Common molecular determinants of flecainide and lidocaine block of heart Na+ channels: evidence from experiments with neutral and quaternary flecainide analogues.
Common molecular determinants of flecainide and lidocaine block of heart Na+ channels: evidence from experiments with neutral and quaternary flecainide analogues.
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DOI：
10.1085/jgp.20028723
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发表时间：
2003-03
期刊：
影响因子：
--
通讯作者：
Kass RS
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作者：
Liu H;Atkins J;Kass RS
氟卡尼（pKa 9.3，pH 7.4时99%带电）和利多卡因（pKa 7.6-8.0，pH 7.4时95%中性）具有相似的结构，但对Na+通道活性的影响明显不同。由于失活状态的稳定，两种药物均引起Na+通道的良好表征的使用依赖性阻滞（UDB），但氟卡尼要求通道在阻滞发生前首先打开，而利多卡因被认为直接结合失活状态。为了测试氟卡尼的电荷是否决定其对Na+通道阻滞的状态特异性，我们开发了两种氟卡尼类似物，NU-FL（pKa 6.4），即在pH 7.4下90%中性，和在生理pH下完全带电的季铵氟卡尼类似物QX-FL。我们检查了氟卡尼、NU-FL、QX-FL、和利多卡因对人胚肾（HEK）293细胞中表达的人心脏Na+通道的作用。在生理pH下，NU-FL与利多卡因（而非氟卡尼）一样，优先与失活通道相互作用，而无需通道开放，并导致最小的UDB。我们发现，UDB的发展主要是由氟卡尼的带电形式证明了QX-FL在生理pH值和NU-FL的研究在更酸性的pH值范围内，其带电分数增加的调查。QX-FL是一种有效的通道阻滞剂，当从细胞内应用时，但与外部应用作用非常弱。QX-FL的UDB与氟卡尼一样，仅在通道开放后才会出现。一旦阻断，通道从QX-FL阻断恢复非常缓慢，显然没有必要的通道开放。我们的数据强烈表明，这是利多卡因和氟卡尼之间的电离度（pKa）的差异，而不是总的结构特征，决定了心脏Na+通道阻滞的区别。数据还表明，这两种药物共享一个共同的受体，但与调制受体假说一致，通过药物分子的电离程度决定的不同途径到达该受体。
Flecainide (pKa 9.3, 99% charged at pH 7.4) and lidocaine (pKa 7.6–8.0, ∼50% neutral at pH 7.4) have similar structures but markedly different effects on Na+ channel activity. Both drugs cause well-characterized use-dependent block (UDB) of Na+ channels due to stabilization of the inactivated state, but flecainide requires that channels first open before block develops, whereas lidocaine is believed to bind directly to the inactivated state. To test whether the charge on flecainide might determine its state specificity of Na+ channel blockade, we developed two flecainide analogues, NU-FL (pKa 6.4), that is 90% neutral at pH 7.4, and a quaternary flecainide analogue, QX-FL, that is fully charged at physiological pH. We examined the effects of flecainide, NU-FL, QX-FL, and lidocaine on human cardiac Na+ channels expressed in human embryonic kidney (HEK) 293 cells. At physiological pH, NU-FL, like lidocaine but not flecainide, interacts preferentially with inactivated channels without prerequisite channel opening, and causes minimal UDB. We find that UDB develops predominantly by the charged form of flecainide as evidenced by investigation of QX-FL at physiological pH and NU-FL investigated over a more acidic pH range where its charged fraction is increased. QX-FL is a potent blocker of channels when applied from inside the cell, but acts very weakly with external application. UDB by QX-FL, like flecainide, develops only after channels open. Once blocked, channels recover very slowly from QX-FL block, apparently without requisite channel opening. Our data strongly suggest that it is the difference in degree of ionization (pKa) between lidocaine and flecainide, rather than gross structural features, that determines distinction in block of cardiac Na+ channels. The data also suggest that the two drugs share a common receptor but, consistent with the modulated receptor hypothesis, reach this receptor by distinct routes dictated by the degree of ionization of the drug molecules.