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1 School of Community Health & Sport Studies, Auckland University of Technology, Auckland, New Zealand; Department of Physiology, University of Auckland, Auckland, New Zealand
2 Department of Physiology, University of Auckland, Auckland, New Zealand
3 Cellular and Molecular Medicine, University of Ottawa, Ottawa, Ontario, Canada
* To whom correspondence should be addressed. E-mail: simeon.cairns{at}aut.ac.nz.
We examined (i) whether the effects of lowered trans-sarcolemmal Na+ gradient on force differed between non-fatigued fast- and slow-twitch muscles of mice, and (ii) whether effects on action potentials could explain the decrease of force. The Na+ gradient was reduced by lowering the extracellular [Na+] ([Na+]o). The peak force - [Na+]o relationships for the twitch and tetanus were the same in non-fatigued extensor digitorum longus and soleus muscles: force was maintained over a large range of [Na+]o, and then decreased abruptly over a much smaller range. However, fatigue was significantly exacerbated at a lower [Na+]o that had little effect in non-fatigued soleus muscle. This suggests that substantial differences exist in the Na+ effect on force between non-fatigued and fatigued muscle. The reduced contractility in non-fatigued muscles at lowered [Na+]o was largely due to (i) increased number of inexcitable fibers and threshold for action potentials, (ii) reduction of action potential amplitude and (iii) a reduced capacity to generate action potentials throughout trains.
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