Na,K-ATPase α2 activity in mammalian skeletal muscle T-tubules is acutely stimulated by extracellular K+.
Na,K-ATPase α2 activity in mammalian skeletal muscle T-tubules is acutely stimulated by extracellular K+.
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DOI：
10.1085/jgp.201511407
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发表时间：
2015-10
期刊：
影响因子：
--
通讯作者：
Heiny JA
中科院分区：
文献类型：
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作者：
DiFranco M;Hakimjavadi H;Lingrel JB;Heiny JA
Na，K-ATP酶α2亚型的K+亲和力使其活性与静息和收缩时T小管中细胞外K+浓度范围相匹配，从而维持活动肌肉的兴奋性。Na，K-ATP酶α2亚型是成人骨骼肌中的主要Na，K-ATP酶，也是横小管中唯一的Na，K-ATP酶。在静止的肌肉中，α2同工酶的作用大大低于其最大转运能力。与α1亚型不同，α2亚型不需要维持静息离子梯度或静息膜电位，这是大多数其他细胞中Na，K-ATP酶的典型作用。然而，α2活性在收缩开始后立即被刺激，并且在工作的肌肉中，其贡献对于维持兴奋和抵抗疲劳至关重要。在这里，我们表明α2活性部分取决于T-小管中的K+浓度，通过其K+底物亲和力。表观K+亲和力测定K+激活泵电流在完整的，电压钳位小鼠趾短屈肌肌纤维的K1/2。α2 Na，K-ATP酶产生的泵电流Ip为K+激活、μ mol哇巴因抑制的外向电流。在所有研究的电位（−90至−30 mV）下，Ip都是外向的，在阈下范围（−90至−50 mV）内，随着去极化的增加而增加。在22-37°C范围内，Q10为2.1。在-90 mV时，Ip的K1/2，K为4.3 ± 0.3 mM，且相对不依赖于电压。这种K+亲和力低于其他细胞类型的报告，但密切匹配的动态范围的细胞外K+浓度的T-小管。在肌肉收缩期间，T-小管腔K+与动作电位放电的频率和持续时间成比例地增加。该K1/2，K预测在静息细胞外K+浓度下K+底物位点的低分数占用，占用率与膜激发频率成比例增加。因此，通过更大的K+位点占据来刺激预先存在的泵提供了增加工作肌肉中α2活性的快速机制。
The K+ affinity of the Na,K-ATPase α2 isoform matches its activity to the range of extracellular K+ concentrations in the T-tubules at rest and during contraction, maintaining the excitability of active muscle. The Na,K-ATPase α2 isoform is the predominant Na,K-ATPase in adult skeletal muscle and the sole Na,K-ATPase in the transverse tubules (T-tubules). In quiescent muscles, the α2 isozyme operates substantially below its maximal transport capacity. Unlike the α1 isoform, the α2 isoform is not required for maintaining resting ion gradients or the resting membrane potential, canonical roles of the Na,K-ATPase in most other cells. However, α2 activity is stimulated immediately upon the start of contraction and, in working muscles, its contribution is crucial to maintaining excitation and resisting fatigue. Here, we show that α2 activity is determined in part by the K+ concentration in the T-tubules, through its K+ substrate affinity. Apparent K+ affinity was determined from measurements of the K1/2 for K+ activation of pump current in intact, voltage-clamped mouse flexor digitorum brevis muscle fibers. Pump current generated by the α2 Na,K-ATPase, Ip, was identified as the outward current activated by K+ and inhibited by micromolar ouabain. Ip was outward at all potentials studied (−90 to −30 mV) and increased with depolarization in the subthreshold range, −90 to −50 mV. The Q10 was 2.1 over the range of 22–37°C. The K1/2,K of Ip was 4.3 ± 0.3 mM at −90 mV and was relatively voltage independent. This K+ affinity is lower than that reported for other cell types but closely matches the dynamic range of extracellular K+ concentrations in the T-tubules. During muscle contraction, T-tubule luminal K+ increases in proportion to the frequency and duration of action potential firing. This K1/2,K predicts a low fractional occupancy of K+ substrate sites at the resting extracellular K+ concentration, with occupancy increasing in proportion to the frequency of membrane excitation. The stimulation of preexisting pumps by greater K+ site occupancy thus provides a rapid mechanism for increasing α2 activity in working muscles.