PARATHYROID-HORMONE ACTION ON PHOSPHATE TRANSPORTER MESSENGER-RNA ANDPROTEIN IN RAT RENAL PROXIMAL TUBULES

The inhibitory action of parathyroid hormone (PTH) on P-i reabsorption in the renal proximal tubule is accompanied by a specific decrease in Na-P-i cotransport at the apical brush-border membrane (BBM). It is n ot known whether this decrease represents decreased activity of Na-P-i cotransporters already present in the BBM or whether the number of co transporters is decreased. The present study of the molecular mechanis m of PTH action made use of a specific cDNA probe and antiserum to a r at renal Na-P-i cotransporter (NaPi-2). Three groups of rats were used : intact controls, chronically parathyroidectomized (PTX), and PTX rat s treated acutely (2 h) with bovine PTH-(1-34). Na-P-i cotransport by isolated renal BBM vesicles was increased to 1,315 +/- 44 in PTX rats, compared with 721 +/- 94 pmol . mg(-1). 10 s(-1) in controls (P < 0.0 02), and was returned to control levels by PTH. Western blots of these BBM showed that PTX caused a 2.8-fold increase in NaPi-2 protein cont ent, which was reduced to control levels by PTH. Immunohistochemistry of perfusion-fixed kidneys showed NaPi-2-specific immunofluorescence e xclusively in apical BBM of proximal tubules. Expression of NaPi-2 pro tein at these sites was increased in PTX rats and decreased after PTH treatment. Northern analysis of total RNA showed that the abundance of NaPi-2-specific mRNA was not changed by PTX but there was a small dec rease in response to PTH. The data indicate that PTH regulation of ren al Na-Pi cotransport is determined by changes in expression of NaPi-2 protein in the renal BBM. PTH may decrease the NaPi-2 protein content of BBM in part by a mechanism that results in endocytic withdrawal int o a cytoplasmic pool.