Abstract
The reabsorption of horseradish peroxidase (HRP) by the proximal tubule cells of rat kidneys was investigated by measuring the concentration of HRP in total particulate fractions of the cortex 1/4 and 1 hr after intravenous injection, and by correlated cytochemical observations. When compared to the corresponding values of the control animals, the concentration of HRP 1 hr after injection was decreased approximately 10-fold in the renal cortex of rats which had received an intravenous injection of hypertonic saline or two subcutaneous injections of mannitol. The plasma clearance and the urinary excretion of HRP were not altered significantly after injection of hypertonic saline, but the plasma clearance was decreased and the urinary excretion increased after injection of mannitol. When the dose of injected HRP was varied, the reabsorption of HRP by the renal cortex was proportional to the dose in the experimental and the control animals. Cytochemical staining for peroxidase activity also showed that the phagosomes and phagolysosomes of the proximal tubule cells contained much less peroxidase in the experimental rats than in the control rats. After injection of mannitol, large vacuoles appeared in the proximal tubule cells. The vacuoles often contained peroxidase-positive granules (phagosomes) which varied in diameter from the limit of microscopic visibility up to several microns. Most of the vacuoles did not react for acid phosphatase activity, but lysosomes were often aggregated around the vacuoles and seemed to release acid phosphatase into the cytoplasm. Certain analogies between the reabsorption of protein and that of water by the proximal tubule cells are discussed.