钠钙交换在心脏发育早期自主膜电活动发生中的作用

钠钙交换是小鼠心脏发育中最早有功能性表达的通道基因。它的功能主要是通过泵出1个钙，泵入3个钠位置细胞内的钙稳态，此外可能参与兴奋收缩偶联。但是，至今钠钙交换在心脏发育过程中的功能性表达及其在细胞早期兴奋形成中的作用还不是很清楚。采用胚胎干细胞分化的心肌细胞为研究对象，发现在发育极早期，电压钳制在35mV的条件下，10mmol／L咖啡因诱导的内向电流的80％能被灌流液中Na^＋被等浓度的Li^＋取代（n=8）。此为钠钙交换电流。所有钳制的细胞单细胞RT-PCR都检测到了NCX1亚型的mRNA表达。进一步研究了钠钙交换的功能，发现等浓度Li^＋取代灌流液中Na^＋及应用高浓度Ni^2＋阻断了膜电位震荡及与震荡相间的动作电位（早期膜兴奋形式）。因此认为钠钙交换（NCX1亚型）在心脏发育极早期的心肌细胞中已有大量功能性表达，它对于早期自主性兴奋活动的发生起着关键性的作用。

Role of Na+/Ca2+ Exchanger in the Initiation of Spontaneous Electrical Activity during Early Cardiogenesis

Na+/Ca2+ exchanger (NCX) is the earliest functional genes in the developing mouse heart. It has been proposed to contribute to intracellular Ca2+ homeostasis and probably excitation- contraction coupling by electrogenic exchange of 1 intracellular Ca2+ ion for 3 extracellular Na+ ions. To date the functional expression of NCX and its correlation with the early spontaneous electrical activity during cardiogenesis are not thoroughly clarified. Using ES cell derived cardiomyocytes, we have found at very early development stage, NCX current (INa/Ca) is the major contributor of the caffeine (10 mmol/ L) induced inward current at a constant holding potential of 35 mV as isomolar Li+ replacement of external Na+ blocked nearly 80% of the evoked inward current (n=8). NCX1 mRNA was identified in all these functionally measured cardiac cells using single-cell RT-PCR. Further functional relevance was investigated. The complete abolishment of membrane fluctuations and the intercalated action potentials (the earlier patterns of spontaneous electrical activity) by isomolar Li+ replacing external Na+ and Ni2+ (5 mmol/L) implicated the essentiality of NCX in the initiation of early membrane excitation. Thus we conclude that NCX1 is highly expressed even in very early stage cardiomycoytes and it plays a pivotal role in set-up of early spontaneous electrical activity.