Sodium ions modulate the binding of sulpride to wild-type dopamine D₂ and D₃ receptors but not to mutant dopamine D₂^D80A and D₃^D75A receptors.

Competition analyses of the inhibition of [³H]spiperone binding to membranes prepared from U-2 OS cells expressing (A) wild-type dopamine D₂ receptors (filled symbols)/dopamine D₂^D80A mutant receptors (open symbols) or (B) wild-type dopamine D₃ receptors (filled symbols)/dopamine D₃^D75A mutant receptors (open symbols), by sulpiride, were performed in the presence of 100 mM NaCl (■/□) or 100 mM NMDG (▲/Δ). Data are mean ± SEM of at least 3 independent experiments.