucose was approximately 1:1. Accordingly, these results indicate that reabsorption of D-glucose across the brush border membranes is coupled to the transmembrane electrochemical gradient of sodium ions.