Effects of GTP on 3H-domperidone binding and its displacement by dopamine in rat striatal homogenates

The potency of dopamine in displacing 3H-domperidone from rat striatal membranes was reduced by GTP and enhanced by Mg++. GTP also increased specific binding, and this effect was larger in the presence of Mg++, which itself inhibited binding. In differently washed and incubated preparations the magnitude of both GTP and Mg++ effects correlated well with the endogenous dopamine content of the membranes. Na+ reduced the potency of dopamine, the GTP effect and specific binding. Saturation experiments revealed that these effects of ions and GTP reflected changes in both the apparent Bmax and KD of 3H-domperidone. It was possible to eliminate the GTP effect on specific binding by extensive washing and incubation, but this treatment may also alter other receptor binding characteristics. It remains unclear whether the enhancing effect of GTP reflects contamination of endogenous dopamine or a genuine property of the D2 receptor.