Binding and dissociation of the LDV-FITC probe in response to cAMP agonists. A, LDV-FITC probe binding and dissociation on U937 cells stably transfected with CXCR4 receptor plotted as mean channel fluorescence (MCF) versus time. Prior to SDF-1 addition, cells were preincubated with forskolin (40 μM), dibutyryl-cAMP (2 mM), or DMSO (control) for 10 min at 37°C. Next, cells were preincubated wth 4 nM LDV-FITC, and treated with SDF-1 (25 nM). Mean and standard error of mean are shown (n = 3). B, C, LDV-FITC probe binding and dissociation on U937 cells stably transfected with the non-desensitizing mutant of FPR plotted as mean channel fluorescence (MCF) versus time. After sequential additions of fluorescent LDV-FITC probe (4 nM), and fMLFF (100 nM), cells were treated with B, forskolin (10 μM), C, different concentration of dibutyryl-cAMP, or DMSO (control). The MCF value corresponding to cell autofluorescence is indicated by the horizontal arrow. One representative experiment out of three experiments is shown.