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Abstract

Publisher Summary: Sodium cotransport systems are present not only in the brush border membranes but also in the basolateral membranes and play an important role of sodium in almost all transport processes occurring in the renal proximal tubule. This chapter describes three aspects of sodium cotransport systems—(1) the intracellular localization of sodium cotransport systems, (2) the stoichiometry of sodium cotransport systems with particular emphasis on sodium anion cotransport systems, and (3) studies on the incorporation of sodium cotransport systems into liposomes. Sodium symport systems in the luminal membrane and sodium antiport systems in the contraluminal membrane (Na⁺/Ca²⁺) are involved in tubular reabsorption; sodium antiport systems (Na⁺/H⁺) in the luminal membrane and sodium symport systems in the contraluminal membrane catalyze sodium-dependent secretion. The cellular distribution and nature of the sodium cotransport system determine—in the presence of a given sodium gradient across the membranes—the direction of the sodium-dependent solute flow. However, the stoichiometry of sodium cotransport systems can vary depending on the sodium concentration. Thus, sodium-dependent transport of L-glutamate seems to involve—at least partially—the influx of two sodium ions together with one glutamate anion coupled to the efflux of one potassium ion.