(A) Internalization of ¹²⁵I-Tf by cells. Cells were incubated with ¹²⁵I-Tf at 4°C to allow for ligand binding to cell surface TfRs. Cells were washed to remove unbound ¹²⁵I-Tf and incubated at 37°C to allow for internalization of ¹²⁵I-Tf. At the specified times, aliquots of cells were removed and centrifuged through an acidic buffer to determine the fraction of ¹²⁵I-Tf internalized. Data shown are representative of three separate experiments. (•) S; (00) R1; (▵) R2 cells. (B) Release of ¹²⁵I-Tf from cells. Cells were allowed to incorporate ¹²⁵I-Tf at 37°C, washed to remove unincorporated radioactivity, and then incubated in serum-free medium containing 100 μg/mL Tf-Fe (nonradioactive). The amount of ¹²⁵I-Tf released by cells at the specified times was determined as described the text. (•) S; (00) R1 cells. Data represent means ± SE (n = 3). Differences between S and R1 cells after the 5-minute time point are significant (P < .004).