rg Biber. Intestinal and renal adaptation to a low-Pi diet of type II NaPi cotransporters in vitamin D receptor-and 1␣OHase-deficient mice.

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 Na + /H + exchanger 3 (NHE3) is expressed in the brush border membrane (BBM) of proximal tubules (PT). Its activity is down-regulated on increases in intracellular cAMP levels. The aim of this study was to investigate the contribution of the protein kinase A (PKA) and the exchange protein directly activated by cAMP (EPAC) dependent pathways in the regulation of NHE3 by adenosine 3′,5′-cyclic monophosphate (cAMP). Opossum kidney cells and murine kidney slices were treated with cAMP analogs, which selectively activate either PKA or EPAC. Activation of either pathway resulted in an inhibition of NHE3 activity. The EPAC-induced effect was independent of PKA as indicated by the lack of activation of the kinase and the insensitivity to the PKA inhibitor H89. Both PKA and EPAC inhibited NHE3 activity without inducing changes in the expression of the transporter in BBM. Activation of PKA, but not of EPAC, led to an increase of NHE3 phosphorylation. In contrast, activation of PKA, but not of EPAC, inhibited renal type IIa Na + -coupled inorganic phosphate cotransporter (NaPi-IIa), another Na-dependent transporter expressed in proximal BBM. PKA, but not EPAC, induced the retrieval of NaPi-IIa from BBM. Our results suggest that EPAC activation may represent a previously unrecognized mechanism involved in the cAMP regulation of NHE3, whereas regulation of NaPi-IIa is mediated by PKA but not by EPAC.

No abstract