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Internalization of renal type IIc Na-P_i cotransporter in response to a high-phosphate diet

¹Nutritional Science, Department of Nutrition, and ²General Laboratory for Medical Research, University of Tokushima School of Medicine, Tokushima; and ³Department of Structural Pathology Institute of Nephrology Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan

Submitted 24 March 2004 ; accepted in final form 13 November 2004

Dietary phosphate levels regulate the renal brush-border type IIa Na-P_i cotransporter. Another Na-P_i cotransporter, type IIc, colocalizes with type IIa Na-P_i cotransporter in the apical membrane of renal proximal tubular cells. The goal of the present study was to determine whether dietary phosphate levels also rapidly regulate the type IIc Na-P_i cotransporter. Type IIa and type IIc transporter protein levels were increased in rats chronically fed a low-P_i diet compared with those fed a normal-P_i diet. Two hours after beginning a high-P_i diet, type IIa transporter levels were decreased, whereas type IIc protein levels remained unchanged. Western blot analysis of brush-border membrane prepared 4 h after beginning a high-P_i diet showed a significant reduction in type IIc transporter protein levels, and immunohistochemistry showed translocation of the type IIc-immunoreactive signal from the entire brush border to subapical membrane. Membrane fractionation studies revealed a decrease in apical membrane type IIc protein without changes in total cortical type IIc protein, which is compatible with redistribution of type IIc protein from the apical membrane to the dense membrane fraction. The microtubule-disrupting reagent colchicine prevented this reduction in apical type IIc transporter at the apical membrane but had no effect on type IIa transporter levels. These data suggest that the type IIc Na-P_i cotransporter level is rapidly regulated by rapid adaptation to dietary P_i in a microtubule-dependent manner. Furthermore, the mechanisms of the internalization of the type IIc transporter are distinct from those of the type IIa transporter.

proximal tubule; dietary phosphate; regulation

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