Breitwieser G E, Szabo G
Nature. 1985;317(6037):538-40. doi: 10.1038/317538a0.
Guanine nucleotide binding proteins, interchangeably called N or G proteins, seem to be the primary signal-transducing components of various agonist-induced cell membrane functions. In the heart, G proteins have been implicated in beta-adrenergic modulation of the slow inward Ca2+ current. We have investigated the role of G proteins in muscarinic activation of an inwardly rectifying, acetylcholine (ACh)-induced K+ current (IACh), and beta-adrenergic activation of an (isoprenaline)-induced Ca2+ current (Isi). Here we report that intracellular application of the non-hydrolysable GTP analogue 5'-guanylylimidodiphosphate (GppNHp) brought about an agonist-induced, antagonist-resistant, persistent activation of IACh and Isi. This functional uncoupling of channel from receptor suggests that the muscarinic receptor and the IACh channel are separate molecular structures. Membrane conductance responses to sequential activation of muscarinic and beta-adrenergic receptors demonstrate that in contrast to the muscarinic inhibition of Isi, muscarinic stimulation of IACh is mediated by a G protein via a pathway that does not involve adenylate cyclase. Taken together, the results support the notion that agonist is required to induce GppNHp binding and/or activation of the G proteins. Once triggered by agonist, the control system remains maximally activated, thereby transforming the cell so that it no longer responds to subsequent homologous receptor-mediated signals.
鸟嘌呤核苷酸结合蛋白，也被称为N蛋白或G蛋白，似乎是各种激动剂诱导的细胞膜功能的主要信号转导成分。在心脏中，G蛋白参与了β-肾上腺素能对缓慢内向Ca2+电流的调节。我们研究了G蛋白在毒蕈碱激活内向整流性乙酰胆碱（ACh）诱导的钾电流（IACh）以及β-肾上腺素能激活异丙肾上腺素诱导的钙电流（Isi）中的作用。在此我们报告，细胞内应用不可水解的GTP类似物5'-鸟苷酰亚胺二磷酸（GppNHp）可导致激动剂诱导的、拮抗剂抵抗的IACh和Isi的持续激活。通道与受体的这种功能解偶联表明毒蕈碱受体和IACh通道是不同的分子结构。对毒蕈碱和β-肾上腺素能受体顺序激活的膜电导反应表明，与毒蕈碱对Isi的抑制作用相反，毒蕈碱对IACh的刺激是由G蛋白通过一条不涉及腺苷酸环化酶的途径介导的。综上所述，这些结果支持了激动剂需要诱导GppNHp与G蛋白结合和/或激活G蛋白这一观点。一旦被激动剂触发，控制系统就会保持最大程度的激活，从而使细胞发生转变，使其不再对随后的同源受体介导的信号作出反应。
