Renal phosphaturia during metabolic acidosis revisited: molecular mechanisms for decreased renal phosphate reabsorption

Abstract

During metabolic acidosis (MA), urinary phosphate excretion increases and contributes to acid removal. Two Na + -dependent phosphate transporters, NaPi-IIa (Slc34a1) and NaPi-IIc (Slc34a3), are located in the brush border membrane (BBM) of the proximal tubule and mediate renal phosphate reabsorption. Transcriptome analysis of kidneys from acid-loaded mice revealed a large decrease in NaPi-IIc messenger RNA (mRNA) and a smaller reduction in NaPi-IIa mRNA abundance. To investigate the contribution of transporter regulation to phosphaturia during MA, we examined renal phosphate transporters in normal and Slc34a1-gene ablated (NaPi-IIa KO) mice acid-loaded for 2 and 7 days. In normal mice, urinary phosphate excretion was transiently increased after 2 days of acid loading, whereas no change was found in Slc34a1−/− mice. BBM Na/Pi cotransport activity was progressively and significantly decreased in acid-loaded KO mice, whereas in WT animals, a small increase after 2 days of treatment was seen. Acidosis increased BBM NaPi-IIa abundance in WT mice and NaPiIIc abundance in WT and KO animals. mRNA abundance of NaPi-IIa and NaPi-IIc decreased during MA. Immunohistochemistry did not indicate any change in the localization of NaPi-IIa and NaPi-IIc along the nephron. Interestingly, mRNA abundance of both Slc20 phosphate transporters, Pit1 and Pit2, was elevated after 7 days of MA in normal and KO mice. These data demonstrate that phosphaturia during acidosis is not caused by reduced protein expression of the major Na/Pi cotransporters NaPi-IIa and NaPi-IIc and suggest a direct inhibitory effect of low pH mainly on NaPi-IIa. Our data also suggest that Pit1 and Pit2 transporters may play a compensatory role.

The phosphate transporter NaPi-IIa determines the rapid renal adaptation to dietary phosphate intake in mouse irrespective of persistently high FGF23 levels

“…Snap-frozen kidneys (five to seven kidneys for each condition) were homogenized in RLT-Buffer (Qiagen, Basel, Switzerland) supplemented with ß-mercaptoethanol to a final concentration of 1%. RNA extraction, reverse transcription, and semi-quantitative realtime PCR were performed as described previously [42].…”

“…After blocking with 5% milk powder in Tris-buffered saline for 60 min, the blots were incubated with the primary antibodies: rabbit polyclonal anti-NaPi-IIa (1:4,000) [19], rabbit polyclonal anti-NaPi-IIc (1:5,000) [42], rabbit polyclonal anti-Pit-2 …”

Acute parathyroid hormone differentially regulates renal brush border membrane phosphate cotransporters

“…A plethora of factors affects renal phosphate reabsorption, among them parathyroid hormone (PTH), dopamine, dietary phosphate intake, glucocorticoids, acid-base status, growth factors, insulin, and FGF-23 (1,3,6,7,31,32). The hormonal regulation of the NaPi-IIa cotransporter has been studied in much detail demonstrating that hormones such PTH, dopamine, FGF-23, sFRP4, MEPE, glucocorticoids cause a reduction of NaPi-IIa expression (7).…”

The SLC34 family of sodium-dependent phosphate transporters

“…Acidosis leads to the release of phosphate from bones as well as to inhibition of renal reabsorption of phosphate, the later contributing to the supply of titrable acids in the urine [2,106]. In contrast, both the intestinal Na/Pi cotransport and the expression of NaPi-IIb are increased in metabolic acidosis [130], suggesting that the gut compensates for the urinary loss of phosphate.…”