Internalization of ¹²⁵I–FGF-2 in the presence of PF-4. For internalization experiments, LEII cells (500,000 cells/dish) were incubated at 37°C with 10 ng/mL¹²⁵I–FGF-2 in the presence (○) or absence (▪) of 2 μg/mL PF-4 or 1 μg/mL unlabeled ligand. At the indicated time points, cell surface–bound and internalized ¹²⁵I–FGF-2 were determined as indicated in Materials and Methods. The figure depicts a representative experiment done in duplicates (data points as mean; SD values: 0 < SD < 0.03 ).