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1 Sport and Recreation, Auckland University of Technology, Auckland, New Zealand
2 Research Pharmacology, Pfizer Global R&D, Groton, Connecticut, United States
3 Cellular and Molecular Medicine, University of Ottawa, Ottawa, Canada
* To whom correspondence should be addressed. E-mail: simeon.cairns{at}aut.ac.nz.
We examined whether electric field stimulation with varying characteristics could excite muscle through different sites in isolated mammalian skeletal muscle. Stimulation with transverse wire or parallel plate electrodes using supramaximal pulses (20V, 0.1ms) evoked similar forces in non-fatigued slow-twitch soleus or fast-twitch extensor digitorum longus (EDL) muscles from mice. D tubocurarine shifted the twitch force-stimulation strength relationship towards higher pulse strengths with both electrode configurations in soleus, suggesting weaker pulses excite muscle via neuromuscular transmission. With wire stimulation a delay appeared between stimulus artifact and action potential peak when the recording electrode was moved along the muscle, consistent with action potential propagation along the sarcolemma. Tetrodotoxin (TTX) abolished all contractions evoked with 20V, 0.1ms pulses suggesting that excitation occurred via voltage dependent Na+channels and hence muscle action potentials. TTX did not prevent force development with pulses 
0.4ms in soleus or 1ms in EDL. Furthermore, myoplasmic calcium (i.e. fura 2 ratio) and sarcomere shortening were greater during tetanic stimulation with 2.0 than 0.5ms pulses in flexor digitorum brevis fibers from rat. TTX prevented all shortening and Ca2+ release with 0.5 but not 2.0ms pulses, indicating that longer pulses can directly trigger Ca2+ release. Hence proper interpretation of mechanistic studies requires precise understanding of how muscles are excited otherwise incorrect conclusions can be made. Using this new understanding we showed that disrupted propagation of action potentials along the surface membrane is a major cause of fatigue in soleus when focally and continuously stimulated at 125 Hz.
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