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Notes: Abstract In order to study the specificity for the contraluminal sulfate transport system the inhibitory potency of sulfate esters and sulfonate compounds on the35SO 4 2− influx from the interstitium into cortical tubular cells in situ has been determined. The following was found: 1. From 10 sulfate monoesters tested 9 inhibited contraluminal sulfate influx with an app.K i between 0.6 and 6 mmol/l; the two sulfate diesters tested, however, did not. 2. Out of 8 aliphatic sulfonate compounds only three, having a NH- or OH-group in a suitable position, exerted a moderate inhibition (app.K i ca. 2–6 mmol/l). 3. Amongst 14 benzene sulfonates tested only 2 compounds (5-nitrobenzene-sulfonate and 2-hydroxy-5-nitrobenzenesulfonate) inhibited with aK i〈5 mmol/l. 4. Out of 10 naphthalene sulfonates tested 8 inhibited with aK i〈5; the highest inhibition was seen with the NH-containing 8-anilinonaphthalene-1-sulfonate (ANS), but no inhibition with 2 compounds containing an amino group. 5. From the polycyclic sulfonates pyrene-3-sulfonate and anthracene-1-sulfonate inhibited with aK i of approximately 2 mmol/l, while no inhibition was seen with anthracene-2-sulfonate. 6. Out of 4 amino-sulfonates tested benzene-1-amino-sulfonate and a similar benzyl-analog inhibited with aK i of 1 mmol/l and smaller; cyclohexyl-1-amino-sulfonate (cyclamate), however, inhibited only slightly (app.K i of 6 mmol/l). The data indicate that sulfate monoesters are well accepted by the contraluminal sulfate transport system. The affinity of sulfonate compounds to this system depends on neighbouring OH-groups −NH-groups, meta-positioned electronegative groups or a hydrophobic moiety in an appropriate position.

Notes: Abstract In order to study the specificity for the contraluminal sulfate transport system the inhibitory potency of disulfonates, di-, tricarboxylates and sulfocarboxylates on the35SO 4 2− influx from the interstitium into cortical tubular cells in situ has been determined. The following was found: 1) Methane- and ethane-disulfonate as well as benzene-1,3-disulfonate inhibit contraluminal35SO 4 2− influx (with an (app.K i of 〈6 mmol/l), while benzene-1,2- and 1,4-disulfonate do not. 2) The inhibitory potency of 1,3-benzene disulfonate is slightly augmented by an additional NH2 − or OH-group in position 4. However, OH-groups at position 4 and 5 or 4 and 6 abolish the inhibitory potency. 3) The naphthalene disulfonates tested inhibit only if they have an OH-group in ortho-position to one SO3H group. 4) The stilbene disulfonates H2DIDS and DNDS inhibit the contraluminal35SO 4 2− influx with high (app.K i≈0.8 mmol/l), DADS with lower potency (app.K i≈6 mmol/l). 5) Amongst the tested aliphatic di- and tricarboxylates inhibition was exerted by oxalate (app.K i 1.1 mmol/l) and maleate (app.K i 3.8 mmol/l), but not by malonate, hydroxymalonate and citrate. 6) Out of the tested benzenedicarboxylates only those inhibit which have the COO−-groups directly on the ring in 1,2 and 1,3 position (app.K i 4.0 and 2.7 mmol/l), but not in the 1,4 position. An additional OH-group in position 4 augments the inhibitory potency of 1,3 benzene-dicarboxylates (app.K i 0.8 mmol/l), while an OH group on position 5 abolishes it. 7) The benzene tricarboxylates (BTC) inhibit in the sequence 1,2,3-BTC〉1,3,5-BTC〉1,2,4-BTC (app.K i 0.9, 1.5 and 4.2 mmol/l, respectively). 8) The carboxy-benzene-sulfonates inhibit also in the 1,2 and 1,3 position only (app.K i 6.7 and 5 mmol/l), but not in the 1,4 position. Addition of an −OH-group to the 3-carboxy-1-benzene-sulfonate forming 4-hydroxy-3-carboxy-1-benzene-sulfate augments the inhibitory potency drastically (app.K i 0.32 mmol/l), while a NH2 substitution at the same position leaves it unchanged (app.K i 4.7 mmol/l). If, however, ethylamine instead of NH2 is used as substituent, the inhibitory potency is almost as high as of 4-hydroxy-3-carboxy-1-benzene-sulfonate (app.K i≈0.6 mmol/l). Amongst the dicarboxy-benzene-sulfonates, 3,4-carboxy-benzene-1-sulfonate inhibits (app.K i ca. 2 mmol/l), while 3,5-carboxy-benzene-1-sulfonate does not. The data indicate that a strong interaction of substrate with the sulfate transporter is given, when two charged groups (COO− and/or SO 3 − ) are present in a distance equivalent to the meta-position on the benzene ring and an additional hydrogen bond forming OH- or −NH-group. Hydrogen bond forming groups and charged groups in other positions usually abolish the inhibitory potency.

Notes: Abstract In order to study the specificity of the contraluminal sulfate transport system the inhibitory potency of salicylate analogs (5 mmol/l each) on the35SO 4 2− influx from the interstitium into cortical tubular cells in situ has been determined. The following was found: 2-hydroxybenzoate (salicylate), per se, did not inhibit contraluminal35SO 4 2− influx. The same holds when an additional NH2-group was introduced in position 4 or 5, or when an additional Cl-group was introduced in position 4. When an additional Cl- or NO2-group was introduced in position 5 a moderate inhibition was seen (app.K i≈4 mmol/l). However, introduction of 2 Cl- or 2 NO2-groups in position 3 and 5 creates compounds with strong inhibitory potency (app.K i≈0.5 mmol/l). 2-hydroxy-3,5-iodobenzoate inhibited too, but with a smaller inhibitory potency (app.K i≈2.3 mmol/l). 2-hydroxybenzoate analogs, which have a carboxy- or sulfo-group in position 5, exerted strong inhibition, those with a acetyl- or butyryl-group exerted moderate inhibition. 1-Naphthol-2-carboxylate did not inhibit, while 1-naphthol-4-sulfamoyl-2-carboxylate did. Amongst the dihydroxybenzoates, 2,3- and 2,5-dihydroxybenzoate did not inhibit contraluminal35SO 4 2− influx, while 2,4- and 2,6-dihydroxybenzoate did. The data indicate that a hydroxy-group in ortho-position and an electro-negative group in the meta-position to the carboxyl group and paraposition to the hydroxy-group are essential for interaction with the contraluminal sulfate transport system. The ability of 2,6-dihydroxybenzoate to inhibit might be explained by its ability to undergo mesomeric conformation.

Notes: Abstract In order to evaluate the specificity for the contraluminal sulfate transport system the inhibitory potency of phenol- and sulfonphthaleins, of sulfamoyl-compounds (diuretics) as well as diphenylamine-2-carboxylates (Cl− channel blockers) on the35SO 4 2− influx from the interstitium into cortical tubular cells in situ has been determined. The following was found: 1) Phenolsulfonphthalein (phenol-red) inhibited with an app.K i-value of 1.7 mmol/l, while analogs which had additional Br-atoms in position 3 and/or 5, i.e. bromphenol-blue, bromcresol-purple and bromcresol-green, inhibited with an apparentK i of 0.1 and 0.5 mmol/l respectively. 2) Phenolphthalein and tetrabromphenolphthalein did not inhibit, while the disulfonate dyes bromsulfalein, fuchsin acid and indigocarmine inhibited with aK i between ≈1 and 3 mmol/l. The highest inhibitory potency in this class of compounds was seen with orange G (app.K i 0.07 mmol/l). The monosulfonate dyes tested, fluoresceinsulfonate and orange I inhibited moderately with an app.K i of ≈5 mmol/l. 3) The 3-sulfamoyl compounds inhibited to a varying degree, when they had a neighbouring −NH-group (furylmethylamino-group), i.e. in position 6 to the COOH or SO3H-group, or when they had a phenoxy-group in position 4. 4) 4-sulfamoylbenzoate and the related compounds probenecid, acetazolamide and hydrochlorothiazide inhibited with an app.K i between 4 and 7 mmol/l. 5) All diphenylamine-2-carboxylate analogs inhibited with an app.K i between 3 and 5 mmol/l, even when the −NH-group was replaced by an =O-group or the benzene ring was replaced by a pyrimidine ring, but not when it was replaced by a thiophen ring. In contrast, 4-phenylaminepyridine-3-sulfonate was ineffective, while diphenylamine-2-amino sulfonate exerted the highest inhibition of this group with an app.K i of 1.4 mmol/l. When, however, the aminosulfonate group was replaced by a methylsulfonamide, the inhibitory potency disappeared. The data can be explained by inhibitory patterns found in previous papers for disulfonates [29], sulfonates with a hydrophobic moiety [28] or neighbouring OH-group [28, 29], carboxylates with a neighbouring −NH- or OH-group in position 2- and an electron-attracting group in position 5 [30].

Notes: 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 interstitial33Pi concentration at different starting concentrations a simple diffusion kinetics with a permeability coefficient of 7.5±1.0 · 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.

Notes: Abstract In order to study contraluminal sulfate transport the influx rate of35SO 4 2− from the interstitium into cortical tubular cells has been determined. Preloading of the rat with sulfate augmented contraluminal35SO 4 2− influx; preperfusion with sulfate-free solutions diminished it. The contraluminal35SO 4 2− influx in sulfate-loaded animals followed two parameter kinetics (K m 1.4 mmol/l,J max 1.2 pmol·s−1·cm−1). The contraluminal35SO 4 2− influx (starting concentration 10 μmol/l) did not change when the K+ concentration was varied between 4 and 40 mmol/l and the Ca2+ concentration from zero to 3 mmol/l. Omission of Na+ from the perfusates augmented contraluminal35SO 4 2− influx markedly. The increase is larger at pH 6 than at pH 7.4. Changes of pH affect contraluminal35SO 4 2− influx only when the solutions are Na+- and K+-free. Under these conditions the35SO 4 2− influx decreased when the ambient pH was raised from pH 6.0 to pH 8.0. Thiosulfate, selenate, molybdate, oxalate, phosphate, arsenate, and bicarbonate exerted competitive inhibition, while formate, 2-oxoglutarate and paraaminohippurate showed a biphasic response: inhibition at 50 mmol/l, no inhibition at 150 mmol/l. Chloride and bicarbonate inhibited35SO 4 2− influx at 10 μmol/l35SO 4 2− , but augmented sulfate influx at 5 mmol/l35SO 4 2− concentration in rats not preloaded with sulfate. The data indicate the presence of a contraluminal sulfate transport system which is shared by a variety of inorganic and organic anions. The biphasic behaviour of some anions suggests parallel pathways leading to a cis-inhibition at small and trans-stimulation at high anion concentrations. Na+ and H+ may be cotransported or interact with the transport system at a modifier site.