Contraluminal phosphate transport in the proximal tubule of the rat kidney

Abstract

In order to study the characteristics of contraluminal phosphate transport the stopped flow microperfusion technique [13] has been applied. By measuring the time-dependent decrease of interstitial 33Pi concentration at different starting concentrations a simple diffusion kinetics with a permeability coefficient of 7.5 +/- 1.0 X 10(-8) cm2 s-1 was found. Such a kinetic was so far only observed with 2-deoxy-D-glucose. This substance, however, is transported in addition by facilitated diffusion as was seen by paraaminohippurate, methylsuccinate and sulfate. The contraluminal transport of phosphate was inhibited by H2-DIDS (5 mmol/l). It was, however, not influenced by omission of Na+ from the perfusates, by addition of sulfate (150 mmol/l), methylsuccinate (50 mmol/l), arsenate (50 mmol/l), the Hg-compound mersalyl (5 mmol/l), high and low phosphate diet and pH changes between 6.0 and 8.0. The data indicate that phosphate, which is reabsorbed from the lumen by a Na+-dependent transport system, leaves the cell by a rather unspecific contraluminal diffusion pathway.

Towards a Molecular View of Renal Proximal Tubular Reabsorption of Phosphate

“…As illustrated in figure 1, there is evi dence that Pj leaves the cell via two different Pj transport systems. Based on experiments with isolated basolateral membranes and intact tubular preparations there is evidence for a sodium-independent DIDS-sensitive anion exchange mechanisms possibly involving bi carbonate ions [14,15]: this transport mecha nisms does not seem to transport sulfate. In addition, a Na-dependent transport pathway for P, has been demonstrated with basolateral membranes isolated from dog kidney [16] and a cultured renal epithelial cell line (OK cells) [17] , In contrast to the apical Na/P, cotran sport, Na-dependent transport of P, through the basolateral membrane is characterized by a low K," value for Pt (millimolar range) and a stoichiometry of 1:1 [6, 17], Heterogeneity o f P, Transport Systems.…”

Cloning and characterization of organic anion systems in the adrenal cortex and their role in steroid release

“…This transport system has wide substrate specificity with a necessity of a negative or partial negative charge on a hydrophobic backbone, but the system also interacts with non-ionisable hydrophobic compounds. The saturable transport of cortisol at physiological concentration was also shown by these studies, and this process was inhibited by probenecid (Ullrich et al 1991). Recently it was demonstrated that adrenocorticotroph hormone (ACTH)-stimulated cortisol release from primary bovine adrenocortical cells was inhibited by probenecid and trans-stimulated by PAH (Steffgen et al 1996 andRohrbach et al 1997).…”

“…However, in vitro studies demonstrated retention of steroids against a concentration gradient at the plasma membrane (Whitehouse et al 1971, Inaba et al 1974, whereas the interaction of steroids with carrier proteins in cell membranes has been shown in several studies (Rao et al 1997, Raven et al 1982, Thompson et al 1995, Ullrich et al 1991, Chen et al 1996. Moreover, the recently cloned organic anion and organic cation transporter families proved to be able to transport different steroids (Burckhardt et al 2000).…”