Kang J X, Li Y, Leaf A
Department of Medicine, Harvard Medical School and Massachusetts General Hospital, Boston 02114, USA.
Proc Natl Acad Sci U S A. 1997 Mar 18;94(6):2724-8. doi: 10.1073/pnas.94.6.2724.
Previous studies have shown that chronic administration of class I antiarrhythmic drugs, which have definite inhibitory action on the fast Na+ channel, result in up-regulation of cardiac Na+ channel expression, and suggest that this effect may contribute to their deleterious effects during chronic administration. Recent studies have shown that the antiarrhythmic effects of free n - 3 polyunsaturated fatty acids (PUFA) are associated with an inhibition of the Na+ channel. Whether the PUFA when used chronically will mimic the effect of the class I drugs on the expression of the Na+ channel is not known. To answer this question, we determined the level of mRNA encoding cardiac Na+ channels and the number of the Na+ channels per cell in cultured neonatal rat cardiac myocytes after supplementation of the cells with the n - 3 PUFA eicosapentaenoic acid (EPA), the class I drug mexiletine, or both EPA and mexiletine for 3-4 days. The number of sodium channels was assessed with a radioligand binding assay using the sodium channel-specific toxin [3H]batrachotoxinin benzoate ([3H]BTXB). The supplementation of myocytes with mexiletine (20 microM) induced a 4-fold increase in [3H]BTXB specific binding to the cells. In contrast, chronic treatment with EPA (20 microM) alone did not significantly affect [3H]BTXB binding. However, the combination of EPA with mexiletine produced a 40-50% reduction in the [3H]BTXB binding, compared with that seen with mexiletine alone. RNA isolated from cardiac myocytes was probed with a 2.5-kb cRNA transcribed with T7 RNA polymerase from the clone Na-8.4, which encodes nucleotides 3361-5868 of the alpha-subunit of the R(IIA) sodium channel subtype. The changes in the level of mRNA encoding sodium channel alpha-subunit were correlated with comparable changes in sodium channel number in the cultured myocytes, indicating that regulation of transcription of mRNA or its processing and stability is primarily responsible for the regulation of sodium channel number. These data demonstrate that chronic EPA treatment not only does not up-regulate the cardiac sodium channel expression but also reduces the mexiletine-induced increase in the cardiac sodium channel expression.
以往的研究表明，长期给予对快速钠通道有明确抑制作用的Ⅰ类抗心律失常药物，会导致心脏钠通道表达上调，并提示这种效应可能是其长期应用时产生有害作用的原因。最近的研究表明，游离的n-3多不饱和脂肪酸（PUFA）的抗心律失常作用与对钠通道的抑制有关。长期使用PUFA是否会模拟Ⅰ类药物对钠通道表达的影响尚不清楚。为了回答这个问题，我们在培养的新生大鼠心肌细胞中补充n-3 PUFA二十碳五烯酸（EPA）、Ⅰ类药物美西律或EPA与美西律两者，持续3至4天后，测定了编码心脏钠通道的mRNA水平以及每个细胞中钠通道的数量。使用钠通道特异性毒素[3H]蝙蝠葛毒碱苯甲酸酯（[3H]BTXB）通过放射性配体结合试验评估钠通道的数量。用美西律（20μM）处理心肌细胞，会使[3H]BTXB与细胞的特异性结合增加4倍。相比之下，单独用EPA（20μM）长期处理对[3H]BTXB结合没有显著影响。然而，与单独使用美西律相比，EPA与美西律联合使用使[3H]BTXB结合减少了40%至50%。用从克隆Na-8.4转录的2.5 kb cRNA探测从心肌细胞分离的RNA，该克隆编码R（IIA）钠通道亚型α亚基的核苷酸3361至5868。编码钠通道α亚基的mRNA水平变化与培养心肌细胞中钠通道数量的相应变化相关，表明mRNA转录调控或其加工及稳定性主要负责钠通道数量的调控。这些数据表明，长期EPA处理不仅不会上调心脏钠通道表达，还会降低美西律诱导的心脏钠通道表达增加。
