⁵⁹Fe release after ⁵⁹Fe-Tf uptake. S, R1, and R2 cells were allowed to incorporate⁵⁹Fe-Tf over 1 or 3 hours and then washed and reincubated in fresh medium. At the specified times, aliquots of cell suspension were harvested and the ⁵⁹Fe in the medium and cells was counted to determine the percent of ⁵⁹Fe released from cells to the medium. (A) ⁵⁹Fe released by cells after a 1-hour uptake. Insert figure shows the amount of ⁵⁹Fe (pmole Fe/10⁶ cells) taken up by cells over the 1-hour incubation before release. Data represent means ± SE (n = 3). (B) Experimental conditions were similar to (A) except that⁵⁹Fe release was examined after a 3-hour uptake of⁵⁹Fe-Tf and the percent ⁵⁹Fe released was measured over 20 hours. Data represent means ± SE (n = 3). Insert figure shows the amount of ⁵⁹Fe (pmole Fe/10⁶cells) taken up by cells over the 3-hour incubation before release.