Binding of conjugated and unconjugated SP to the rSPR. Competition binding of rSPR transfected CHO cells was performed with 50 pM [125I] SP and increasing concentrations (1 pM, 100 pM, 1 nM, 100 nM and 1 μM) of each fluorescent SP analog. The calculated IC50 values for BODIPY Fl-SP, fluorescein-SP, Oregon Green 488-SP, tetramethylrhodamine-SP and unconjugated SP are 18.0 nM, 44.5 nM, 6.39 nM, 4.20 nM and 1.98 nM, respectively. Note that Alexa 488-SP was unable to displace the radioisotope. Each data point represents an n of at least 4 from 2 or 3 separate experiments.