Contraluminal para-aminohippurate (PAH) transport in the proximal tubule of the rat kidney

Abstract

In order to study the specificity for contraluminal para-aminohippurate (PAH) transport, the inhibitory potency of aliphatic dicarboxylates on³H-PAH influx, as well as the inhibitory effect on³⁵SO ²⁻₄ - and³H-succinate influx, from the interstitium into cortical tubular cells in situ has been determined. The following was found:

• 1. Testing a homologous series of dicarboxylates-ranging from the 2 C oxalate to the 10 C sebacate — PAH transport was inhibited by succinate (app.K _i 1.35 mmol/l), and all longer dicarboxylates, with high potency (app.K _i 0.05–0.35 mmol/l). Sulfate transport was inhibited only by oxalate (app.K _i 1.1 mmol/l), while dicarboxylate transport was inhibited by succinate, glutarate, adipate and pimelate with decreasing potency (app.K _i 0.04, 0.24, 0.91, 4.0 mmol/l, respectively).

• 2. PAH transport was inhibited by succinate and glutarate with high potency (app.K _i 1.35 and 0.05 mmol/l), by the correspondent monomethylester to a lesser extent (app.K _i 1.7 and 0.74 mmol/l), but not by the dimethylester. On the other hand, the semialdehyde of succinate with aK _i-value of 1.2 mmol/l, had the same inhibitory potency as succinate itself, while the dialdehyde of glutarate (app.K _i 1.4 mmol/l) was much less potent as glutarate.

• 3. Introduction of an oxo-, methyl- or sulfhydroxylgroup onto the 2-position of succinate, or of an oxo-group onto the 2-position of glutarate moderately augmented the inhibitory potency against PAH-uptake. However, introduction of a 2-hydroxy group onto succinate or glutarate in thel-position reduced the inhibitory potency more than in thed-position. Introduction of two methyl-, sulfhydryl- or hydroxyl-groups in the 2–3-position of succinate reduced or abolished its inhibitory potency. The introduction of a 2-amino group onto succinate or glutarate abolished its effect on PAH transport. However, N-acetylation or N-benzoylation led to a restitution in inhibitory potency.

• 4. The trans-isomers fumarate and mesaconate inhibited PAH- and methylsuccinate transport, while the cis-isomers maleate and citraconate did so to a lesser extent or not at all. The effect was reversed with the tricarboxylic aconitates, because cis-aconitate bears a CH₂-extended COOH-group in trans-position and trans-aconitate in cis-position.

The data indicate that there exist three different anion transport systems at the contraluminal cell side of the proximal renal tubule: 1. a sulfate-oxalate transporter, 2. a sodium-dependent dicarboxylate transporter, and 3. a paraaminohippurate transporter. The PAH transport system accepts dicarboxylates with chain length higher than 7.5 Å (=distance between the terminal oxygen atoms), while the dicarboxylate transport interacts with dicarboxylates with a chain length between 6.5 and 10 Å. Both transport systems prefer the transconfiguration. The effect of side groups on the interaction of dicarboxylates with the PAH-transport system is due mainly to hydrophobicity and electron configuration.