Distinct roles of Gi/o protein subunits in signaling by dopamine D2S receptors in rat pituitary adenoma cells.

In GH4ZR7 lactotroph cells dopamine-D2S receptor activation inhibited forskolin-induced cAMP production, BayK8644-activated calcium influx, basal B-raf kinase and blocked TRH-mediated c-Raf activity, phosphorylation of MEK 1/2 and p42/44-MAPK. These actions were blocked by pretreatment with PTX, indicating mediation by Gi/o proteins. Following PTX treatment, D2S receptor signaling was rescued in cells stably transfected with individual PTX-insensitive Galpha mutants. Inhibition of adenylyl cyclase (AC) was partly rescued by Galphai2 or Galphai3. By contrast, Galphao alone completely reconstituted D2S-mediated inhibition of L-type calcium channels. Galphao and Galphai3 mediated D2S inhibition of p42/44-MAPK. In cells transfected with GRK-carboxylterminal domain to inhibit Gbetagamma signaling, only D2S-mediated inhibition of calcium influx was blocked, but not inhibition of adenylyl cyclase or MAPK. These results indicate that the dopamine-D2S receptor couples to distinct Gi/o proteins depending on the pathway addressed, and suggest a novel Galphai3/Galphao-dependent inhibition of MAPK that is independent of Gbetagamma signaling