Chapter 21 Electrogenic and Electroneutral Na+ Gradient-Dependent Transport Systems in the Renal Brush Border Membrane Vesicle

Abstract

Publisher Summary This chapter presents a study in which the Na + gradient-dependent uptakes of D-glucose, L-proline, and L-glutamate were examined. Membrane potentials across the brush border membrane were modified by the use of anions of different modes of permeability and ionophores—such as valinomycin—to generate large K + diffusion potentials. In addition, the responses of fluorescent probes that monitored changes in polarity concomitant with the uptakes were determined. Preliminary experiments using L-proline instead of D-glucose indicated that the cotransport of Na + and L-proline resulted in analogous membrane responses; while, contrary to the transient increases in fluorescence with the addition of D-glucose and L-proline, L-glutamate caused a nonspecific, nontransient change in fluorescent intensity. No distinction was found between L- and D-glutamate or between L-glutamate in an NaCl or KCl medium. Although an explanation for this nonspecific increase in fluorescence seen with glutamate is not evident, it is clear that the cotransport of Na + and L-glutamate did not lead to a net transient increase in positive charge. The chapter provides evidence that both electrogenic and electroneutral Na + gradient-dependent transport systems were present in the renal brush border membrane, the Na + -D-glucose and Na + -L-proline cotransport systems being electrogenic and the Na + -L-glutamate cotransport system being electroneutral.