Comparison of the association kinetics of GM-CSF binding with the low- and high-affinity GM-CSF receptors. Time courses of specific ¹²⁵I-labeled GM-CSF binding to the COS cells expressing human low-affinity GMR and HL-60 promyelocytic cells expressing only human high-affinity GMR/β were measured as described in Materials and Methods. Results of representative experiments performed at 23°C and 4°C are shown. (A) GM-CSF association with low-affinity GMR at 23°C. (B) GM-CSF association with high-affinity GMR/β at 23°C. (C) Analyses of association rate constants (see Eq. 1 in Materials and Methods) for low-affinity (□) and high-affinity (▪) GM-CSF receptors shown in (A) and (B); k_on (GMR) = 1.48 × 10⁻⁶pmol/L⁻¹ min⁻¹ and k_on(GMR/β) = 1.41 × 10⁻⁴ pmol/L⁻¹min⁻¹. (D) GM-CSF association with low-affinity GMR at 4°C. (E) GM-CSF association with high-affinity GMR/β at 4°C. (F) Analyses of association rate constants (see Eq. 1 in Materials and Methods) for low-affinity (□) and high-affinity (▪) GM-CSF receptors shown in (D) and (E); k_on (GMR) = 8.44 × 10⁻⁷ pmol/L⁻¹ min⁻¹and k_on (GMR/β) = 8.89 × 10⁻⁵pmol/L⁻¹ min⁻¹.