Internalization of proximal tubular type IINa-Pi cotransporter by PTH:: immunogold electron microscopy

Physiological/pathophysiological alterations in proximal tubular P-i reabsorption are associated with an altered brush-border membrane (BBM) expression of type II Na-P-i cotransporter molecules. Reduction is achieved by an internalization and lysosomal degradation and an increase in P-i reabsorption by new synthesis and BBM insertion of type II Na-P-i cotransporters. In the present study, we investigated by immunohistochemistry and immunogold electron microscopy the routing of internalized rat type II Na-P-i cotransporters (NaPi-2). In kidney of rats on a chronic low-P-i diet, NaPi-2 is mainly localized in the BBM, in cisterns of the Golgi apparatus and sparsely also in large endocytotic vacuoles and lysosomes. Fifteen minutes after the injection of the 1-34 analog of parathyroid hormone (PTH), the amount of NaPi-2 was decreased in the BBM and increased in endocytotic vesicles. NaPi-2 molecules colocalized with horseradish peroxidase injected prior to the injection of PTH. Vesicles labeled for NaPi-2 were occasionally also labeled for clathrin or the adaptor protein AP2. We conclude that NaPi-2 molecules enter the subapical compartment from where NaPi-2-containing vesicles are segregated off and directed to the lysosomes. A clathrin-mediated pathway may contribute to the PTH-induced internalization of NaPi-2.