Gassanov Natig, Er Fikret, Endres-Becker Jeannette, Wolny Martin, Schramm Christoph, Hoppe Uta C
Department of Internal Medicine III, University of Cologne, Kerpener Str. 62, 50937, Cologne, Germany.
Pflugers Arch. 2009 Oct;458(6):1061-8. doi: 10.1007/s00424-009-0691-x. Epub 2009 Jul 22.
Thyroid hormone (TH) markedly modulates cardiovascular function and heart rate. The pacemaker current I(f) and encoding hyperpolarization-activated cation (HCN) genes have been identified as TH targets. To analyze the specific contribution and functional significance of thyroid receptor isoforms responsible for HCN gene transactivation, we generated transgenic neonatal rat cardiomyocytes with adenovirus-mediated overexpression of the thyroid receptors alpha1 (TR alpha 1) and beta1 (TR beta 1), and analyzed native I(f) current and expression levels of the underlying molecular components HCN2 and HCN4. Initial results revealed that spontaneous beating activity was higher in TR alpha 1- and lower in TR beta 1-expressing cardiomyocytes. This was associated with accelerated depolarization velocity and abbreviated action potential duration in cells overexpressing TR alpha 1, while TR beta 1 suppressed phase 4 depolarization and prolonged action potentials. Consistently, TR alpha 1-infected myocytes exhibited larger I(f) current densities along with increased HCN2 and HCN4 mRNA and protein levels. In contrast, HCN2 gene expression was not significantly affected by TR beta 1. TR beta 1 exclusively suppressed HCN4 transcription. T3 application led to significant effects only in controls and TR alpha 1-infected cardiomyocytes; whereas, no ligand-dependent actions were observed in TR beta 1-expressing neonatal cardiomyocytes. Our results demonstrate that TR alpha 1 and TR beta 1 divergently regulate cardiac pacing activity. TH-induced positive chronotropic effects are likely to be mediated by TR alpha 1 through enhanced expression of I(f) pacemaker current and its underlying genes.
甲状腺激素（TH）显著调节心血管功能和心率。起搏电流I(f)以及编码超极化激活阳离子（HCN）的基因已被确定为TH的作用靶点。为了分析负责HCN基因反式激活的甲状腺受体亚型的具体作用和功能意义，我们利用腺病毒介导的甲状腺受体α1（TRα1）和β1（TRβ1）过表达，构建了转基因新生大鼠心肌细胞，并分析了天然I(f)电流以及相关分子成分HCN2和HCN4的表达水平。初步结果显示，表达TRα1的心肌细胞自发搏动活性较高，而表达TRβ1的心肌细胞自发搏动活性较低。这与过表达TRα1的细胞中去极化速度加快和动作电位时程缩短有关，而TRβ1则抑制4期去极化并延长动作电位。一致地，感染TRα1的心肌细胞表现出更大的I(f)电流密度，同时HCN2和HCN4的mRNA及蛋白水平增加。相比之下，TRβ1对HCN2基因表达没有显著影响。TRβ1仅抑制HCN4转录。T3处理仅在对照组和感染TRα1的心肌细胞中产生显著效应；而在表达TRβ1的新生心肌细胞中未观察到配体依赖性作用。我们的结果表明，TRα1和TRβ1对心脏起搏活动的调节作用不同。TH诱导的正性变时效应可能由TRα1通过增强I(f)起搏电流及其相关基因的表达介导。
