Parathyroid hormone action on phosphate transporter mRNA and protein in rat renal proximal tubules

Abstract

The inhibitory action of parathyroid hormone (PTH) on Pi reabsorption in the renal proximal tubule is accompanied by a specific decrease in Na-Pi cotransport at the apical brush-border membrane (BBM). It is not known whether this decrease represents decreased activity of Na-Pi cotransporters already present in the BBM or whether the number of cotransporters is decreased. The present study of the molecular mechanism of PTH action made use of a specific cDNA probe and antiserum to a rat renal Na-Pi cotransporter (NaPi-2). Three groups of rats were used: intact controls, chronically parathyroidectomized (PTX), and PTX rats treated acutely (2 h) with bovine PTH-(1--34). Na-Pi 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 < 0.002), 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 content, which was reduced to control levels by PTH. Immunohistochemistry of perfusion-fixed kidneys showed NaPi-2-specific immunofluorescence exclusively in apical BBM of proximal tubules. Expression of NaPi-2 protein 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 decrease in response to PTH. The data indicate that PTH regulation of renal Na-Pi cotransport is determined by changes in expression of NaPi-2 protein in the renal BBM.(ABSTRACT TRUNCATED AT 250 WORDS)