Displacement curve for the inhibition of [³H]Ins(1,4,5)P₃ binding to human platelet membranes by inositol phosphates. Platelet membranes were incubated with 20 nM [³H]Ins(1,4,5)P₃ in the presence of increasing concentrations of D-Ins(1,4,5)P₃ (▲), D-Ins(2,4,5)P₃ (●), GPIP₂ (♦), and L-Ins(1,4,5)P₃ (■) at 4°C for 10 minutes. The data are the mean S.E.M. of three independent experiments, each run in duplicate. The IC50 values were calculated using the displacement curve (Table 1).