Sun X H, Protasi F, Takahashi M, Takeshima H, Ferguson D G, Franzini-Armstrong C
Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, Philadelphia 19104-6058, USA.
J Cell Biol. 1995 May;129(3):659-71. doi: 10.1083/jcb.129.3.659.
Peripheral couplings are junctions between the sarcoplasmic reticulum (SR) and the surface membrane (SM). Feet occupy the SR/SM junctional gap and are identified as the SR calcium release channels, or ryanodine receptors (RyRs). In cardiac muscle, the activation of RyRs during excitation-contraction (e-c) coupling is initiated by surface membrane depolarization, followed by the opening of surface membrane calcium channels, the dihydropyridine receptors (DHPRs). We have studied the disposition of DHPRs and RyRs, and the structure of peripheral couplings in chick myocardium, a muscle that has no transverse tubules. Immunolabeling shows colocalization of RyRs and DHPRs in clusters at the fiber's periphery. The positions of DHPR and RyR clusters change coincidentally during development. Freeze-fracture of the surface membrane reveals the presence of domains (junctional domains) occupied by clusters of large particles. Junctional domains in the surface membrane and arrays of feet in the junctional gap have similar sizes and corresponding positions during development, suggesting that both are components of peripheral couplings. As opposed to skeletal muscle, membrane particles in junctional domains of cardiac muscle do not form tetrads. Thus, despite their proximity to the feet, they do not appear to be specifically associated with them. Two observations establish the identify of the structurally identified feet arrays/junctional domain complexes with the immunocytochemically defined RyRs/DHPRs coclusters: the concomitant changes during development and the identification of feet as the cytoplasmic domains of RyRs. We suggest that the large particles in junctional domains of the surface membrane represent DHPRs. These observations have two important functional consequences. First, the apposition of DHPRs and RyRs indicates that most of the inward calcium current flows into the restricted space where feet are located. Secondly, contrary to skeletal muscle, presumptive DHPRs do not show a specific association with the feet, which is consistent with a less direct role of charge movement in cardiac than in skeletal e-c coupling.
外周偶联是肌浆网（SR）与表面膜（SM）之间的连接点。脚状结构占据SR/SM连接间隙，并被确定为SR钙释放通道，即兰尼碱受体（RyRs）。在心肌中，兴奋-收缩（e-c）偶联过程中RyRs的激活始于表面膜去极化，随后表面膜钙通道（二氢吡啶受体，DHPRs）开放。我们研究了DHPRs和RyRs的分布以及鸡心肌中外周偶联的结构，鸡心肌是一种没有横管的肌肉。免疫标记显示RyRs和DHPRs在纤维周边的簇中共定位。在发育过程中，DHPR和RyR簇的位置同时发生变化。表面膜的冷冻断裂显示存在由大颗粒簇占据的区域（连接区域）。表面膜中的连接区域和连接间隙中的脚状结构阵列在发育过程中具有相似的大小和相应的位置，这表明两者都是外周偶联的组成部分。与骨骼肌不同，心肌连接区域的膜颗粒不形成四联。因此，尽管它们靠近脚状结构，但似乎与脚状结构没有特异性关联。两项观察结果确定了结构上确定的脚状结构阵列/连接区域复合物与免疫细胞化学定义的RyRs/DHPRs共簇的一致性：发育过程中的伴随变化以及将脚状结构确定为RyRs的胞质结构域。我们认为表面膜连接区域中的大颗粒代表DHPRs。这些观察结果有两个重要的功能后果。首先，DHPRs和RyRs的并置表明大部分内向钙电流流入脚状结构所在的受限空间。其次，与骨骼肌相反，推测的DHPRs与脚状结构没有特异性关联，这与电荷运动在心肌e-c偶联中比在骨骼肌中作用不那么直接一致。
