Yatani A, Brown A M
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, Texas 77030.
Am J Physiol. 1990 Jun;258(6 Pt 2):H1947-51. doi: 10.1152/ajpheart.1990.258.6.H1947.
We have proposed that voltage-gated ionic channels are regulated by receptor-guanine-nucleotide binding (G) protein pathways that are membrane delimited (direct) or cytoplasmic (indirect). Since indirect pathways are known to couple muscarinic and beta-adrenergic receptors to hyperpolarization-activated current (If) channels, we tested our hypothesis that direct pathways might also be present by measuring If currents in rabbit sinoatrial node cells using the gigaseal patch clamp method. Whole cell If currents were increased by the beta-agonist isoproterenol (Iso) and decreased by the muscarinic agonist carbachol (Carb) at rates that were similar to the rate at which carbachol increased muscarinic K(+)-acetylcholine channel (K+[ACh]) current in the same cells. K+[ACh] currents are directly gated by a G protein, and the results therefore supported our hypothesis. Much stronger support came after we established for the first time that If could be recorded from excised inside out membrane patches in the absence of cytoplasmic substrates. Under these conditions Iso increased and Carb decreased cell-free patch If provided guanosine 5'-triphosphate and Mg2+ were present. Thus G proteins couple autonomic receptors to If within the plasma membrane, probably by a direct action.
我们提出电压门控离子通道受受体 - 鸟嘌呤核苷酸结合（G）蛋白途径调控，这些途径是膜限定的（直接的）或胞质的（间接的）。由于已知间接途径将毒蕈碱和β - 肾上腺素能受体与超极化激活电流（If）通道偶联，我们通过使用千兆封接膜片钳方法测量兔窦房结细胞中的If电流来检验我们的假设，即直接途径可能也存在。在相同细胞中，β - 激动剂异丙肾上腺素（Iso）使全细胞If电流增加，毒蕈碱激动剂卡巴胆碱（Carb）使其降低，其速率与卡巴胆碱增加毒蕈碱K⁺ - 乙酰胆碱通道（K⁺[ACh]）电流的速率相似。K⁺[ACh]电流由G蛋白直接门控，因此这些结果支持了我们的假设。在我们首次证实可以在没有胞质底物的情况下从切除的内向外膜片记录If之后，获得了更强有力的支持。在这些条件下，如果存在鸟苷5'-三磷酸和Mg²⁺，Iso会增加且Carb会降低无细胞膜片的If。因此，G蛋白可能通过直接作用将自主受体与质膜内的If偶联。
