title: Excitability of the T-tubular system in rat skeletal muscle: roles of K+ and Na+ gradients and Na+-K+ pump activity.
abstract: Strenuous exercise causes an increase in extracellular [K(+)] and intracellular Na(+) ([Na(+)](i)) of working muscles, which may reduce sarcolemma excitability. The excitability of the sarcolemma is, however, to some extent protected by a concomitant increase in the activity of muscle Na(+)-K(+) pumps. The exercise-induced build-up of extracellular K(+) is most likely larger in the T-tubules than in the interstitium but the significance of the cation shifts and Na(+)-K(+) pump for the excitability of the T-tubular membrane and the voltage sensors is largely unknown. Using mechanically skinned fibres, we here study the role of the Na(+)-K(+) pump in maintaining T-tubular function in fibres with reduced chemical K(+) gradient. The Na(+)-K(+) pump activity was manipulated by changing [Na(+)](i). The responsiveness of the T-tubules was evaluated from the excitation-induced force production of the fibres. Compared to control twitch force in fibres with a close to normal intracellular [K(+)] ([K(+)](i)), a reduction in [K(+)](i) to below 60 mM significantly reduced twitch force. Between 10 and 50 mM Na(+), the reduction in force depended on [Na(+)](i), the twitch force at 40 mM K(+) being 22 +/- 4 and 54 +/- 9% (of control force) at a [Na(+)](i) of 10 and 20 mM, respectively (n= 4). Double pulse stimulation of fibres at low [K(+)](i) showed that although elevated [Na(+)](i) increased the responsiveness to single action potentials, it reduced the capacity of the T-tubules to respond to high frequency stimulation. It is concluded that a reduction in the chemical gradient for K(+), as takes place during intensive exercise, may depress T-tubular function, but that a concomitant exercise-induced increase in [Na(+)](i) protects T-tubular function by stimulating the Na(+)-K(+) pump.
authors.affiliation: Department of Physiology, University of Aarhus, DK-8000 Arhus C, Denmark. obn@fi.au.dk
authors.forename: O B
authors.initials: OB
authors.lastname: Nielsen
authors.forename: N
authors.initials: N
authors.lastname: Ørtenblad
authors.forename: G D
authors.initials: GD
authors.lastname: Lamb
authors.forename: D G
authors.initials: DG
authors.lastname: Stephenson
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D004558:Electric Stimulation; D004563:Electrochemistry; D066298:In Vitro Techniques; D008297:Male; D008870:Microtubules / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D020318:Rats, Long-Evans; D012508:Sarcolemma / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D012996:Solutions
chemical_list: D012996:Solutions; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium