Moore Edwin D, Voigt Tilman, Kobayashi Yvonne M, Isenberg Gerrit, Fay Fred S, Gallitelli Maria F, Franzini-Armstrong Clara
Department of Physiology, University of British Columbia, Vancouver, British Columbia, Canada.
Biophys J. 2004 Sep;87(3):1836-47. doi: 10.1529/biophysj.104.044123.
Ca(2+) release from internal stores (sarcoplasmic reticulum or SR) in smooth muscles is initiated either via pharmaco-mechanical coupling due to the action of an agonist and involving IP3 receptors, or via excitation-contraction coupling, mostly involving L-type calcium channels in the plasmalemma (DHPRs), and ryanodine receptors (RyRs), or Ca(2+) release channels of the SR. This work focuses attention on the structural basis for the coupling between DHPRs and RyRs in phasic smooth muscle cells of the guinea-pig urinary bladder. Immunolabeling shows that two proteins of the SR: calsequestrin and the RyR, and one protein the plasmalemma, the L-type channel or DHPR, are colocalized with each other within numerous, peripherally located sites located within the caveolar domains. Electron microscopy images from thin sections and freeze-fracture replicas identify feet in small peripherally located SR vesicles containing calsequestrin and distinctive large particles clustered within small membrane areas. Both feet and particle clusters are located within caveolar domains. Correspondence between the location of feet and particle clusters and of RyR- and DHPR-positive foci allows the conclusion that calsequestrin, RyRs, and L-type Ca(2+) channels are associated with peripheral couplings, or Ca(2+) release units, constituting the key machinery involved in excitation-contraction coupling. Structural analogies between smooth and cardiac muscle excitation-contraction coupling complexes suggest a common basic mechanism of action.
平滑肌中从内部储存库（肌浆网或SR）释放Ca(2+)，要么是由于激动剂的作用通过药理-机械偶联启动，涉及IP3受体，要么是通过兴奋-收缩偶联启动，主要涉及质膜中的L型钙通道（二氢吡啶受体，DHPRs）、兰尼碱受体（RyRs）或SR的Ca(2+)释放通道。这项工作聚焦于豚鼠膀胱相平滑肌细胞中DHPRs与RyRs偶联的结构基础。免疫标记显示，SR的两种蛋白：肌集钙蛋白和RyR，以及质膜的一种蛋白，即L型通道或DHPR，在位于小窝结构域内的众多周边位点相互共定位。来自超薄切片和冷冻蚀刻复制品的电子显微镜图像显示，在含有肌集钙蛋白的周边小SR囊泡中有脚状结构，并且在小膜区域内有独特的大颗粒聚集。脚状结构和颗粒簇都位于小窝结构域内。脚状结构和颗粒簇的位置与RyR和DHPR阳性灶的位置之间的对应关系表明，肌集钙蛋白、RyRs和L型Ca(2+)通道与周边偶联或Ca(2+)释放单位相关，构成了兴奋-收缩偶联的关键机制。平滑肌和心肌兴奋-收缩偶联复合物之间的结构相似性提示了一种共同的基本作用机制。
