Comparison of the forms of the dopamine D2 receptor expressed in GH4C1 cells

The effects of dopamine (DA) on prolactin (PRL) secretion, phosphoinositide metabolism, cytosolic calcium concentrations ([Ca2+]i), and cAMP production in GH4C1 cells expressed either the short (GH4ZR7 cells) or long (GH4I12 cells) form of the rat DA D2 receptor were compared in this study. The GH4C1 cell line is derived from the rat anterior pituitary gland and lacks functional DA receptors. The GH4ZR7 and GH4I12 cell lines have been transfected with either the short or the long form of the D2 receptor, respectively. In this study, the functional coupling of these receptors to both basal and stimulated PRL secretion and to common second messenger systems was examined. Both cell types expressing DA receptors exhibited similar saturable binding to the D2 antagonist [3H]spiperone (Kd GH4ZR7 = 96 +/- 8 pM, Kd GH4I12 = 107 +/- 49 pM). In GH4ZR7 cells, 1 and 10 microM DA inhibited basal PRL secretion by 37% and 58%, respectively. In GH4I12 cells, 1 and 10 microM DA inhibited basal PRL secretion by 63% and 54%, respectively. In GH4ZR7 cells, 10 microM DA completely reversed the stimulatory effects of 1-100 nM thyrotropin-releasing hormone (TRH), and 1 microM DA attenuated the stimulatory effects of 10 and 100 nM TRH. Interestingly, GH4I12 cells were not responsive to TRH. In both cell lines, the inhibitory effects of DA were blocked by the specific D2 antagonist, eticlopride. The stimulatory effects of TRH on [Ca2+]i were dose dependent and could be blocked (at least in GH4ZR7 cells) by prior treatment of the cells with 1 microM DA. The ability of dopamine to block the TRH-mediated increase in [Ca2+]i was attenuated by eticlopride. DA (1 microM) had no effect on resting [Ca2+]i in either cell line expressing DA receptor. TRH (100 nM) maximally stimulated total inositol phosphate (IP) accumulation to values approximately three times greater than controls in GH4C1 and GH4ZR7 cells only. DA had no effect on basal or TRH-stimulated IP accumulation in any of the cell lines. DA (1 and 10 microM) inhibited cAMP production by 40% and 39%, respectively, in GH4ZR7 cells. Similarly, in GH4I12 cells, DA (1 and 10 microM) inhibited cAMP production by 48% and 60%, respectively. These data indicate that both the long and short forms of the D2 receptors play similar roles when expressed in GH4C1 cells. However, maximal inhibition of PRL in these cell lines is approximately 20% less than that of normal lactotrophs.