The effect of varying 3H-shiperone concentration on its binding parameters

Higher concentrations of spiperone were required to compete against the binding of ³H-spiperone to calf caudate when higher concentrations of ³H-spiperone were used. The Cheng-Prusoff equation did not adequately apply to this hydrophobic ligand, since the K_i values varied directly with the ³H-spiperone concentration. The equation was only adequate and gave a constant K_i when the total concentrations of ³H-spiperone were replaced by the measured free concentrations. It is suggested that IC₅₀ values are best reported as such, unless the free concentrations are actually determined directly.