Abstract
Alteration of the dietary intake of phosphate (Pi) leads to rapid changes in renal Pi transport activity. The present study, examined the underlying cellular mechanisms of the rapid regulation, with special reference to renal P, cotransporter. Rats were fed either a low-Pi (0. 02%) diet (CLP rats), the low-Pi diet followed by a high-P, (1. 2%) diet (AHP rats), or a normal (0. 6%) diet (control rats). Nat-dependent Pi transport activity in the brush border membrane was significantly increased in CLP rats compared with control rats, and this activity decreased rapidly within 2 h after the change of diet in AHP rats. Kinetic analysis of Pi transport in the AHP rats indicated that the reduction was accompanied by a decrease in the apparent Vmax, for Na+-dependent Pi uptake. Northern blot analysis showed no difference in the abundance of NaPi-2 mRNA of the kidney between AHP and CLP rats. In contrast, 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 were detected in lysosomal fractions of AHP and CLP rats. Immunohistochemical analysis showed that, NaPi-2 immunoreactivity in AHP rats was largely reduced in the apical membrane of the proximal tubular epithelial cells. Neither cycloheximide nor actinomycin D affected high-Pi-induced reduction of NaPi-2 protein in the brush border membrane of AHP rats, indicating that de novo protein synthesis of an unidentified regulator protein was not involved in the mechanism of this reduction. In contrast, treatment with colchicine, which disrupts microtubulers, abolished the effect of high-Pi diet on NaP, -2 expression. These results suggested that rapid en-docytotic internalization of NaP, -2 may occur specificallyin the brush border membrane following an acute increase in dietary Pi intake.
