RENAL AND SMALL-INTESTINAL SODIUM-DEPENDENT SYMPORTERS OF PHOSPHATE AND SULFATE

Homeostasis of inorganic phosphate (P-i) and sulphate (S-i) is largely achieved by absorption in the mammalian small intestine and by reabso rption in the proximal tubule of the kidney. Under normal physiologica l conditions, the kidney appears to play the major role in maintaining the extracellular concentration of these anions. In both epithelia, r eabsorption of P-i and to some extent also of S-i underlie a variety o f regulatory acute and chronic control mechanisms. Acute regulatory me chanisms are predominantly found in renal P-i reabsorption, whereas ch ronic regulation of transepithelial P-i transport is observed in both tissues. Also, in both epithelia, apically located sodium-dependent tr ansport systems (Na+/P-i and Na+/S-i symport) represent major targets for known regulatory factors. By expression cloning using oocytes of X enopus laevis, renal and small intestinal Na+-dependent phosphate and sulphate transport systems have been identified. Evidence has been obt ained that cloned Na+/P-i and Na+/S-i symporters are localized in the apical membrane of proximal tubular or small intestinal epithelial cel ls respectively. Furthermore, recent results indicate that one of the cloned Na+/P-i symporters is involved in the physiological and pathoph ysiological regulation of proximal tubular P-i reabsorption.