Reducing chloride conductance prevents hyperkalaemia-induced loss of twitch force in rat slow-twitch muscle

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 hyperkalaernia. 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 hyperkalaernia. Isolated rat soleus muscles were kept at 25 degreesC and twitch contractions were evoked by current pulses. Reducing [Cl-] 0 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-](o). However, hereafter twitch force recovered completely. The recovery of force was absent at [K+](o) exceeding 14 mm. In addition, reducing [Cl-](o) increased membrane excitability by 24%, as shown by a shift in the relationship between force and current level. Measurements of Vm 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 mum, 9-AC reduced the loss of force due to hyperkalaemia, while at 200 mum, 9-AC completely prevented loss of force. To study the role of the Na+- K+ - 2Cl(-) cotransporter (NKCC1) in this matter, we added 400 mum 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 hyperkalaernia.