Low-Pi diet increases the abundance of an apical protein in rat proximal-tubular S3 segments.

Dietary phosphate (Pi) restriction is associated with an adaptive increase in proximal-tubular apical brush-border membrane (BBM) sodium-dependent Pi transport (Na-Pi cotransport). Adaptation to Pi restriction is dependent on de novo protein synthesis; however, it is not known whether the proteins involved represent newly synthesized Na-Pi cotransporters or some other (regulatory) proteins. Recently the cDNA for a Na-Pi cotransport system of rabbit kidney cortex (system NaPi-1) has been identified by expression cloning. The purpose of this study was to determine if the adaptive increase in Na-Pi cotransport in response to dietary Pi restriction in the rat is associated with an increase in the abundance of a NaPi-1-related protein. To answer this question we took advantage of the cross-reactivity of polyclonal antibodies raised against a C-terminal peptide of the NaPi-1 protein with a protein of BBM isolated from rat kidney cortex. On Western blots, a positive reaction with a protein with an apparent molecular mass of 100 kDa was observed in BBM isolated from juxtamedullary cortex and, to a lesser extent, in BBM isolated from superficial cortex. In immunohistochemical studies anti-(NaPi-1)-antiserum-mediated immunofluorescence was observed predominantly in S3 segments where the immunoreaction was restricted to the brush borders. Compared to control BBM, in BBM isolated from the juxtamedullary cortex of rats fed a low-Pi diet, there was a twofold increase in the abundance of the 100-kDa protein. In the same membrane vesicles Na-Pi cotransport was increased threefold. The results of this study demonstrate specific expression of a 100-kDa apical protein in S3 cells of rat proximal tubules. Increased abundance of the 100-kDa protein due to chronic Pi restriction suggests an involvement of this protein in the (chronic) adaptive response of S3 cells to a low-Pi diet.