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

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Abstract
Physiological/pathophysiological alterations in proximal tubular Pireabsorption are associated with an altered brush-border membrane (BBM) expression of type II Na-Pi cotransporter molecules. Reduction is achieved by an internalization and lysosomal degradation and an increase in Pi reabsorption by new synthesis and BBM insertion of type II Na-Pi cotransporters. In the present study, we investigated by immunohistochemistry and immunogold electron microscopy the routing of internalized rat type II Na-Picotransporters (NaPi-2). In kidney of rats on a chronic low-Pi 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.

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