Effects of charge on membrane processing in the proximal nephron

Receptor-mediated endocytosis of albumin in cultured opossum kidney cells: a model for proximal tubular protein reabsorption

The accumulation of fluorescein (FITC)-labelled bovine albumin was measured against the extracellular-fluid-phase marker FITC-inulin within confluent monolayers of the opossum kidney cell line OK. Fluorescence and electron microscopic pictures show that FITC-albumin is taken up by endocytosis and appears in a vesicular intracellular distribution. The uptake of FITC-albumin was quantified by measuring the cell-adherent fluorescence fluorimetrically. FITC-albumin uptake shows a time- and concentration-dependent saturation kinetics in contrast to the non-saturable FITC-inulin uptake, and exceeds the latter more than tenfold at low concentrations. Half-maximum saturation occurs at 20-30 mg/l. Initial FITC-albumin uptake/mg protein is stimulated by cell maturation, being six-to sevenfold higher in the confluent than in the subconfluent state, while FITC-inulin uptake is unchanged. Both an elevation of ambient osmolality to 600-750 mOsm/kg and disruption of the cytoskeleton by cytochalasin B (0.1 mmol/l) reduce initial FITC-albumin uptake by 50%-60% in a non-additive fashion. Albumin endocytosis is reduced both in acidic (pH 5.4) and alkaline (pH 8.4) medium, but does not depend on extracellular sodium, calcium or chloride. High concentrations of fetal calf serum or unlabelled albumin reduce FITC-albumin endocytosis dose-dependently. The present study is the first to investigate both the protein uptake and the fluid-phase endocytosis in a cultured proximal tubular cell line, using these cells as a model system-for proximal tubular protein reabsorption.

Investigations in intracellular drug storage: localization of disobutamide in lysosomal and nonlysosomal vesicles

In the investigation of the dynamic nature of intracellular drug-induced storage disorder associated with clear cytoplasmic vacuoles (CCV), rat urinary bladder carcinoma cells (RBT CC-8) were cultured with [14C]disobutamide. Cell monolayers were then harvested and analytical cell fractionation techniques were employed to examine the association of disobutamide with the various subcellular fractions. Disobutamide distributed into two modes: as a free, organelle-independent fraction and as a light membrane-associated fraction that overlapped with markers for the plasma membrane, endoplasmic reticulum, and lysosomes. The similarity in buoyant densities of these organelles derived from RBT CC-8 cells precluded resolution of these structures by isopycnic centrifugation. In additional experiments, disobutamide was incubated in vitro with a suspension of isolated rabbit renal proximal tubules. In these cells, analytic fractionation showed that the drug localized predominantly to lysosomes and to a separate light membrane fraction that was clearly resolved from the markers for the endoplasmic reticulum, brush border, mitochondria, and peroxisomes; this fraction overlapped with the most buoyant aspects of the Golgi apparatus and basolateral plasma membrane. The buoyant density of this disobutamide-associated nonlysosomal fraction was 1.11 g/ml. Electron microscopy of the disobutamide-exposed tubules showed a substantial formation of apical vesicles, especially small, smooth-surfaced vesicles, typical of the endocytic apparatus. From these findings and based on the physicochemical properties of the cationic moiety of disobutamide, we conclude that the drug localizes in lysosomal and nonlysosomal acidic vesicles.