Polarity of the Proximal Tubular Cell: Comparison of Luminal and Contraluminal Transport Systems for Hexoses, Dicarboxylates, and Sulfate

Abstract

Until 1970 almost nothing was known about the individual steps of transepithelial transport. Epithelia have therefore to be considered as “black box.” In studies of the proximal tubule it was first realized that solutes move either paracellularly and then passively or transcellularly and then usually actively [18]. By analyzing the latter, Na⁺-coupled transport was detected, which functions as follows (Fig. 1): Na⁺ ions are pumped out of the cell into the interstitial space by (Na⁺ + K⁺)-ATPase, which is localized at the basolateral cell side. In this process, the intracellular Na⁺ concentra­tion falls far below that of the lumen or interstitium and Na⁺ ions tend to run back into the cell. In the cell membrane, especially at the luminal side, there do exist transport systems in which Na⁺ ions can flow, but only in cotransport with other solutes, i.e., hexoses, amino acids, phosphate, monocarboxylates, sulfate, bile acids, or dicarboxy­lates. The transport systems for each have been well characterized by transport studies in situ, by experiments with membrane vesicles, and by electrophysiological measurements [19]. The transport systems on the contraluminal cell side, by contrast, are less well investigated. There are several reasons for this. Electrophysiological techniques cannot be applied, because of electroneutral coupling. Futhermore, it is more difficult to obtain defined membrane vesicles of this cell side. An in situ method is therefore very welcome, and one such was recently devised by our group [20]. Together with vesicle studies, it allows a good characterization of contraluminal transport mechanisms. In the following I will present data on luminal and contralumi­nal transport of:

• 1. Glucose and other hexoses

• 2. Dicarboxylates, including all intermediates of the Krebs cycle

• 3. Sulfate and related compounds, including oxalate.