Role of assay conditions in determining agonist potency at D2 dopamine receptors in striatal homogenates.

The affinity of dopamine agonists for rat striatal D2 receptors labelled by [3H]spiperone was enhanced by 5 and 10 mM magnesium ions. Preparation and assay of membranes with 10 mM magnesium ions yielded displacement data better described by a model where agonists competed for [3H]spiperone at two sites. The high affinity component (Kd 0.2-10 nM) accounted for 80% of the total sites and 100 microM guanosine-5'-triphosphate shifted 25-30% of the sites to a low affinity form, independent of the agonist. These data are consistent with the D2 receptor being coupled to a guanine nucleotide regulatory protein, with the proportion of high affinity agonist sites being influenced by membrane preparation procedures, magnesium ions and guanosine-5'-triphosphate.