Internalization and degradation kinetics of the cell surface EpoRs.(A) Degradation kinetics of the cell surface EpoRs in resting (▪) and Epo-stimulated cells (▴). Resting UT-7 cells were incubated with 500 μM cycloheximide (CHX) for the indicated times, and cell surface EpoRs were quantified by a 10-minute incubation with 1 nM 125I-Epo. Quantification of cell surface EpoRs during Epo stimulation was performed by preincubating resting UT-7 cells for 15 minutes with cycloheximide before Epo stimulation. At the indicated times, cells were sampled for the determination of cell surface–associated radioactivity as described in “Materials and methods.” (B) Kinetics of 125I-Epo internalization. Resting UT-7 cells were preincubated for 15 minutes with 100 μM monensin (closed symbols) or with solvent alone (open symbols) before stimulation with 125I-Epo. At the indicated times, cells were sampled for the determination of cell surface–associated radioactivity (squares, thin lines) and internalized radioactivity (triangles, dashed lines). Total cell-associated radioactivity is also presented on the graph (circles, thick lines).