Carrier-mediated transfer of D-glucose in brush border vesicles derived from rabbit renal tubules. Na+-dependent versus Na+-independent transfer

Abstract

A brush border preparation from rabbit renal tubules containing a high yield of vesicles has been used to study the transfer of D-glucose through the brush border membrane. In the presence of an Na+ gradient across the vesicular membrane, the vesicles could concentrate D-glucose to a factor of 1.5, whereas in the absence of an Na+ gradient, only equilibrium with the medium was achieved. Two types of transfer could be distinguished by their requirement of Na+, their sensitivity to phlorizin and their pH optimum. The Na+-independent transfer was about 100 times less sensitive to phlorizin than the Na+-dependent path and exhibited a pH optimum between 7 and 8, whereas the Na+-dependent transfer was highest at a pH between 8 and 9. The brush border preparation could be freed of most of the contaminating material derived from the basal and lateral tubular cell membrane by a discontinuous density gradient centrifugation. It still showed both forms of transfer to a similar extent, indicating that both are located in the brush border membrane. A study of the sensitivity of D-glucose transfer to phlorizin, in the presence and absence of Na+ at different temperature, suggests a single carrier species functioning in two interchangeable conformational states with different affinities for phlorizin rather than two transfer systems working independently.