title: Effects of 8 wk of voluntary unloaded wheel running on K+ tolerance and excitability of soleus muscles in rat.
abstract: During intense exercise, efflux of K(+) from working muscles increases extracellular K(+) ([K(+)](o)) to levels that can compromise muscle excitability and hence cause fatigue. In this context, the reduction in the exercise-induced elevation of [K(+)](o) observed after training in humans is suggested to contribute to the increased performance after training. Although a similar effect could be obtained by an increase in the tolerance of muscle to elevated [K(+)](o), this possibility has not been investigated. To examine this, isolated soleus muscles from sedentary (sedentary) rats and from rats that had voluntarily covered 13.1 ± 0.7 km/day in an unloaded running wheel for 8 wk (active) were compared. In muscles from active rats, the loss of force induced by exposure to an elevated [K(+)](o) of 9 mM was 42% lower than in muscles from sedentary rats (P < 0.001). This apparent increase in K(+) tolerance in active rats was associated with an increased excitability as evident from a 33% reduction in the electrical current needed to excite individual muscle fibers (P < 0.0009). Moreover, muscles from active rats had lower Cl(-) conductance, higher maximal rate of rise of single-fiber action potentials (AP), and higher Na(+)/K(+) pump content. When stimulated intermittently at 6.5 mM K(+), muscles from active rats displayed better endurance than muscles from sedentary rats, whereas no difference was found when the muscles were stimulated continuously at 30 or 120 Hz. We conclude that voluntary running increases muscle excitability, leading to improved tolerance to elevated [K(+)](o).
authors.affiliation: Department of Physiology and Biophysics, Ole Worms Allé 4 8000 Aarhus C, Aarhus University, Aarhus, Denmark. mbl@fi.au.dk
authors.forename: Martin
authors.initials: M
authors.lastname: Broch-Lips
authors.forename: Frank
authors.initials: F
authors.lastname: de Paoli
authors.forename: Thomas Holm
authors.initials: TH
authors.lastname: Pedersen
authors.forename: Kristian
authors.initials: K
authors.lastname: Overgaard
authors.forename: Ole Bækgaard
authors.initials: OB
authors.lastname: Nielsen
mesh_terms: D000200:Action Potentials; D000818:Animals; D001522:Behavior, Animal; D005260:Female; D009043:Motor Activity; D009119:Muscle Contraction; D018763:Muscle Fatigue; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000294:innervation / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D010807:Physical Endurance; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012420:Running; D057185:Sedentary Behavior; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013997:Time Factors; D014836:Volition
chemical_list: D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium