Roche J P, Treistman S N
Department of Pharmacology and Molecular Toxicology, Program in Neuroscience, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
J Neurosci. 1998 Feb 1;18(3):878-86. doi: 10.1523/JNEUROSCI.18-03-00878.1998.
We have shown previously that the Ca2+ channel beta3 subunit is capable of modulating tonic G-protein inhibition of alpha1A and alpha1B Ca2+ channels expressed in oocytes. Here we determine the modulatory effect of the Ca2+ channel beta3 subunit on M2 muscarinic receptor-activated G-protein inhibition and whether the beta3 subunit modulates the G-protein sensitivity of alpha1A and alpha1B currents equivalently. To compare the relative inhibition by muscarinic activation, we have used successive ACh applications to remove the large tonic inhibition of these channels. We show that the resulting rebound potentiation results entirely from the loss of tonic G-protein inhibition; although the currents are temporarily relieved of tonic inhibition, they are still capable of undergoing inhibition through the muscarinic pathway. Using this rebound protocol, we demonstrate that the inhibition of peak current amplitude produced by M2 receptor activation is similar for alpha1A and alpha1B calcium currents. However, the contribution of the voltage-dependent component of inhibition, characterized by reduced inhibition at very depolarized voltage steps and the relief of inhibition by depolarizing prepulses, was slightly greater for the alpha1B current than for the alpha1A current. After co-expression of the beta3 subunit, the sensitivity to M2 receptor-induced G-protein inhibition was reduced for both alpha1A and alpha1B currents; however, the reduction was significantly greater for alpha1A currents. Additionally, the difference in the voltage dependence of inhibition of alpha1A and alpha1B currents was heightened after co-expression of the Ca2+ channel beta3 subunit. Such differential modulation of sensitivity to G-protein modulation may be important for fine tuning release in neurons that contain both of these Ca2+ channels.
我们之前已经表明，Ca2+通道β3亚基能够调节卵母细胞中表达的α1A和α1B Ca2+通道的紧张性G蛋白抑制作用。在此，我们确定Ca2+通道β3亚基对M2毒蕈碱受体激活的G蛋白抑制作用的调节效应，以及β3亚基是否等效地调节α1A和α1B电流的G蛋白敏感性。为了比较毒蕈碱激活引起的相对抑制作用，我们采用连续应用乙酰胆碱来消除这些通道的大量紧张性抑制。我们发现，由此产生的反弹增强完全源于紧张性G蛋白抑制作用的丧失；尽管电流暂时摆脱了紧张性抑制，但它们仍然能够通过毒蕈碱途径受到抑制。使用这种反弹方案，我们证明M2受体激活对α1A和α1B钙电流产生的峰值电流幅度抑制作用相似。然而，以在非常去极化电压阶跃时抑制作用降低以及去极化预脉冲解除抑制作用为特征的抑制作用的电压依赖性成分，对α1B电流的贡献比对α1A电流略大。在共表达β3亚基后，α1A和α1B电流对M2受体诱导的G蛋白抑制作用的敏感性均降低；然而，α1A电流的降低更为显著。此外，在共表达Ca2+通道β3亚基后，α1A和α1B电流抑制作用的电压依赖性差异增大。这种对G蛋白调节敏感性的差异调节对于微调同时含有这两种Ca2+通道的神经元中的递质释放可能很重要。
