Subcellular localization of renal kallikrein by ultrastructural immunocytochemistry

The subcellular distribution of immunoreactive kallikrein was described in the rat nephron using ultrastructural immunocytochemistry. The renal tissue was fixed with a mixture of buffered picric acid-paraformaldehyde-glutaraldehyde and immunostained with the peroxidase-antiperoxidase method for the electron microscope with the following steps: antikallikrein antiserum, anti-IgG serum, peroxidase-antiperoxidase complex, 3-3' diaminobenzidine-H2O2, and post-staining with osmium tetroxide. Preabsorption of the primary antiserum with purified rat urinary kallikrein and substitution with normal serum were used as controls. As we have described previously, kallikrein was present exclusively in the connecting tubule cell of the distal nephron. Subcellularly, kallikrein was distributed in luminal membranes, basal membranes, rough endoplasmic reticulum, Golgi apparatus, and vesicles. The immunoreactive vesicles were present in the proximity of the Golgi apparatus and in the cytoplasm in the way between the Golgi and the luminal and basal plasma membranes. No immunostaining was observed in other subcellular components of the connecting tubule cell or in the other type of cell. With the description of kallikrein in subcellular organelles involved in the synthesis, processing, and transport of glycoproteins, we have advanced an hypothetical intracellular processing pathway for renal kallikrein.