Membrane potential-sensitive fluorescence changes during Na+-dependent D-glucose transport in renal brush border...

When D-glucose was added to a suspension of renal brush border membrane vesicles equilibrated in a Na+-containing medium, there was a rapid transient increase in the fluorescence of the probe, 3,3'-dipropyl thiodicarbocyanine iodide (DiS-C3-(5)). This sugar-induced response was stereospecific for the D isomer, dependent on Na+, inhibited by phlorizin, and blocked by ionophores, valinomycin plus nigericin, which dissipate ionic gradients. The enhancement in fluorescence suggests the entrance into the vesicle of Na+, cotransported with the sugar. This would lead to the interior of the membrane vesicle becoming more positive, resulting in depolarization of the membrane potential. That the sugar induced the transport of Na+ was confirmed by direct measurement of 22Na+ uptake. Thus, the Na+-sugar co-transport system provides a mechanism for D-glucose to stimulate the flux of Na+ as well as for the Na+ electrochemical gradient to enhance the transport of D-glucose.