Effects of phosphate intake on distribution of type II Na/P(i) cotransporter mRNA in rat kidney

Abstract

Background. Renal phosphate (Pi) reabsorption is regulated by dietary Pi intake, as well as in other ways. Changes in Pi reabsorption are associated with the modulation of sodium/Pi cotransporter type II (NaPi-2) protein abundance in the brush border membrane (BBM) of proximal tubules (PTs) and of renal NaPi-2 mRNA levels. In this study, we address whether the NaPi-2 protein and NaPi-2 mRNA distribution patterns in the renal cortex vary in parallel with changes of dietary Pi intake. Methods. We investigated in cryosections of perfusion-fixed rat kidneys by in situ hybridization (ISH) and immunohistochemistry (IHC) the distribution patterns of NaPi-2 mRNA and of NaPi-2 protein one week, two hours, and four hours after changes in dietary Pi intake. Results. NaPi-2 mRNA and NaPi-2 protein were present in PTs exclusively. In rats adapted to one week of high Pi intake, signals for NaPi-2 mRNA and NaPi-2 protein in cortical PTs were weak, except in the convoluted parts of PTs of juxtamedullary nephrons. After one week of low Pi intake, the ISH and IHC signals for NaPi-2 were high in PT segments in all cortical levels. The switch from a chronic high to a low Pi intake within two and four hours induced no increase and a slight increase, respectively, in the NaPi-2 mRNA signal in PTs of midcortical and of superficial nephrons, whereas in the BBM of these nephrons, NaPi-2 protein was markedly up- regulated. Two and four hours after switching from low to high Pi intake, the overall high ISH signal for NaPi-2 mRNA was unchanged, whereas NaPi-2 protein staining was drastically down-regulated in the BBM of PTs from superficial and midcortical nephrons. Conclusions. The marked changes in NaPi-2 protein abundance in the BBM, following altered dietary Pi intake, precede corresponding changes at the RNA level by several hours. Thus, the early adaptation to altered Pi intake involves mRNA-independent mechanisms. The up- or down-regulation of NaPi-2 protein abundance in the BBM and NaPi-2 mRNA in PT affects mainly midcortical and superficial nephrons.