CA Permeable Na Channels & Cardiac Cell Excitation

说明(申请人提供)：ICA(TTX)是一种钠电流成分，见于许多神经和心脏制剂中。在每种情况下，都发现ICA(TTX)通道表现出与该细胞类型的钠电流主体不同的门控和通透性特性。对于大鼠心室肌细胞，我们已经证明了ICA(TTX)通道是由一个不同于编码经典心脏钠电流的基因编码的。正如预期的那样，存在明确的电流分量时，ICA(TTX)通道确实会影响心室细胞的电行为，甚至可以产生动作电位。与经典的心脏钠电流相比，ICA(TTX)在更负的电位范围内激活。因此，它应该起到放大传递给心室细胞的去极化作用，从而为心脏动作电位提供直接触发。由于这一作用，ICA(TTX)在心律失常及其控制中可能具有相当重要的作用。据报道，在人的心房和心室细胞中都有这种情况。于是，我们有了一个新的通道，它由一个独特的基因编码，它对心脏细胞的电行为做出了贡献。这一通道对正常和病理心脏电生理学的潜在重要性需要广泛的研究。我们建议：(I)确定ICA(TTX)的慢失活特性，并将其与经典钠电流的特性进行比较。缓慢的失活过程对于确定可用钠通道的静止状态是至关重要的。ICA(TTX)作为心脏动作电位直接触发器的预期作用表明，它的缓慢失活特性(及其缺陷)可能对心脏动作电位的产生和传导产生不成比例的强烈影响。(Ii)定量测定ICA(TTX)通道对单价和二价离子的选择性顺序。电流分量的特性由它的选择性以及它的选通特性决定。这将是首次测定表现出高钙通透性的天然钠离子通道的碱性离子选择性，并可能对典型钠通道离子选择性结构基础的出现具有重要意义。(Iii)通过使用针对已知在心肌细胞中表达的几种钠通道亚型的反义寡核苷酸，开始ICA(TTX)通道的分子鉴定。(4)比较正常和衰竭心脏ICA(TTX)的性质，看看其性质的变化是否与病理条件有关。

DESCRIPTION (provided by applicant): ICa(TTX) is a sodium current component seen in a number of neural and cardiac preparations. In each case, ICa(TTX) channels were found to express both different gating and different permeability properties from the main body of sodium current in that cell type. For rat ventricular cells we have shown that ICa(TTX) channels are encoded by a different gene from that encoding the classical cardiac sodium current. As expected from the very existence of a clear current component, ICa(TTX) channels do affect the electrical behavior of ventricular cells and can even generate action potentials. ICa(TTX) activates over a more negative range of potentials than the classical cardiac sodium current. It should, then, act to amplify the depolarization delivered to a ventricular cell and so provide the immediate trigger for the cardiac action potential. Owing to this role, ICa(TTX) could be of considerable importance in cardiac arrhythmias and in their control. It has been reported in both human atrial and ventricular cells. We have, then, a new channel encoded by a distinct gene that contributes to cardiac cell electrical behavior. The potential importance of this channel for both normal and pathological cardiac electrophysiology calls for its extensive study. We propose to: (i) Determine the slow inactivation properties of ICa(TTX) and compare them to those of the classical sodium current. The slow inactivation process is critical for determining the stationary state pool of available sodium channels. The expected role of ICa(TTX) as the immediate trigger for the cardiac action potential suggests that its slow inactivation properties (and defects in them) could have disproportionately strong effects on the generation and conduction of the cardiac action potentials. (ii) Quantitatively determine the selectivity sequence of ICa(TTX) channels to both mono- and divalent ions. The characteristics of a current component are determined by it selectivity as well as its gating properties. This will be the first determination of the alkali ion selectivity of a native sodium channel that expresses high calcium permeability and could be of importance for the emerging picture of the structural basis for ion selectivity in typical sodium channels. (iii) Start the molecular identification of ICa(TTX) channels by use of antisense oligonucleotides directed against the several sodium channel isoforms known to be expressed in cardiac cells. (iv) Compare the properties of ICa(TTX) in normal and failing human hearts to see if alterations in its properties correlate with pathological conditions.