E-selectin is rapidly internalized in clathrin-coated pits of transfected CHO cells and cytokine-stimulated HUVECs. (A,B) The internalization rate of wild-type or tail-less E-selectin in transfected CHO cells was measured in isotonic medium or in hypertonic medium, which disrupts clathrin-coated pits. (C,D) The internalization rate of E-selectin in IL-1β–stimulated HUVECs was measured in isotonic or hypertonic medium. The internalization rate was measured by the ability of acidic buffer to remove surface-bound ¹²⁵I-labeled anti-E-selectin mAb (A-C) or by the ability of sodium 2-mercaptoenthanesulfonate to remove biotin from cell surface E-selectin (B-D), after warming cells to 37°C for the indicated intervals. The data represent the mean plus or minus SEM from 3 to 16 experiments for each experimental condition.