Effects of reduced electrochemical Na+ gradient on contractility in skeletal muscle: role of the Na+-K+ pump.
Continued excitation of skeletal muscle may induce a combination of a low extracellular Na+ concentration ([Na+]o) and a high extracellular K+ concentration ([K+]o) in the T-tubular lumen, which may contribute to fatigue. Here, we examine the role of the Na+-K+ pump in the maintenance of contractility in isolated rat soleus muscles when the Na+, K+ gradients have been altered. When [Na+]o is lowered to 25 mM by substituting Na+ with choline, tetanic force is decreased to 30% of the control level after 60 min. Subsequent stimulation of the Na+-K+ pump with insulin or catecholamines induces a decrease in [Na+]i and hyperpolarization. This is associated with a force recovery to 80-90% of the control level which can be abolished by ouabain. This force recovery depends on hyperpolarization and is correlated to the decrease in -Na+-i (r = 0. 93; P<0.001). The inhibitory effect of a low -Na+-o on force development is considerably potentiated by increasing [K+]o. Again, stimulation of the Na+-K+ pump leads to rapid force recovery. The Na+-K+ pump has a large potential for rapid compensation of the excitation-induced rundown of Na+, K+ gradients and contributes, via its electrogenic effect, to the membrane potential. We conclude that these actions of the Na+-K+ pump are essential for the maintenance of excitability and contractile force.
Overgaard
K
K
Department of Physiology, University of Aarhus, Ole Worms Alle 160, 8000 Arhus C, Denmark.
Nielsen
O B
OB
Clausen
T
T
D000818:Animals; D004563:Electrochemistry; D005110:Extracellular Space / Q000378:metabolism; D005260:Female; D008297:Male; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D009994:Osmolar Concentration; D011188:Potassium / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology*; D013268:Stimulation, Chemical
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium