Macromolecules which are filtered in the renal glomeruli in general are taken up by adsorptive endocytosis by the renal proximal tubule cells. This process and the following transport into the lysosomes has been studied by histochemistry and at the electron microscope level for a variety of proteins. It has been generally believed that the uptake was characterized by a low-affinity high-capacity system and different studies have shown a certain specificity in the reabsorption. In this study we demonstrate 2 different specific receptors for endocytosis of protein to be located in the endocytic invaginations of rat renal proximal tubule cells (1,2), and in addition a similar localization of a receptor for endocytosis of folate, a low molecular weight vitamin B (3).
For immunohisto-and cytochemistry, the kidneys of male Wistar rats were fixed by retrograde perfusion with either 8% formaldehyde in 0.1 M sodium cacodylate or with 2% formaldehyde and 0.1% glutaraldehyde in the same buffer. The tissue was then infiltrated in sucrose, quickly frozen and prepared for cryosectioning. Immunostaining for light microscopy was performed with secondary antibodies conjugated either with horseradish peroxidase or with FITC and for electronmicroscopy with protein A-gold or secondary antibodies conjugated to colloidal gold particles. For autoradiographic studies, after i.v. infusion or microinfusion of radioactive labeled tracers the kidneys were either fixed as above but with 1% glutaraldehyde or the tubules were fixed by micropuncture with 1% glutaraldehyde. The tissue was then prepared for light or electronmicroscope autoradiography.
Immuno-and ligand blotting was performed using homogenized rat renal cortical tissue supernates applied to sodium dodecyl sulfonate gels.