Wellner-Kienitz Marie Cécile, Bender Kirsten, Meyer Thomas, Pott Lutz
Department of Physiology, Ruhr-University Bochum, D-44780 Bochum, Germany.
Biochim Biophys Acta. 2003 Sep 23;1642(1-2):67-77. doi: 10.1016/s0167-4889(03)00101-0.
The predominant histamine receptor subtype in the supraventricular and ventricular tissue of various mammalian species is the H2 receptor (H2-R) subtype, which is known to couple to stimulatory G proteins (Gs), i.e. the major effects of this autacoid are an increase in sinus rate and in force of contraction. To investigate histamine effects in H2-R-transfected rat atrial myocytes, endogenous GIRK currents and L-type Ca2+ currents were used as functional assays. In H2-R-transfected myocytes, exposure to His resulted in a reversible augmentation of L-type Ca2+ currents, consistent with the established coupling of this receptor to the Gs-cAMP-PKA signalling pathway. Mammalian K+ channels composed of GIRK (Kir3.x) subunits are directly controlled by interaction with betagamma subunits released from G proteins, which couple to seven-helix receptors. In mock-transfected atrial cardiomyocytes, activation of muscarinic K+ channels (IK(ACh)) was limited to Gi-coupled receptors (M2R, A1R). In H2-R-overexpressing cells, histamine activated IK(ACh) via Gs-derived betagamma subunits since the histamine-induced current was insensitive to pertussis toxin. These data indicate that overexpression of Gs-coupled H2-R results in a loss of target specificity due to an increased agonist-induced release of Gs-derived betagamma subunits. When IK(ACh) was maximally activated by GTP-gamma-S, histamine induced an irreversible inhibition of the inward current in a fraction of H2-R-transfected cells. This inhibition is supposed to be mediated via a G(q/11)-PLC-mediated depletion of PIP2, suggesting a partial coupling of overexpressed H2-R to G(q/11). Dual coupling of H2-Rs to Gs and Gq is demonstrated for the first time in cardiac myocytes. It represents a novel mechanism to augment positive inotropic effects by activating two different signalling pathways via one type of histamine receptor. Activation of the Gs-cAMP-PKA pathway promotes Ca2+ influx through phosphorylation of L-type Ca2+ channels. Simultaneous activation of Gq-signalling pathways might result in phosphoinositide turnover and Ca2+ release from intracellular stores, thereby augmenting H2-induced increases in [Ca2+]i.
在各种哺乳动物的心室和室上组织中，主要的组胺受体亚型是H2受体（H2-R）亚型，已知其与刺激性G蛋白（Gs）偶联，即这种自分泌物质的主要作用是窦性心率增加和收缩力增强。为了研究组胺对转染H2-R的大鼠心房肌细胞的影响，内源性GIRK电流和L型Ca2+电流被用作功能检测指标。在转染H2-R的肌细胞中，暴露于组胺会导致L型Ca2+电流可逆性增强，这与该受体与Gs-cAMP-PKA信号通路的既定偶联一致。由GIRK（Kir3.x）亚基组成的哺乳动物K+通道直接受与从G蛋白释放的βγ亚基相互作用的控制，这些G蛋白与七螺旋受体偶联。在mock转染的心房心肌细胞中，毒蕈碱K+通道（IK(ACh)）的激活仅限于与Gi偶联的受体（M2R、A1R）。在过表达H2-R的细胞中，组胺通过Gs衍生的βγ亚基激活IK(ACh)，因为组胺诱导的电流对百日咳毒素不敏感。这些数据表明，由于激动剂诱导的Gs衍生的βγ亚基释放增加，与Gs偶联的H2-R的过表达导致靶标特异性丧失。当IK(ACh)被GTP-γ-S最大程度激活时，组胺在一部分转染H2-R的细胞中诱导内向电流的不可逆抑制。这种抑制被认为是通过G(q/11)-PLC介导的PIP2消耗介导的，表明过表达的H2-R与G(q/11)存在部分偶联。H2-R与Gs和Gq的双重偶联首次在心肌细胞中得到证实。它代表了一种通过经由一种组胺受体激活两种不同信号通路来增强正性肌力作用的新机制。Gs-cAMP-PKA途径的激活通过L型Ca2+通道的磷酸化促进Ca2+内流。Gq信号通路的同时激活可能导致磷酸肌醇周转和Ca2+从细胞内储存释放，从而增强组胺诱导的细胞内Ca2+浓度（[Ca2+]i）升高。
