Vallar L, Muca C, Magni M, Albert P, Bunzow J, Meldolesi J, Civelli O
Department of Pharmacology, University of Milano, Italy.
J Biol Chem. 1990 Jun 25;265(18):10320-6.
Dopaminergic D2 receptors are widely regarded as typical inhibitory receptors, as they both inhibit adenylyl cyclase and decrease the cytosolic free Ca2+ concentration ([Ca2+]i) by activating K+ channels. A D2 receptor has recently been cloned (Bunzow, J. R., Van Tol, H. H. M., Grandy, D. K., Albert, P., Salon, J., Christie, M. D., Machida, C. A., Neve, K. A., and Civelli, O. (1988) Nature 336, 783-787) and expressed in two different cell lines, pituitary GH4C1 cells and Ltk- fibroblasts, where it has been shown to induce inhibition of adenylyl cyclase. We have investigated the additional effector systems coupled to this receptor. The responses observed in the two cells lines, which express similar levels of receptors (0.5-1 x 10(5)/cell), were surprisingly different. In GH4C1 cells D2 receptors failed to affect phosphoinositide hydrolysis and induced a decrease of [Ca2+]i. This latter effect appears to be mediated by hyperpolarization, most likely due to the activation of K+ channels. In striking contrast, in Ltk- fibroblasts the D2 receptor induced a rapid stimulation of inositol(1,4,5)-trisphosphate (+73% at 15 s) followed by the other inositol phosphates, and an immediate increase of [Ca2+]i due to both Ca2+ mobilization from internal stores and influx from the extracellular medium. In both GH4C1 and Ltk- cells, the D2 receptor response was mediated by G protein(s) sensitive to pertussis toxin. The increases of inositol trisphosphate and [Ca2+]i observed in Ltk- cells required dopamine concentrations only slightly higher than those inhibiting adenylyl cyclase (EG50 = 25, 29, and 11 nM, respectively) and were comparable in magnitude to the responses induced by the endogenous stimulatory receptor agonists, thrombin and ATP. The results demonstrate that in certain cells D2 receptors are efficiently coupled to the stimulation of phosphoinositide hydrolysis. The nature of receptor responses appears therefore to depend on the specific properties not only of the receptor molecule but also of the cell type in which it is expressed.
多巴胺能D2受体被广泛认为是典型的抑制性受体，因为它们既能抑制腺苷酸环化酶，又能通过激活钾通道降低胞质游离钙离子浓度（[Ca2+]i）。最近已克隆出一种D2受体（Bunzow, J. R., Van Tol, H. H. M., Grandy, D. K., Albert, P., Salon, J., Christie, M. D., Machida, C. A., Neve, K. A., and Civelli, O. (1988)《自然》336, 783 - 787），并在两种不同的细胞系，即垂体GH4C1细胞和Ltk-成纤维细胞中进行了表达，在这些细胞中已证明它能诱导对腺苷酸环化酶的抑制作用。我们研究了与该受体偶联的其他效应系统。在表达相似受体水平（0.5 - 1×10(5)/细胞）的这两种细胞系中观察到的反应惊人地不同。在GH4C1细胞中，D2受体未能影响磷酸肌醇水解，并导致[Ca2+]i降低。后一种效应似乎是由超极化介导的，很可能是由于钾通道的激活。与之形成鲜明对比的是，在Ltk-成纤维细胞中，D2受体诱导肌醇(1,4,5)-三磷酸迅速增加（15秒时增加73%），随后其他肌醇磷酸也增加，并且由于细胞内储存钙离子的释放和细胞外介质中钙离子的内流，[Ca2+]i立即升高。在GH4C1和Ltk-细胞中，D2受体反应均由对百日咳毒素敏感的G蛋白介导。在Ltk-细胞中观察到的肌醇三磷酸和[Ca2+]i的增加所需的多巴胺浓度仅略高于抑制腺苷酸环化酶的浓度（EG50分别为25、29和11 nM），并且在幅度上与内源性刺激受体激动剂凝血酶和ATP诱导的反应相当。结果表明，在某些细胞中，D2受体能有效地偶联到磷酸肌醇水解的刺激作用。因此，受体反应的性质似乎不仅取决于受体分子的特定特性，还取决于其表达所在的细胞类型的特定特性。
