Expression of Na-P(i) cotransport in rat kidney: localization by RT-PCR and immunohistochemistry

We have recently identified a rat kidney cortex Na-dependent transport system for phosphate (P(i)) by expression cloning (NaP(i)-2) (S. Magagnin, A. Werner, D. Markovich, V. Sorribas, G. Stange, J. Biber, and H. Murer. Proc. Natl. Acad. Sci. USA 90: 5979, 1993). In this study we have used reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry to establish the sites of expression of the NaP(i)-2-related mRNA and protein. RT-PCR was performed with single microdissected nephron segments. From these experiments we conclude that NaP(i)-2 mRNA is predominantly expressed in the proximal tubules of superficial and deep nephrons. No NaP(i)-2 mRNA was detected in the thick ascending limb of Henle's loop; however, faint NaP(i)-2 related PCR products were also observed in collecting ducts. Expression of the NaP(i)-2 protein was examined with the use of polyclonal antibodies raised against synthetic NaP(i)-2-derived peptides. Strong specific anti-NaP(i)-2 antiserum-mediated immunofluorescence was found in the convoluted part of proximal tubules and gradually decreased along the straight part. Immunofluorescence indicated that the NaP(i)-2 protein is present in the brush border of proximal tubular cells. In addition, NaP(i)-2-specific immunofluorescence was also observed in subapical vesicles. The described distribution of the NaP(i)-2 protein is in agreement with previously described nephron sites of P(i) reabsorption in the rat kidney and therefore suggests that the NaP(i)-2 transport system represents an Na-P(i) cotransporter involved in proximal tubular apical transport of phosphate.