Substrate specificity of the luminal Na⁺-dependent sulphate transport system in the proximal renal tubule as compared to the contraluminal sulphate exchange system

The efflux of [35S]sulphate from the lumen of the proximal renal tubule into tubular cells of rats was measured by the stop-flow tubular-lumen microperfusion technique. The transport parameters obtained and the apparent Ki values of competing substrates were compared with those of the contraluminal influx of [35-S]sulphate from the interstitium into tubular cells. For the luminal sulphate efflux a Km(l, SO2−4) of 0.8 mmol/l and a Jmax(l, SO2−4) of 0.2 pmol s−1 cm−1 were found. The corresponding contraluminal values were Km(cl,SO2−4) 1.4 mmol/l and Jmax(cl, SO2−4) 1.2 pmol s−1 cm−1. Omission of Na+ from the perfusates reduced the luminal efflux of sulphate by 83%, while the contraluminal influx of sulphate was not changed. Increase in HCO−3 concentration inhibited both luminal efflux and contraluminal influx of sulphate, while a change of pH from 6.0 to 8.0 was without effect. Comparing the apparent Ki(SO2−4) values for luminal and contraluminal sulphate transport, a relationship close to 1∶1 was seen for some inorganic substrates with tetrahedral molecular structure (thio-sulphate, sulphate, molybdate and selenate). The same holds for phosphate, while for oxalate the contraluminal Ki(SO2−4) value was lower than the luminal one (1.2 and 4.5 mmol/l). Some of the dicarboxylates and disulphonates tested show the same affinity to the luminal Na+-dependent sulphate transporter and the contraluminal sulphate exchange system, whereas most of the benzene carboxylate and benzenesulphonate derivatives tested exhibit higher luminal than contraluminal ki values. The inhibitory potency increased with rising numbers of substituents on the benzene ring. This effect was more pronounced for the contraluminal sulphate transporter. In general, only disulphonates and analogues as well as similarly structured compounds (5-sulphosalicylate, 2-hydroxy-5-nitrobenzenesulphonate, eosine-5-isothiocyanate) have a good inhibitory potency toward the luminal sulphate transporter [apparent Ki 0.9–3.1 mmol/l]. All the tested sulphamoyl and phenoxy diuretics, and fluorescein and phenolphthalein dyes showed no or a smaller inhibitory potency to the luminal sulphate transport system than to the contraluminal. The most effective inhibitors of both sulphate transport systems are 8-anilino-1-naphthalenesulphonate, orange G, and H2-DIDS. The data indicate that the Na+-dependent luminal and the Na+-independent contraluminal sulphate transport systems accommodate a similar spectrum of anionic substrates, whereby the inhibitory potency against the luminal Na+-dependent sulphate transport system is identical or smaller than against the contraluminal transporter