EFFECT OF P-I RESTRICTION ON RENAL NA+-P-I COTRANSPORTER MESSENGER-RNA AND IMMUNOREACTIVE PROTEIN IN X-LINKED HYP MICE

Although renal Na+-P-i cotransporter gene expression is decreased in X-linked Hyp mice, the mutants do respond to P-i restriction with an adaptive increase in Na+-P-i cotransport maximal velocity in renal brush-border membrane vesicles. In the present study, we examined the mechanism for the adaptive increase in Na+-P-i cotransport in P-i-deprived Hyp mice and normal littermates, using a cDNA probe encoding a rat, renal-specific Na+-P-i cotransporter (NaPi-2) and a rabbit polyclonal antibody raised against a synthetic NaPi-2-derived peptide. The low-P-i diet elicited an increase in Na+-P-i cotransport in normal (141 +/- 13 to 714 +/- 158) and Hyp mice (59 +/- 6 to 300 +/- 62 pmol . mg protein(-1). 6 s(-1); means +/- SE, n = 3, P < 0.01) that was accompanied by an increase in brush-border membrane NaPi-2 protein, relative to ecto-5'-nucleotidase, in normal (1.0 +/- 0.1 to 7.6 +/- 1.5) and Hyp mice (0.3 +/- 0.1 to 7.7 +/- 1.4) (means +/- SE, n = 4; P < 0.01). The low-P-i diet also elicited an increase in the abundance of NaPi-2 mRNA, relative to the 18S RNA, in normal (157 +/- 9% of control diet, P < 0.05) and Hyp mice (194 +/- 10% of control diet, P < 0.01). Immunohistochemistry revealed that NaPi-2 protein was localized to the brush-border membrane of the proximal tubule and that both intensity of the signal and number of immunostained proximal tubules were increased in renal sections from normal and Hyp mice fed the low-P-i diet. In summary, i) the adaptive increase in Na+-P-i cotransport is the result of an increase in NaPi-2 mRNA and protein in both P-i-deprived normal and Hyp mice; 2) abundance of NaPi-2 protein is similar in both mouse strains following P-i deprivation, despite persistence of the P-i transport defect in Hyp mice under these conditions; and 3) the increase in NaPi-2 protein far exceeds the increase in NaPi-2 mRNA, suggesting that translational and/or posttranslational mechanisms play a significant role in the adaptive response to P-i restriction.