van Emst Maarten Geert, Klarenbeek Sjoerd, Schot Arend, Plomp Jaap Jan, Doornenbal Arie, Everts Maria Elisabeth
Department of Pathobiology, Division of Anatomy and Physiology, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80.158, 3508 TD Utrecht, The Netherlands.
J Physiol. 2004 Nov 15;561(Pt 1):169-81. doi: 10.1113/jphysiol.2004.071498. Epub 2004 Sep 2.
Exercise-induced loss of skeletal muscle K(+) can seriously impede muscle performance through membrane depolarization. Thus far, it has been assumed that the negative equilibrium potential and large membrane conductance of Cl(-) attenuate the loss of force during hyperkalaemia. We questioned this idea because there is some evidence that Cl(-) itself can exert a depolarizing influence on membrane potential (V(m)). With this study we tried to identify the possible roles played by Cl(-) during hyperkalaemia. Isolated rat soleus muscles were kept at 25 degrees C and twitch contractions were evoked by current pulses. Reducing Cl(-) to 5 mM, prior to introducing 12.5 mM K(o), prevented the otherwise occurring loss of force. Reversing the order of introducing these two solutions revealed an additional effect, i.e. the ongoing hyperkalaemia-related loss of force was sped up tenfold after reducing Cl(-). However, hereafter twitch force recovered completely. The recovery of force was absent at K(+) exceeding 14 mM. In addition, reducing Cl(-) increased membrane excitability by 24%, as shown by a shift in the relationship between force and current level. Measurements of V(m) indicated that the antagonistic effect of reducing Cl(-) on hyperkalaemia-induced loss of force was due to low-Cl(-)-induced membrane hyperpolarization. The involvement of specific Cl(-) conductance was established with 9-anthracene carboxylic acid (9-AC). At 100 microm, 9-AC reduced the loss of force due to hyperkalaemia, while at 200 microm, 9-AC completely prevented loss of force. To study the role of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) in this matter, we added 400 microm of the NKCC inhibitor bumetanide to the incubation medium. This did not affect the hyperkalaemia-induced loss of force. We conclude that Cl(-) exerts a permanent depolarizing influence on V(m). This influence of Cl(-) on V(m), in combination with a large membrane conductance, can apparently have two different effects on hyperkalaemia-induced loss of force. It might exert a stabilizing influence on force production during short periods of hyperkalaemia, but it can add to the loss of force during prolonged periods of hyperkalaemia.
运动诱导的骨骼肌钾离子（K⁺）流失可通过膜去极化严重阻碍肌肉性能。到目前为止，人们一直认为氯离子（Cl⁻）的负平衡电位和大膜电导可减轻高钾血症期间的力量损失。我们对这一观点提出质疑，因为有一些证据表明Cl⁻本身可对膜电位（Vₘ）产生去极化影响。通过本研究，我们试图确定Cl⁻在高钾血症期间可能发挥的作用。将离体大鼠比目鱼肌置于25℃，通过电流脉冲诱发抽搐收缩。在引入12.5 mM细胞外钾离子（Kₒ）之前，将细胞外氯离子浓度（[Cl⁻]ₒ）降至5 mM可防止原本会发生的力量损失。颠倒引入这两种溶液的顺序揭示了另一种效应，即降低[Cl⁻]ₒ后，正在进行的与高钾血症相关的力量损失加快了十倍。然而，此后抽搐力量完全恢复。当细胞外钾离子浓度（[K⁺]ₒ）超过14 mM时，力量无法恢复。此外，如力量与电流水平关系的变化所示，降低[Cl⁻]ₒ可使膜兴奋性增加24%。膜电位（Vₘ）测量表明，降低[Cl⁻]ₒ对高钾血症诱导的力量损失的拮抗作用是由于低Cl⁻诱导的膜超极化。用9-蒽甲酸（9-AC）确定了特定Cl⁻电导的参与。在100 μM时，9-AC减少了高钾血症导致的力量损失，而在200 μM时，9-AC完全防止了力量损失。为研究钠-钾-2氯共转运体（NKCC1）在此过程中的作用，我们向孵育培养基中添加了400 μM的NKCC抑制剂布美他尼。这并未影响高钾血症诱导的力量损失。我们得出结论，Cl⁻对Vₘ施加永久性去极化影响。Cl⁻对Vₘ的这种影响，与大膜电导相结合，显然可对高钾血症诱导的力量损失产生两种不同的影响。它可能在高钾血症短时间内对力量产生发挥稳定作用，但在高钾血症长时间内可能会加剧力量损失。
