Mechanisms of E-C Coupling in Atrial Cells

描述（由申请人提供）：哺乳动物心脏细胞的正常兴奋-收缩（EC）偶联需要钙（Ca2+）从肌浆网（SR）的协调释放。在哺乳动物心室细胞中，广泛的横管（t-管）系统将电去极化迅速传导到细胞内部，触发SR几乎同步释放C2+，随后激活整个细胞的肌原纤维。由于心房细胞缺乏广泛的t管网络，心房细胞中SR Ca2+释放和收缩的协调必须依赖于一个完全不同的细胞过程。拟议研究的目的是回答这个问题，“在心脏心房细胞中，细胞膜去极化转导到随后的收缩的细胞机制是什么？”本研究的总体假设是：(1)心房细胞外周偶联处Ca2+通过l型Ca2+通道进入和Ca2+从SR释放的机制与心室细胞t管-连接SR区域发生的机制相同；(2)由于心房细胞中缺乏组织良好的t管系统，SR Ca2+从外周偶联处释放的正常生理机制完全不同。特异性目的1检验了心房细胞外周偶联处钙离子通过l型钙离子通道进入和钙离子从SR释放之间的关系与心室细胞t管连接SR区相同的假设。特异性目的2检验了心房细胞中增殖和非增殖Ca2+释放的假设，Ca2+释放的类型是由SR Ca2+负荷、ryanodine受体（RyR） Ca2+敏感性以及Ca2+通过l型Ca2+通道进入的幅度和持续时间决定的。具体目标3测试了收缩的幅度和速度取决于Ca2+释放的类型（非扩散性或扩散性）和细胞的“扩散”大小的假设。我们假设细胞周长的增加触发了非繁殖和繁殖Ca2+释放之间的切换，从而提供了一种方法来维持收缩的速度和幅度，尽管细胞大小不同。具体来说，我们测试(1)，非传播Ca2+释放可以引起心房细胞快速和大的收缩与小周长。(2)在周长较大的心房细胞中，需要传播Ca2+波引起快速而大的收缩。(3)横轴管系统（TATS），通过减少扩散距离，允许非传播Ca2+释放引起大心房细胞快速和大的收缩。该建议独特地结合了数学建模、细胞电生理学和Ca2+成像，并将提供心脏心房细胞Ca2+稳态的定量理解。

DESCRIPTION (provided by applicant): Normal excitation-contraction (EC) coupling in mammalian cardiac cells requires the coordinated release of calcium (Ca2+) from the sarcoplasmic reticulum (SR). In mammalian ventricular cells, the extensive transverse-tubular (t-tubular) system conducts electrical depolarization rapidly to the cell interior that triggers a near synchronous release of C2+ from the SR and subsequent activation of the myofibrils throughout the cell. Because atrial cells lack an extensive t-tubular network, the coordination of SR Ca2+-release and contraction in atrial cells must depend on an entirely different cellular process. The aim of the proposed research is to answer the question, "What is (are) the cellular mechanism(s) responsible for the transduction of membrane depolarization to subsequent contraction in cardiac atrial cells?" The overall hypotheses of this proposed project are (1) that the mechanism of coupling Ca2+-entry through L-type Ca2+ channels and Ca2+-release from the SR at the peripheral couplings of atrial cells is the same as that which occurs at the t-tubular-junctional SR region in ventricular cells, and (2) that because of the absence of a well-organized t-tubular system in atrial cells, the normal physiological mechanisms of SR Ca2+-release away from the peripheral couplings are entirely different. Specific aim 1 tests the hypothesis that the relationship between Ca2+-entry via L-type Ca2+ channels and Ca2+-release from the SR at the peripheral couplings in atrial cells is identical to that in the t-tubular-junctional SR region of ventricular cells. Specific aim 2 tests the hypothesis that both propagated and non-propagated Ca2+-release occur in atrial cells and that the type of Ca2+-release is determined by SR Ca2+-load, ryanodine receptor (RyR) Ca2+-sensitivity, and the magnitude and duration of the Ca2+-entry through L-type Ca2+ channels. Specific aim 3 tests the hypothesis that the magnitude and velocity of contraction depends on the type of Ca2+-release (non-propagating or propagating) and the "diffusional" size of the cell. We hypothesize that an increase in the cell circumference triggers a switch between non-propagating and propagating Ca2+-release, thereby providing a means for maintaining the speed and magnitude of contraction despite differences in cell size. Specifically, we test (1) that non-propagating Ca2+-release can elicit rapid and large contractions in atrial cells with small circumference. (2) Propagating Ca2+-waves are needed to cause rapid and large contractions in atrial cells with a large circumference. (3) The transverse axial tubular system (TATS), by reducing diffusional distances, allows non-propagating Ca2+-release to cause rapid and large contractions in large atrial cells. This proposal uniquely combines mathematical modeling, cellular electrophysiology and Ca2+ imaging and will provide a quantitative understanding of Ca2+ homeostasis in cardiac atrial cells.