Background: A decrease of proximal tubular reabsorption of phosphate (Pi), which can be provoked by parathyroid hormone (PTH) or by a high Pi-diet, has been shown to correlate with a decrease of the number of type II Na/Pi-cotransporters residing in the brush border membrane. While both PTH and a high Pi-diet lead to an internalization of type II cotransporters, the further cellular routing of internalized cotransporters has not been established unequivocally.
Methods: To prevent lysosomal degradation, rats were treated with leupeptin prior to the injection of PTH or feeding acutely with a high Pi-diet. Kidney cortex were recovered and used for immunohistochemistry. In parallel, brush border membranes and lysosomes were isolated and analyzed by Western blotting.
Results: Under both conditions (PTH and high Pi-diet), a strong overlap of internalized type II cotransporters with the late endosomes/lysosomes was observed by immunohistochemistry. In agreement, the content of type II Na/Pi-cotransporters was increased in lysosomes isolated from the corresponding tissues.
Conclusions: These results suggest that in proximal tubular cells type II Na/Pi-cotransporters internalized due to the action of PTH and acute high Pi-diet are routed to the lysosomes, and likely do not enter a recycling compartment.