title: Reducing chloride conductance prevents hyperkalaemia-induced loss of twitch force in rat slow-twitch muscle.
abstract: 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(-)](o) 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 V(m) indicated that the antagonistic effect of reducing [Cl(-)](o) 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.
authors.affiliation: Department of Pathobiology, Division of Anatomy and Physiology, Faculty of Veterinary Medicine, Utrecht University, P.O. Box 80.158, 3508 TD Utrecht, The Netherlands. m.vanemst@vet.uu.nl
authors.forename: Maarten Geert
authors.initials: MG
authors.lastname: van Emst
authors.forename: Sjoerd
authors.initials: S
authors.lastname: Klarenbeek
authors.forename: Arend
authors.initials: A
authors.lastname: Schot
authors.forename: Jaap Jan
authors.initials: JJ
authors.lastname: Plomp
authors.forename: Arie
authors.initials: A
authors.lastname: Doornenbal
authors.forename: Maria Elisabeth
authors.initials: ME
authors.lastname: Everts
mesh_terms: D000818:Animals; D000873:Anthracenes / Q000494:pharmacology; D002034:Bumetanide / Q000494:pharmacology; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000502:physiology*; D002712:Chlorides / Q000493:pharmacokinetics*; D004232:Diuretics / Q000494:pharmacology; D006947:Hyperkalemia / Q000378:metabolism / Q000503:physiopathology*; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D018482:Muscle, Skeletal / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism; D064506:Solute Carrier Family 12, Member 2
chemical_list: D000873:Anthracenes; D018118:Chloride Channels; D002712:Chlorides; D004232:Diuretics; C578879:Slc12a2 protein, rat; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters; D064506:Solute Carrier Family 12, Member 2; D002034:Bumetanide; C010030:9-anthroic acid