Takeshima H, Komazaki S, Hirose K, Nishi M, Noda T, Iino M
Department of Pharmacology, Faculty of Medicine, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan.
EMBO J. 1998 Jun 15;17(12):3309-16. doi: 10.1093/emboj/17.12.3309.
The ryanodine receptor type 2 (RyR-2) functions as a Ca2+-induced Ca2+ release (CICR) channel on intracellular Ca2+ stores and is distributed in most excitable cells with the exception of skeletal muscle cells. RyR-2 is abundantly expressed in cardiac muscle cells and is thought to mediate Ca2+ release triggered by Ca2+ influx through the voltage-gated Ca2+ channel to constitute the cardiac type of excitation-contraction (E-C) coupling. Here we report on mutant mice lacking RyR-2. The mutant mice died at approximately embryonic day (E) 10 with morphological abnormalities in the heart tube. Prior to embryonic death, large vacuolate sarcoplasmic reticulum (SR) and structurally abnormal mitochondria began to develop in the mutant cardiac myocytes, and the vacuolate SR appeared to contain high concentrations of Ca2+. Fluorometric Ca2+ measurements showed that a Ca2+ transient evoked by caffeine, an activator of RyRs, was abolished in the mutant cardiac myocytes. However, both mutant and control hearts showed spontaneous rhythmic contractions at E9.5. Moreover, treatment with ryanodine, which locks RyR channels in their open state, did not exert a major effect on spontaneous Ca2+ transients in control cardiac myocytes at E9.5-11.5. These results suggest no essential contribution of the RyR-2 to E-C coupling in cardiac myocytes during early embryonic stages. Our results from the mutant mice indicate that the major role of RyR-2 is not in E-C coupling as the CICR channel in embryonic cardiac myocytes but it is absolutely required for cellular Ca2+ homeostasis most probably as a major Ca2+ leak channel to maintain the developing SR.
2型兰尼碱受体（RyR-2）作为细胞内钙储存上的钙诱导钙释放（CICR）通道发挥作用，分布于除骨骼肌细胞外的大多数可兴奋细胞中。RyR-2在心肌细胞中大量表达，被认为介导由通过电压门控钙通道的钙内流触发的钙释放，从而构成心脏型兴奋-收缩（E-C）偶联。在此，我们报告了缺乏RyR-2的突变小鼠。突变小鼠在大约胚胎第10天死亡，心脏管出现形态异常。在胚胎死亡前，突变心肌细胞中开始出现大的空泡状肌浆网（SR）和结构异常的线粒体，空泡状SR似乎含有高浓度的钙。荧光钙测量显示，RyRs激活剂咖啡因诱发的钙瞬变在突变心肌细胞中消失。然而，在胚胎第9.5天，突变心脏和对照心脏均表现出自发的节律性收缩。此外，用使RyR通道处于开放状态的兰尼碱处理，对胚胎第9.5至11.5天对照心肌细胞中的自发钙瞬变没有产生主要影响。这些结果表明，在胚胎早期阶段，RyR-2对心肌细胞中的E-C偶联没有重要贡献。我们对突变小鼠的研究结果表明，RyR-2的主要作用不是作为胚胎心肌细胞中CICR通道参与E-C偶联，而是最有可能作为维持发育中的SR的主要钙泄漏通道，对细胞钙稳态是绝对必需的。
