Pierobon Nicola, Renard-Rooney Dominique C, Gaspers Lawrence D, Thomas Andrew P
Department of Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103, USA.
J Biol Chem. 2006 Nov 10;281(45):34086-95. doi: 10.1074/jbc.M607788200. Epub 2006 Sep 13.
The ryanodine receptor has been mainly regarded as the Ca2+ release channel from sarcoplasmic reticulum controlling skeletal and cardiac muscle contraction. However, many studies have shown that it is widely expressed, with functions not restricted to muscular contraction. This study examined whether ryanodine receptor plays a role in calcium signaling in the liver. RT-PCR analysis of isolated hepatocytes showed expression of a truncated type 1 ryanodine receptor, but no type 2 or type 3 message was detected. We also detected binding sites for [3H]ryanodine in the microsomal cellular fraction and in permeabilized hepatocytes. This binding was displaced by caffeine and dantrolene, but not by ruthenium red, heparin or cyclic ADP-Ribose. Ryanodine, by itself, did not trigger Ca2+ oscillations in either primary cultured hepatocytes or hepatocytes within the intact perfused rat liver. In both preparations, however, ryanodine significantly increased the frequency of the cytosolic free [Ca2+] oscillations evoked by an alpha1 adrenergic receptor agonist. Experiments in permeabilized hepatocytes showed that both ryanodine and cyclic ADP-ribose evoked a slow Ca2+ leak from intracellular stores and were able to increase the Ca2+-released response to a subthreshold dose of inositol 1,4,5-trisphosphate. Our findings suggest the presence of a novel truncated form of the type 1 ryanodine receptor in rat hepatocytes. Ryanodine modulates the pattern of cytosolic free [Ca2+] oscillations by increasing oscillation frequency. We propose that the Ca2+ released from ryanodine receptors on the endoplasmic reticulum provides an increased pool of Ca2+ for positive feedback on inositol 1,4,5-trisphosphate receptors.
雷诺丁受体主要被视为控制骨骼肌和心肌收缩的肌浆网钙释放通道。然而，许多研究表明它广泛表达，其功能并不局限于肌肉收缩。本研究检测了雷诺丁受体是否在肝脏钙信号传导中发挥作用。对分离的肝细胞进行逆转录聚合酶链反应（RT-PCR）分析显示存在截短型1雷诺丁受体的表达，但未检测到2型或3型的信息。我们还在微粒体细胞组分和透化肝细胞中检测到了[3H]雷诺丁的结合位点。这种结合可被咖啡因和丹曲林取代，但不能被钌红、肝素或环ADP-核糖取代。单独使用雷诺丁，在原代培养的肝细胞或完整灌注大鼠肝脏内的肝细胞中均未引发钙振荡。然而，在这两种制剂中，雷诺丁均显著增加了由α1肾上腺素能受体激动剂诱发的胞质游离钙振荡频率。在透化肝细胞中进行的实验表明，雷诺丁和环ADP-核糖均能诱发细胞内钙库缓慢释放钙，并能够增强对亚阈值剂量肌醇1,4,5-三磷酸的钙释放反应。我们的研究结果表明大鼠肝细胞中存在一种新型的截短型1雷诺丁受体。雷诺丁通过增加振荡频率来调节胞质游离钙振荡模式。我们提出，从内质网上的雷诺丁受体释放的钙为肌醇1,4,5-三磷酸受体提供了一个增加的钙池，用于正反馈。
