Dissociation kinetics of the low- and high-affinity GM-CSF receptors. COS cells expressing human low-affinity GMR and HL-60 promyelocytic cells expressing only high-affinity GMR/β were pre-equilibrated with ¹²⁵I-labeled GM-CSF. The time courses of dissociation of radioactive ligand were determined by addition of 2 μmol/L of competing nonradioactive GM-CSF, as described in the experimental procedures. Representative results obtained at 23°C and 4°C are shown. (A) Time courses of GM-CSF dissociation from low-affinity GMR and high-affinity GMR/β at 23°C. (B) Analyses of dissociation rate constants (see Eq. 4 in Materials and Methods) of the low-affinity (□) and high-affinity (▪) GM-CSF receptors shown in (A); k_off (GMR) = 4.74 × 10⁻² min⁻¹, loose complex k_off (GMR/β) = 5.02 × 10⁻²min⁻¹, and tight complex k_off (GMR/β) = 1.41 × 10⁻³ min⁻¹. (C) Time courses of GM-CSF dissociation from low-affinity GMR and high-affinity GMR/β at 4°C. (D) Analyses of dissociation rate constants (see Eq. 4 in Materials and Methods) of the low-affinity (□) and high-affinity (▪) GM-CSF receptors shown in (C); k_off(GMR) = 2.22 × 10⁻² min⁻¹, loose complex k_off (GMR/β) = 2.24 × 10⁻²min⁻¹, and tight complex k_off (GMR/β) = 4.09 × 10⁻³ min⁻¹.