Cloning of a rabbit renal Na-Pi cotransporter, which is regulated by dietary phosphate

Previously, we isolated a cDNA (NaPi-1) related to a rabbit renal proximal tubular Na-Pi cotransporter (A. Werner, M.L. Moore, N. Mantei, J. Biber, G. Semenza, and H. Murer. Proc. Natl. Acad. Sci. USA 88:9608-9612, 1991.). In this study, we isolated an additional (rabbit renal) cDNA (NaPi-6), which induces Na-dependent Pi uptake in Xenopus laevis oocytes. Substrate specificity and kinetic properties corresponded to those known for rabbit renal brush-border membrane (BBM) Na-Pi cotransport. NaPi-6 was cloned by homology using NaPi-2 cDNA, a rat renal BBM Na-Pi cotransporter (S. Magagnin, A. Werner, D. Markovich, V. Sorribas, G. Stange, J. Biber, and H. Murer. Proc. Natl. Acad. Sci. USA 90: 5979-5983, 1993). NaPi-6 encodes a protein of 642 amino acids, exhibiting at least eight transmembrane domains. NaPi-6 mRNA and protein in kidneys of rabbits fed a low-Pi diet (LPD; 0.11% Pi) for 1 wk were increased by 1.5- and 4-fold, respectively, compared with those of rabbits fed a high-Pi diet (HPD; 1.20% Pi). This effect was correlated with an increase in Na-Pi cotransport of BBM vesicles isolated from animals adapted to LPD (2.5-fold with respect to HPD). In contrast, NaPi-1 mRNA and protein were not altered in response to LPD. Thus rabbit proximal tubular BBMs contain two different Na-Pi cotransport systems: NaPi-1 (type I) and NaPi-6 (type II). Only the type II transport system seems to be under regulatory control in response to low-Pi dietary intake.