title: Excitation-induced exchange of Na+, K+, and Cl- in rat EDL muscle in vitro and in vivo: physiology and pathophysiology.
abstract: In skeletal muscle, excitation leads to increased [Na(+)](i), loss of K(+), increased [K(+)](o), depolarization, and Cl(-) influx. This study quantifies these changes in rat extensor digitorum longus (EDL) muscles in vitro and in vivo using flame photometric determination of Na(+) and K(+) and (36)Cl as a tracer for Cl(-). In vitro, 5-Hz stimulation for 300 s increased intracellular Na(+) content by 4.6 ± 1.2 µmol/g wet wt (P < 0.002) and decreased intracellular K(+) content by 5.5 ± 2.3 µmol/g wet wt (P < 0.03). This would increase [K(+)](o) by 28 ± 12 mM, sufficient to cause severe loss of excitability as the result of inactivation of Na(+) channels. In rat EDL, in vivo stimulation at 5 Hz for 300 s or 60 Hz for 60 s induced significant loss of K(+) (P < 0.01), sufficient to increase [K(+)](o) by 71 ± 22 mM and 73 ± 15 mM, respectively. In spite of this, excitability may be maintained by the rapid and marked stimulation of the electrogenic Na(+),K(+) pumps already documented. This may require full utilization of the transport capacity of Na(+),K(+) pumps, which then becomes a limiting factor for physical performance. In buffer containing (36)Cl, depolarization induced by increasing [K(+)](o) to 40-80 mM augmented intracellular (36)Cl by 120-399% (P < 0.001). Stimulation for 120-300 s at 5-20 Hz increased intracellular (36)Cl by 100-188% (P < 0.001). In rats, Cl(-) transport in vivo was examined by injecting (36)Cl, where electrical stimulation at 5 Hz for 300 s or 60 Hz for 60 s increased (36)Cl uptake by 81% (P < 0.001) and 84% (P < 0.001), respectively, indicating excitation-induced depolarization. Cl(-) influx favors repolarization, improving K(+) clearance and maintenance of excitability. In conclusion, excitation-induced fluxes of Na(+), K(+), and Cl(-) can be quantified in vivo, providing new evidence that in working muscles, extracellular accumulation of K(+) is considerably higher than previously observed and the resulting depression of membrane excitability may be a major cause of muscle fatigue.
authors: Department of Biomedicine, Aarhus University, DK-8000 Aarhus C, Denmark. t­c­@­f­i­.­a­u­.­d­k­
authors: Torben
authors: T
authors: Clausen
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D002713:Chlorine / Q000378:metabolism*; D004558:Electric Stimulation / Q000379:methods*; D008657:Metabolic Clearance Rate; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism*
chemical_list: D002713:Chlorine; D012964:Sodium; D011188:Potassium