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Am J Physiol Cell Physiol (February 20, 2002). doi:10.1152/ajpcell.00549.2001
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Articles in PresS, published online ahead of print February 20, 2002
Am J Physiol Cell Physiol, 10.1152/ajpcell.00549.2001
Submitted on November 15, 2001
Accepted on February 15, 2002

The K+-induced twitch potentiation is not due to longer action potential

Craig Yensen1, Whadi Matar1, and Jean-Marc Renaud1*

1 Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario, Canada

* To whom correspondence should be addressed. E-mail: jmrenaud{at}uottawa.ca.

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°C. Twitch contractions were elicited by field stimulation and action potentials were measured with conventional microelectrodes. For mouse 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+. The time to peak and the 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. The 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 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.




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