Department of Human Health and Nutritional Sciences, University of Guelph, Guelph, ON, Canada N1G 2W1.
J Physiol. 2011 Jun 1;589(Pt 11):2887-99. doi: 10.1113/jphysiol.2011.206730. Epub 2011 Apr 11.
Controversy exists as to whether mammalian skeletal muscle is capable of volume regulation in response to changes in extracellular osmolarity despite evidence that muscle fibres have the required ion transport mechanisms to transport solute and water in situ. We addressed this issue by studying the ability of skeletal muscle to regulate volume during periods of induced hyperosmotic stress using single, mouse extensor digitorum longus (EDL) muscle fibres and intact muscle (soleus and EDL). Fibres and intact muscles were loaded with the fluorophore, calcein, and the change in muscle fluorescence and width (single fibres only) used as a metric of volume change. We hypothesized that skeletal muscle exposed to increased extracellular osmolarity would elicit initial cellular shrinkage followed by a regulatory volume increase (RVI) with the RVI dependent on the sodium–potassium–chloride cotransporter (NKCC). We found that single fibres exposed to a 35% increase in extracellular osmolarity demonstrated a rapid, initial 27–32% decrease in cell volume followed by a RVI which took 10-20 min and returned cell volume to 90–110% of pre-stimulus values. Within intact muscle, exposure to increased extracellular osmolarity of varying degrees also induced a rapid, initial shrinkage followed by a gradual RVI, with a greater rate of initial cell shrinkage and a longer time for RVI to occur with increasing extracellular tonicities. Furthermore, RVI was significantly faster in slow-twitch soleus than fast-twitch EDL. Pre-treatment of muscle with bumetanide (NKCC inhibitor) or ouabain (Na+,K+-ATPase inhibitor), increased the initial volume loss and impaired the RVI response to increased extracellular osmolarity indicating that the NKCC is a primary contributor to volume regulation in skeletal muscle. It is concluded that mouse skeletal muscle initially loses volume then exhibits a RVI when exposed to increases in extracellular osmolarity. The rate of RVI is dependent on the degree of change in extracellular osmolarity, is muscle specific, and is dependent on the functioning of the NKCC and Na+, K+-ATPase.
尽管有证据表明肌肉纤维具有原位运输溶质和水的必需离子转运机制，但哺乳动物骨骼肌是否能够响应细胞外渗透压的变化进行体积调节仍存在争议。我们通过研究在诱导高渗应激期间骨骼肌调节体积的能力来解决这个问题，使用单个、小鼠伸趾长肌（EDL）肌纤维和完整肌肉（比目鱼肌和 EDL）。纤维和完整肌肉加载荧光染料钙黄绿素，并用肌肉荧光强度和宽度的变化（仅用于单纤维）作为体积变化的度量。我们假设暴露于增加的细胞外渗透压的骨骼肌会引起初始细胞收缩，然后是调节性体积增加（RVI），RVI 依赖于钠-钾-氯协同转运蛋白（NKCC）。我们发现，暴露于细胞外渗透压增加 35%的单纤维迅速出现初始 27-32%的细胞体积减少，然后是 RVI，RVI 持续 10-20 分钟，使细胞体积恢复到刺激前值的 90-110%。在完整肌肉中，暴露于不同程度的增加细胞外渗透压也会引起快速的初始收缩，然后是逐渐的 RVI，随着细胞外张力的增加，初始细胞收缩的速度更快，RVI 发生的时间更长。此外，与快肌 EDL 相比，慢肌比目鱼肌的 RVI 更快。用布美他尼（NKCC 抑制剂）或哇巴因（Na+，K+-ATP 酶抑制剂）预处理肌肉会增加初始体积损失并损害对增加细胞外渗透压的 RVI 反应，表明 NKCC 是骨骼肌体积调节的主要贡献者。结论是，当暴露于细胞外渗透压增加时，小鼠骨骼肌最初会失去体积，然后表现出 RVI。RVI 的速度取决于细胞外渗透压变化的程度，是肌肉特异性的，并且依赖于 NKCC 和 Na+，K+-ATP 酶的功能。
