Differential Regulation of the Renal 1 Sodium / Phosphate Co - Transporters NaPi - IIa , NaPi - IIc 2 and PiT - 2 in Dietary Potassium Deficiency 3

25 Dietary potassium (K)-deficiency is accompanied by phosphaturia, and decreased renal 26 brush border membrane (BBM) vesicle sodium (Na)-dependent phosphate (Pi) transport 27 activity. We previously showed that K-deficiency in rats leads to increased abundance in 28 the proximal tubule BBM of the apical Na/Pi co-transporter NaPi-IIa, but that the activity, 29 diffusion and clustering of NaPi-IIa could be modulated by the altered lipid composition 30 of the K-deficient BBM (Zajicek et al., Kidney Int. 60, 694-704, 2001; Inoue et al., J. Biol. 31 Chem. 279, 49160-49171, 2004). Here we investigated the role of the renal Na/Pi co32 transporters NaPi-IIc and PiT-2 in K-deficiency. Using Western blotting, 33 immunofluorescence and quantitative real-time PCR we found that in rats and in mice 34 K-deficiency is associated with a dramatic decrease in the NaPi-IIc protein abundance 35 in proximal tubular BBM and in NaPi-IIc mRNA. In addition, we documented the 36 presence of a third Na-coupled Pi transporter in the renal BBM, PiT-2, whose 37 abundance is also decreased by dietary K-deficiency in rats and in mice. Finally, 38 electron microscopy showed subcellular redistribution of NaPi-IIc in K-deficiency: in 39 control rats, NaPi-llc immunolabel was primarily in BBM microvilli whereas in K-deficient 40 rats, NaPi-IIc BBM label was reduced and immunolabel was prevalent in cytoplasmic 41 vesicles. In summary, our results demonstrate that decreases in BBM abundance of the 42 phosphate transporter NaPi-IIc and also PiT-2 might contribute to the phosphaturia of 43 dietary K-deficiency, and that the three renal BBM phosphate transporters characterized 44 so far can be differentially regulated by dietary perturbations. 45 46