INHIBITION OF ADENYLYL-CYCLASE ACTIVITY BY A HOMOGENEOUS POPULATION OF DOPAMINE-RECEPTORS - SELECTIVE BLOCKADE BY ANTISERA DIRECTED AGAINSTG(I1) AND OR G(I2)/

The 7315c pituitary tumor cell expresses a homogeneous population of d opamine receptors that are functionally similar to brain dopamine D-2 receptors. [H-3]-Sulpiride binding to 7315c cell homogenates was speci fic and saturable, and K-I Values for compounds to compete for these s ites were highly correlated with values for the same compounds at D-2 receptors in brain. Dopamine maximally inhibited similar to 65% of for skolin-stimulated cyclase activity in cell membranes. Some D-2 agonist s had lower efficacies, suggesting that some compounds are partial ago nists at this receptor. Removal of GTP from the assay buffer or pretre atment of the tissue with pertussis toxin abolished the Inhibition of adenylyl cyclase by dopamine. Immunodetection of most of the known G a lpha subunits revealed that G(i1), G(i2), G(i3), G(o) G(q) and G(s) ar e present in the 7315c membrane. Pretreatment with the AS antibody (wh ich recognizes the C-terminal regions of G alpha(i1) and G alpha(I2)) significantly attenuated the inhibition of adenylyl cyclase activity b y dopamine, whereas antibodies to C-terminal regions of the other G al pha subunits had no effect. These findings suggest that the dopamine D -2 receptor regulates cyclase inhibition predominantly via G(i1) and/o r G(i2) and that the 7315c tumor cells provide a useful model for stud ying naturally expressed dopamine D-2 receptors in the absence of othe r dopamine receptor subtypes.