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Abstract

Some N-containing xenobiotics were recently shown to behave as bisubstrates; that is, they interact with and are transported by both the contraluminal transport system for organic anions (PAH) and the contraluminal transport system for organic cations (NMeN). Thus we determined whether other classes of N-containing substrates, such as sulfamoyl-, sulfonylurea-, thiazide- and benzeneamino-carboxylate (nicotinate) compounds, amongst them diuretics and other drugs, also interact with both transporters. To test this, we applied the stop-flow peritubular capillary perfusion method with initial flux measurements and determined app. Ki values for these substrates on PAH, sulfate and NMeN transport. We found that the following compounds interact with 1) the PAH transporter: benzene carboxylates, benzenesulfonylureas and benzenesulfonamides (as long as their acid pKa value is below 9.5). 2) the sulfate transporter: 2-anilinobenzoates, benzenesulfonylureas, polysubstituted sulfamoylbenzoates and some sulfamoylthiazides with electronegative charge accumulation around an anionic site. 3) the NMeN transporter: anilinobenzoates, sulfamoylbenzoates and benzenesulfonamides, if they bear an N-containing pyridine, pyrrolidine, furylmethylamino or thiazide group. There are, however, exceptions when H-bond formation might be responsible for interaction with that transporter. The data confirm the specificity rule for each transporter and the concept that one and the same substrate can match the requirements for several transporters. Thus the loop diuretics furosemide and piretanide, the thiazide diuretics hydrochlorothiazide, cyclopenthiazide and bendroflumethiazide and the sulfonylureas tolbutamide, chlorpropamide and torasemide interact with all three tested transport systems for PAH, sulfate and NMeN. Therefore, they are able to accomplish complex transport interactions with different transporters.