CXCR7 does not trigger calcium mobilization in response to CXCL12 and modulates that of CXCR4. (A) Representative Ca²⁺ responses triggered by 1 μM CXCL12 in HEK-293T cells transfected with a G_α16-apoaequorin encoding vector and either CXCR4 or CXCR7. (B) Calcium mobilization assay in CHO-K1-CXCR4 cells expressing or not either CXCR7-GFP or CCR5-GFP (left panel). Cells were loaded with Coelenterazine h, and [Ca²⁺]_i levels were measured after exposure to the indicated CXCL12 concentrations. Values (mean ± SEM, n = 3) are expressed as percentage of the maximal response. CXCR7 and CCR5 cell surface expression levels were determined by fluorescence intensity measurements (right panel). EC₅₀ values for CXCL12 were significantly different in the presence of CXCR7 (CXCR4/CXCR7⁺ or CXCR4/CXCR7⁺⁺ cells) compared with cells expressing CXCR4 alone (P < .05).