The CXCR2 ligand KC desensitizes marrow neutrophil response to SDF-1. The effects of pretreatment with KC on neutrophil calcium response to SDF-1α were examined by using murine neutrophils loaded with Indo-1, as described, that were exposed to 10 nM SDF-1 with or without 10 nM KC pretreatment. (A) Untreated cells exposed to SDF-1 alone (blue line) show a robust calcium transient with a prolonged plateau phase because of extracellular calcium influx. Pretreatment with KC (red line) 5 minutes before exposure to SDF1 results in an SDF-1 response lacking the secondary plateau phase. (B) Response of cells exposed to 0.5 nM SDF-1 for 45 minutes at 37°C before analysis. Cells subsequently stimulated with 10 nM SDF-1 alone (blue line) show an attenuated response (compare with blue line in panel A), but, if treated with 10 nM KC 5 minutes before (red line), they show a decrement in the peak calcium transient. Note that exposure to 0.5 nM SDF-1 for 45 minutes had no effect on the response to KC, but the scale of the y-axis was changed to better depict the changes in response to SDF-1.