Swift Fredrik, Tovsrud Nils, Enger Ulla H, Sjaastad Ivar, Sejersted Ole M
Institute for Experimental Medical Research, Ullevaal University Hospital, University of Oslo, Oslo, Norway.
Cardiovasc Res. 2007 Jul 1;75(1):109-17. doi: 10.1016/j.cardiores.2007.03.017. Epub 2007 Mar 24.
The presence of both alpha1- and alpha2-isoforms of the Na+/K+-ATPase (NKA) in cardiomyocytes indicates different functions. We hypothesized that preferential localization of the alpha2-isoform to the t-tubules, locally controlling the Na+/Ca2+-exchanger (NCX), underlies a specific role in Ca2+ handling.
We studied NKA isoform distribution in isolated cardiomyocytes from Wistar rats using immunocytochemistry. NKA pump and NCX currents (I(pump) and I(NCX)) were measured in control and detubulated cardiomyocytes. Intracellular Na+ concentration [Na+]i was assessed with the fluorescent dye SBFI.
The alpha2-isoform abundance was higher in the t-tubules than in the surface sarcolemma. We established that 0.3 microM ouabain specifically blocked the alpha2-isoform in isolated rat cardiomyocytes. This low concentration blocked 10.7+/-0.6% of I(pump) in control, but only 6.0+/-0.5% in detubulated cardiomyocytes. Moreover, measured and calculated alpha1-specific and alpha2-specific I(pump) in control (547+/-29 pA and 66 pA, respectively) and in detubulated cells (495+/-30 pA and 31 pA, respectively) showed that 53% of the alpha2-isoform, but only 9.5% of the alpha1-isoform, were localized to the t-tubules. Despite the small abundance of the alpha2-isoform (approximately 11% of total NKA), selective inhibition of this isoform induced a 40% increase in contractility in field stimulated cardiomyocytes, but no increase in global [Na+]i. However, inhibition of the alpha2-isoform increased I(NCX) indicating local subsarcolemmal accumulation of Na+ near NCX.
The alpha2-isoform of the NKA is functionally coupled to the NCX and can regulate Ca2+ handling without changing global [Na+]i.
心肌细胞中钠钾ATP酶（NKA）的α1和α2亚型的存在表明其功能不同。我们推测α2亚型优先定位于横小管，局部控制钠钙交换体（NCX），这是其在钙处理中发挥特定作用的基础。
我们使用免疫细胞化学研究了Wistar大鼠分离心肌细胞中NKA亚型的分布。在对照和去横小管的心肌细胞中测量NKA泵电流和NCX电流（I(pump)和I(NCX)）。用荧光染料SBFI评估细胞内钠离子浓度[Na+]i。
α2亚型在横小管中的丰度高于表面肌膜。我们确定0.3微摩尔哇巴因可特异性阻断分离的大鼠心肌细胞中的α2亚型。这种低浓度在对照中阻断了10.7±0.6%的I(pump)，但在去横小管的心肌细胞中仅阻断了6.0±0.5%。此外，对照（分别为547±29 pA和66 pA）和去横小管细胞（分别为495±30 pA和31 pA）中测量和计算的α1特异性和α2特异性I(pump)表明，53%的α2亚型，但仅9.5%的α1亚型定位于横小管。尽管α2亚型丰度较小（约占总NKA的11%），但对该亚型的选择性抑制导致场刺激心肌细胞的收缩力增加40%，但细胞内总[Na+]i没有增加。然而，对α2亚型的抑制增加了I(NCX)，表明NCX附近肌膜下局部钠离子积累。
NKA的α2亚型在功能上与NCX偶联，可在不改变细胞内总[Na+]i的情况下调节钙处理。
