Characterization of the dopaminergic regulation of human prolactin-secreting cells cultured on extracellular matri

Human PRL-secreting adenoma cells were cultured on an extracellular matrix (ECM) secreted by bovine corneal endothelial cells, and their sensitivity to dopamine and dopaminergic agonists was examined. Cells plated on ECM attach rapidly, adopt a flattened morphology, and maintain stable PRL secretion for several weeks. Dopamine inhibited PRL secretion from tumor and nontumor cells on ECM in a dosedependent fashion, with IC50 values of 243 ± 29 and 76 ± 3.3 nM, respectively, and a maximal inhibition of 80% at 10^-5 M. Apomorphine (APO), dihydroergocryptine (DHE), and bromocriptine (CB-154) also inhibited PRL secretion from adenoma cells in a dose-dependent fashion, with IC₅₀ values of 24 ± 13, 0.35 ± 0.1, and 0.05 ± 0.01 nM, respectively. Thus, the rank order of potency of the dopamine agonists in inhibiting PRL secretion from adenoma cells was consistent with an interaction at a dopamine receptor: CB-154 > DHE > APO > dopamine. The dopamine antagonists, D-butaclamol, spiperone, domperidone, chlorpromazine, and Z-butaclamol, reversed the inhibitory effect of DHE on PRL secretion with IC₅₀ values of 0.8 nM, 0.9 nM, 1.3 nM, 80 nM, and 30 μM, respectively. The inhibition of PRL secretion was stereoselective, since d-butaclamol was approximately 2000 times more potent than the inactive i-isomer in reversing the effect of DHE. Pieces of adenomas incubated with dopamine before dispersion responded similarly to the dispersed cells plated on ECM, indicating that the ECM did not appear to alter the dopamine-mediated inhibition of PRL. In summary, human PRL-secreting adenoma cells cultured on ECM respond to dopamine in a dose-dependent fashion, but with an IC₅₀ slightly greater than that observed with normal mammotrophs. The rank order of potency of the dopamine agonists to inhibit PRL secretion (CB-154 > DHE > APO > dopamine) and the rank order of potency of the dopamine antagonists to reverse the inhibitory effect of DHE (d-butaclamol ≃ spiperone ≥ domperidone > chlorpromazine > l-butaclamol) are consistent with an interaction at a dopamine receptor and correlate with the ability of these agents to displace [³H]DHE in membrane-binding preparations. These data would argue that human PRLsecreting adenoma cells possess a dopamine receptor, both of high affinity and stereoselective, which mediates the inhibition of PRL secretion.