Uptake and intracellular distribution of ferritin in the rat distal convoluted tubule.

The purpose of this study was to determine the uptake and intracellular distribution of anionic ferritin (AF) and cationic ferritin (CF) in the distal convoluted tubule (DCT) of the rat kidney. Male Sprague-Dawley rats were prepared for micropuncture, and individual distal tubules were perfused with 5 or 10 mg/ml of AF or 0.1 or 0.5 mg/ml of CF in isotonic saline for either 30 sec or 3 min. The tubules were fixed by perfusion with 6.25% glutaraldehyde either immediately or at different time intervals after exposure to ferritin. Electron microscopy of tubules fixed immediately after perfusion showed no binding of AF to the apical cell membrane, and only traces of AF were observed in small apical structures. In contrast, CF was extensively bound to the apical cell membrane and located in apical vesicles and tubules, and in multivesicular bodies. Occasionally, CF was observed in Golgi vesicles and cisternae. Sixty min after perfusion with ferritin, traces of AF were present in multivesicular bodies and lysosome-like structures. Thirty and 60 min after perfusion, large concentrations of CF were located in multivesicular bodies and lysosome-like bodies. This study reveals that in the DCT, CF is bound to the apical cell membrane and taken up into the tubule cells, whereas only traces of AF are taken up, indicating that the charge of a protein molecule may determine whether or not the protein is reabsorbed by the DCT. The demonstration of CF in the Golgi complex is compatible with the existence of membrane recycling in cells of the DCT.