Jarvis W D, Judd A M, MacLeod R M
Department of Internal Medicine, University of Virginia School of Medicine, Charlottesville 22908.
Endocrinology. 1988 Dec;123(6):2793-9. doi: 10.1210/endo-123-6-2793.
We have examined the influences of dopamine and the D2 receptor agonist bromocriptine on phosphoinositide metabolism in primary cultures of rat anterior pituitary cells, monitoring changes in the levels of phosphatidylinositol (PtdIns), phosphatidylinositol-4-phosphate [PtdIns(4)P], and phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2]. Basal incorporation of [3H]inositol ([3H]Ins) into phosphoinositides was progressive, and radioisotopic equilibrium was attained in all three species within 48 h. The inclusion of dopamine or bromocriptine in the incubation medium promoted concentration-dependent reductions in the rate, but not the magnitude, of phosphoinositide radiolabeling. The onset of this effect was rapid; inhibition of [3H]Ins incorporation by dopamine (500 nM) and bromocriptine (100 nM) could be detected within 2 h. This treatment also produced a comparable reduction in the incorporation of [32P]orthophosphate into PtdIns(4,5)P2. In extended time-course studies, bromocriptine dramatically retarded the radiolabeling of PtdIns(4)P and PtdIns(4,5)P2, and apparent equilibria in these species were attained only after 96 h. We also assessed the ability of dopamine to modify the concentration-response characteristics of [3H]Ins-labeled inositol phosphate ([3H]InsPx) production by TRH, angiotensin II (AII), neurotensin (NTS), bombesin (BBS), and vasoactive intestinal polypeptide (VIP). Neither dopamine nor bromocriptine altered the rate or magnitude of TRH-, AII-, NTS-, or BBS-related InsPx generation. VIP was completely ineffective in stimulating InsPx generation. PRL release was significantly reduced in all dopamine-treated groups. That the InsPx concentration-response relationships for each of these peptides remained unimpaired by exposure to dopamine or bromocriptine extends our previous observation that the phosphoinositide-specific phospholipase-C is insensitive to dopaminergic tone. Consistent with our earlier findings, these data indicate that activation of the D2 dopamine receptor attenuates the activity of mechanisms associated with the serial phosphorylations of PtdIns and PtdIns(4)P, reactions that give rise to PtdIns(4)P and PtdIns(4,5)P2, respectively. It is our conclusion that dopamine, in addition to its other actions, attenuates the phosphorylation, rather than the hydrolysis, of anterior pituitary phosphoinositide. This attenuation appears to be mediated by an inhibitory coupling of the D2 receptor with the phosphoryltransferase activities that catalyze PtdIns(4)P and PtdIns(4,5)P2 formation.(ABSTRACT TRUNCATED AT 400 WORDS)
我们研究了多巴胺和D2受体激动剂溴隐亭对大鼠垂体前叶细胞原代培养物中磷酸肌醇代谢的影响，监测磷脂酰肌醇（PtdIns）、磷脂酰肌醇-4-磷酸[PtdIns(4)P]和磷脂酰肌醇-4,5-二磷酸[PtdIns(4,5)P2]水平的变化。[3H]肌醇（[3H]Ins）向磷酸肌醇的基础掺入是渐进性的，并且在48小时内所有三种物质都达到了放射性同位素平衡。在孵育培养基中加入多巴胺或溴隐亭会促进磷酸肌醇放射性标记速率的浓度依赖性降低，但不影响其幅度。这种作用起效迅速；多巴胺（500 nM）和溴隐亭（100 nM）对[3H]Ins掺入的抑制在2小时内即可检测到。这种处理还使[32P]正磷酸盐掺入PtdIns(4,5)P2的量产生了类似程度的减少。在延长的时间进程研究中，溴隐亭显著延迟了PtdIns(4)P和PtdIns(4,5)P2的放射性标记，并且这些物质的明显平衡仅在96小时后才达到。我们还评估了多巴胺改变促甲状腺激素释放激素（TRH）、血管紧张素II（AII）、神经降压素（NTS）、蛙皮素（BBS）和血管活性肠肽（VIP）诱导的[3H]Ins标记的肌醇磷酸（[3H]InsPx）产生的浓度-反应特性（的能力）。多巴胺和溴隐亭均未改变TRH、AII、NTS或BBS相关的InsPx生成速率或幅度。VIP在刺激InsPx生成方面完全无效。所有多巴胺处理组的催乳素释放均显著降低。这些肽各自的InsPx浓度-反应关系在暴露于多巴胺或溴隐亭后仍未受损，这扩展了我们之前的观察结果——磷酸肌醇特异性磷脂酶C对多巴胺能张力不敏感。与我们早期的发现一致，这些数据表明D2多巴胺受体的激活减弱了与PtdIns和PtdIns(4)P的系列磷酸化相关机制的活性，这些反应分别产生PtdIns(4)P和PtdIns(4,5)P2。我们的结论是，多巴胺除了其其他作用外，还减弱了垂体前叶磷酸肌醇的磷酸化而非水解。这种减弱似乎是由D2受体与催化PtdIns(4)P和PtdIns(4,5)P2形成的磷酸转移酶活性的抑制性偶联介导的。（摘要截短至400字）
