ACUTE REGULATION BY DIETARY PHOSPHATE OF THE SODIUM-DEPENDENT PHOSPHATE TRANSPORTER (NAPI-2) IN RAT-KIDNEY

Alteration of the dietary intake of phosphate (P-i) leads to rapid cha nges in renal P-i transport activity. The present study, examined the underlying cellular mechanisms of the rapid regulation, with special r eference to renal P-i cotransporter. Rats were fed either a low-P-i (0 .02%) diet (CLP rats), the low-P-i diet followed by a high-P-i (1.2%) diet (AHP rats), or a normal (0.6%) diet (control rats). Na+-dependent P-i transport activity in the brush border membrane was significantly increased in CLP rats compared with control rats, and this activity d ecreased rapidly within 2 h after the change of diet in AHP rats. Kine tic analysis of P-i transport in the AHP rats indicated that the reduc tion was accompanied by a decrease in the apparent V-max for Na+-depen dent P-i uptake. Northern blot analysis showed no difference in the ab undance of NaPi-2 mRNA of the kidney between AHP and CLP rats. In cont rast, Western blot analysis of renal brush border membrane proteins of AHP rats indicated a significant decrease in the abundance of NaPi-2 protein as compared with CLP rats. Immunoreactive signals for NaPi-2 w ere detected in lysosomal fractions of AHP and CLP rats. Immunohistoch emical analysis showed that, NaPi-2 immunoreactivity in AHP rats was l argely reduced in the apical membrane of the proximal tubular epitheli al cells. Neither cycloheximide nor actinomycin D affected high-P-i-in duced reduction of NaPi-2 protein in the brush border membrane of ARP rats, indicating that de novo protein synthesis of an unidentified reg ulator protein was not involved in the mechanism of this reduction. In contrast, treatment with colchicine, which disrupts microtubulers, ab olished the effect of high-P-i diet on NaPi-2 expression. These result s suggested that rapid endocytotic internalization of NaPi-2 may occur specifically in the brush border membrane following an acute increase in dietary P-i intake.