Relation between extracellular [K+], membrane potential and contraction in rat soleus muscle: modulation by the Na+-K+ pump.
An increased extracellular K+ concentration ([K+]0) is thought to cause muscle fatigue. We studied the effects of increasing [K+]0 from 4mM to 8-14mM on tetanic contractions in isolated bundles of fibres and whole soleus muscles from the rat. Whereas there was little depression of force at a [K+]0 of 8-9mM, a further small increase in [K+]0 to 11-14mM resulted in a large reduction of force. Tetanus depression at 11mM [K+]0 was increased when using weaker stimulation pulses and decreased with stronger pulses. Whereas the tetanic force/resting membrane potential (EM) relation showed only moderate force depression with depolarization from -74 to -62mV, a large reduction of force occurred when EM fell to-53mV. The implications of these relations to fatigue are discussed. Partial inhibition of the Na+-K+ pump with ouabain (10(-6 )M) caused additional force loss at 11mM [K+]0. Salbutamol, insulin, or calcitonin gene-related peptide all stimulated the Na+-K+ pump in muscles exposed to 11mM [K+]0 and induced an average 26-33% recovery of tetanic force. When using stimulation pulses of 0.1ms, instead of the standard 1.0-ms pulses, force recovery with these agents was 41-44% which was significantly greater (P < 0.025). Only salbutamol caused any recovery of EM (1.3mV). The observations suggest that the increased Na+ concentration difference across the sarcolemma, following Na+-K+ pump stimulation, has an important role in restoring excitability and force.
Cairns
S P
SP
Institute of Physiology, University of Aarhus, DK-8000 Arhus-C, Denmark.
Flatman
J A
JA
Clausen
T
T
D000818:Animals; D004558:Electric Stimulation; D005110:Extracellular Space / Q000378:metabolism*; D006614:Hindlimb; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000187:drug effects; D018482:Muscle, Skeletal / Q000502:physiology*; D009994:Osmolar Concentration; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology*
D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium