Sulphate and phosphate transport in the renal proximal tubule

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Abstract
Experiments performed on microperfused proximal tubules and brush-border membrane vesicles revealed that inorganic phosphate is actively reabsorbed in the proximal tubule involving a 2 Na$^+$-HPO$^{2-}_4$ or H$_2$PO$^-_4$ co-transport step in the brush-border membrane and a sodium-independent exit step in the basolateral cell membrane. Na$^+$-phosphate co-transport is competitively inhibited by arsenate. The transtubular transport regulation is mirrored by the brush-border transport step: it is inhibited by parathyroid hormone intracellularly mediated by cyclic AMP. Transepithelial inorganic phosphate (P$_i$) transport and Na$^+$-dependent P$_i$ transport across the brushborder membrane correlates inversely with the P$_i$ content of the diet. Intraluminal acidification as well as intracellular alkalinization led to a reduction of transepithelial P$_i$ transport. Data from brush-border membrane vesicles indicate that high luminal H$^+$ concentrations reduce the affinity for Na$^+$ of the Na$^+$-phosphate co-transport system, and that this mechanism might be responsible for the pH dependence of phosphate reabsorption. Contraluminal influx of P$_i$ from the interstitium into the cell could be partly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). It is not, however, changed when dicarboxylic acids are present or when the pH of the perfusate is reduced to pH 6. Sulphate is actively reabsorbed, involving electroneutral 2 Na$^+$-SO$^{2-}_4$ co-transport through the brush-border membrane. This transport step is inhibited by thiosulphate and molybdate, but not by phosphate or tungstate. The transtubular active sulphate reabsorption is not pH dependent, but is diminished by the absence of bicarbonate. The transport of sulphate through the contraluminal cell side is inhibited by DIDS and diminished when the capillary perfusate contains no bicarbonate or chloride. The latter data indicate the presence of an anion exchange system in the contraluminal cell membrane like that in the erythrocyte membrane.

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