Endocytosis and phosphate transport in OK epithelial cells.

Endocytosis was studied in OK epithelial cells, an established cell line from opossum kidney. The presence of fluid-phase endocytosis in these cells was demonstrated by measuring cell uptake of lucifer yellow and horseradish peroxidase. The intracellular distribution of lucifer yellow fluorescence was consistent with uptake by endocytosis. Endocytosis was inhibited in medium made hyperosmolar by addition of sucrose. In hyperosmolar medium the action of parathyroid hormone on Na+/phosphate cotransport was significantly diminished. We suggest that an intact endocytic mechanism is required for the full inhibitory effect of parathyroid hormone on Na+/phosphate cotransport.