K+-induced twitch potentiation is not due to longer action potential.
The objective of this study was to determine whether an increased duration of the action potential contributes to the K+-induced twitch potentiation at 37 degrees C. Twitch contractions were elicited by field stimulation, and action potentials were measured with conventional microelectrodes. For mouse extensor digitorum longus (EDL) muscle, twitch force was greater at 7-13 mM K+ than at 4.7 mM (control). For soleus muscle, twitch force potentiation was observed between 7 and 11 mM K+. Time to peak and half-relaxation time were not affected by the increase in extracellular K+ concentration in EDL muscle, whereas both parameters became significantly longer in soleus muscle. Decrease in overshoot and prolongation of the action potential duration observed at 9 and 11 mM K+ were mimicked when muscles were respectively exposed to 25 and 50 nM tetrodotoxin (TTX; used to partially block Na+ channels). Despite similar action potentials, twitch force was not potentiated by TTX. It is therefore suggested that the K+-induced potentiation of the twitch in EDL muscle is not due to a prolongation of the action potential and contraction time, whereas a longer contraction, especially the relaxation phase, may contribute to the potentiation in soleus muscle.
Yensen
Craig
C
Department of Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario, Canada K1H 8M5.
Matar
Wadih
W
Renaud
Jean-Marc
JM
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D005260:Female; D006614:Hindlimb; D066298:In Vitro Techniques; D051379:Mice; D008815:Mice, Inbred Strains; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000502:physiology*; D009994:Osmolar Concentration; D011188:Potassium / Q000494:pharmacology*; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology
D013779:Tetrodotoxin; D011188:Potassium