Mechanisms of E-C Coupling in Atrial Cells

描述(由申请人提供)：哺乳动物心肌细胞的正常兴奋-收缩(EC)耦合需要从肌浆网(SR)协调释放钙(Ca~(2+))。在哺乳动物的心室细胞中，广泛的横管(t管)系统将电去极化迅速传导到细胞内部，从而触发从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-管状网络，心房细胞中SR钙释放和收缩的协调必须依赖于完全不同的细胞过程。这项研究的目的是要回答这样一个问题：“是什么细胞机制(S)负责转导心肌细胞膜去极化到随后的收缩？”这一项目的总体假设是：(1)心房细胞外周偶联处的钙通道偶联Ca~(2+)进入和SR的Ca~(2+)释放的机制与发生在心室细胞t-管连接的SR区的机制相同；(2)由于心房细胞中没有一个组织良好的T-管系统，SR钙释放离开外周偶联的正常生理机制是完全不同的。特异性目标1验证了这样一种假设，即心房细胞通过L型钙通道的钙内流与心房细胞外周偶联的SR释放钙的关系与心室细胞T管连接SR区的关系相同。特异性目标2验证了一种假设，即心房细胞同时存在增殖性和非传播性钙释放，并且钙释放的类型由肌浆网钙负荷、兰尼定受体(RyR)的钙敏感性以及通过L型钙通道进入钙的幅度和持续时间决定。具体目标3验证了这样一种假设，即收缩的幅度和速度取决于钙离子释放的类型(非繁殖或繁殖)和细胞的“扩散”大小。我们假设，细胞周长的增加触发了非传播性和传播性钙释放之间的切换，从而提供了一种尽管细胞大小不同，但仍保持收缩速度和幅度的手段。具体地说，我们测试了(1)非增殖性钙释放能引起周长较小的心房细胞快速而剧烈的收缩。(2)在较大周长的心房细胞中，需要传播钙波才能引起快速而大幅度的收缩。(3)横轴小管系统(TATS)通过缩短扩散距离，使非传播性钙离子释放在大的心房细胞上引起快速而大的收缩。这一建议独特地结合了数学建模、细胞电生理学和钙离子成像，并将提供对心肌细胞内钙稳态的定量了解。

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.