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.
D000818:Animals; D001058:Apomorphine / Q000494:pharmacology; D001667:Binding, Competitive; D003342:Corpus Striatum / Q000187:drug effects / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism / Q000502:physiology*; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006220:Haloperidol / Q000494:pharmacology; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D011869:Radioligand Assay / Q000379:methods; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000187:drug effects*; D013134:Spiperone / Q000378:metabolism
D011954:Receptors, Dopamine; D013134:Spiperone; D006160:Guanosine Triphosphate; D008274:Magnesium; D006220:Haloperidol; D001058:Apomorphine; D004298:Dopamine